Relative Nitrogen Limitation at Steady-state Nutrition as a Determinant of Plasticity in Five Grassland Plant Species

Abstract

Partitioning of biomass between roots and different shoot parts has often been used to explain the response of plants to variations in resource availability. There are still many uncertainties in the importance of this trait for plant performance, and clear guidelines on how partitioning should be quantified in relation to growth rate and resource supply are of fundamental importance for such an understanding. This paper reports an attempt to show how plant nitrogen status relates to root:shoot partitioning and other plastic 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 forbs Polygala vulgaris and Crepis praemorsa, and the grasses Danthonia decumbens, Agrostis capillaris and Dactylis glomerata. The experiment was conducted in a climate chamber where the plants were grown hydroponically (1) under non-limiting nutrient conditions and (2) at a steady-state nitrogen limitation, 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 effects on net assimilation rate (NAR) were very small. Despite large differences in maximum relative growth rate, the species showed remarkable similarities in dry matter partitioning between root and shoot. It is concluded that root: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 hydroponic systems may be the most important reason for the divergent results in earlier studies. Better knowledge of soil-root interactions, and plant responses to the whole span of resource-supply levels, is required for a thorough understanding of how nutrients limit growth.