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 dry 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 white 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 and infrequent), and two soil water regimes (wet and dry) were determined on various parameters associated with root growth and plant water uptake. A microscopy study was also undertaken to measure the relative size and abundance of xylem vessels in the primary roots of frequently defoliated low (P0) and high P (P150) plants. Increased P supply increased the rate of water loss per pot, which was better related to the coarse root length density compared with the fine root length density in dry soil. Coarse root length density, 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 P0 and P150 treatments. It also appeared that leaf area and rate of water loss were dependent on the mean xylem diameter and associated hydraulic conductance. Increased P supply improved the ability of frequently defoliated white clover plants to tolerate dry soil conditions by lowering the resistance to water flow in roots and increasing the water uptake.