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. This revised version was published online in June 2006 with corrections to the Cover Date.