Nitrogen budgets of fully expanded young leaves of Spinacia oleracea L. grown under three growth irradiances at four nitrate concentrations, were compared in relation to photosynthesis. The proportion of nitrogen allocated to thylakoid membranes was 24% of total leaf nitrogen irrespective of the growth conditions. The composition of the photosynthetic components in thylakoid membranes was affected by growth irradiance but unaffected by nitrogen levels. The proportion of total leaf nitrogen allocated to soluble protein and RuBP carboxylase (RuBPCase) increased with the increases in nitrogen and in irradiance levels. Some ultrastructural properties of chloroplasts and their intra-leaf gradients were also compared. The results suggest that nitrogen nutrition affects the amount of thyalkoids per unit leaf area but neither the properties of thyalkoids nor their intra-leaf gradient. Growth irradiance, however, controls both the properties and the amount of thylakoids. The ratio of in vitro RuBPCase activity to electron transport/photophosphorylation activity increased with the increase in nitrogen level, but decreased with the increase in growth irradiance. The change in the ratio of in vitro activities may serve to balance the in vivo activities, given that the in vivo efficiency of RuBPCase declines with the increase in volume of a chloroplast due to the increased liquid phase resistance to CO2 diffusion.
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Terashima et al. (1988) studied this question.