Responses of Rhizobium-inoculated and nitrogen-supplied Phaseolus vulgaris and Vigna unguiculata plants to root volume restriction

Abstract

Present investigation was undertaken to analyse the effect of root volume restriction on the physiological performance of Rhizobium-inoculated (R⁺ ) and nitrogen-supplied (N⁺) cowpea (Vigna unguiculata (L.) Walp. var. Tuy.) and bean (Phaseolus vulgaris L. var. Montalbán.) plants. Restriction of root volume (RRV) was imposed via an 8-fold reduction in the rooting space. Plants were grown under controlled conditions and an unlimited availability of water and nutrients. For both plant species, RRV reduced the dry weight of roots, shoots and nodules, retarded the rate of leaf initiation and development, and increased the leaf chlorophyll concentration, regardless of the source of nitrogen. In contrast, RRV did not alter the shoot/root, leaf/root and nodule/root ratios, the concentration of total reducing sugars in shoots, roots and nodules, the leaf relative water content or the concentration of Rubisco in mature leaves of R⁺ and N⁺ bean and cowpea plants. Concentration of ureide and α-amino-N in shoots and nodules of R⁺ plants as well as of α-amino-N in shoots of N⁺ plants were also not affected by RRV, and abscission of trifoliolated leaves was not recorded in any of the treatment combinations. We conclude that dwarfism of RRV plants was not due to water stress, decreased rates of assimilation, increased abscisic acid, sink/source alterations, or to a reduction on the effectiveness of the symbiotic process or of nitrogen uptake in R⁺ and N⁺ plants, respectively.