Short‐term effects of Al on nutrient uptake, H+ efflux, root respiration and nitrate reductase activity of two sorghum genotypes differing in al‐susceptibility

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Abstract
To investigate whether effects of Al in the sorghum genotypes SCO283 (Al‐tolerant) and NB9040 (Al‐sensitive) are primary effects of Al toxicity, a short‐term experiment was carried out. After 21 days of pregrowth in an Al‐free nutrient solution, plants were exposed to nutrient solutions without or with 10 mg 1‐1 Al for 96 h. Nitrogen was supplied as NH4NO3. Twenty four h after Al supply, the relative uptake of all nutrient cations and anions, except NH4 +, declined to values varying between 0.8 (K+) and 0.2 (Ca2+, Mg2+). In contrast, the uptake of NH4 + was stimulated by Al in both sorghum lines. Total cation uptake and uptake of total nitrogen were balanced by the stimulation of NH4 + uptake. As a consequence, excess cation uptake and proton efflux were increased by Al, especially in the Al‐sensitive line, NB9040. Time relationships of nutrient uptake (except for NH4 +) and root respiration showed close resemblance, i.e. a decline within 24 h followed by a stable phase. This could indicate that effects of Al on nutrient uptake were energy‐linked. Increased NH4 + preference in the presence of Al was the result of stimulated NH4 + uptake and inhibited NO3 ‐ absorption. Decreased NO3 ‐ absorption was observed within 24 h after Al supply and resulted in reduced NO3 ‐xylem transport and lower shoot NRA. The results of this short‐term experiment confirm earlier findings of an Al‐induced increase in NH4 + preference as well as enhanced net proton efflux. Effects on nutrient uptake, root respiration and NRA may be primary Al effects since changes were observed 24 h after Al supply.