Impact of High Irradiance on Tomato Photoreceptor Mutants at Different Red/Far-Red Light Ratios

When plants are constantly exposed to changing light conditions, the preceding spectral conditions influence their physiological parameters and stress sensitivity. Although the role of red (RL) and far-red light (FRL) in plant photomorphogenesis is well studied, the effects of preexposure to white LED light with specific RL/FRL ratios on subsequent plant resistance to high-intensity light (HIL) remain poorly understood. The effects of different RL/FRL ratios on 45-d-old Solanum lycopersicum under HIL were studied, with a special focus on the roles of the PHYB1, PHYB2 and CRY1 photoreceptors. Wild-type (WT) and mutant (phyB1, phyB2, cry1) plants were grown under three defined RL/FRL regimes (0.5; 1.2; 2.0) and without FRL (–FRL) and then exposed to HIL (1000 μmol photons/m2 s; 4 and 48 h). Photosynthetic activity, evaluated by PIABS, and Fv/Fm parameters reflecting PSII activity and photosynthesis rate (Pn), as well as stomatal conductance and the various pigment contents were assessed both before and after HIL exposure. The exposure of all the plants to HIL decreased both the Pn and PSII activity of the WT and the mutants, with the most noticeable decreases in these values in cry1 and phyB1 at all ratios after 4 h of irradiation and at lower ratios (1.2 and 0.5) after 48 h of irradiation, especially for phyB1. These findings indicate the new key role of cry1 and phyB1 photoreceptors in the response of the photosynthetic apparatus and antioxidant levels to HIL. The resistance of the WT photosynthetic apparatus to HIL was the highest at –FRL option and the lowest at predominance of FRL. In addition, a difference among spectral options in photosynthetic activity after HIL exposure was the lowest in the WT. Hence, the growth of WT plants without addition of FRL or at its low fraction confers the greatest HIL tolerance.