Photosynthesis impairment in cassava leaves in response to nitrogen deficiency

Plants of cassava ( Manihot esculenta Crantz cv. Cigana Preta) grown in a sand root medium were watered with nutrient solutions containing either 3 mM nitrate (low N) or 12 mM nitrate (high N). Chlorophyll concentration, chlorophyll a/b ratio, stomatal conductance, photorespiration rate and net carbon assimilation rate ( on an area and a mass basis, but not on a chlorophyll basis) all decreased in low-N plants as compared with high-N ones. By contrast, photosynthetic nitrogen-use efficiency increased in low-N plants. As indicated by chlorophyll a fluorescence data, these plants exhibited increases in both excitation pressure on Photosystem II and thermal energy dissipation, with a corresponding decrease in quantum yield of electron transport, when contrasted with high-N plants. This decrease paralleled an unchanged maximal Photosystem II photochemical efficiency, suggesting a down-regulation of the Photosystem II photochemistry. It is proposed that decline in biochemical capacity for carboxylation, rather than stomatal limitation or electron transport, were the major constraints associated to the reduced photosynthetic rates induced by nitrogen deficiency in cassava plants.