Horticultural Reviews, Volume 43 E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Horticultural Reviews
• Sprache: Englisch

Seitenzahl: 775

Veröffentlichungsjahr: 2015

Beschreibung

Horticultural Reviews presents state-of-the-art reviews on topics in horticultural science and technology covering both basic and applied research. Topics covered include the horticulture of fruits, vegetables, nut crops, and ornamentals. These review articles, written by world authorities, bridge the gap between the specialized researcher and the broader community of horticultural scientists and teachers.

Leseprobe

CONTENTS

Cover

Series Page

Title Page

Copyright

Contributors

Dedication: Chad E. Finn

Chapter 1: Light-Emitting Diodes in Horticulture

Abbreviations

I. Introduction

II. Properties of LEDs

III. Design Considerations

IV. Historical Overview of LED Use in Horticulture

V. Summary of Plant Experiments in Space with LEDs

VI. Horticultural Applications of LEDs

VII. LED Lighting and Plant Health

VIII. LEDs and Light Pollution

IX. LED Light Distribution Issues

X. LED Environmental and Health Issues

XI. Adoption of LED Technology by Horticultural Industries

XII. The Future of Plant Applications for LEDs

Literature Cited

Chapter 2: Chayote: Pre-Columbian Origins and Dispersal

I. Introduction

II. Botany and Horticulture

III. Linguistic History of Chayote

IV. Metaphor and Ethnography

V. Mesoamerican Distribution of Chayote

VI. Archaeology and Iconography

VII. Folk Taxonomy

VIII. Conclusion

Acknowledgments

Literature Cited

Chapter 3: Ragwort: Invasive Weed and Potential Pharmaceutical

I. Introduction

II. Botany

III. Chemistry

IV. Control Measures

V. Commercial Potential

VI. Conclusions

Literature Cited

Chapter 4: Advances and Trends in Organic Fruit and Vegetable Farming Research

I. Principles, History, and Standards of Organic Horticulture

II. Global Expansion of Organic Fruit and Vegetable Production

III. Published Research on Organic Fruit and Vegetable Farming

IV. Benefits of Organic Horticulture

V. Constraint of Organic Horticulture

VI. Proposed Research Area for Organic Fruit and Vegetable Production

VII. Conclusion

Literature Cited

Chapter 5: Western Botanical Gardens: History and Evolution

I. Introduction

II. Ancient Roots

III. Hortus Conclusus: Precursor of the Hortus Botanicus

IV. The Rise of Botanical Studies

V. European Botanical Gardens

VI. Botanical Gardens in Great Britain

VII. Botanical Gardens in the United States

VIII. The Future of the Botanical Garden

Literature Cited

Chapter 6: The Future of Global Banana Production

I. Introduction

II. Origins, Taxonomy, and Production

III. Production, Trade, and Marketing

IV. Past and Current Threats to Production

V. Banana Diseases and Pests

VI. An Outlook for Future Production

Literature Cited

Subject Index

Cumulative Subject Index

Cumulative Contributor Index

End User License Agreement

List of Tables

Table 1.1

Table 2.1

Table 2.2

Table 2.3

Table 3.1

Table 3.2

Table 3.3

Table 3.4

Table 4.1

Table 4.2

Table 4.3

Table 4.4

Table 4.5

Table 4.6

Table 4.7

Table 4.8

Table 4.9

Table 4.10

Table 6.1

Table 6.2

Table 6.3

List of Illustrations

Fig. 1.1

Fig. 1.2

Fig. 1.3

Fig. 1.4

Plate 1.1

Fig. 1.5

Fig. 2.1

Fig. 2.2

Fig. 2.3

Fig. 2.4

Plate 2.1

Plate 2.2

Fig. 3.1

Fig. 3.2

Fig. 3.3

Fig. 3.4

Fig. 3.5

Fig. 4.1

Fig. 5.1

Guide

Cover

Table of Contents

Chapter 1

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Horticultural Reviews is sponsored by:

American Society for Horticultural Science

International Society for Horticultural Science

Editorial Board, Volume 43

Timothy K. Broschat

Martine Dorais

D. Michael Glenn

Horticultural Reviews

Volume 43

edited by

Jules Janick

Purdue University

A John Wiley & Sons, Inc. Publication

Copyright © 2015 by Wiley-Blackwell. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

ISBN 978-1-119-10775-0 (cloth)

ISSN 0163-7851

Contributors

Beatrix Alsanius, Department of Biosystems and Technology, Microbial Horticulture Laboratory, Swedish University of Agricultural Sciences, Alnarp, Sweden

Arend J. Both, Department of Environmental Sciences, Rutgers University, New Brunswick, NJ, USA

Christopher M. Bourget, Orbital Technologies Corporation, Madison, WI, USA

John F. Burr, Krannert School of Business Management, Purdue University, West Lafayette, IN, USA

John R. Clark, Department of Horticulture, University of Arkansas, Fayetteville, AR, USA

Christopher J. Currey, Department of Horticulture, Iowa State University, Ames, IA, USA

Martine Dorais, Agriculture and Agri-Food Canada, Horticulture Research and Development Centre, Laval University, Quebec City, Quebec, Canada

Michael P. Dzakovich, Department of Horticulture & Landscape Architecture, Purdue University, West Lafayette, IN, USA

Edward A. Evans, Tropical Research & Education Center, University of Florida, Homestead, FL, USA

Celina Gómez, Department of Horticulture & Landscape Architecture, Purdue University, West Lafayette, IN, USA

Richardo Hernández, School of Plant Sciences, The University of Arizona, Tucson, AZ, USA

Chieri Kubota, School of Plant Sciences, The University of Arizona, Tucson, AZ, USA

Sharon A. Lee, Sharon Lee and Associates, Swarthmore, PA, USA

Roberto Lopez, Department of Horticulture & Landscape Architecture, Purdue University, West Lafayette, IN, USA

Michelle McKeon-Bennett, CELLS Research Group, Shannon Applied Biotechnology Centre, Limerick Institute of Technology, Limerick, Ireland

Quingwu Meng, Department of Horticulture, Michigan State University, East Lansing, MI, USA

Cary A. Mitchell, Department of Horticulture & Landscape Architecture, Purdue University, West Lafayette, IN, USA

Fernando A. Moreira, University of Calgary, Calgary, Alberta, Canada

Robert C. Morrow, Orbital Technologies Corporation, Madison, WI, USA

Catherina M. O'Keeffe, CELLS Research Group, Shannon Applied Biotechnology Centre, Limerick Institute of Technology, Limerick, Ireland

