Date of Award

Fall 2014

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Horticulture and Landscape Architecture

First Advisor

Cary A. Mitchell

Committee Chair

Cary A. Mitchell

Committee Member 1

Roberto Lopez

Committee Member 2

Yiwei Jiang

Committee Member 3

Chieri Kubota


Intensive year-round local production of greenhouse-grown tomatoes (Solanum lycopersicum L.) requires the use of supplemental lighting (SL) to complement solar radiation in light-limited seasonal climates. However, SL represents a large expense to greenhouse-vegetable production. Currently, energy is second only to labor as the most expensive indirect cost of production. Thus, the greenhouse industry is interested in cost-effective, energy-efficient sources of supplemental photosynthetic light to sustain steady supplies of high-quality produce during the off-season. Overhead (OH) high-pressure sodium (HPS) lamps are considered the industry standard in greenhouse SL because of their capability to deliver adequate photosynthetically active radiation (PAR) to crops. However, HPS lamps are inefficient consumers of electrical energy with a high life-cycle cost, an intense environmental impact, and an orange-biased, blue-deficient emission spectrum. Light-emitting diodes (LEDs) offer an exciting opportunity to improve energy efficiency in greenhouse lighting because their relatively low surface temperature allows them to operate in close proximity to plant tissue without overheating or scorching plants, thereby increasing availablePAR at leaf level using less input power than HPS lamps. In addition, unlike traditional light sources used in commercial greenhouses today, LEDs are solid state, robust, long-lasting, and can be designed to emit narrow-band wavelengths that can be selected to maximize photosynthesis and growth for specific crops. ^ The goal of our research is to enable U.S. greenhouse growers to transition from HPS lighting to LED technologies for supplemental photosynthetic lighting. The specific objective of this research was to evaluate LEDs as alternative SL sources for greenhouse tomato propagation and production. Three research goals were established to support my objective: 1) to compare seasonal growth responses to three red:blue ratios of LED SL vs. HPS SL vs. ambient light for the propagation of six tomato cultivars; 2) quantify plant growth, yield, and energy consumption using intracanopy lighting (ICL) with LEDs (ICL-LED) or OH-HPS lamps as different SL sources and positions for high-wire greenhouse tomato production; 3) compare crop physiological responses to different SL sources and positions [ICL-LED vs. OH-HPS vs. hybrid lighting (ICL-LED + OH-HPS)] within an indeterminate high-wire tomato canopy. ^ Supplemental lighting increased hypocotyl diameter, epicotyl length, shoot dry weight, leaf number, and leaf expansion relative to control, whereas hypocotyl elongation decreased when SL was applied. For all cultivars tested, the combination of red and blue in SL typically increased growth of tomato seedlings. Our results indicate that blue light in SL has potential to increase overall seedling growth compared to blue-deficient LED SL treatments in overcast, variable-DLI climates. Further production studies showed that the ICL-LED technology supports similar growth and yield compared to OH-HPS but at lower electrical costs (from SL only). Additionally, we found that CO2 assimilation measured under ambient environmental conditions (A), photosynthetic quantum yield (&thetas;), maximum gross CO2 assimilation (Amax) and the light-saturation point of photosynthesis were good indicators of how ICL diminishes the top-to-bottom decline in photosynthetic activity that typically occurs with OH SL. However, we did not find any yield differences among SL treatments, indicating that higher source activity from ICL does not necessarily lead to yield increases. Based on the lower energy consumption measured for ICL-LED, and, to a lesser extent, for hybrid SL, compared to OH-HPS, we concluded that replacing OH-HPS lamps with ICL-LED or hybrid SL has great potential for energy savings during high-wire greenhouse tomato production. However, our results showed that higher total canopy photosynthesis did not lead to higher yields, most likely due to a redistribution of photoassimilate partitioning to non-harvested, vegetative plant parts.