Nitrate Nutrition of Grasses from Steady-State Supplies in Flowing Solution Culture following Nitrate Deprivation and/or Defoliation

Two Lolium genotypes, i.e. Lolium perenne cv. S23 and a L. perenne × L. multiflorum hybrid cv. Augusta were, in one treatment, grown in flowing solution culture with a maintained supply of $NO3-$ at 10 mmol m−3 $NO3-$ throughout. In another treatment, the plants were deprived of $NO3-$ for 11 d, $NO3-$ was then re-supplied and both control and N-deprived plants were either defoliated or left entire. Effects of the stresses imposed by N deprivation or defoliation on subsequent net uptake of $NO3-$, shoot and root growth and their interactions were examined. There were no major differences in the behaviour of the two genotypes throughout. The removal of NO3 supply did not alter total dry matter production during the 11 d period of deprivation but the rate of root growth accelerated markedly. There was little effect of N deprivation on the specific growth rates of the recovering defoliated shoots. Cumulative net uptake of $NO3-$ closely paralleled the pattern of growth and there were obvious and marked effects of both N deprivation and defoliation. Effects on $NO3-$ uptake were more distinctly demonstrated when uptake was expressed on a per unit root or shoot basis, i.e. specific unit absorption rates (SNO₃). In the control plants with sustained $NO3-$ supply, there was a progressive decline with time in SNO₃, in spite of an increasing shoot: root ratio. After both N deprivation and defoliation treatments there was a rapid initial increase in SNO₃, followed by an oscillatory cycle of increasing and decreasing rates with a 10–11 d period. The relationships between growth and $NO3-$ uptake were complex and did not conform to a previously predicted linear relationship between SNO₃, and the ratio total fresh plant weight: root fresh weight. The results are discussed in relation to the possible mechanisms involved. It is suggested that the oscillations in unit absorption rate indicated that the roots of N-deprived or defoliated plants retained a high capacity for absorption immediately after re-supply or defoliation and that the stressed plants were unable to assimilate or utilize $NO3-$ taken up; net uptake then decreased through a shift in favour of efflux over influx until the influx mechanism was able to dominate and the pattern was then repeated.