Diurnal Variation of Photosynthetic Efficiency in C_3 Plants
XU Da-Quan, XU Bao-Ji and SHEN Yun-Gang(Shanghai Institute of Plant Physiology, Academia Sinica, Shanghai 200032)
Leaf photosynthetic efficiency was measured by using ADC leaf chamber analyser (LCS) system in many plants. On clear days photosynthetic efficiency in many C_3 plants often decreased around noon under field conditions, displaying a significant diurnal variation (Fig. 1). Based on the experimental resuits carried out with soybean leaves, it is deduced that the main cause of the midday decline of photosynthetic efficiency in C_3 plants is likely photoinhibition rather than changes in air CO_2 concentration, stomatal conductance Table 1) or photorespiration. The following facts are in favour of the deduction: 1. the intercellular CO_2 concentration around noon was higher than that in the morning when measurements were made under weak light on a clear day (Table 1), and the midday decline was also observed when measured in saturating CO_2 and at the same temperature; 2. saturating light intensity of leaf photosynthesis was much lower than sunlight intensity around noon on clear days (Fig. 3); 3. preshading the leaves with cheesecloth for about 4 hours could increase photosynthetic efficiency of leaves as compared with that of control leaves exposed to full sunlight (Fig. 4); 4. on cloudy days the midday decline did not occur (Table 2).