Concurrent changes in net CO2 assimilation and chloroplast ultrastructure in nitrogen deficient citrus leaves

To elucidate a mechanistic basis for the reductions in chlorophyll (Chl) and inhibition of CO2 assimilation (ACO2) in nitrogen (N) starved citrus leaves, changes in Chl concentration and net gas exchange rates were examined together with chloroplast ultrastructure in N-deficient (125 mmol N m−2) and N-sufficient (286 mmol N m−2) citrus leaves. Two-year-old 'Hamlin' orange (Citrus sinensis L.) trees were field-grown with and without N. N-deficient leaves exhibited smaller chloroplasts (3.30 μm in length) with reduced Chl concentration per unit area (0.06 mmol N m−2), but a greater Chl a/b ratio (3.7) than the N-sufficient leaves. The reductions in Chl and ACO2 of N-deficient leaves paralleled with high intercellular CO2 concentration (Ci; 259 ppm) in the mesophyll and prodigious accumulation of starch granules (2.71 μm2) in the chloroplasts. Starch granules significantly reduced stroma size and caused disassembly and disruption of the internal membrane system, the grana and stroma lamellae. Concomitant with destruction of membrane assembly was the presence of numerous large plastoglobuli (15 in number). In contrast, ACO2 increased and Ci declined in N-sufficient leaves. Furthermore, chloroplasts from N-sufficient leaves had well-developed grana and stroma lamellae in parallel alignment with a few small or no starch granules in the large stroma. Thus, it appeared that the loss of chloroplast ultrastructural integrity brought about by starch accumulation facilitated reductions in Chl concentration and CO2 assimilation in N-deficient citrus leaves.