Randy C. Ploetz, Tropical Research & Education Center, University of Florida, Homestead, FL, USA

Donald A. Rakow, Section of Horticulture, School of Integrative Plant Science, Cornell College of Agriculture and Life Sciences, Ithaca, NY, USA

Erik S. Runkle, Department of Horticulture, Michigan State University, East Lansing, MI, USA

Gary W. Stutte, CELLS Research Group, Shannon Applied Biotechnology Centre, Limerick Institute of Technology, Limerick, Ireland

Chad E. Finn

Dedication: Chad E. Finn

Volume 43 of Horticultural Reviews celebrates the exceptionally productive career of Dr. Chad Elliott Finn. One of the five kids of D. Francis “Mickey” and Gabrielle “Gay”, Chad grew up in Potomac, Maryland in the Washington, DC area. He spent much of his youth exploring the local creek, took over the family vegetable garden at age 10, and a couple of years later had a “eureka moment” when perusing the Burpee Seed Co. catalogue as he realized people could actually have a career developing new cultivars. He attained his B.S. degree in horticultural production at Purdue University (1983) where he was fortunate to be taken under the wings of Jules Janick, Frank Emerson, and Dick Hayden. During his summers away from Purdue, he received his first taste of berry research working under the guidance of renowned breeders Gene Galletta and Arlen Draper. He obtained M.S. (1986) and Ph.D. (1989) in horticulture with a minor in plant breeding from the University of Minnesota, where he was James Luby's first graduate student. His M.S. research focused on the inheritance of late bloom and early ripening in northern highbush, lowbush, and half-high blueberries. He was co-advised by soil scientist Carl Rosen for his Ph.D. thesis, and identified and characterized the response of Vaccinium species to varying pH levels and the interaction between pH level and nutrient uptake.

Chad's first career stop was at the University of Missouri where he served as State Fruit Extension Specialist from 1989 to 1993 working with home gardeners and commercial fruit growers. Growers Bob and Ronnie Hershey were introduced to Chad at a chilly blueberry field day and described him as the “redhead with an infectious smile” and recognized a young man enthusiastic about his job. They later visited the home of Chad and wife Barbara Fick and their sons Elliot and Ian, and marveled at the grapes, berries, fruit trees, and flowers reflecting their joint love affair with plants. Chad, sometimes referred to the “fruit geek,” is in reality one of the greatest small fruit breeders in the United States.

After his stint in Missouri, Dr. Finn took over the leadership of the USDA-ARS small fruit breeding program in Corvallis. This was a “dream job” for Chad, and he has lived his dream, developing what is probably the most diverse berry breeding program in the world with significant efforts in the major small fruit crops. Initially, he developed very active programs in germplasm and cultivar development for strawberries, blackberries, and red raspberries. More recently, in response to grower input, he added blueberries and black raspberries to his portfolio of breeding programs. Dr. Finn's germplasm development program is the largest and most productive of its kind in the world, extending from collection and evaluation of traits in wild species to incorporation of desirable traits into new cultivars. He has developed cooperative research with other breeders, other scientists for trait evaluations, commodity groups, and growers in the Pacific Northwest and throughout the world.

Dr. Finn led or co-led collection trips for germplasm in the Pacific Northwest, Ecuador, China, and the eastern United States and for Rubus and Vaccinium materials and incorporated new valuable traits into his breeding materials. He has cooperated with colleagues at various universities to evaluate Fragaria germplasm leading to a greater characterization of wild species and the discovery of new traits for cultivar development. The research on R. occidentalis from eastern North America has identified multiple sources of aphid resistance, Verticillium tolerance, and novel anthocyanin profiles, which he is now incorporating into cultivar material. The goal is to minimize the impact of Verticillium wilt and aphid transmitted viruses, which have reduced the productive life of black raspberry plantings in the Pacific Northwest to 2–3 years.

Dr. Finn has developed a multipronged approach in many of his breeding programs through collaborations with other scientists, including molecular biologists, food/flavor chemists, plant pathologists, virologists, horticulturists, and other breeders. In this way, he has been able to evaluate a wide range of traits and develop molecular markers for traits of interest in the berry crops. These efforts have been funded through the Specialty Crops Research Initiative (SCRI) grants program, with major efforts on strawberry through RosBREED, and a blueberry and two Rubus grants. He led a black raspberry SCRI grant, which was funded based on the preliminary work he and his student did on germplasm evaluations.

Dr. Finn has released or co-released (with USDA, Agriculture and Agri-Food Canada, Washington State University, and University of Arkansas) 37 new cultivars including 11 trailing, 1 semi-erect, and 2 primocane-fruiting, erect blackberries, 8 red raspberries, 11 strawberries, and 4 blueberries, as well as 2 germplasm releases. Among the most important of his many cultivar releases are five thornless blackberries, and ‘Black Diamond’ has been the most widely planted blackberry in the Pacific Northwest in recent years. The latest, ‘Columbia Star’, is anticipated to be as good as or better than ‘Marion’, the processing industry standard. Fruit sales from cultivars that Dr. Finn has released were greater than $120 million over the past 5 years.

Dr. Finn has authored or co-authored 162 scientific papers, 6 patents/patent applications, 30 book chapters, 34 extension publications, 89 proceedings, and over 85 abstracts as well as given over 180 invited presentations. Dr. Finn has obtained, with teams, over $14 million in competitive grants with over $3.75 million going to his program. The innovativeness and impact of Dr. Finn's research program have been recognized by the scientific community and small fruit industries as demonstrated by his election as Fellow in the American Society for Horticultural Science; Distinguished Alumni Award from the Department of Horticulture, Purdue University; a USDA-ARS Technology Transfer award; Wilder Medal by the American Pomological Society; and numerous international and domestic requests received for information, invitations to discuss his research programs, successful grant proposals, and requests to assist in development and evaluation of plant materials from other breeding programs. He is a courtesy professor in the Department of Horticulture at Oregon State University and has supervised/mentored 6 M.S. and 2 Ph.D. students and has served on 15 graduate student committees.

