© 1993 Heron Publishing—Victoria, Canada
Changes in photosynthesis and water status of developing leaves of Brachystegia spiciformis Benth.
J. S. Choinski, Jr. (1, 2) and J. M. Johnson (1, 3, 4)
1. Department of Biological Sciences, University of Zimbabwe, P.O. Box MP167, Mount Pleasant, Harare, Zimbabwe / 2. Department of Biology, University of Central Arkansas, Conway, AR 72032, USA / 3. Centre for Arid Zone Studies, University of Wales, Bangor, Gwynedd LL57 2UW, Wales, U.K. / 4. Formerly J. M. Tuohy / Received August 7, 1992
Summary
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 CO2 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 µmol m–2 s–1, whereas flushing leaves had positive net photosynthesis only at PPFDs greater than 300 µmol 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.
Keywords:
anthocyanins, carbon assimilation, chlorophyll, gas exchange, leaf development, water relations, Zimbabwe.
