Shoot labelling with 14CO2: a technique for assessing total root carbon exudation under phosphorus stress

Abstract

Root exudation is one of the mechanisms adopted by plants to acquire fixed phosphorus (P) from soil. We report a rapid method for evaluating P-efficient genotypes under low-P availability by measuring root exudation capacity after labelling shoots with 14CO2. Forty-four greengram (Vigna radiata) genotypes were raised with low (2 μM) and sufficient (100 μM) P levels for 15 days in hydroponics. Shoots were exposed to 14CO2 and root exudate sampling was done at 24, 48 and 96 h after exposure. On the basis of relative 14C exudation and biomass, genotypes were categorized into four groups viz. efficient and responsive, inefficient and responsive, efficient and non-responsive and inefficient and non-responsive. Carboxylic acids were measured in exudates and root apices of selected genotypes. Amount of carboxylates in root exudate was significantly correlated with total 14C exudation under low-P. Highest proportion of oxalic acid was detected in genotype PDM-139 belonging to efficient and responsive category under low-P. This suggests that total 14C count can be used as a rapid method as compared to quantification of carboxylates using chromatographic techniques to measure the root exudation capacity of genotypes. Thus, this simple technique can be employed to screen genotypes at an early stage of plant growth.