SHOOT NITROGEN DYNAMICS IN TROPICAL COMMON BEAN

The common bean (Phaseolus vulgaris L.) is an important food crop in the tropics, where its productivity is often limited by low soil N availability. An ideotype for efficient N-use in bean would distribute N in time and space so that leaf N (and therefore leaf photosynthetic capacity) is optimized with respect to reproductive N sinks. To define parameters and estimate genetic variability for N use, we examined shoot N dynamics in three tropical bean genotypes of similar phenology believed to differ in N acquisition and allocation. Seeds were sown in a high-N soil in two plantings separated by 20 d. Shoots of BAT 477 acquired considerably more N than Porrillo Sintetico, but both genotypes yielded well. G 5059 acquired an intermediate amount of N, partitioned more N to leaves, and matured slightly later, but had a lower final N harvest index (NHI) than the other two genotypes. Time of planting had a substantial effect on shoot N uptake and partitioning. After flowering, N was lost from leaves and stems, rose and fell in young pods and pod walls, and accumulated in seeds. Harvest index (HI) and NHI increased almost linearly from flowering to physiological maturity at the same rate in all three genotypes. Seed N duration dominated post-flowering shoot N duration. Leaf N duration represented 25% of post-flowering shoot N duration on average. Leaf CO2 exchange rate (CER) was highly correlated with leaf N content, and both declined during mid and late pod filling. Instantaneous leaf photosynthetic N-use efficiency (CER per unit leaf N) declined with time. Our data are consistent with the hypothesis that N allocation to seeds limits leaf photosynthesis in bean grown in the tropics. Variation in N translocation patterns was associated with phenology, which must be taken into account in breeding for N-use efficiency.