Root Hairs and Plant Growth Atlow Nitrogen Availabilities

S ummary Four grass species, Holcus lanatus L., Deschampsia ftexuosa (L.) Trin., Poa annua L. and Lolium perenne L. cv. S.23 and cv. ‘Norlea’ were grown for 28 d in continuous flow nutrient solution containing nitrogen (N) at one of three concentrations (0.02, 0.21 or 4.0 mol m −3 ) as nitrate or ammonium. Growth, N uptake and root hair production were measured. Root hair growth in D. flexuosa and P. annua was the most responsive to low N availability. These responses were generally greater to ammonium than nitrate. The possible effects of the absence of root hairs on growth were tested via empirical relationships between relative growth rates and N inflow rates to estimate the contributions made to growth by root hairs. Only in D. flexuosa at the lowest N concentration were significant contributions found. Nitrate and ammonium had no differential effect in this respect.