Physiological adaptations for nitrogen use efficiency in sorghum[dagger]

Known high nitrogen utilization efficiency (NUE^sub 1^, biomass per unit plant N) China lines of sorghum, China 17 and San Chi San, were compared with relatively low NUE^sub 1^ 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 NUE^sub 1^ 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, NUE^sub 1^ China line, China 17, retained a significantly greater PEPcase activity than the less-NUE^sub 1^ U.S. lines, and also the NUE^sub 1^ 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 δ^sup 13^C 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.[PUBLICATION ABSTRACT]