Relative nitrogen limitation at steady-state nutrition as a determinant ofplasticity in five grassland plant species

Partitioning of biomass between roots and different shoot parts has often b een used to explain the response of plants to variations in resource availa bility. There are still many uncertainties in the importance of this trait for plant performance, and clear guidelines on how partitioning should be q uantified in relation to growth rate and resource supply are of fundamental importance for such an understanding. This paper reports an attempt to sho w how plant nitrogen status relates to root:shoot partitioning and other pl astic responses, in a manner that can be used for quantitative predictions. The reactions to nitrogen limitation of five grassland plant species, with different ecological demands, were compared. The species used were the for bs Polygala vulgaris and Crepis praemorsa, and the grasses Danthonia decumb ens. Agrostis capillaris and Dactylis glomerata. The experiment was conduct ed in a climate chamber where the plants were grown hydroponically (1) unde r non-limiting nutrient conditions and (2) at a steady-state nitrogen limit ation, which enabled the plants to express half of their growth potential. The relative growth rate (RGR) of the species was strongly related to plant nitrogen concentration (PNC) and leaf area ratio (LAR), whereas the effect s on net assimilation rate (NAR) were very small. Despite large differences in maximum relative growth rate, the species showed remarkable similaritie s in dry matter partitioning between root and shoot. It is concluded that r oot:shoot partitioning can be treated as a direct function of the relative resource limitation of the plant. The difficulty of attaining well-defined levels of resource limitation in soil. other solid substrates and many hydr oponic systems may be the most important reason for the divergent results i n earlier studies. Better knowledge of soil-root interactions, and plant re sponses to the whole span of resource-supply levels, is required for a thor ough understanding of how nutrients limit growth. (C) 1999 Annals of Botany Company.