Short-term interactions between nitrate and iron nutrition in cucumber

Cucumber (Cucumis sativus L.) plants were precultured for 7 days in either optimal (10 µm) or low (0.5 µm) Fe conditions and then grown for further 5 days in a N-free nutrient solution with (+Fe) or without (-Fe) 10 µm Fe. Thereafter NO₃^- (4 mm) was added to the nutrient solution for 24 h and, concomitantly, half of the -Fe plants were treated with 1 µm Fe complexed to water extractable humic substances (WEHS). Supply of NO₃^- to +Fe-N-deprived plants caused a large induction in the capacity to take up the anion by roots, which was accompanied by a rise in root-shoot NO₃^- concentration. The -Fe plants showed a lower level of induction of NO₃^- uptake and hence a lower accumulation of the anion in the tissues, these effects being reversed by supply of Fe-WEHS. Supply of either NO₃^-- or NH₄⁺-N (+/- Fe-WEHS) to -Fe plants promoted the development of the root Fe^III-chelate reductase activity, but the capacity of roots to take up the Fe²⁺ remained unaffected. Results show that an inadequate Fe supply can limit the acquisition of NO₃^-, whereas NO₃^- supply can affect Fe uptake by influencing the development and maintenance of a high Fe^III-chelate reducing capacity.