LED and HPS Supplementary Light Differentially Affect Gas Exchange in Tomato Leaves.

Onofrio Davide Palmitessa,Elias Kaiser,Aina E Prinzenberg,Ep Heuvelink

doi:10.3390/plants10040810

Abstract

Using light emitting diodes (LED) instead of conventionally used high pressure sodium (HPS) lamps as a supplemental light source in greenhouses results in a higher efficacy (µmol light per J electricity) and makes it possible to customize the light spectrum. To explore the effects of LED and HPS on gas exchange, thermal relations, photosynthesis, and water status of young tomato plants, seven genotypes were grown in a greenhouse under LED (95% red, 5% blue) or HPS lamps in four experiments differing in the fraction of lamp light over natural light. HPS lights emit a broader spectrum of red (40%), green–yellow (50%), blue (5%), and far-red (5%) and a substantial amount of infrared radiation (heat). Young tomato plants grown under LED showed lower leaf temperature and higher stomatal density, stomatal conductance (gs) and transpiration rate (E) than plants grown under HPS; this may be due to the different supplemental light spectrum. The young plants grown under LED tended to have increased photosynthetic capacity. Furthermore, the water stress indices CWSI and IG, which were obtained using thermal imaging, were positively correlated with gas exchange-derived gs and E, putting forward the use of thermal imaging for the phenotyping of transpiration. Under LED light, photosynthetic gas exchange was generally increased, which agreed with the water stress indices. The extent of this increase was genotype-dependent. All differences between LED and HPS were smaller in the experiments where the fraction of lamp light over natural light was smaller.

Highlights

In the northern hemisphere, tomato nurseries must meet the peak demand for transplants during a period when the daily light integral (DLI; mol photons m−2 s−1 ) of natural light is lowest [1]
Light emitting diodes (LED) have been investigated as a possible alternative for high-pressure sodium lamps (HPS) in greenhouses. Their efficacy is higher compared to HPS lamps [8], with less radiant heat output and a longer lifetime compared to HPS
The ratio of gs under light emitting diodes (LED) divided by gs under HPS across experiments showed no significant differences between genotypes (Table S1)

Summary

Introduction

Tomato nurseries must meet the peak demand for transplants during a period when the daily light integral (DLI; mol photons m−2 s−1 ) of natural light is lowest [1]. Light emitting diodes (LED) have been investigated as a possible alternative for HPS in greenhouses. Their efficacy (μmol per J) is higher compared to HPS lamps [8], with less radiant heat output and a longer lifetime compared to HPS. While LED fixtures dissipate a large part of heat through natural or forced convection, HPS lamps have a high operating temperature and emit longwave radiation (3000–100,000 nm) in the same direction as photosynthetically active radiation, i.e., towards the plants [9]. Apart from the light spectrum, plants will likely need to adapt differentially to the reduced heat radiation when grown under LED compared to HPS light. Nelson and Bugbee [9] found that

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