PHOTOSYNTHETIC NITROGEN-USE EFFICIENCY IN RELATION TO LEAF LONGEVITY IN COMMON BEAN

Nitrogen deficiency is an important limitation to bean (Phaseolus vulgaris L.) production in the tropics. The ability of a leaf to utilize N for photosynthetic C assimilation (leaf N-use efficiency) may be an element of genetic adaptation to low N soils. We tested the hypotheses that (i) leaf N-use efficiency is related to leaf longevity, and (ii) shoot architecture influences the relationship of leaf N-use efficiency and leaflongevity. Four genotypes that have contrasting shoot architectures (determinate bush, indeterminate bush, indeterminate prostrate, and indeterminate climbing) were grown in duplicate experiments in Colombia. At frequent intervals, the third and eighth leaves to appear on the main stem were analyzed for CO2 assimilation and N content. Daily CO2 assimilation and leaf N content, which were highly correlated, peaked soon after leaf expansion and declined steadily until leaf abscission. Leaf N duration and cumulative leaf CO2 assimilation were highly correlated. Leaf longevity was highly correlated with leaf N duration and significantly correlated with cumulative leaf CO2 assimilation. Leaf longevity was highly negatively correlated with leaf N-use efficiency expressed as cumulative CO2 assimilation per unit leaf N duration but slightly positively correlated with leaf N-use efficiency expressed per unit of average leaf N content. Shoot architecture had little impact on these variables and relationships. We conclude that leaf longevity is an important factor in leaf N-use efficiency but that genetic variation for shoot architecture in common bean is not important in determining the N-use efficiency of individual leaves.