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 i ncrease in atmospheric partial pressure of CO2 (p(CO2)) may depend on P ava ilability. 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 chan ge in the partitioning of P between the root and shoot. To test these hypot heses, 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 o f transpiration but did not impair the total P uptake per plant. However, a t twice ambient p(CO2) and a moderate to high supply of P, concentrations o f structural P and soluble P (Pi) were lower in the leaves and higher in th e 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 ro ots. At the highest P concentration, twice ambient p(CO2) stimulated photos ynthesis and the growth rate of the plant without affecting the concentrati on of nonstructural carbohydrates in the leaves. However, at the lower P co ncentrations, plants at twice ambient p(CO2) lost their stimulation of phot osynthesis in the afternoon, they accumulated nonstructural carbohydrates i n the leaves and their growth rate was not stimulated; indicating C-sink li mitation of growth. P nutrition will be crucial to the growth of white clov er under the expected future conditions of increased p(CO2).