Defoliation effects on isoprene emission from Populus deltoides.

Isoprene emission from plants is one of the principal ways in which plant processes alter atmospheric chemistry. Despite the importance of this process, few long-term controls over basal emission rates have been identified. Stress-induced changes in carbon allocation within the entire plant, such as those produced by defoliation, have not been examined as potential mechanisms that may control isoprene production and emission. Eastern cottonwood (Populus deltoides) saplings were partially defoliated and physiological and growth responses were measured from undamaged and damaged leaves 7 days following damage. Defoliation reduced isoprene emission from undamaged and damaged leaves on partially defoliated plants. Photosynthetic rates and leaf carbon and nitrogen pools were unaffected by damage. Photosynthetic rate and isoprene emission were highly correlated in undamaged leaves on undamaged plants and damaged leaves on partially defoliated plants. There was no correlation between photosynthetic rate and isoprene emission in undamaged leaves on partially defoliated plants. Isoprene emission was also highly correlated with the number of source leaves on the apical shoot in damage treatments. Increased carbon export from source leaves in response to defoliation may have depleted the amount of carbon available for isoprene synthesis, decreasing isoprene emission. These results suggest that while isoprene emission is controlled at the leaf level in undamaged plants, emission from leaves on damaged plants is controlled by whole-branch allocation patterns.