Guntur V. Subbarao

Genotypic variation in iron-, and aluminum-phosphate solubilizing activity of pigeonpea root exudates under P deficient conditions

Abstract The internal factors controlling the release of the solubilizing activity (for Fe-P and Al-P) in the root exudates of pigeonpea were investigated. Four pigeonpea genotypes with differential abilities in taking up P from Fe-P were grown in nutrient solution under optimum P and P-free conditions. Root exudates and plant samples were collected at weekly intervals; the solubilizing activity for Fe-P and Al-P was estimated. Plant samples were analyzed for total P and also for P fractions. The 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 negatively correlated with tissue P levels. Various P fractions in leaves i.e. inorganic-P, soluble organic-P, and insoluble organic-P showed a significant negative correlation with the Fe-P and Al-P solubilizing activity. The Al-P solubilizing activity was correlated positively with the Fe-P solubilizing activity. In ...

Biological nitrification inhibition by Brachiaria humidicola roots varies with soil type and inhibits nitrifying bacteria, but not other major soil microorganisms

Abstract The tropical pasture grass Brachiaria humidiola (Rendle) Schweick releases nitrification inhibitory compounds from its roots, a phenomenon termed ‘biological nitrification inhibition’ (BNI). We investigated the influence of root exudates of B.u2009humidicola on nitrification, major soil microorganisms and plant growth promoting microorganisms using two contrasting soil types, Andosol and Cambisol. The addition of root exudates (containing BNI activity that is expressed in Allylthiourea unit (ATU) was standardized in a bioassay against a synthetic inhibitor of nitrification, allylthiourea, and their function in soil was compared to inhibition caused by the synthetic nitrification inhibitor dicyandiamide. At 30 and 40u2009ATUu2009g−1soil, root exudates inhibited nitrification by 95% in fresh Cambisol after 60u2009days. Nitrification was also similarly inhibited in rhizosphere soils of Cambisol where B.u2009humidicola was grown for 6u2009months. Root exudates did not inhibit other soil microorganisms, including gram-negative bacteria, total cultivable bacteria and fluorescent pseudomonads. Root exudates, when added to pure cultures of Nitrosomonas europaea, inhibited their growth, but did not inhibit the growth of several plant growth promoting microorganisms, Azospirillum lipoferum, Rhizobium leguminosarum and Azotobacter chroococcum. Our results indicate that the nitrification inhibitors released by B.u2009humidicola roots inhibited nitrifying bacteria, but did not negatively affect other major soil microorganisms and the effectiveness of the inhibitory effect varied with soil type.

Genetic variation in acquisition, and utilization of phosphorus from iron-bound phosphorus in pigeonpea

Abstract In earlier experiments it was observed that pigeonpea is one of the few crop species that can utilize fixed forms of soil-P, particularly iron-bound P (Fe-P). The present investigation was aimed at assessing the genotypic variation in P uptake from Fe-P in pigeonpea. Fifteen pigeonpea germplasm lines that comprised genotypes of different maturity groups and three wild relatives were grown in vermiculite supplied with 15 mg P either as Fe-P or Ca-P for 60 d. Significant genetic variation in P uptake from Fe-P was observed; also P-use efficiency varied substantially among the genotypes. Dry matter production was positively correlated with P uptake both under Fe-P and Ca-P treatments (r=0.78**; r=0.63*; n=15). There was no significant relation between P-use efficiency and dry matter production. Wild related species did not show higher P uptake from Fe-P than the cultivated pigeonpea. From Fe-P, the P uptake was only 36% of that from Ca-P. The best genotypes in the present study took up an amount of ...