Dr. Finn is recognized internationally as a leading authority on small fruit crops, especially in the areas of breeding, germplasm, and cultivar performance as well as in production and processing. He has hosted visiting scientists from Argentina, Australia, Canada, Chile, China, Ecuador, France, Germany, Greece, Italy, Japan, Korea, Mexico, the Netherlands, New Zealand, Pakistan, Poland, Portugal, Russia, Serbia, Scotland, Serbia, South Africa, Spain, Sweden, Turkey, Ukraine, and United Kingdom, as well as U.S. scientists from more than 30 states. Dr. Finn is also active in the American Society for Horticultural Science (ASHS) and the International Society for Horticultural Science (ISHS), serving in Working Groups and on the Scientific Committees for the publication of the Acta Horticulturae for Rubus and Ribes, Vaccinium, and strawberry symposia since 2001. He has given invited keynote addresses at the Rubus and Ribes (2001, 2005) and Vaccinium (2012) Symposia of ISHS and was co-convener of the ISHS Vaccinium Symposium held in Corvallis in 2008 and the ISHS Berry Fruit Symposium held in Brisbane, Australia. He has been invited to present his research results in Argentina, Brazil, Canada, Chile, China, Italy, Mexico, the Netherlands, Scotland, United Kingdom, Uruguay, and at multiple universities, grower's meetings, and ASHS meetings in the United States. Dr. Finn has been involved in the Small Fruit Crop Germplasm Committee for the USDA-ARS National Plant Germplasm System since 1993. He is a member of the American Pomological Society, having served on advisory committees from 1996 to 1998 and on the Executive Board since 2011. He served as co-editor for the American Pomological Society/ASHS Fruit and Nut Cultivar List for 2005–2012 and as registrar/contributor for the List for blackberry and hybrid berry (1999–2014) and strawberry (1999–2009). He also served as co-editor for the Journal of Berry Research.

One of the Chad's closest colleagues at Oregon State University, Dr. Bernadine Strik, offers the following tribute: “Chad is the kind of colleague you dream about—one who is very passionate about his job, hardworking, giving, fair, innovative, and productive. He does his job with a sense of humor and no matter what the circumstances, he makes people feel at ease; he is a great speaker—knowledgeable, humorous, and animated; he is extremely well respected by peers and industry nationally and internationally. I couldn't imagine a better collaborator and friend.”

Fellow USDA-ARS researcher and Research Leader (Chad's boss) for the Horticultural Crops Research Laboratory at Corvallis, Dr. Robert Martin, comments: “Chad is a bright, optimistic, jovial, helpful, enthusiastic colleague and friend. We have worked together on many projects and he is a great collaborator in every respect. Although unlike Bernadine, I don't dream about him.” Dr. Martin and Chad share enological enthusiasm along with their annual vintage of “Bottled Optimism.”

Another longtime colleague, Jim Hancock at Michigan State University, shared his relationship with Chad: “I have worked closely with Chad for probably 25 years on a wide array of projects involving small fruit genetics, and seen him in action with growers, marketers, and scientists. Simply stated, he is a tremendous joy to work with and is the consummate professional. He is thoughtful, caring, articulate, thorough, willing, dependable, productive, and a particularly fine human being. The small fruit community is a much better place because of his accomplishments and warm, giving personality.”

Dr. Finn has had a worldwide impact. Professor Bruno Mezzetti of Universitá Politecnica delle Marche, Ancona, Italy shared that he can “surely confirm his recognition as an international leader in the field of genetic and breeding studies applied to all major berries.” Dr. Mezzetti further comments “For his personality, friendship, and sincerity, I consider Chad one of the greatest colleagues and collaborators ever, and hope to have more cooperation in the future.”

Dr. Rex M. Brennan of the James Hutton Institute, Invergowrie, Scotland is another admirer of Dr. Finn. “I've known Chad for more than 20 years, and have the greatest admiration for him and his achievements in fruit breeding. He has a tremendous depth of knowledge about germplasm and breeding, and can both apply and communicate these things in an inspirational way. I know he takes very seriously the mentoring of younger people starting out in the fruit breeding world. Chad is so generous with his knowledge and with his time—not just with people like me but with absolutely everyone. He treats everyone with the same generosity of spirit. And as I am sure everyone will say, he's a really great guy to spend time with, in person or even just on email!”

From Chile, Dr. Jorge Retamales with the University of Talca: “Chad and I have interacted ‘fruitfully’ for more than two decades on small fruit crop physiology and breeding. The most outstanding characteristics of Chad are his wisdom and happiness…he has taught us that the joy is not only found in reaching the target, or the finish line (i.e., published paper or registered cultivar), but throughout his life Chad has shown us that the process of doing research in horticulture can be (and should be) exciting and a joy. I feel blessed for having Chad as a colleague and friend. He has been a light that shines very brightly and leads us to excellent and enjoyable science (as well as life in general).”

Dr. Chad Finn has achieved an immense amount in his career, a result of his love for his family, friends, colleagues, and the plants he cherishes. All who come in contact with Chad Finn recognize that he is special. He continues to inspire horticulturists everywhere.

John R. Clark

Department of Horticulture

University of Arkansas

Fayetteville, AR, USA

1Light-Emitting Diodes in Horticulture

Cary A. Mitchell, Michael P. Dzakovich, Celina Gomez, and Roberto Lopez

Department of Horticulture & Landscape Architecture Purdue University West Lafayette, IN USA

