Defoliation effects on carbon and nitrogen substrate import and tissue‐bound efflux in leaf growth zones of grasses

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Abstract
Grassland plants suffer regular defoliation, causing loss of photosynthetic activity and internal resources. Consequently, re‐foliation may be substrate‐limited. The present study was undertaken to test the hypothesis that decreased C import in leaf growth zones is (partially) compensated by: (i) mobilization of substrate within growth zones; and (ii) increased efficiency of substrate use in leaf area expansion; but (iii) that these processes depend on the C status of growth zones at defoliation. Mixtures of a C3 (Lolium perenne L.) and a C4 grass (Paspalum dilatatum Poir.) were grown at 15 °C (C3 dominance) and 23 °C (C4 dominance). Individual plants thus grew in contrasting (light and temperature) environments before being defoliated. Defoliation caused a drastic and immediate decrease in C import, but effects on leaf area expansion were buffered by biomass mobilization in the growth zone and increases in specific leaf area of produced tissue. Thus, over the first 2 d post‐defoliation, the amount of leaf area produced per unit imported C increased by 39 to 102% depending on treatment. The magnitude of these buffering responses was correlated with the concentration of water soluble carbohydrates in the growth zone at defoliation. Similar responses were observed for N, although defoliation effects were smaller and delayed relative to those on C import. This study demonstrates refoliation is sustained by short‐term mobilization of reserves within the growth zone and reduced costs of produced leaf area, but that these mechanisms depend on growth zone C status at defoliation.

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