Nitrogen Concentrations in Legumes Respond Differently to Defoliation and Ontogenesis.

The nutrients a plant allocates to vegetative tissue are determined by costs and benefits.  We determined how N concentrations in leaves of two warm-season perennial herbaceous legumes were affected by simulated herbivory and plant maturity.  Desmodium paniculatum (panicled tick-clover; PTC) and Lespedeza cuneata (sericea lespedeza; SL) were reared in a greenhouse and subjected to one of four treatments: previously un-defoliated plants and three repeated defoliations of the upper 50, 75, and 100% of herbage regrowth at 145, 181 , and 222  d after seeding, approximately equivalent to vegetative, early reproductive and seed set stages.  The two species responded differently to simulated herbivory.  All PTC treatment groups experienced stable or increased (P≤0.05) N concentration from vegetative to flowering stage, and all treatments underwent a decrease (P≤0.05) in N between flowering and seeding.  Ontogenesis and defoliation treatment did not affect SL N concentration.  Results suggest PTC might have experienced nutrient stress after the second defoliation and was unable to compensate in subsequent leaf development for N lost to defoliation.  Seeds usually contain the greatest proportion of legume N, and in a N-stressed situation, PTC might have mobilized N from vegetative tissue to reproductive organs to meet N demands for seed fill.  By maintaining stable leaf N concentrations across multiple defoliation events, SL appears to be able to utilize N resources more efficiently than PTC at seed set when demands for N are high.  Regenerated PTC leaves with lower nutrient concentrations may be less attractive to herbivores. Different nutrient allocation strategies provide varying challenges and opportunities that are legume species specific.