Abstract:

Diurnal variation of photosynthesis under natural conditions and shading treatment in summer in the leaves of field Tree Peony(Paeonia suffruticosa) was studied using the technology of gas exchange and analyzing chlorophyll fluorescence. The results indicated that the curve of diurnal variation of net photosynthetic rate (P_n) was demonstrated single peak in spring,two peaks in clear days in summer,with midday depression of photosynthesis. The apparent quantum yield (AQY) and the carboxylation efficiency (CE) displayed significant midday decline. The maximal PSⅡ efficiency (F_v/F_m) and maximal fluorescence (F_m) demonstrated reverse single peaks, which decreased obviously in the midday. Through shading treatment in summer, the curve of diurnal variation of P_n was demonstrated single peak. Photosynthetic active radiation (PAR) and limit of stomatal (L_s) decreased, but stomatal conductance (G_s) and the internal leaf CO₂ pressure (C_I) increased compared with those in sun-light. And the decreased amplitude of F_v/F_m and F_m declined obviously. Shading treatment hardly affected the F_o. These results showed that PSⅡ reaction center exhibited a dynamic change on revisable inactivation; PSⅡ function exhibited obviously down-regulation and photoinhibition under strong midday light in summer. Shading treatment in summer can improve photosynthetic function and accelerate products by abating photoinhibition.

Key words: Tree Peony(Paeonia suffruticosa), photosynthesis, photoinhibition, chlorophyll fluorescence parameters, shading treatment