Elevation of atmospheric CO 2 and N-nutritional status modify nodulation, nodule-carbon supply, and root exudation of Phaseolus vulgaris L.

Abstract

Increased root exudation and a related stimulation of rhizosphere-microbial growth have been hypothesised as possible explanations for a lower nitrogen- (N-) nutritional status of plants grown under elevated atmospheric CO 2 concentrations, due to enhanced plant-microbial N competition in the rhizosphere. Leguminous plants may be able to counterbalance the enhanced N requirement by increased symbiotic N 2 fixation. Only limited information is available about the factors determining the stimulation of symbiotic N 2 fixation in response to elevated CO 2. In this study, short-term effects of elevated CO 2 on quality and quantity of root exudation, and on carbon supply to the nodules were assessed in Phaseolus vulgaris, grown in soil culture with limited (30 mg N kg −1 soil) and sufficient N supply (200 mg N kg −1 soil), at ambient (400 μmol mol −1) and elevated (800 μmol mol −1) atmospheric CO 2 concentrations. Elevated CO 2 reduced N tissue concentrations in both N treatments, accelerated the expression of N deficiency symptoms in the N-limited variant, but did not affect plant biomass production. 14CO 2 pulse-chase labelling revealed no indication for a general increase in root exudation with subsequent stimulation of rhizosphere microbial growth, resulting in increased N-competition in the rhizosphere at elevated CO 2. However, a CO 2-induced stimulation in root exudation of sugars and malate as a chemo-attractant for rhizobia was detected in 0.5–1.5 cm apical root zones as potential infection sites. Particularly in nodules, elevated CO 2 increased the accumulation of malate as a major carbon source for the microsymbiont and of malonate with essential functions for nodule development. Nodule number, biomass and the proportion of leghaemoglobin-producing nodules were also enhanced. The release of nod-gene-inducing flavonoids (genistein, daidzein and coumestrol) was stimulated under elevated CO 2, independent of the N supply, and was already detectable at early stages of seedling development at 6 days after sowing.