John F. Burr

Krannert School of Business Management Purdue University West Lafayette, IN USA

Richardo Hernández and Chieri Kubota

School of Plant Sciences The University of Arizona Tucson, AZ USA

Christopher J. Currey

Department of Horticulture Iowa State University Ames, IA USA

Quingwu Meng and Erik S. Runkle

Department of Horticulture Michigan State University East Lansing, MI USA

Christopher M. Bourget and Robert C. Morrow

Orbital Technologies Corporation Madison, WI USA

Arend J. Both

Department of Environmental Sciences Rutgers University New Brunswick, NJ USA

Abstract

Light-emitting diodes (LEDs) have great potential to revolutionize lighting technology for the commercial horticulture industry. Unique LED properties of selectable, narrow-spectrum emissions, long life spans, cool photon-emitting surfaces, and rapidly improving energy use efficiency encourage novel lighting architectures and applications with promising profitability potential. In greenhouses, such unique properties can be leveraged for precise control of flowering and product quality for the floriculture industry, for energy-efficient propagation of ornamental and vegetable transplants, and for supplemental lighting of high-wire greenhouse vegetable crops for all-year production. In a sole-source lighting mode, LEDs can also be used for transplant production, as well as for production of rapid-turning vegetable and small fruit crops. Evidence is accumulating that nutritional and health attributes of horticultural products may be enhanced by specific wavelength combinations of narrow-spectrum light from LEDs. During periods of seasonally limited solar light, LEDs have potential to enhance daily light integral in greenhouses by providing supplemental photosynthetic radiation, particularly of red and blue light. The cool photon-emitting surfaces of LEDs permit their novel placement relative to crop foliar canopies, including close-canopy overhead lighting as well as within-canopy lighting, which greatly reduces electrical energy requirements while maintaining adequate incident photon fluxes. Because of the small size of individual LEDs and narrow beam angles from LED arrays, light distribution can be highly targeted and waste of light from LEDs minimized compared with other light sources traditionally used for horticulture. Prescriptions of spectral blends (e.g., red:far-red and red:blue ratios) can be developed for LEDs to accomplish specific photomorphogenic goals for seedling development, flowering, and possibly yield and produce quality. LED light quality may also be useful to control pest insects and to avoid physiological disorders otherwise caused by low-intensity or narrow-spectrum lighting. Complex factors such as rapidly improving LED luminous efficacy, favorable mass-manufacturing costs, local costs of electrical energy, and capital investment will interact to determine for which applications and when LEDs become the dominant lighting technology in horticulture.

KEYWORDS: energy savings; greenhouse; intracanopy; light quality; night interruption; photomorphogenesis; photoperiod; propagation; sole-source lighting; solid-state lighting; supplemental lighting

Abbreviations

I. Introduction

II. Properties of LEDs

A. What Are LEDs?

B. LEDs as a Horticultural Lighting System

C. LED Packaging

D. Wavebands of Interest

E. Performance Trends and Outlook

F. Misconceptions About LED Lighting

III. Design Considerations

A. General Design Requirements

B. Thermal Management

C. Control

1. Warm-Up and Restrike Times

2. “Smart” Control Systems

D. LED Lighting Systems

1. Intracanopy Lighting

2. Overhead Point Source

3. Overhead Distributed Source

E. Strategies for Maximizing Life and Maintaining Output

IV. Historical Overview of LED Use in Horticulture

V. Summary of Plant Experiments in Space with LEDs

VI. Horticultural Applications of LEDs

A. Providing Photosynthetic Light for Young Ornamental Plants

1. Introduction

2. Supplemental Lighting

3. Sole-Source Lighting

B. Photoperiodic Lighting with LEDs

1. Historical Background

2. Red and Far-Red Light

3. Blue Light

4. Green Light

5. Growth Response Parameters

6. Comparison of LEDs with Traditional Light Sources

C. Propagation of Vegetable Transplants Under LED Lighting

1. Introduction and Brief History

2. Improving Transplant Morphology with LED Lighting

3. Improving Transplant Photosynthesis and Growth

4. Considerations in Evaluating Electric Lighting for Greenhouses

5. LEDs for Sole-Source Lighting of Vegetable Transplants

D. LED Applications for Indoor Crop Production

1. Full-Coverage Sole-Source Lighting

2. Targeted Close-Canopy Lighting

E. LED Applications for Greenhouse Vegetable Crop Production

1. Current Standard

2. Sole-Source Lighting Pretreatments

3. Supplemental Lighting

4. Current Status and Challenges

F. The Potential of LEDs to Enhance Produce Quality

1. Strawberry

2. Salad and Microgreens

3. Tomato

4. Postharvest

5. Summary

VII. LED Lighting and Plant Health

A. Physiological Disorders

B. Insect Pests

VIII. LEDs and Light Pollution

A. Control of Spectral Output

B. High Light Intensity

C. High-Resolution Control

IX. LED Light Distribution Issues

X. LED Environmental and Health Issues

A. Disposal

B. Optical Safety for LEDs

XI. Adoption of LED Technology by Horticultural Industries

A. Economics

B. Evolution of Design and Industry

XII. The Future of Plant Applications for LEDs

A. Improvements in Technology

B. Improved Use of Light to Achieve Specific Horticultural Goals

Literature Cited

Abbreviations

ABRS

Advanced Biological Research System

AC

Alternating current

ASHS

American Society for Horticultural Science

B

Blue

BF

Blue fluorescent

CEWG

Controlled Environments Working Group

DC

Direct current

DE

Day extension

DIF

Day temperature − night temperature

DLC

Dynamic lighting control

DLI

Daily light integral

DOE

Department of Energy

DPPH

2,2-Diphenyl-1-picrylhydrazyl

EOD

End of day

ESD

Electrostatic discharge

FL

Fluorescent

FR

Far-red

G

Green

HID

High-intensity discharge

HPS

High-pressure sodium

HR

Hyper-red

IC

Integrated circuit

ICL

Intracanopy lighting

INC

Incandescent

ISS

International Space Station

kWh

Kilowatt hour

LD

Long day

LDP

Long-day plant

LED

Light-emitting diode

lm

Lumen

MH

Metal halide

NASA

National Aeronautics and Space Administration

NBL

Narrowband lighting

NCERA-101

North-Central Extension and Research Activity-101

NI

Night interruption

OH

Overhead

PAR

Photosynthetically active radiation

PBB

Polybrominated biphenyl

PBDE

Polybrominated diphenyl ether

P

FR

Far-red-absorbing form of phytochrome

PPF

Photosynthetic photon flux

P

R

Red-absorbing form of phytochrome

PS

Photosynthesis

PWM

Pulse-width modulation

QI

Quality index

R

Red

RDM

Root dry mass

RWB

Red + white + blue

SD

Short day

SDP

Short-day plant

SL

Supplemental lighting

SPAD

Relative chlorophyll content

SSBRP

Space Station Biological Research Program

UV

Ultraviolet

VOC

Volatile organic compound

W

Watt

WF

White fluorescent

I. Introduction

Horticultural lighting long has borrowed technology from the lighting industry that was not originally designed or intended for plant growth and development. As a consequence, horticulturists and plant physiologists learned to “make do” with the range of lamps that were available for supplemental or sole-source lighting of horticultural crops. Incandescent lamps became the standard for photoperiod control in greenhouses (Downs et al. 1958). Fluorescent (FL) ± incandescent (INC) lamps were widely used to achieve “normal” plant growth and development in growth chambers (Biran and Kofranek 1976; Bickford 1979), and when high-intensity discharge (HID) lamps came along, they quickly became the standard for supplemental lighting (SL) in greenhouses and for sole-source lighting in phytotrons and some growth chambers (Warrington et al. 1978; Tibbitts et al. 1983). All of these light sources do the job, but also have serious limitations. At the time they were adopted, there were no good alternatives. Incandescent lamps are highly wasteful of energy, are very short-lived (Bickford and Dunn 1972), and are rapidly disappearing from the marketplace. Fluorescent lamps have limited photon output and a short effective life span (Sager and McFarlane 1997). High-intensity discharge lamps require high voltage, emit intense radiant heat (McCree 1984), and require wide spatial separation from plants and/or thermal barriers. Light-emitting diodes (LEDs) were first tested with plants more than 20 years ago (Bula et al. 1991; Barta et al. 1992), and a revolution in lighting technology for horticulture has been underway ever since. This chapter compiled by a multi-institutional team of researchers investigating the feasibility of adopting LED technology for commercial specialty crop production (Mitchell et al. 2012) summarizes the state of knowledge regarding LED technology for horticulture and plant responses to various spectral combinations of LED lighting as of 2015.

