Choinski J S, Johnson J M
Department of Biological Sciences, University of Zimbabwe, P.O. Box MP167, Mount Pleasant, Harare, Zimbabwe.
Tree Physiol. 1993 Jul;13(1):17-27. doi: 10.1093/treephys/13.1.17.
Changes in net carbon assimilation and water status were studied during leaf development in the deciduous, tropical species Brachystegia spiciformis Benth. In this upland savanna African tree, bud-burst and leaf development occur approximately two months before the rainy season. The newly formed leaves synthesize anthocyanin until the fully expanded leaves of the whole canopy are red. This foliage is referred to as "spring flush" foliage. Subsequently, the anthocyanins are metabolized and the pre-rain leaves become green. Carbon dioxide assimilation exhibited a bimodal diurnal pattern and was similar for pre-rain green leaves and fully expanded flushing leaves, although pre-rain green leaves showed a net uptake of carbon throughout the daylight period, whereas flushing leaves exhibited only brief periods of net photosynthesis in the morning and early afternoon. Measurements of leaf water potential and relative water content showed a diurnal pattern with considerable variation throughout the day. Leaf water potential and relative water content values decreased soon after sunrise reaching a minimum at a time corresponding to the afternoon peak in CO(2) assimilation. Stomatal conductance was closely related to transpiration rate in both flushing and pre-rain green leaves, although flushing leaves had lower stomatal conductances than pre-rain green leaves. Pre-rain green leaves exhibited a compensation irradiance of approximately 180 micro mol m(-2) s(-1), whereas flushing leaves had positive net photosynthesis only at PPFDs greater than 300 micro mol m(-2) s(-1). Rate of photosynthesis (expressed per leaf area or chlorophyll unit) increased as anthocyanin concentration decreased, although the photosynthetic rate continued to increase long after the leaf anthocyanins had been degraded to low, visually undetectable amounts. Post-rain green leaves had chlorophyll concentrations, transpiration rates and stomatal conductances similar to those of pre-rain green leaves; however, photosynthetic rates in post-rain leaves were more than three times higher. Thus, during the early stages of the spring flush, carbon asimilation rates of the flushing leaves were inversely related to leaf anthocyanin concentrations. In pre-rain green leaves, photosynthesis was limited by other non-stomatal factors.
对落叶热带树种短盖豆（Brachystegia spiciformis Benth.）叶片发育过程中的净碳同化和水分状况变化进行了研究。在这片非洲高地稀树草原的树木中，芽萌发和叶片发育大约在雨季前两个月发生。新形成的叶片会合成花青素，直到整个树冠完全展开的叶片都呈红色。这种 foliage 被称为“春季新梢” foliage。随后，花青素被代谢，雨前叶片变为绿色。二氧化碳同化呈现双峰日变化模式，雨前绿叶和完全展开的新梢叶片相似，尽管雨前绿叶在整个白天都表现出净碳吸收，而新梢叶片仅在上午和下午早些时候有短暂的净光合作用时期。叶片水势和相对含水量的测量显示出日变化模式，全天有相当大的变化。日出后不久，叶片水势和相对含水量值下降，在与二氧化碳同化下午峰值对应的时间达到最低值。气孔导度在新梢叶片和雨前绿叶中都与蒸腾速率密切相关，尽管新梢叶片的气孔导度低于雨前绿叶。雨前绿叶的补偿光照强度约为180微摩尔·米⁻²·秒⁻¹，而新梢叶片仅在光合光子通量密度大于300微摩尔·米⁻²·秒⁻¹时才有正的净光合作用。光合作用速率（以每叶面积或叶绿素单位表示）随着花青素浓度的降低而增加，尽管在叶片花青素降解到低水平、肉眼无法检测到之后，光合速率仍持续增加。雨后绿叶的叶绿素浓度、蒸腾速率和气孔导度与雨前绿叶相似；然而，雨后叶片的光合速率高出三倍多。因此，在春季新梢的早期阶段，新梢叶片的碳同化速率与叶片花青素浓度呈负相关。在雨前绿叶中，光合作用受到其他非气孔因素的限制。
