Physiological adaptations for nitrogen use efficiency in sorghum†

Known high nitrogen utilization efficiency (NUE1, biomass per unit plant N) China lines of sorghum, China 17 and San Chi San, were compared with relatively low NUE1 U.S. lines, CK60 and Tx623, for both their physiological and biochemical adaptations to tolerate an imposed N stress in the greenhouse. Assimilation efficiency indices (ACi) were significantly greater for the China lines than the U.S. lines at both low and high soil nitrogen levels by about two-fold. Chlorophyll levels in leaves of high NUE1 lines were lower at both soil N treatments. Immunoblots of leaf extracts of sorghum subjected to N stress indicated reduced levels of both phosphoenolpyruvate carboxylase (PEPcase) and ribulose 1,5-bisphosphate carboxylase (Rubisco) while NADP-malic enzyme levels, in general, appear not to be affected. However, NUE1 China line, China 17, retained a significantly greater PEPcase activity than the less-NUE1 U.S. lines, and also the NUE1 China line San Chi San, when grown under N stress conditions. This suggests that PEPcase and enzymes associated with phosphoenolpyruvate synthesis, perhaps, are significant factors in maintaining relatively high photosynthesis under N stress. Carbon isotope ratios of leaves from sorghum genotypes, as indicated by δ13C values, became less negative when sorghum plants were grown under N stress, but a genotypic variation either at a low or high N was not observed.