Nitrogen Partitioning and Mobilization Patterns in Bean Plants¹

Abstract

The assimilation and distribution of N in the vegetative and reproductive plant parts of edible grain legumes are important processes determining final seed and protein yields. Limited information is available describing these processes for beans ( Phaseolus vulgaris L.). The objective of this study was to measure the N partitioning and mobilization patterns in this important grain legume. Two dry bean cultivars, ‘3512’ and ‘3591’, were labeled with either K 15 NO 3 or 15 N 2 at the late vegetative (V5), early pod development (R2‐R3), and seed filling (R6) developmental stages in the greenhouse. Initial N partitioning was evaluated on plants harvested 48 h after the start of labeling, while the N mobilization patterns were evaluated on the 15 NO 3 ‐labeled plants harvested at subsequent developmental stages and at physiological maturity (R9). Both cultivars had similar dry weight and N distribution patterns throughout development, but 3512 had a larger final seed yield while the final seed N concentrations were greater for 3591. These differences developed from R6 to R9 where the dry weight increase in 3591 had a N concentration of 28.1 g kg −1 compared with 9.9 g kg −1 for 3512. The relative proportion of 15 N in each plant part was dependent upon the growth stage and 15 N source, and was independent of cultivar. Greater proportions were found in the mature leaves and roots of plants labeled with 15 NO 3 , and in the seed and nodules in those plants labeled with 15 N 2 at all growth stages. Pods and seeds were major sinks from both 15 N sources when applied at R2‐R3 and R6, respectively. The seed at R9 contained 64, 73, and 84% of the labeled‐N applied at the V5, R2‐R3, and R6 growth stages, respectively. The seed contained an average of 68% of the total plant N and 53% of the total plant dry weight at R9. These data indicate that the photosynthetic and N 2 ‐fixation activities during seed filling can have a significant influence on the final seed N concentrations and yield.