Growth and potential conductivity of white clover roots in dry soil with increasing phosphorus supply and defoliation frequency

Shallow-rooted white clover can be exposed to severe water stress during dr y periods, leading to poor growth and persistence in pastures, Root growth is a primary determinant of drought tolerance and water uptake in dry soils , which may interact with P supply and defoliation Frequency for grazed whi te clover in legume-based pastures. Effects of four levels of P supply (0, 17, 50, and 150 mg P kg(-1) soil), two defoliation frequencies (frequent an d infrequent), and two soil water regimes (wet and dry) were determined on various parameters associated with root growth and plant water uptake. A mi croscopy study was also undertaken to measure the relative size and abundan ce of xylem vessels in the primary roots of frequently defoliated low (P-0) and high P (P-150) plants. Increased P supply increased the rate of water loss per pot, which was better related to the coarse root length density co mpared with the fine root length density in dry soil. Coarse root length de nsity, leaf area, mean number and diameter of xylem vessels, and potential root hydraulic conductance increased 6,2-, 12.4-, 2.4-, 1.5-, and 12.7-fold , respectively, for the frequently defoliated plants in dry soil between P- 0 and P-150 treatments. It also appeared that leaf area and rate of water l oss were dependent on the mean xylem diameter and associated hydraulic cond uctance. Increased P supply improved the ability of frequently defoliated w hite clover plants to tolerate dry soil conditions by lowering the resistan ce to water now in roots and increasing the water uptake.