Nitrogen Partitioning and Mobilization Patterns in Bean Plants

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.J. 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 K15NO3 or 15N2 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 15NO3 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 15N in each plant part was dependent upon the growth stage and 15N source, and was independent of cultivar. Greater proportions were found in the mature leaves and roots of plants labeled with 15NO3, and in the seed and nodules in those plants labeled with 15N2 at all growth stages. Pods and seeds were major sinks from both 15N 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 N2-fixation activities during seed filling can have a significant influence on the final seed N concentrations and yield