Plasticity in grass species in relation to nitrogen supply

Responses of phenotypic features of grass species (Holcus lanatus L., Deschampsia flexuosa (L.) Trin., Poa annua L. and Lolium perenne L. cv. S. 23 and cv. 'Norlea') to the availability of nitrogen (0.02, 0.21 or 4.0 mol m(-3) nitrate or ammonium) were measured after 28 days' growth in solution culture. Plasticity was measured by relating phenotypic variations to the state of the plant growing at a maximum relative growth rate (R). The degree of plasticity of a phenotypic feature was independent of its level of organization but was species-dependent. Mean root radius was always the least plastic feature. Different degrees of plasticity occurred in different features of the same plants. There was little difference between species in overall plasticity. R was attained by each species adopting a unique phenotype which always included optimal values for root/shoot ratio and of total nitrogen concentrations in roots and shoots, other features being optimized, minimized or maximized depending on species. Root/shoot ratio was stabilized close to the optimum in D. flexuosa but varied widely in other species. At low concentrations of either nitrate or ammonium, the nitrogen concentrations in shoots of D. flexuosa and L. perenne cv. 'Norlea' were maintained optimally for growth but those in the roots were reduced.