Structural and physiological responses of the C4 grass Sorghum bicolor to nitrogen limitation
Yutaro Makino | Osamu Ueno
Nitrogen (N) is one of the major nutrients influencing photosynthesis and productivity of C4 plants as well as C3 plants. C4 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 C4 plants is regulated under N limitation remains uncertain. We investigated structural and physiological responses of leaves of the NADP-ME-type C4 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 δ13C 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 δ13C values, and fewer plasmodesmata in the BS cells. These data reveal structural and physiological responses of C4 plants to N limitation, most of which would be related to cellular N allocation, light use, CO2 diffusion and leakiness, and metabolite transport under N limitation.
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