CHANGES IN PHOTOSYNTHESIS AND WATER STATUS OF DEVELOPING LEAVES OF BRACHYSTEGIA-SPICIFORMIS BENTH

Changes in net carbon assimilation and water status were studied durin g leaf development in the deciduous, tropical species Brachystegia spi ciformis Benth. In this upland savanna African tree, bud-burst and lea f development occur approximately two months before the rainy season. The newly formed leaves synthesize anthocyanin until the fully expande d leaves of the whole canopy are red. This foliage is referred to as ' 'spring flush'' foliage. Subsequently, the anthocyanins are metabolize d and the pre-rain leaves become green. Carbon dioxide assimilation ex hibited a bimodal diurnal pattern and was similar for pre-rain green l eaves and fully expanded flushing leaves, although pre-rain green leav es showed a net uptake of carbon throughout the daylight period, where as flushing leaves exhibited only brief periods of net photosynthesis in the morning and early afternoon. Measurements of leaf water potenti al and relative water content showed a diurnal pattern with considerab le variation throughout the day. Leaf water potential and relative wat er content values decreased soon after sunrise reaching a minimum at a time corresponding to the afternoon peak in CO2 assimilation. Stomata l conductance was closely related to transpiration rate in both flushi ng and pre-rain green leaves, although flushing leaves had lower stoma tal conductances than pre-rain green leaves. Pre-rain green leaves exh ibited a compensation irradiance of approximately 180 mumol m-2s-1, wh ereas flushing leaves had positive net photosynthesis only at PPFDs gr eater than 300 mumol m-2s-1. Rate of photosynthesis (expressed per lea f area or chlorophyll unit) increased as anthocyanin concentration dec reased, although the photosynthetic rate continued to increase long af ter the leaf anthocyanins had been degraded to low, visually undetecta ble amounts. Post-rain green leaves had chlorophyll concentrations, tr anspiration rates and stomatal conductances similar to those of pre-ra in green leaves; however, photosynthetic rates in post-rain leaves wer e more than three times higher. Thus, during the early stages of the s pring flush, carbon asimilation rates of the flushing leaves were inve rsely related to leaf anthocyanin concentrations. In pre-rain green le aves, photosynthesis was limited by other non-stomatal factors.