Photons at the ultraviolet-visible interface : Effects on leaf expansion and photoinhibition

Summary

High energy ultraviolet-A (UV-A) and violet photons are at the edge of photosynthetically active radiation and may induce damage to sensitive metabolic pathways. The increasing use of light emitting diodes (LEDs) that provide these photons makes it important to better understand their effects on horticultural crops. We compared the effects of UV-A/violet photons to blue photons on short-term photosynthetic efficiency and long-term growth and morphological responses of lettuce ‘Rex’ and cucumber ‘Straight Eight’ under high and low photon flux densities (PFD). All plants received 67 % of the total PFD from white LEDs with the remaining 33 % of the photons from either blue (peak at 442 nm) or UV-A/violet LEDs (peak at 404 nm). Under a high PFD (500 μmol m⁻² s⁻¹; 16 hr photoperiod), the UV-A/violet photon treatment increased leaf area (by 24 %) and dry mass (by 17 %) in cucumber. In contrast, ‘Rex’ lettuce plants grown with UV-A/violet photons had significantly reduced chlorophyll content, leaf area, leaf photosynthetic rate, F_v/F_m ratio (indication of photoinhibition) and dry mass. The photoinhibition induced by UV-A/violet photons was reversible. Chlorophyll concentration of ‘Rex’ lettuce dramatically increased 2 days after being transferred from the UV-A/violet + white treatment to the blue + white light treatment, and plants regained full photosynthetic capacity after one week. The photoinhibition was also dose-dependent and did not occur when the photon flux density of UV-A/violet photons was reduced by 50 % at a lower daily light integral. We further evaluated the effects of UV-A/violet photons on three additional lettuce cultivars. Contrary to the detrimental effects in ‘Rex’ lettuce, UV-A/violet photons significantly increased leaf area and dry mass of ‘Marshall’ and tended to improve growth of ‘Parris Island’ and ‘Red Salad Bowl’ (although not statistically significant). Our analysis revealed that ‘Rex’ lettuce contained much lower levels of photoprotective pigments carotenoids and anthocyanins than other cultivars. We conclude that differences in photoprotective pigments are likely responsible for the contrasting responses to UV-A/violet photons.