Phosphorus requirement for biomass accumulation is higher compared to photosynthetic biochemistry for three ornamental shrubs

Abstract

Ornamental nursery producers grow a variety of plant taxa in soil-less substrates and rely on frequent water and nutrient applications to maximize plant growth and quality. This study was performed to understand the morpho-physiological basis of plant response to phosphorus (P) additions and to identify the optimum P concentration required for three common woody ornamental taxa: Hydrangea quercifolia ‘Queen of Hearts’, Cornus obliqua ‘Powell Gardens’ and Physocarpus opulifolius ‘Seward’. In a greenhouse experiment, all plants were watered with a complete nutrient solution that varied in P concentration (0, 0.7, 1.3, 2.5, 3.7, 6.5 mg L−1). Optimum P concentration for photosynthetic biochemistry was dependent on taxa and ranged between 2.5 and 3.7 mg L−1. For total dry biomass, the optimum P concentration was approximately 4.0 mg L−1 for all three taxa. Phosphorus concentration below 2.5 mg L−1 reduced leaf size and resulted in greater partitioning of biomass and P to root growth. Analysis of responses of photosynthesis to intercellular carbon dioxide concentration (A/Ci curves) indicated a continuous increase in photosynthetic parameters to increasing P concentration. Rate of rubisco for carboxylation (Vcmax), RuBP regeneration rate (J), and the rate of triose phosphate use (TPU) limited photosynthesis in P-deficient plants for all three taxa. Light-harvesting efficiency (Fv’/Fm’) for all three taxa was less sensitive to P addition than photosynthetic biochemistry or plant growth. The optimal P concentrations identified in this study are lower than common recommendations and less than the amounts provided by typical commercial fertilizers. Thus, for these three taxa, application of P above 4 mg L−1 in combination with excess irrigation resulting in leachate could have negative environmental consequences without improving crop growth or physiology.