II. Properties of LEDs

A. What Are LEDs?

An LED is a light source that, unlike traditional lamps, does not use a filament or gas discharge. Illumination is produced solely by movement of electrons in a semiconductor material (Held 2009). Electrons cross a semiconductor junction and recombine with electron holes, releasing energy as photons (electroluminescence) in a narrow waveband. The color of a specific LED is determined by the energy gap of the semiconductor used, which is based on the semiconductor chemical composition.

LEDs are available in a variety of wavebands ranging from the ultraviolet (UV)-C (about 250 nm) to the near-infrared range (about 1,000 nm), with half-peak bandwidths generally ranging from 25 to 50 nm. Broad-spectrum white LEDs are also available—these create white light by using a blue (400–500 nm) LED combined with a phosphor coating. LEDs can also be used to create white light by mixing appropriate amounts of light from individual red (600–700 nm), green (500–600 nm), and blue LEDs.

Unlike traditional lamps, LEDs do not radiate heat directly in the light beam. However, a significant amount of heat is still produced and this heat must be conducted out of the device to prevent premature failure. Modern, high-power LEDs have a thermal pad directly connected to the light-emitting (and heat-generating) substrate. This pad moves heat from the junction to the solder point, through the circuit board, and to the heat sink by conduction, and then from the heat sink to the environment by convection and radiation.

B. LEDs as a Horticultural Lighting System

Solid-state lighting using narrow-waveband LEDs represents a fundamentally different technology from the broad-spectrum gaseous discharge-type lamps currently used in horticulture (Sager and McFarlane 1997). The semiconductor nature of LEDs makes them potentially one of the most significant advances in horticultural lighting since the development of HID lamps (Morrow 2008). The specific advantages of LEDs include capability to control spectral output and light intensity and to provide high or low light levels. Because LEDs can be rapidly turned on and off, and easily incorporated into electronic circuits, they can respond to complex control protocols. LEDs also provide the potential for reducing lighting operational costs through their long operating life and ability to operate directly adjacent to plant tissues due to their low radiant heat output (thereby reducing power use). Light-emitting diodes lack glass envelopes and toxic materials such as mercury, have low touch temperatures, and generally are operated at low direct current (DC) voltages, making them safer than current lamp types. Other benefits include their thin cross section, rugged construction, and flexibility for assembly into lighting systems with specialized configurations. Their use of DC would be an advantage in a setting using DC power generated from alternative power systems such as batteries or solar panels.

Disadvantages of LEDs compared with existing lamp types include currently high hardware costs. Since LEDs operate most effectively using DC, implementation requires conversion of standard alternating current (AC) to DC (using AC-to-DC power converters).

C. LED Packaging

Light-emitting diode lighting systems are generally used as groupings of many individual LED devices, each device being approximately 2 mm × 3 mm in size. The device includes the actual LED semiconductor chip, a lens, and components to provide mechanical support and transfer of heat away from the chip. Components are included to allow integration of the LED into an electronic circuit (Fig. 1.1).

Fig. 1.1 Cross section of example LED package, about 2 mm × 3 mm in size. This package is designed to be soldered to a circuit board. The ceramic substrate provides a means to remove heat from the LED chip through the thermal pad to the circuit board. The transient voltage suppressor protects against electrostatic discharge and the silicone lens shapes light and shields the chip.

D. Wavebands of Interest

Several wavebands of interest to horticulturists are available in LEDs (Olle and Virsile 2013). Commonly available red wavebands include 627 and 660 nm, whose spectra are close to the maximum chlorophyll absorption peak. Red light of 660 nm also matches a phytochrome absorption peak, as does 735 nm. Ultraviolet and blue wavebands, including 365, 400, 450, and 470 nm, are absorbed by cryptochrome pigments, which also impact plant development and physiological functions. Green wavebands (i.e., 540 nm) may have some utility due to improved foliar penetration increasing canopy photosynthesis. Other colors are used for specialized functions such as providing excitation for visualization of fluorescing proteins. In addition, several phosphors are available that can be used with blue LEDs to provide broader spectrum light in a variety of colors (Mills 2004), which is the primary technique used to produce white LEDs.

E. Performance Trends and Outlook

Light-emitting diode technology (both for research and for general area lighting in homes and businesses) has improved significantly in terms of physical shapes and designs, number of color wavebands available, reduced power use per unit light output, higher light output per unit power input, and reduced cost per unit light output (Morkoc and Mohammad 1995; Norlux Corporation 2004; Philips Lumileds Lighting Company 2004, 2005, 2006, 2007, 2008a, 2011, 2012/2013). The technical development of LEDs is said to follow Haitz's law, named after Dr. Roland Haitz, who states that every decade the cost per unit of useful light emitted for a given waveband of light falls by a factor of 10 and the amount of light generated per LED package increases by a factor of 20 (Haitz et al. 1999; Haitz and Tsao 2011). LED lighting applications may ultimately be limited by market forces (e.g., achievable light levels are already in excess of what is needed to meet large commercial market requirements).

F. Misconceptions About LED Lighting

With the great interest in LED lighting systems, a number of misconceptions about their capabilities have become commonplace. One of the primary misconceptions is about LED inherent luminous efficacy. It is widely discussed how much more efficient LEDs are than currently used lamp types. Interestingly, this has been a common statement for many years, even when LED efficiency was actually substantially less than current sources (whose efficiency has also improved over the last several years), and it is only recently that some LED devices (e.g., blue LEDs) are approaching or exceeding the best of the fluorescent and high-intensity discharge lamps (Nelson and Bugbee 2014). Although LEDs are projected to exceed all other current lighting technologies in the next few years (DOE 2013a), it should be emphasized that the potential for large improvements in power efficiency in horticultural settings is not so much related to the LED semiconductor die composition per se, but to the fact that their solid-state nature and physical characteristics allow implementation of unique configurations and operating protocols that can bring about large efficiency gains (Fig. 1.2).

