Relationship Between Diurnal Variations of Photosynthetic Efficiency and Midday Depression of Photosynthetic Rate in Wheat Leaves Under Field Conditions

We have reported that, under field conditions, photosynthetic rate of wheat flag leaves displays significant midday depression on bright sunny days in early summer (XU et al. 1984), and that on clear days photosynthetic efficiency in many C_3 plants, including wheat, often decreases around noon (XU et al. 1990). Naturally, the question arises whether the decreased efficiency is an important cause of the midday depression of photosynthetic rate. To answer this question, we made observations on the diurnal variation in apparent quantum yield (AQY), net photosynthetic rate (P_n), and stomatal conductance (G_S) for the flag leaves of wheat plants grown in pots using a portable ADC gas analysis system during early grain-filling stage. Also, based on gas exchange data, we calculated the intercellular CO_2 concentration (C_i) and stomatal limiting value (L_S), analyzed the correlaton between P_n and G_S, P_n and C_i, and P_n and AQY.It was found that midday depression of photosynthetic rate was accompanied by decreases in stomatal conductance and intercellular CO_2 concentration and by an increase in stomatal lim- lting value (Figs. 1,2). The results confirm the deduction presented in a previous paper that midday closure of stomata is an important cause of midday depression of photosynthesis. Midday decline of photosynthetic efficiency was not accompanied invariably by midday depression of photosynthetic rate (Fig. 5). Intercellular CO_2 concentratin decreased when midday depression of both the efficiency and the rate occurred simultaneously (Fig. 2). Furthermore , significant levels of correlation between photosynthetic rate and stomatal conductance as well as intercellular CO_2 concentration were higher than the correlation between photosynthetic rate and photosynthetic efficiency. These facts suggest that midday decline of photosynthetic efficiency may be, if at all, a less important cause for midday depression of photosynthetic rate than midday closure of stomata, at least under the conditions reported here.Midday decline of photosynthetic efficiency may be related to photoinhibition (Figs. 4, 3). Its mechanism needs further study.