C_3植物光合效率的日变化

多种田间C_3植物在晴天的光合效率常有明显的日变化,中午前后光合效率降低。C_3植物大豆叶片光合效率中午降低的主要原因,不是空气CO_2浓度和气孔导度及光呼吸的变化,而可能是光抑制。因为:1.在饱和CO_2中测定仍可观测到这种中午降低;2.光合作用的饱和光强远低于晴天中午的太阳光强;3.用纱布预遮阴可以提高叶片的光合效率;4.阴天时叶片光合效率不发生中午降低。

Diurnal Variation of Photosynthetic Efficiency in C_3 Plants

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).