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; translocation