田间密植诱导抽穗期玉米叶片衰老时的光合作用机制

为理解田间密植是否诱导抽穗期玉米叶片衰老以及衰老叶片的光合作用规律和机制,本研究以玉米"先玉335"为材料研究了抽穗期栽培密度对穗位叶和穗下第4叶的光环境、比叶重、氮素含量、叶绿素含量、气体交换以及叶绿素荧光诱导动力学的影响。结果表明,随着密度的增加玉米冠层内的光强大幅降低,尤其穗下第4叶;穗位叶和穗下第4叶的比叶重降低。同时,穗位叶和穗下第4叶的氮素含量和叶绿素含量均随栽培密度增加而下降。不同栽培密度下穗位叶荧光诱导动力学曲线(OJIP)未发生明显改变,而高密度下穗下第4叶OJIP的J和I相的相对荧光产量较低密度有提高趋势。高密度下,穗位叶和穗下第4叶叶片的光合速率、气孔导度和蒸腾速率均降低;不过,穗位叶胞间CO2浓度降低,穗下第4叶胞间CO2浓度增加。我们认为,田间密植条件下异质性光环境可以迅速诱导抽穗期玉米冠层下部叶片(如穗下第4叶)衰老;该过程中,光合作用的限制因素不是光能吸收和电子传递,而可能是碳同化。

Photosynthetic characteristics of senescent leaf induced by high planting density of maize at heading stage in the field

A field experiment was conducted using maize variety “Xianyu 335” at heading stage to study the influences of planting density (15,000 plants ha ^-1, 75,000 plants ha ^-1, and 135,000 plants ha ^-1) on canopy light intensity, specific leaf weight, nitrogen content, chlorophyll content, gas exchange, and chlorophyll a fluorescence induction kinetics of the ear leaf and the fourth leaf below ear. With increasing the density, the light intensity within the canopy significantly reduced, especially around the fourth leaf below ear, and the specific leaf weight and content of nitrogen and chlorophyll in the ear leaf and the fourth leaf below ear reduced. The fluorescence induction kinetics of the ear leaf revealed little changes under different planting densities. Compared with low density, the J and I phases of the fluorescence induction kinetics curves increased slightly in the fourth leaf below ear under high density. In addition, at high planting density, the photosynthetic rate, stomatal conductance and transpiration rate of the both leaves decreased. The intercellular CO₂ concentration decreased in the ear leaf, while increased in the fourth leaf below ear. Therefore, we believe that the heterogeneity of light environment can rapidly induce senescence in the lower leaves of the canopy (such as the fourth leaf below ear) at heading stage under high planting density in the field. In the process of leaf senescence within canopy, the limiting factor of photosynthesis is not light energy capture and electron transport, while may be the decrease of carbon assimilation.