田间大豆叶片成长过程中的光合特性及光破坏防御机制

田间大豆叶片在成长进程中光饱和光合速率持续提高,但气孔导度的增加明显滞后.尽管叶片在成长初期就具有较高的最大光化学效率,但是仍略低于发育成熟的叶片.随着叶片的成长,光下叶片光系统Ⅱ实际效率增加;非光化学猝灭下降.幼叶叶黄素总量与叶绿素之比较高,随着叶面积的增加该比值下降,在光下,幼叶的脱环氧化程度较高.因此认为大豆叶片成长初期就能够有效地进行光化学调节;在叶片生长过程中依赖叶黄素循环的热耗散机制迅速建立起来有效抵御强光的破坏.

Photosynthetic Characteristics and Photoprotective Mechanisms During Leaf Development of Soybean Plants Grown in the Field

Gas exchange, chlorophyll a fluorescence and HPLC analysis were used to explore photosynthesis and dissipation of excited energy in soybean leaves from emergence to full development. During leaf development, both photosynthetic rate and stomata conductance increased gradually, whereas the increase in stomata conductance significantly lagged behind that of photosynthetic rate. Considering stomatal limiting value, it can be easily deduced that photosynthesis was considerably limited by low stomatal conductance during leaf development. Though the maximum quantum yield of photosystem II (PSII) photochemistry (F(v)/F(m)) was quite high at the stages of leaf development, it is appreciably lower than that in fully developed leaves. Reversible decrease in F(v)/F(m) occurred caused by high irradiance during daily courses at all stages of leaf development, indicating that no severe photoinhibition occurred when exposed to high light. Under high irradiance, a substantial increase in the actual PSII efficiency (Phi(PSII)) together with a marked decreased in non-photochemical quenching (NPQ) occurred during the process of leaf development. Compared with the values obtained at 9:00 AM, Phi(PSII) in developing leaves was drastically down-regulated after midday, whereas NPQ was enhanced significantly. Compared with fully expanded leaves, those developing leaves, with higher xanthophyll pool size, exhibited a much higher ratio of zeaxanthin (Z) + antheraxanthin (A) to Chl under irradiation. In addition, the relative xanthophyll pool size (V+A+Z)/Chl was found to decrease with leaf development. Our experiments revealed that the decline of xanthophyll cycle pool during leaf development was due to the fact that the chlorophyll content increased faster than the xanthophyll cycle pigment content. We suggest that as soon as leaf emerged, photoprotective mechanism was developed preferentially which can effectively protect leaves against excessive irradiance. Thermal dissipation depending on xanthophyll cycle is a very important photoprotective mechanism for dealing with high irradiance during leaf development.