Carbon and nitrogen deposition in expanding tissue elements of perennial ryegrass (Lolium perenne L.) leaves during non-steady growth after defoliation

The effect of defoliation on the deposition of carbon (C) and nitrogen (N) and the contribution of reserves and current assimilates to the use of C an d N in expanding leaf tissue of severely defoliated perennial ryegrass (Lol ium perenne L.) was assessed with a new material element approach. This inc luded C-13/C-12-and N-15/N-14-steady-state labelling of all post-defoliatio n assimilated C and N, analysis of tissue expansion and displacement in the growth zone, and investigation of the spatial and temporal changes in subs trate and label incorporation in the expanding elements prior to and after defoliation. The relationship between elemental expansion and C deposition was not altered by defoliation, but total C deposition in the growth zone w as decreased due to decreased expansion of tissue at advanced developmental stages and a shortening of the growth zone. The N deposition per unit expa nsion was increased following defoliation, suggesting that N supply did not limit expansion. Transition from reserve- to current assimilation-derived growth was rapid (<1 d for carbohydrates and approximately 2 d for N), more rapid than suggested by label incorporation in growth zone biomass. The N deposition was highest near the leaf base, where cell division rates are gr eatest, whereas carbohydrate deposition was highest near the location of mo st active cell expansion. The contribution of reserve-derived relative to c urrent assimilation-derived carbohydrates (or N) to deposition was very sim ilar for elements at different stages of expansion.