Shoot P status regulates cluster-root growth and citrate exudation in Lupinus albus grown with a divided root system

The present study was carried out to investigate whetherthe P concentration in the roots or the shoots controls thegrowth and citrate exudation of cluster roots in white lupin(Lupinus albusL). Foliar P application indicated that lowP concentration in the shoots enhanced cluster-root growthand citrate-exudation rate more so than low P concentrationin the roots. In the split-root study, the P concentrationin the shoots increased with increased P supply (1, 25 or75 mmol m-3P), to the ‘privileged’ root halves. Roots‘deprived’ of P invariably had the same low P concentrations,whereas those in the ‘privileged’ roots increased withincreasing P supply (1, 25 or 75 mmol m-3P). Nevertheless,the proportion of the total root mass allocated to clusterroots, and the citrate-exudation rates from the root halveswere always similar on both root halves, irrespective of Psupply, and decreased with increasing shoot P concentrations.Peak citrate exudation rates from developing clusterroots were significantly faster from cluster roots on the‘deprived’ root halves when the ‘privileged’ half wasexposed to 1 mmol m-3P as compared with 25 or75 mmol m-3P. The possibility that changes in the concentrationsof P fractions in the root halves influenced clusterrootgrowth and citrate exudation was discounted, becausethere were no significant differences in insoluble organic P,ester-P and inorganic P among all ‘deprived’ root halves.The results indicate that cluster-root proportions and citrateexudation rates were regulated systemically by the Pstatus of the shoot, and that P concentrations in the rootshad little influence on growth and citrate exudation of clusterroots inL. albus.