Responses to shading of seedlings of savanna grasses (with different C4 photosynthetic pathways) in Botswana

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Abstract
Different C4 photosynthesis pathways may influence plant response to shade and/or moisture stress. In the savanna of south-eastern Botswana, seedlings of PCK C4 type grass species were frequently observed to emerge in shaded habitats. The species Digitaria eriantha subsp. pentzii, which shows the NADP-ME pathway, was also found to be associated with the tree canopy. Species emerging in habitats exposed to full sun often showed the NAD-ME C4 pathway. Seedlings or Aristida congesla, Eragrostis rigidior and Tragus berteronianus which normally emerge in full light, showed higher relative growth rates and/or unit leaf rates compared with seedlings of Panicum maximum and Urochloa panicoides which are associated with shaded habitats. Seedlings of the latter species, however, survived low light conditions. Relative growth rate of the seedlings appeared to be negatively related to caryopsis weight. Light response analysis of leaves of Digitaria eriantha subsp. pentzii, grown under low light intensities, showed similar trends in light saturation to those reported for the PCK type in Panicum maximum. Mesophyll rather than stomatal limitations appeared to induce light saturation at higher light intensities in leaves grown under shade conditions.