Root system growth and nodule establishment on pea (Pisum sativumL.)

Abstract

Development of the root system, appearance of nodules, and relationships between these two processes were studied on pea (Pisum sativum L., cv. Solara). Plants were grown in growth cabinets for 4 weeks on a nitrogen-free nutrient solution inoculated with Rhizobium leguminosarum. Plant stages, primary root length, distance from the primary root base to the most distal first-order lateral root, and distance from the root base to the most distal nodule, were recorded daily. Distribution of nodules along the primary root and distribution of laterals were recorded by sampling root systems at two plant stages. Primary root elongation rate was variable, and declined roughly in conjunction with the exhaustion of seed reserves. First-order laterals appeared acropetally on the primary root. A linear relationship was found between the length of the apical unbranched zone and root elongation rate, supporting the hypothesis of a constant time lag between the differentiation of first-order lateral's primordia and their emergence. Decline of the primary root elongation rate was preceded by a reduction in density and length of first-order laterals. Nodules appeared not strictly but roughly acropetally on the primary root. A linear relationship was found between the length of the apical zone without nodule and root elongation rate, supporting the hypothesis of a constant time lag between infection and appearance of a visible nodule. A relationship was found between the presence/absence of nodules on a root segment and the root elongation rate between infection and appearance of nodules on the considered root segment. Regulation of both processes by carbohydrate availability, as a causal mechanism, is proposed.