EFFECTS OF NITRATE WITHDRAWAL AND RESUPPLY ON THE ASSIMILATION OF NITRATE IN LEAVES OF TOMATO

Nitrate reduction in leaves of tomato occurred at the same rate in plants grown in 8.0 mol m-3 nitrate as in plants grown in 2.0 mol m-3 nitrate, but at a much slower rate in plants grown in 0.1 mol m-3 nitrate. However, the plants grown in 8.0 mol m-3 nitrate had a larger leaf system than the plants grown in 2.0 mol m-3 nitrate, and so the total capacity to assimilate nitrate was greater in the plants grown in the higher concentration. It was shown that plants grown in 8.0 mol m-3 nitrate were better buffered against nitrate withdrawal than plants grown in 2.0 mol m-3 nitrate as the rate of nitrate reduction declined more slowly when plants were transferred to 0.1 mol m-3 nitrate from the higher concentration than from the lower concentration. Furthermore, leaf expansion continued in the plants transferred from the higher concentration, whereas it ceased abruptly in the plants transferred from the lower concentration. It was concluded that both continuing expansion and continuing nitrate reduction were accompanied, and possibly caused by, a release of nitrate from storage pools in the lower part of the stem or the roots. During withdrawal of nitrate the leaves were shown to maintain potential activity of the enzyme nitrate reductase although there was no nitrate to be reduced. When nitrate was resupplied it could be reduced very quickly and reduction in the leaves was seen to increase within 5 h of resupply. By 3 d after resupply further enzyme activity had been induced.