Phosphorus supply and the growth of frequently defoliated white clover (Trifolium repens L.) in dry soil

A previous study found that increased phosphorus (P) supply to frequently d efoliated white clover plants, growing in a low-P, dry soil, alleviated wat er stress symptoms and increased plant recovery on rewatering. In this stud y we determined how these stresses influence white clover growth. Measureme nts were made of the leaf canopy, stolon infrastructure and root system of the white clover plants growing in a low-P soil. Treatments included the fa ctorial combination of four levels of P supply, two defoliation frequencies and two soil water treatments. White clover growth declined markedly when P-deficient plants were exposed to frequent defoliation and dry soil condit ions. Leaf area was more affected than other parameters, in that the combin ation of stresses reduced leaf area to 2% of maximum observed for infrequen tly defoliated plants growing in high-P soil, with adequate water. Increase d P supply generally increased the growth of all plant parts. Frequently de foliated plants growing in dry soil produced similar or greater leaf mass a nd leaf area as plants from similar treatments growing in wet soil, when th e P supply increased to 50 mg P kg(-1) soil. Higher P rates were able to ne gate the effect of dry soil on these frequently defoliated plants, as a res ult of larger water and P uptake. Also, the frequently defoliated plants wi th restricted root growth did not respond to a small increase in P supply ( 17 mg P kg(-1) soil) for the leaf growth, irrespective of whether they were growing in wet or dry soil. Infrequently defoliated plants with greater ro ot growth, compared to frequently defoliated plants, more than doubled thei r leaf mass with this P treatment.