GENOTYPIC VARIATION IN IRON-PHOSPHATE, AND ALUMINUM-PHOSPHATE SOLUBILIZING ACTIVITY OF PIGEONPEA ROOT EXUDATES UNDER P DEFICIENT CONDITIONS

The internal factors controlling the release of the solubilizing activ ity (for Fe-P and Al-P) in the root exudates of pigeonpea were investi gated. Four pigeonpea genotypes with differential abilities in taking up P from Fe-P were grown in nutrient solution under optimum P and P-f ree conditions. Root exudates and plant samples were collected at week ly intervals; the solubilizing activity for Fe-P and Al-P was estimate d. Plant samples were analyzed for total P and also for P fractions. T he solubilizing activity in root exudates was revealed mostly under P- deficient conditions. Under P-optimum conditions, there was a negative activity. The solubilizing activity (for Fe-P and Al-P) was negativel y correlated with tissue P levels. Various P fractions in leaves i.e. inorganic-P, soluble organic-P, and insoluble organic-P showed a signi ficant negative correlation with the Fe-P and Al-P solubilizing activi ty. The Al-P solubilizing activity was correlated positively with the Fe-P solubilizing activity. In ICPL 88032, there was a 12 fold increas e in the solubilizing activity under internal P deficiency, compared t o the P-optimum status. Though, various genotypes used in the present study reached similar internal P levels by 9 weeks after sowing, there were significant differences in their solubilizing activities for Fe- P. In the genotypes ICPL 88032 and C 11 the Fe-P solubilizing activity was significantly higher compared to ICPL 88039 and ICPL 87. The solu bilizing activity showed a declining trend towards pod-filling (i.e. 1 2 WAS). Genotypic differences in the Fe-P solubilizing activity of the root exudates could not account for their differential P uptake from Fe-P. The results indicated that the Fe-P solubilizing activity of the root exudates in pigeonpea is under genetic control and that its expr ession is possibly regulated by the internal P status and internal P d emand which to some extent depend on the growth stages.