PHENOTYPIC PLASTICITY IN RESPONSE TO NITRATE SUPPLY OF AN INHERENTLY FAST-GROWING SPECIES FROM A FERTILE HABITAT AND AN INHERENTLY SLOW-GROWING SPECIES FROM AN INFERTILE HABITAT

The aim of the present study was to investigate possible differences i n plasticity between a potentially fast-growing and a potentially slow -growing grass species. To this end, Holcus lanatus (L.) and Deschamps ia flexuosa (L.) Trin., associated with fertile and infertile habitats , respectively, were grown in sand at eight nitrate concentrations. Wh en plants obtained a fresh weight of approximately 5 g, biomass alloca tion, specific leaf area, the rate of net photosynthesis, the organic nitrogen concentration of various plant parts and the root weight at d ifferent soil depths were determined. There were linear relationships between the morphological and physiological features studied and the I n-transformed nitrate concentration supplied, except for the specific leaf area and root nitrogen concentration of H. lanatus, which did not respond to the nitrate concentration. The root biomass of H. lanatus was invariably more evenly distributed over the soil layers than that of D. flexuosa. However, D. flexuosa allocated more root biomass to lo wer soil depths with decreasing nitrate concentration, in contrast to H. lanatus, which did not respond. The relative response to nitrate su pply, i.e. the value of a character at a certain nitrate level relativ e to the value of that character at the highest nitrate supply, was us ed as a measure for plasticity. For a number of parameters (leaf area ratio, root weight ratio, root nitrogen concentration, vertical root b iomass distribution and rate of net photosynthesis pei unit leaf weigh t) the potentially slow-growing D. flexuosa exhibited a higher phenoty pic plasticity than the potentially fast-growing H. lanatus. These fin dings are in disagreement with current literature. Possible explanatio ns for this discrepancy are discussed in terms of differences in exper imental approach as well as fundamental differences in specific traits between fast- and slow-growing grasses.