[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.