Compensatory changes in Photosystem II electron turnover rates protect photosynthesis from photoinhibition

Exposure of algae or higher plants to bright light can result in a photoinhibitory reduction in the number of functional PS II reaction centers (n) and a consequential decrease in the maximum quantum yield of photosynthesis. However, we found that light-saturated photosynthetic rates (P(max)) in natural phytoplankton assemblages sampled from the south Pacific ocean were not reduced despite photoinhibitory decreases in n of up to 52%. This striking insensitivity of P(max) to photoinhibition resulted from reciprocal increases in electron turnover (1/tau(PSII)) through the remaining functional PS II centers. Similar insensitivity of P(max) was also observed in low light adapted cultures of Thalassiosira weissflogii (a marine diatom), but not in high light adapted cells where P(max) decreased in proportion to n. This differential sensitivity to decreases in n occurred because 1/tau(PSII) was close to the maximum achievable rate in the high light adapted cells, whereas 1/tau(PSII) was initially low in the low light adapted cells and could thus increase in response to decreases in n. Our results indicate that decreases in plant productivity are not necessarily commensurate with photoinhibition, but rather will only occur if decreases in n are sufficient to maximize 1/tau(PSII) or incident irradiance becomes subsaturating.