Effects of growth under UVA radiation on CO2 assimilation, carbon partitioning, PSII photochemistry and resistance to UVB radiation in Brassica napus cv. Topas

The effects of growth of Brassica napus L. cv. Topas under PAR or PAR+UVA radiation was assessed with respect to sensitivity to subsequent exposure to UVB radiation. Despite the fact that growth under PAR+UVA induced minimal effects in photosystem II (PSII) photochemistry, growth under PAR+UVA inhibited the accumulation of'the photosynthetic end products, sucrose and starch. This was associated with a decreased capacity for ribulose 1,5-bisphosphate (RuBP) regeneration, a decreased capacity for light- and CO 2 -saturated rates of CO 2 assimilation, a decrease in the apparent quantum yield for CO 2 assimilation, an over-reduction of chloroplast stroma, an increased susceptibility to the feedback effects on photosynthesis and a stimulation of glycolysis compared to controls grown under PAR. Subsequent exposure to UVB decreased the maximum Rubisco activity in leaves of both PAR- and PAR+UVA-grown plants. However, the decrease in the capacity for CO 2 assimilation in PAR-grown plants exposed to UVB did not appear to be associated with limitations at the level of PSII linear electron transport, but rather with a decreased capacity for sucrose biosynthesis, limited triose-P utilization and a decreased capacity for RuBP regeneration. In contrast, the decreased capacity for CO 2 assimilation in PAR+UVA-grown plants exposed to UVB was associated with an inhibition of PSII photochemistry and a decreased supply of ATP. Thus, growth under UVA radiation appears to induce feedback-limited photosynthesis and does not enhance resistance of Brassica napus to UVB radiation.