Aluminum effects on nitrate uptake and reduction in sorghum

The effects of Al on nitrate uptake and on the activity of the nitrate reductase (NR) in two hybrid cultivars of sorghum (Sorghum bicolor (L.) Moench) differing in Al tolerance were studied. The nitrate uptake by intact root system was strongly reduced by Al in both cultivars, but mainly in the Al‐sensitive cultivar. The kinetic constants also changed in the presence of Al: Vmax decreased 98% and 71% and Km increased 267% and 42% in the Al‐sensitive and Al‐tolerant cultivar, respectively. Aluminum reduced the in vitro NR activity on the roots and shoots of both cultivars, especially of the Al‐sensitive cultivar. Aluminum added to the nutrient solution or to the reaction mixture, however, inhibited differentially the NR of the roots and shoots, indicating marked differences between the enzymes from these two tissues. Aluminum reduced the Vmax but did not affect the Km of nitrate activation of the shoot NR. Therefore, Al inhibition of the NR was non‐competitive and could not be reversed by increasing nitrate concentration. Aluminum not only reduced the nitrate uptake but also had a direct effect on the NR and consequently on nitrate reduction. A correlation between NR tolerance to Al and plant tolerance to Al was observed.