ROOT-SYSTEM GROWTH AND NODULE ESTABLISHMENT ON PEA (PISUM-SATIVUM L.)

Development of the root system, appearance of nodules, and relationshi ps between these two processes were studied on pea (Pisum sativum L., cv. Solara). Plants were grown in growth cabinets for 4 weeks on a nit rogen-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 ro ot base to the most distal nodule, were recorded daily, Distribution o f nodules along the primary root and distribution of laterals were rec orded by sampling root systems at two plant stages. Primary root elong ation 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 leng th of the apical unbranched zone and root elongation rate, supporting the hypothesis of a constant time lag between the differentiation of f irst-order lateral's primordia and their emergence, Decline of the pri mary root elongation rate was preceded by a reduction in density and l ength of first-order laterals, Nodules appeared not strictly but rough ly acropetally on the primary root. A linear relationship was found be tween the length of the apical zone without nodule and root elongation rate, supporting the hypothesis of a constant time lag between infect ion and appearance of a visible nodule, A relationship was found betwe en the presence/absence of nodules on a root segment and the root elon gation rate between infection and appearance of nodules on the conside red root segment. Regulation of both processes by carbohydrate availab ility, as a causal mechanism, is proposed.