Photoresponse to different lighting strategies during red leaf lettuce growth

Abstract

The objective of the study was to investigate the effects of growth-stage specific lighting for the physiological homeostasis of red leaf lettuce (Lactuca sativa L. cv. Red Cos), by measuring the productivity of photosynthesis and primary metabolism. In the experiments, the main photosynthetic photon flux consisted of red (R) and blue (B) light, supplemented with blue, green (G) or UV-A wavelengths. Decrease of fructose, accompanied by significant decrease of stomatal conductance (gs), the ratio of intracellular to ambient CO₂ concentration (C_i/C_a), photosynthetic rate (Pr), light adapted actual quantum yield of PSII photochemistry (Φ_PSII), biomass formation and significant increase of transpiration rate (Tr) suggest that supplemental UV-A during maturity stage, after supplemental green irradiation during seedling stage (BRG to BRUV) was the least favourable condition for red leaf lettuce. However, constant irradiation with supplemental green (BRG) or supplemental green irradiation after increased blue exposure (B^↑R to BRG) resulted in significant increase of Pr, gs, Ci/Ca, and light use efficiency(LUE), and decrease of Tr and Water use efficiency (WUE). Significant increase of leaf area was observed under supplemental green in both seedlings (BR; BRG) and matured plants (B^↑R to BRG). Significant increase of specific leaf area was found under supplemental green (BRG) for seedlings and under increased blue (B^↑R) for matured plants. Accordingly, the most favourable growth-stage specific lighting spectrum strategy for red leaf lettuce, based on photosynthetic and primary metabolite response, is supplemental green irradiation after increased blue exposure (B^↑R to BRG), whereas, the most favourable condition for seedlings is BRG. According to the PCA correlation matrix, associations among the measured data indicate that WUE negatively correlated with gs and C_i/C_a, while LUE positively correlated with gs and Pr. However, weak correlations between Ф_PSII, LUE and photochemical reflectance index (PRI) suggest that selected light conditions were not optimal for red leaf lettuce.

Keywords: Lettuce; Light-emitting diode; Photochemical and non-photochemical efficiency; Photosynthesis.