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 (P_n), stomatal conductance (G_s) 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 P_n and G_s in the afternoon, the ratio of intercellular CO₂ concentration (C_i) to atmospheric CO₂ concentration (C_a) 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 (F_m) and maximal photochemical efficiency of PSII (F_v/F_m) decreased continuously until it reached its minimum at 13:00 h, while the reverse occurred in the later afternoon. The quantum yield of PSII (Φ_PSII) declined after 09:00 h; in contrast, initial fluorescence (F_o) increased. A decrease in the rate of Q_A 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 F_v/F_m, Φ_PSII, qS and rate of Q_A 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.