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. Ill 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 (Phi(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, Phi(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. (C) 2009 Published by Elsevier B.V.