Nitrogen-15 partitioning within a three generation tiller sequence of the bunchgrass Schizachyrium scoparium: response to selective defoliation.

Nitrogen partitioning among three generations of tillers within the bunchgrass Schizachyrium scoparium var. frequens was investigated in a controlled environment as a potential mechanism of herbivory tolerance. Nitrogen-15 was transported from the labelled primary tiller generation to both shoots and roots of nondefoliated secondary and tertiary tiller generations within 24 h. Partial defoliation increased shoot nitrogen concentration of secondary and tertiary generation tillers by 110 and 120%, respectively, 24 h following defoliation. Shoot nitrogen concentration was preferentially increased by partial defoliation of tertiary generation tillers throughout the 120 h experimental period, but diminished to concentrations comparable to nondefoliated tillers within shoots of the secondary generation at 72 h. In contrast to nitrogen concentration, the total amount of nitrogen imported by secondary and tertiary generation tillers decreased 62 and 73%, respectively, 24 h following partial defoliation and did not attain values comparable to respective nondefoliated tillers. Consequently, preferential nitrogen concentration occurred in response to partial tiller defoliation without an increase in total nitrogen import based on the reduction in the total nitrogen requirement per tiller generation associated with defoliation. Estimates of both the total amount of nitrogen import and nitrogen concentration are necessary to accurately interpret the dynamics of intertiller nitrogen allocation.