Grain Legumes

Abstract

Previous chapters have described the general pattern of plant N% decrease with biomass accumulation in certain crops. In contrast to these crops, grain legumes have specific traits that could modify this pattern. Growth of the pods and of seeds with a high N concentration after flowering could compensate partly or totally for the decline in plant N%, caused by the decrease of the leaf: stem ratio and the remobilization of N from the shaded to the illuminated parts of the canopy. Theoretically, after the growth of pods and seeds, plant N% changes according to the number of reproductive organs that are growing. If seed number is high, nitrogen can be remobilized from roots, and shoot N% increases. Low seed number corresponds to a reduced shoot N%, as the capacity for N in the seeds is limited. Since the number of pods and seeds depends on many factors other than N nutrition, plant N% can decrease without N deficiency, simply because a large proportion of pods and seeds has aborted. Thus, the comparison between observed plant N% and critical plant N% could be inadequate to diagnose N nutrition deficiency after pod emergence.