NUTRIENT-LIMITED GROWTH-RATES - QUANTITATIVE BENEFITS OF STRESS RESPONSES AND SOME ASPECTS OF REGULATION

Young sunflower plants (Helianthus annuus L.) under stress of low nitrate or phosphate availability exhibited increases in root:shoot ratio and in kinetic parameters for uptake. They showed no significant changes in photosynthetic utilization of either nutrient. Increases in root:shoot ratio were achieved by early and persistent suppression of shoot growth, but not root growth. Affinity for phosphate uptake, 1/K-m(P), increased with phosphate stress, as did affinity for nitrate uptake, 1/K-m(N), with nitrate stress. Maximal uptake rate, V-max, for phosphate uptake increased with phosphorus stress; V-max for nitrate did not increase with nitrogen stress. Phosphate V-max was related strongly to root nutrient status. Decreases in V-max with plant age were not well explained by changes in age structure of roots. Estimated benefits of acclimatory changes in root: shoot ratio and uptake kinetics ranged up to 2-fold increases in relative growth rate, RGR. The relation of RGR to uptake physiology followed predictions of functional balance moderately well, with some systematic deviations. Analyses of RGR using growth models imply no significant growth benefit from regulating V-max specifically, not from down-regulating it at high nutrient availability. Quantitative benefits of increases in root:shoot ratio and uptake parameters are predicted to be quite small under common conditions wherein nutrient concentrations are significantly depleted by uptake. The root:shoot response is estimated to confer the smallest benefit under nondepleting conditions and the largest benefit under depleting conditions. Even then, the absolute benefit is predicted to be small, possibly excepting the case of heterogeneous soils. Depleting and non-depleting conditions are addressed with very different experimental techniques. We note that a theoretical framework is lacking that spans both these cases, other than purely numerical formulations that are not readily interpreted.