Abstract

Involvement of photosynthetic capacity and D1 protein turnover in the susceptibility of photosystem II (PSII) to photoinhibition was investigated in leaves of Chenopodium album L. grown at different combinations of irradiance and nitrogen availability: low light and high nitrogen (LL-HN); high light and low nitrogen (HL-LN); and high light and high nitrogen (HL-HN). To test the importance of photosynthetic capacity in the susceptibility to photoinhibition, we adjusted growth conditions so that HL-HN plants had the highest photosynthetic capacity, while that of LL-HN and HL-LN plants was lower but similar to each other. Photoinhibition refers here to net inactivation of PSII determined by the balance between gross inactivation (photoinactivation) and concurrent recovery of PSII via D1 protein turnover. Leaves were illuminated both in the presence and absence of lincomycin, an inhibitor of chloroplast-encoded protein synthesis. Susceptibility to photoinhibition was much higher in plants grown in low light (LL-HN) than those grown in high light (HL-HN and HL-LN). Susceptibility to photoinhibition was similar in HL-LN and HL-HN plants, suggesting that higher photosynthetic energy consumption alone did not mitigate photoinhibition. Experiments with and without lincomycin showed that high-light-grown plants had a lower rate of photoinactivation and a higher rate of concurrent recovery, and that these rates were not influenced by nitrogen availability. These results indicate that turnover of D1 protein plays a crucial role in photoprotection in high-light-grown plants, irrespective of nitrogen availability. For low-nitrogen-grown plants, higher light energy dissipation by other mechanisms may have compensated for lower energy utilization by photosynthesis.