Optimality in (sub)optimal conditions; Leaf stoichiometry in response to contrasting CO2 and phosphorus fertilization

Theorethical Eco-Evolutionary Optimality (EEO) hypotheses are proving helpful in representing leaf-level processes, but are scarcely applied to whole plant levels. Applying EEO approaches at plant level can provide simple ways of representing plant- and ecosystem interactions and dynamics, especially when incorporating anthropogenic environmental impacts. An increase in plant productivity related to, for example, increased emission of CO₂ and Nitrogen (N), will likely increase limitation by other essential nutrients and minerals, such as phosphorus (P). Interacting effects of elevated CO₂ and limitation of essential nutrients are thought to affect plant tissue concentrations, organ growth rates, and photosynthetic capacities. However, it remains uncertain how plant-level reactions to varying nutritional resources affects optimality in plant functioning. Here we used plants with contrasting nutrient limitations to test EEO theoretical optimal photosynthetic traits and the corresponding internal nutrient allocation. It is hypothesised that (I) relative allocation of N and P towards the leaf will decrease under rising CO₂ to optimize photosynthesis in relation to transpiration and (II) effects of P deficiency on growth will be relatively stronger in plants grown in high CO₂ conditions compared to lower CO₂ concentrations. Preliminary data was collected from a phytotron experiment focussing on the combined effect of P limitation and CO₂ fertilization. In this experiment, plant photosynthetic traits (e.g. photosynthetic maximum carboxylation rate, Vcmax, and electron transport rate, Jmax) were measured on three different plant species, Holcus lanatus, Panicum miliaceum, and Solanum dulcamara (a C3 grass, a C4 grass, and a C3 herb respectively). They were grown at either low (150ppm), ambient (450ppm), or high (800ppm) CO₂ concentrations, and given one of either treatments; sufficient P in an N:P ratio of 1:1, or severely limiting P in an N:P ratio of 45:1 with a similar supply of N. Preliminary results suggest that decreased availability of P limits Vcmax and Jmax, constraining the maximum photosynthesis rate. This effect is amplified in low CO₂ conditions, as this triggers plants to increase their photosynthetic capacities when nutrients are sufficiently available. Measured leaf N and P concentrations, alongside Vcmax and Jmax, will be additionally used to determine leaf stoichiometry and photosynthetic P-use efficiency as a result of fertilization. Applied N and P will be compared with leaf concentrations to evaluate their relative allocation. Results will be used to validate EEO model predictions on optimality in suboptimal conditions.