The effect of phosphate nutrition of young rape plants on nitrate reductase activity and xylem exudation,and their relation to H ion efflux from the roots

Levels of nitrate reductase activity (N.R.A.) were measured in shoots and roots of P sufficient and P deficient rape plants and changes in N.R.A. examined in relation to the onset of H ion efflux from the roots. Rates of xylem exudation were measured and the sap analysed for nitrate, amino-N and phosphate content. The optimum concentration of phosphate in the leaves for N.R.A. was about 0.7%. Both high and low concentrations of phosphate within the leaves inhibited N.R.A in those leaves. This inhibition of N.R.A led to the accumulation of nitrate in the older parts of the shoots of P sufficient plants. Less accumulation of nitrate occurred in the P deficient plants since nitrate uptake by the plants decreased before any fall in N.R.A. Xylem exudation rates halved within 18 hours of depriving the plants of phosphate, and, since the composition of the sap remained constant, this indicated a reduced flux of nitrate into the xylem. The rate of xylem exudation continued to fall and by the end of the experiment was approximately one tenth of the rate in the P sufficient plants. The onset of H ion efflux from the terminal portions of the root preceded any effect on N.R.A by 2 days.