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 NO (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 NO 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 NO concentration. The -Fe plants showed a lower level of induction of NO uptake and hence a lower accumulation of the anion in the tissues, these effects being reversed by supply of Fe-WEHS. Supply of either NO- or NH-N (+/- Fe-WEHS) to -Fe plants promoted the development of the root Fe-chelate reductase activity, but the capacity of roots to take up the Fe remained unaffected. Results show that an inadequate Fe supply can limit the acquisition of NO, whereas NO supply can affect Fe uptake by influencing the development and maintenance of a high Fe-chelate reducing capacity.