RESPONSES TO SHADING OF SEEDLINGS OF SAVANNA GRASSES (WITH DIFFERENT C4 PHOTOSYNTHETIC PATHWAYS) IN BOTSWANA

Different C4 photosynthesis pathways may influence plant response to s hade and/or moisture stress. In the savanna of south-eastern Botswana, seedlings of PCK C4 type grass species were frequently observed to em erge in shaded habitats. The species Digitaria eriantha subsp. pentzii , which shows the NADP-ME pathway, was also found to be associated wit h the tree canopy. Species emerging in habitats exposed to full sun of ten showed the NAD-ME C4 pathway. Seedlings of Aristida congesta, Erag rostis rigidior and Tragus berteronianus which normally emerge in full light, showed higher relative growth rates and/or unit leaf rates com pared 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 se edlings appeared to be negatively related to caryopsis weight. Light r esponse analysis of leaves of Digitaria eriantha subsp. pentzii, grown under low light intensities, showed similar trends in light saturatio n to those reported for the PCK type in Panicum maximum. Mesophyll rat her than stomatal limitations appeared to induce light saturation at h igher light intensities in leaves grown under shade conditions.