Nitrogen (N) is one of the major nutrients influencing photosynthesis and productivity of C 4 plants as well as C 3 plants. C 4 photosynthesis operates through close coordination between mesophyll (M) and bundle sheath (BS) cells. However, how the development of structural and physiological traits in leaves of C 4 plants is regulated under N limitation remains uncertain. We investigated structural and physiological responses of leaves of the NADP-ME-type C 4 grass Sorghum bicolor to N limitation. Plants were grown under four levels of N supply (.05 to .6 g N per 5-L pot). Decreasing N supply resulted in decreases in net photosynthetic rate, stomatal conductance, leaf N and chlorophyll contents, and the activity ratio of phosphoenolpyruvate carboxylase to ribulose 1,5-bisphosphate carboxylase/oxygenase and increases in δ 13 C values and photosynthetic N use efficiency. Low-N leaves were thinner and had smaller photosynthetic cells, especially in M, resulting in lower M/BS tissue area ratio, and contained smaller and fewer chloroplasts. The BS chloroplasts in the low-N leaves accumulated abundant starch grains. The number of thylakoids per granal stack was reduced in M chloroplasts but not in BS chloroplasts. The low-N leaves had thicker cell walls, especially in the BS cells, which might be associated with less negative δ 13 C values, and fewer plasmodesmata in the BS cells. These data reveal structural and physiological responses of C 4 plants to N limitation, most of which would be related to cellular N allocation, light use, CO 2 diffusion and leakiness, and metabolite transport under N limitation.
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