The effects of low, regulated supplies of nitrate and ammonium nitrogen on the growth and composition of perennial ryegrass

Summary

Perennial ryegrass was grown in flowing solution culture with nitrogen supplied in amounts that increased exponentially, i.e. in parallel with the rate of increase in growth. Nitrogen was supplied as either NO ^-₃ or NH ⁺₄ , and the amounts to be added were calculated on the basis of extrapolated values for dry weights obtained from fitted curves. There were two rates of addition for each form of N aimed at providing adequate (5.0 per cent) and less than adequate (2.75 per cent) contents in the plants in each case. Measured plant weights and N concentrations were in close agreement with predicted values over a four week experimental period. There was no effect of N-form at high N, and these plants produced 46 per cent more dry matter than the plants at low N. Only minor differences in overall growth occurred with NO ^-₃ or NH ⁺₄ plants at low N, but the NH ⁺₄ plants had a greater shoot:root ratio. The absorption rate (m mol N g^- root d^-1) for NH ⁺₄ -N was therefore greater than for NO ^-₃ -N. The cation/anion composition of the plants was affected in a predictable way, and to a greater or lesser extent at high or low N, respectively, in NO ^-₃ or NH ⁺₄ plants. The major changes in cation composition came through effects on potassium absorption. Plants with low NO ^-₃ appeared to be under greater N stress than those with low NH ⁺₄ because of the lower shoot:root ratio and the greater C:N ratio in the shoots.