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 photoinh ibitory reduction in the number of functional PS II reaction centers (n) an d a consequential decrease in the maximum quantum yield of photosynthesis. However, we found that light-saturated photosynthetic rates (P-max) in natu ral 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 increas es in electron turnover (1/tau(PSII)) through the remaining functional PS I I centers. Similar insensitivity of P-max was also observed in low light ad apted cultures of Thalassiosira weissflogii (a marine diatom), but not in h igh light adapted cells where P-max decreased in proportion to n. This diff erential sensitivity to decreases in n occurred because 1/tau(PSII) was clo se 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 decrease s in plant productivity are not necessarily commensurate with photoinhibiti on, but rather will only occur if decreases in n are sufficient to maximize 1/tau(PSII) or incident irradiance becomes subsaturating.