Fig. 1.2 Stacking different LED attributes illustrates the potential for developing LED horticultural lighting systems with very high operational efficiency (numbers are approximate and shown as additive for illustrative purposes). Refer to the text for detailed explanation for basis of graph.

Efficiency of a specific LED package (a single LED with mount) is related to factors such as semiconductor composition and doping, and mounting package configuration (DOE 2014). Efficacy of an LED package differs for each color, with blue LEDs being most efficient, while other colors, such as green, have room for improvement. Increases in photosynthetic efficiency by matching wavelengths to chlorophyll absorption peaks have been shown in some plant testing (Stutte et al. 2009). Luminaire configuration relates to reflector design, lenses, how the LEDs are arranged, and how the luminaire is positioned (operating in close proximity to plants can significantly reduce light loss because it is falling on walls and walkways rather than on plant tissue) (Morrow 2008; Nelson and Bugbee 2014). Control protocols can also be used to optimize energy consumption. For example, control protocols have been developed that detect the locations of plant tissue and only provide light to those locations (Massa et al. 2005b; Morrow and Bourget 2009).

Another common perception is that LED systems have an extensive operating life. While manufacturer's literature provides a conservative figure, often around 50,000 h of operation (DOE 2006), the actual life of the devices is dependent on a number of parameters when used in a real system. The output and operating life of an LED can be adversely impacted by high LED junction temperatures, poor current regulation, manufacturing quality (e.g., soldering quality), component quality, excessive shock and vibration (though LEDs are more resistant to shock and vibration damage than traditional lamp types), and the operating environment (humidity, cleanliness, etc.). Also, different color devices may degrade differently, and phosphors used with some LEDs may degrade faster than the LED die itself.

An LED lighting system is composed of a number of other electronic components (such as capacitors) that may have a shorter inherent life span than the LEDs themselves. Other components such as power supplies (which are complex devices with many electronic components), fans, connectors, and temperature or light sensors may also impact operating life; therefore, the effective operating life of an LED lighting system is based on interaction between an array of operating conditions and individual hardware characteristics. Based on experience, it is likely that LED devices themselves are not usually the life-limiting component in an integrated LED lighting system.

One other major misconception is that “LEDs don't generate heat.” Light-emitting diodes certainly do generate heat, just in a different fashion from currently used lamps. While INC and HID lamps lose heat primarily as radiant heat, and FL lamps through radiation and convection, LEDs lose heat primarily through conduction (Table 1.1). Because LEDs produce very little radiant heat, they will not significantly heat plant tissue and can thus be operated in close proximity to the plant surface, allowing target photon fluxes to be achieved using much less operating power compared with traditional lighting devices. Nevertheless, LED systems need to be designed with effective conduction/convection-based cooling systems.

Table 1.1 Cooling mechanism for different lamp types.

Source: http://www.ledtransformations.com/Lightfair_5-28-08.pdf (accessed September 3, 2014).

Heat loss (%)

Source

Efficacy (lm W

−1

)

Radiation

Convection

Conduction

Incandescent

15

90

5

5

Fluorescent

90

40

40

20

HID

100

90

5

5

III. Design Considerations

A. General Design Requirements

A total system approach to LED lighting design is necessary; this includes the type and arrangement of LED devices used, mechanical packaging, thermal management, power and control electronics, and optical considerations. Light-emitting diode systems should take advantage of LED solid-state features including rapid on/off capabilities, easy dimming, low radiant heat output, ruggedness, long life, and small size allowing for easy integration into a wide variety of shapes, designs, and forms.

B. Thermal Management

To operate an LED system at high light outputs, heat generated at the backplane must be effectively managed to ensure long life and high performance, especially with high-power LEDs (≥1 W). Cooling mechanisms of LED systems all use direct conduction from the LED to the LED mounting surface, and then to a heat sink, which then dissipates the heat by mechanisms such as natural convection, forced convection (using fans), or liquid cooling.

C. Control

The light output of an LED is proportional to the amount of electrical current flowing through it. Therefore, LEDs should be operated in a current-controlled manner; that is, the driver circuit is actively controlling current flow through the LED rather than the voltage across it. This allows very precise control over the range of LED light intensity. Two common methods of control include linear and pulse-width modulation (PWM). Linear dimming simply reduces the DC current flowing through the LED: as the current is lowered, the LED gets dimmer. However, some LEDs exhibit a wavelength shift of up to 10 nm as the current changes, so a different strategy is often used (Gu et al. 2006). In PWM dimming, the LEDs are pulsed at a constant frequency (typically a few hundred hertz). When the LED is on, it is always at the same current level. To dim the LED, the percentage of time that the pulse is “on” (duty cycle) is reduced, thereby reducing the total photon output. The human eye cannot discern the rapid pulses, so it appears that the LED is dimming.

1. Warm-Up and Restrike Times

LEDs can switch on and off instantaneously, essentially having no warm-up and restrike times. This provides a high degree of flexibility in control of a lighting system, allowing rapid, precise changes in light levels. Conversely, gas-discharge lamps like FL lamps require about 3 min to reach at least 80% of full light output (warm-up time), and HID sources require 3–7 min to reach full brightness and several minutes to cool down before they can be switched on again.

2. “Smart” Control Systems

Control systems can range from simple manual adjustments of intensity to complex computerized controls. Light-emitting diodes are ideal for use with sensor feedback. For example, it is easy to integrate ambient light sensors to control LEDs for supplemental daily light integral (DLI) control systems that measure the amount of light received by a crop and then make up any deficit in lighting by applying SL at a precise level and duration (Seginer et al. 2006; Torres and Lopez 2010). Another example is the use of LEDs to enable an adaptive control system to prevent the unproductive practice of providing lighting over areas where there are no plants. Traditional electric overhead lighting illuminates significant “empty space” both above the crop height and between plants before the crop canopy fills in. To increase productivity per unit area or per volume of crop space, some approaches have changed plant spacing over time. This approach requires either extensive automation or labor, and the opposite approach, of changing the position of lights rather than that of plants (Wheeler et al. 1992), previously has been difficult to do with hot, bright HID lamps. Coupling precision LED lighting arrays with plant detection techniques (e.g., reflectance or imaging) can eliminate loss of light by illuminating space occupied only by photosynthetic tissue (Morrow and Bourget 2009). This strategy has the potential to increase energy use efficiency significantly. Intelligent controls can also compensate for reduced LED output as they age, and automatically compensate for failed LEDs or driver circuitry. Future developments in solid-state lighting will enable other new techniques to improve overall plant production efficiency, or improve marketable properties.

