Partitioning of P and the activity of root acid phosphatase in white clover (Trifolium repens L.) are modified by increased atmospheric CO2 and P fertilisation

The growth response of white clover (Trifolium repens L.) to the expected increase in atmospheric partial pressure of CO2 (p(CO2)) may depend on P availability. A decrease in the rate of transpiration due to increased p(CO2) may reduce the amount of P transported to the shoot, thereby causing a change in the partitioning of P between the root and shoot. To test these hypotheses, four concentrations of P in the nutrient solution, combined with two p(CO2) treatments, were applied to nodulated white clover plants. Compared to ambient p(CO2) (35 Pa), twice ambient p(CO2) (70 Pa) reduced the rate of transpiration but did not impair the total P uptake per plant. However, at twice ambient p(CO2) and a moderate to high supply of P, concentrations of structural P and soluble P (Pi) were lower in the leaves and higher in the roots. The activity of root acid phosphatase was lower at twice ambient p(CO2) than at ambient p(CO2); it depended on the Pi concentration in the roots. At the highest P concentration, twice ambient p(CO2) stimulated photosynthesis and the growth rate of the plant without affecting the concentration of nonstructural carbohydrates in the leaves. However, at the lower P concentrations, plants at twice ambient p(CO2) lost their stimulation of photosynthesis in the afternoon, they accumulated nonstructural carbohydrates in the leaves and their growth rate was not stimulated; indicating C-sink limitation of growth. P nutrition will be crucial to the growth of white clover under the expected future conditions of increased p(CO2).