Abstract: To investigate the senescence among different position leaves at booting stage in rice plant, gas exchange and chlorophyll fluorescence properties for different leaf positions of six rice cultivars were examined. The net photosynthetic rate (P_n), stomatal conductance (g), transpiration rate (E) and water use efficiency (WUE) decreased significantly with lowering of leaf positions, while intercellular CO₂ concentration (C_i) had little change, which indicated that the decrease of photosynthetic rate did not result from the decrease of g. The decrease of SPAD reading which had a close correlation with chlorophyll content was one of the apparent reasons resulted in decease of P_n. Further evidence by chlorophyll fluorescence showed that the photochemical efficiency of PSⅡ and its electron transport rate (ETR) decreased substantially with lowering of the leaf positions but the variable-to-maximum fluorescence ratio (F _v/F _m) and efficiency of energy conversion of open PSⅡ (F′ _v/F′ _m) maintained rather stable, suggesting that primary limitation of P_n was suppressed by electron transport of PSⅠ, and probably associated with carbon assimilation.

Key words: rice, leaf position, chlorophyll fluorescence, photosynthesis, electron transport

摘要： 研究了6个水稻品种孕穗期不同叶位的气体交换和叶绿素荧光特性，结果表明净光合速率([i]P[/i][sub]n[/sub]）、气孔导度（[i]g[/i]）、蒸腾速率（[i]E[/i]）和水分利用效率（[i]WUE[/i]）均随叶位降低而明显下降；而胞间CO[sub]2[/sub]浓度（[i]C[/i][sub]i[/sub]）基本维持不变，说明叶片衰老时[i]P[/i][sub]n[/sub]的下降主要不是气孔因素引起的。叶绿素含量降低与[i]P[/i][sub]n[/sub]下降的表现一致，叶绿素荧光分析表明[i]F[/i][sub]v[/sub]/[i]F[/i][sub]m[/sub]和[i]F[/i]'[sub]v[/sub]/[i]F[/i]'[sub]m[/sub]没有随叶位发生变化，而PSⅡ的光化学效率[i]Ф[/i][sub]PSⅡ[/sub]和电子传递速率([i]ETR[/i]）随叶位降低明显下降，这表明可能是PSⅠ电子传递受阻导致CO[sub]2[/sub]同化下降。

关键词: 水稻, 叶位, 叶绿素荧光, 光合作用, 电子传递