Low phosphorus (P) availability elicits efflux of organic substances viz. carboxylic acids, phenolics, proteins, amino acids, sugars and other low molecular weight compounds in many leguminous crops including soybean (Glycine max (L.) Merr.). The potential for root exudation varies widely among soybean genotypes, as synthesis and secretion of root exudates place additional burden on the carbon demand of the plant. Hence, efficient photosynthetic machinery may attribute to the differential root exudation potential of soybean genotypes in response to low soil P availability. An attempt was made to understand the varietal differences in photo-biochemical processes of soybean genotypes identified previously with contrasting root exudation potential under low P (Vengavasi and Pandey, 2016). Genotypes EC-232019 (P-efficient) and EC-113396 (P-inefficient) were grown in soil with low (2 mg P kg(-1) soil) and sufficient (25 mg P kg(-1) soil) P levels under natural environment and observations were recorded at anthesis. The genotype EC-232019 exhibited higher maximal carboxylation rate (V-cmax), maximal photosynthesis (A(max)), apparent quantum efficiency (Phi), mesophyll conductance (g(m)), triose phosphate utilization rate (TPU), photochemical quenching (qP) and electron transport rate (ETR), along with higher chlorophyll a, total chlorophyll and total carotenoid concentration as compared to the P-inefficient EC-113396. Low P-induced reduction in maximal electron transport rate (J(max)) and Phi was higher in EC-113396 rather than EC-232019, suggesting superior photo-biochemical efficiency in the latter. The observed variation in P uptake and growth responses might be attributed in part to the improved photo-biochemical processes exhibited by the P-efficient genotype EC-232019.