D. LED Lighting Systems

There are essentially three physical LED configurations used for horticultural applications, each with variations. These configurations can be applied either as sole-source lighting or as supplemental or photoperiodic lighting.

1. Intracanopy Lighting

Intracanopy lighting (ICL) consists of a linear LED luminaire that is placed within a plant canopy either as a sole source of light or as a source of light supplementing solar or overhead electric lighting in a greenhouse or similar protected agriculture, controlled-environment setting. Planophile crop stands, within which leaves present themselves perpendicular to overhead light, eventually close off their inner canopy to light resulting in mutual shading of lower leaves by those above, which in turn leads to net carbon loss via respiration, premature leaf drop, and often flower bud and fruit abortion within the canopy. Intracanopy lighting can provide light distribution throughout the canopy of a crop, allowing a much greater percentage of available leaf surface to be utilized for photosynthesis. This should increase biomass output per unit of input energy (Massa et al. 2006). Light-emitting diodes provide an opportunity for horticulturalists to utilize ICL protocols because of their low radiant heat output, ease of cooling, small volume, and high light output capabilities.

Intracanopy lighting can be implemented in either vertical or horizontal configurations. Clear benefits of one configuration over the other have not yet been determined. Vertical orientation results in a reduced chance of entanglement with plant tissue and facilitates the use of adaptable lighting approaches (i.e., providing light only where it is needed). Horizontal orientation avoids interference with watering systems and may require less hardware. The total linear length of lighting required is thought to be similar between the two configurations. Applications of vertical and horizontal ICL are included in Section VI.E.3.

2. Overhead Point Source

Overhead point source lighting consists of a light fixture with a tight grouping of LED devices that provides light output in a cone pattern meant to cover a broad growing area. This configuration most closely matches HID lamps in form and can be used as sole-source lighting, SL, or for photoperiod lighting. When using LEDs, overhead point source lighting is effective for photoperiodic lighting, but for applications requiring higher light output it fails to take advantage of the unique properties of LEDs (e.g., low radiant heat output) and can result in significant light loss due to scattering. In addition, it can be difficult to remove heat efficiently from a dense grouping of LEDs, reducing device efficiency and operating life.

3. Overhead Distributed Source

This is essentially the opposite of a point source. In this configuration, the LEDs are distributed over a broad area and provide very diffuse irradiance. This configuration can be applied as the sole lighting source in growth chambers and growth rooms, or as SL in a greenhouse setting. Diffuse overhead systems better utilize the inherent advantages of LEDs than do point sources. For example, their low radiant heat output allows them to be operated in close proximity to plants while retaining a uniform light level across the growing area. A point source moved close to the plant tissue would not provide uniform lighting. A broad array of LEDs also facilitates the use of precision lighting techniques, where various segments of the array can be set to different colors and turned on or off independently.

E. Strategies for Maximizing Life and Maintaining Output

For LEDs, as with other lamp types, careful design, manufacturing, and operation are required to maximize lamp life and to minimize decline of light output. A factor critical to maximizing lamp life and light output is keeping the temperature of the LEDs per se as low as possible (Keeping 2011). To accomplish this, it is necessary to use heat sinks attached to the back of the LEDs (Comerford 2011). The heat sinks can be cooled passively through natural air convection, by forced convection, or by using other cooling fluids such as chilled water (conduction). Passive cooling techniques tend to be insufficient except at very low light levels. Liquid-cooled systems can be unwieldy and expensive to maintain, but are highly effective and provide a mechanism to capture heat from the lamps for other uses. Forced convection is currently the most common cooling technique used, although fans add to the cost of the light fixture. An integrated temperature sensor can be used to monitor the temperature of an LED luminaire and cut off power if the temperature becomes too high, preventing damage to the LEDs and other electronics in case the cooling system fails. Another technique that can be used to maintain lower operating temperatures is adding more LEDs to a luminaire than are required to meet light output goals, and then operate these devices at a lower power. This reduces thermal degradation of the devices, improving electrical efficiency and extending life span. The primary disadvantage is the additional component cost. For SL fixtures in greenhouse applications, the use of surfaces that reflect solar (heat) irradiance can also help reduce LED operating temperatures.

Light-emitting diode luminaire design is critical. The fixture should provide protection from physical abrasion or impact and from moisture (including condensation), which can cause corrosion. It is also important to design the fixtures with good circuit protection to prevent propagation of electronic failures as LEDs are at higher risk than traditional lamps for damage from electrostatic discharge (ESD) and electrical transients caused by power supply problems (Publitek 2013). In addition, temperature cycling and vibration need to be accounted for, as both can damage wire bonds. Light fixture components should be made accessible as LED systems are relatively straightforward to repair, which offers the potential to greatly extend operating life without LED fixture replacement.

As discussed earlier, LEDs themselves may not be the life-limiting component in an LED lighting system, where components such as connectors, integrated circuit (IC) chips, op-amps, voltage regulators, driver chips, resistors, and capacitors may also be life limiting. Electronic components are generally quite reliable when circuitry is properly designed (proper grounding paths, circuit protection, etc.). Connectors, however, can be consistent failure points, if they are exposed to harsh environments, such as might be encountered in a greenhouse. Cabling is another weak point, as any external cables are susceptible to harsh environments, along with pulling, grabbing, and physical damage. Components of the cooling systems may also have a more limited lifetime than the LEDs, particularly fans, although cooling pumps and valves (used in fluid-cooled systems) may also fail before electronic components do. Most of these items are parts of the active cooling systems needed to keep LEDs within their proper operating temperature range, but should be easily replaceable.

IV. Historical Overview of LED Use in Horticulture

Testing of LEDs for plant growth applications in the United States was concomitant with the development of the first terrestrial LED arrays in the late 1980s and space-based arrays in the early 1990s. An overview of this early development history is discussed by Morrow (2008). The first work with LEDs for plant lighting used red LED (peak wavelength 660 nm) arrays to produce a light intensity adequate for plant growth. These arrays were made of individually lensed devices often referred to as discrete LEDs. Over time, as new LED chip technologies became available, LED modules were developed using high-density chip-on-board or “surface-mount” designs. Early surface-mount LED modules, often referred to as light engines, might contain hundreds of low-to-moderate-output LED chips in a variety of colors (Emmerich et al. 2004). This technology was too expensive for large-scale use, but ideal for specialty or research applications that required high light output with several independently controllable spectral bands. In the late 1990s, high-output LEDs that could be manufactured in an automated process (Philips Lumileds Lighting Company 2008b) were developed, making the fabrication of solid-state lighting arrays of more than several square meters in area feasible.

