Effects of defoliation and atmospheric CO2 depletion on nitrate acquisition, and exudation of organic compounds by roots of Festuca rubra

The aim of this study was to investigate the physiological basis of increased root exudation from Festuca rubra, in response to defoliation. The hypothesis, that assimilate supply to roots is a key determinant of the response of root exudation to defoliation was tested by imposing CO2-deplete (< 50 μmol mol(-1)) atmospheres to F. rubra. This was done as a non-destructive means of preventing supply of new assimilate to roots of intact and defoliated plants. F. rubra was grown in axenic sand systems, with defoliation and CO2-depletion treatments applied to plants at 14 and 35 days after planting. Root exudation and NO3- uptake were quantified throughout, and post-treatment uptake and allocation of N were determined from the distribution of N-15 label, supplied as (NO3-)-N-15. Defoliation of F. rubra resulted in significantly (P < 0.01) increased root exudation, CO2-depletion did not result in increased exudation from plants of either age. When treatments were applied to F. rubra after 14 days, defoliation and CO2-depletion each reduced NO3- uptake significantly (P < 0.05). However, in older plants, uptake of NO3- was less sensitive to defoliation and CO2-depletion. The results indicate that increased root exudation following defoliation is not related directly to reduced assimilate supply to roots. This was evident from the lack of effect of CO2-depletion on root exudation, and the absence of correlation between root-C efflux and the rate of NO3- uptake. The physiological basis of increased exudation following defoliation remains uncertain, but may be dependent on physical damage, either directly or as a consequence of systemic responses to wounding.