NUTRIENT-LIMITED GROWTH-RATES - ROLES OF NUTRIENT-USE EFFICIENCY AND OF ADAPTATIONS TO INCREASE UPTAKE RATE

When stressed by low nutrient availability, young sunflower plants (Helianthus annuus) showed responses seen in many other species: increases in root uptake capacity (V(max), 1/K(m)), root: shoot ratio, and putative nutrient-use efficiency, nUE= 1/(tissue nutrient content). A straightforward mechanistic model is derived for relative growth rate (RGR) in solution culture in terms of these factors. A linear regression based on the model indicates a negative role for nUE, which violates a premise of the model. A revised model proposes that primary adaptations are only in uptake rate and growth or nutrient allocations, and these act through the photosynthetic utility of nutrient. The tissue nutrient content and associated nUE become dependent quantities. The predictions for RGR, as tested by linear regression, arc improved. The model predicts that nUE can increase as external solution concentration decreases, but decreases with increased uptake adaptations in one given environment. The decrease in nUE compromises potential gains in RGR from uptake adaptations, and makes increases in root: shoot ratio a nearly insignificant contributor to early RGR. The model and associated regression analyses are generalized for additional adaptations such as increased root fineness and for different quantitative ways that a nutrient may limit photosynthesis. The model and analyses are also generalized to plant growth in soil and growth without functional balance between root and shoot.