The physical and operational flexibility of solid-state lighting has enabled the development of many alternative lighting configurations, such as ICL (Massa et al. 2005a,b, 2006), and new control protocols such as adaptive or “smart” lighting (Morrow and Bourget 2009). A timeline of some developmental milestones critical in the development of LED-based horticultural lighting systems is shown in Fig. 1.3.

Fig. 1.3 Timeline of developments impacting the use of LED lighting for horticultural applications.

Massa et al. (2008) provide a comprehensive historical overview of work related to growing plants under LEDs from the early 1990s through the mid-2000s. A review of five scientific horticulture journals showed that a sharp increase in the number of LED plant growth research papers occurred during and after that mid-2000s time frame (Fig. 1.4). Much recently published work is cited in other sections of this chapter.

Fig. 1.4 Frequency of LED plant lighting-related journal articles between 1985 and 2013. Journals examined include HortScience, HortTechnology, Journal of the American Society of Horticultural Science, Chronica Horticulturae, and Scientia Horticulturae.

V. Summary of Plant Experiments in Space with LEDs

The space program was the primary driver behind the development of the first LED plant lighting developments in the United States. Light-emitting diode systems were first used in microgravity spaceflight experiments in 1994 to support the growth of dwarf wheat (Triticum aestivum) and rapid-cycling Brassica in the Astroculture plant growth chamber (Astroculture 4 experiment; Morrow et al. 1995). They were also used in subsequent Astroculture and Advanced Astroculture experiments (Zhou 2005). Light-emitting diode prototype plant growth lighting hardware was developed and tested for the Plant Research Unit (Emmerich et al. 2004) that was to be part of the Space Station Biological Research Program (SSBRP) prior to that program's termination by NASA in 2005/2006. Another plant growth system called the Advanced Biological Research System (ABRS) was placed on the International Space Station (ISS) in 2009. The ABRS used LEDs for photosynthetic lighting and also for fluorescent protein imaging (Levine et al. 2009). As of 2014, the Vegetable Production System (Veggie), on board the ISS, and the Plant Habitat, being developed for ISS by Kennedy Space Center and ORBITEC, both utilize complex LED-based plant lighting systems (Massa et al. 2014; Morrow 2014). Terrestrial LED research in support of space research has been summarized in the general LEDs-in-horticulture history discussion.

VI. Horticultural Applications of LEDs

A. Providing Photosynthetic Light for Young Ornamental Plants

1. Introduction

Ornamental plants are commonly propagated either sexually from seeds or asexually from unrooted cuttings, or by micropropagation. Seedlings growing in plug trays are commonly propagated in greenhouses, though potential exists for production of plugs in more controlled environments similar to the “plant factories” used for vegetable transplant production. Production of rooted cuttings as liners typically occurs in a greenhouse, whereas tissue culture production is primarily performed in sole-source-lighted environments. High-intensity discharge lamps or LEDs may provide photosynthetic light as a supplement to sunlight in a greenhouse or as a sole source of light in a controlled environment for propagation of ornamental young plants. Here we review literature associated with the use of supplemental or sole-source lighting for sexual and asexual propagation of ornamental young plants in both greenhouse and more controlled environments.

2. Supplemental Lighting

Ornamental young plants typically are propagated from late winter to early spring in northern latitudes for spring sales. During this period, solar DLIs are at seasonally low levels (Korczynski et al. 2002). Light transmission into a greenhouse may also be reduced by 40–70% due to glazing materials, changing angles of solar incidence, cleanliness, infrastructure (i.e., lights, shade curtains, etc.), and hanging baskets (Hanan 1998; Lopez and Runkle 2008), further lowering DLI at canopy level.

Daily light integral affects growth and quality of seedlings, as well as subsequent development (Pramuk and Runkle 2005; Oh et al. 2010; Torres and Lopez 2011; Randall 2014). A high-quality seedling is one that is compact, fully rooted with a large stem caliper and high root dry mass (RDM). Compact seedlings with a large stem caliper and RDM are less likely to be damaged during shipping and transplant (Pramuk and Runkle 2005). Pramuk and Runkle (2005) reported that shoot dry mass per internode of celosia (Celosia argentea var. plumosa), bedding impatiens (Impatiens walleriana), salvia (Salvia splendens), marigold (Tagetes patula), and pansy (Viola × wittrockiana) grown under DLIs ranging from 4.1 to 14.2 mol m−2 day−1 increased by 47% (bedding impatiens) to 68% (pansy) as the DLI increased by 10.1 mol m−2 day−1. Similarly, Torres and Lopez (2011) reported that increasing DLI from 0.75 to 25.2 mol m−2 day−1 during seed propagation of tecoma (Tecoma stans) enhanced biomass accumulation of roots and shoots by 2,388%.

Cuttings also respond favorably to increasing DLI during propagation (Lopez and Runkle 2008; Currey et al. 2012). Lopez and Runkle (2008) reported that after 16 days in propagation, root mass of petunia (Petunia × hybrida) and New Guinea impatiens (Impatiens hawkeri) increased linearly as DLI increased from 1.2 to 8.4 mol m−2 day−1 and from 1.3 to 10.7 mol m−2 day−1, respectively. In addition, Currey et al. (2012) reported that increasing DLI during root development of argyranthemum (Argyranthemum frutescens), diascia (Diascia barberae), lantana (Lantana camara), nemesia (Nemesia fruticans), osteospermum (Osteospermum ecklonis), scaevola (Scaevola hybrid), bacopa (Sutera cordata), and verbena (Verbena × hybrida) cuttings in propagation from 1.2 to 12.3 mol m−2 day−1 increased root and shoot mass by 156–1,137% and 110–384%, respectively.

In a greenhouse environment, the only way to appreciably increase DLI is to provide SL. Currently, HPS lamps are the most frequently employed source of supplemental photosynthetic light for use inside greenhouses. However, LEDs are an emerging technology with application as supplemental sources of photosynthetic light in greenhouses (Plate 1.1a). The main selection criteria for SL are investment capital for the fixture, energy cost to power the light source, and spectral composition and irradiance capability of the SL source. Other potential opportunities as well as challenges associated with the use of overhead LED lighting are similar to those outlined in Sections II.C III.D.2 and III.D.3.

Plate 1.1