Midday depression of photosynthesis is related with carboxylation efficiency decrease and D1 degradation in bayberry (Myrica rubra) plants

Diurnal patterns of photosynthesis in two-year-old Myrica rubra young trees under natural conditions were studied by measuring gas exchange, chlorophyll fluorescence parameters and D1 protein. When measured on a clear day, the diurnal changes of net photosynthetic rate (Pn), stomatal conductance (Gs) and apparent quantum yield (AQY) show two daily maxima; the maximum value occurred at about 09:00 h, then declined, and reached the lowest values at about 13:00 h then increased to reach the second maxima at about 15:00 h. However, with the consistent decline of Pn and Gs in the afternoon, the ratio of intercellular CO2 concentration (Ci) to atmospheric CO2 concentration (Ca) increased. In addition, carboxylation efficiency (CE) and RUBP regeneration declined as the afternoon progressed. In the morning, the maximum yield of fluorescence after dark adaptation (Fm) and maximal photochemical efficiency of PSII (Fv/Fm) decreased continuously until it reached its minimum at 13:00 h, while the reverse occurred in the later afternoon. The quantum yield of PSII (ΦPSII) declined after 09:00 h; in contrast, initial fluorescence (Fo) increased. A decrease in the rate of QA reduction and an increase in inactive PSII reaction centers was observed as the day progressed. Non-photochemical quenching (qN) and its slow-relaxing (qS) component increased at about midday, while the fast-relaxing component (qF) declined. The amount of inactive PSII centers was significantly enhanced, while Fv/Fm, ΦPSII, qS and rate of QA reduction were significantly reduced by DTT (dithiothreitol), an inhibitor of the xanthophyll cycle. The D1 protein was significantly degraded at 13:00 h relative to that at 09:00 h during the course of the day. These results suggest that stomatal and non-stomatal limitations, which decreased carboxylation and photochemical efficiency, may cause the midday depression; and that non-stomatal limitations may be due to the decrease in RuBPCase activity and degradation of D1 protein, which causes decreased photoprotection at midday.