The regulation of starch accumulation in Panicum maximum Jacq, by nitrogen.

Electron microscopic studies of Panicum maximum (PCK C4 photosynthetic type) when grown under controlled growth conditions as low nutrient nitrogen (20 ppm N, KNO3) showed interesting second order bundle sheath cell organisation: the cells were several times bigger than the mesophyll cells and were packed with starch grains. Under higher nutrient nitrogen (200 ppm N, KNO3) the number of starch grains were drastically reduced, suggesting that at the higher nitrogen levels the fate of fixed carbon is different to that at the lower level of nitrogen supply. This suggests that nitrogen may regulate photosynthetic carbon assimilation after carboxylation. At low nitrogen the carbon is stored as starch at the site of CO2 fixation, the bundle sheath cells. It is argued that the nitrogen may play a role in translocation of metabolites to mesophyll cells, and thus decrease the starch level. These observations are discussed in relation to the photosynthetic characteristics of P. maximum.Keywords: accumulation; botany; carbon assimilation; co2 fixation; growth conditions; mesophyll; metabolites; nitrogen; nitrogen levels; nitrogen supply; panicum maximum; plant physiology; starch; translocatio

Intraspecifíc variability in <i>Alloteropsis semialata</i> (R. Br.) Hitchc.

Intraspecific variability in Alloteropsis semialata (R. Br.) Hitchc. is shown ultrastructurally with particular regard to the structure of the bundle sheath and the development of the dimorphic chloroplasts in both the C3 and C4 forms of this species. A. semialata is known to possess two distinct anatomical forms — Kranz and non-Kranz — within one species and occurring within a single ecological niche. Kranz and non-Kranz anatomy is known to be correlated with C3 and C4 physiology respectively. Transverse sections of leaf portions taken at midsheath, at the ligule and at midlamina show plastids with different morphologies at different ontogenetic stages. Plastid form is related to the stage of development, the influence of light on the emerging leaf and the C3 or C4 form of A. semialata. Stages from amyloplast to chloroplast are investigated with regard to fine structure. Leaf transverse sections are examined microscopically and formation of new bundles, chiefly in the lamina is traced. Differences in anatomy and distribution of vascular bundles are more evident in the lamina than in the colourless leaf sheath. Both C3 and C4 forms of A. semialata are found to show chloroplast dimorphism in vascular bundle sheath and mesophyll cells. This is shown to differ in the two forms. The specialized chloroplasts of the Kranz sheath are shown to develop in the inner or mestome sheath, and not in the parenchyma sheath as in some other members of the Kranz Panicoideae. Features of A. semialata,  such as the double bundle sheath, granal chloroplasts and large numbers of mitochondria in bundle sheath cells, in the C4 form, question the current classification of this grass as a malate former