LOSS OF PSBS EXPRESSION REDUCES VEGETATIVE GROWTH, REPRODUCTIVE OUTPUT, AND LIGHT-LIMITED, BUT NOT LIGHT-SATURATED, PHOTOSYNTHESIS IN ARABIDOPSIS THALIANA (BRASSICACEAE) GROWN IN TEMPERATE LIGHT ENVIRONMENTS

Plants protect themselves against the deleterious effects of high light intensities by inducing a mechanism ubiquitous among plants known as energy dissipation, which safely converts excess light to heat before it can lead to the formation of free radicals. Mutants possessing a deletion of the psbS gene, such as the npq4 mutant, cannot perform energy dissipation and thus offer an opportunity to assess the importance of this process to plant function. In a temperate light environment, greenhouse-grown npq4 mutants of Arabidopsis thaliana had smaller rosette diameters and leaf numbers. The reduction in size observed in npq4 plants was associated with fewer floral stalks, fewer fruits, lower whole-plant and individual seed masses, and lower germination rates. In the field, npq4 mutants developed fewer fruits. After a controlled exposure to high light stress, both PSII efficiency and CO2 assimilation were more significantly compromised in npq4 mutants at low light intensities, but not at high light intensities. Thus, the protective nature of energy dissipation manifests in light environments that include periods of high light, which predispose plants to PSII photoinactivation, and periods of low light, when PSII photoinactivation decreases the rate of photosynthesis.