Defoliation effects on isoprene emission from Populus deltoides

Isoprene emission from plants is one of the principal ways in which plant p rocesses alter atmospheric chemistry. Despite the importance of this proces s, few long-term controls over basal emission rates have been identified. S tress-induced changes in carbon allocation within the entire plant, such as those produced by defoliation, have not been examined as potential mechani sms 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 fo llowing damage. Defoliation reduced isoprene emission from undamaged and da maged leaves on partially defoliated plants. Photosynthetic rates and leaf carbon and nitrogen pools were unaffected by damage. Photosynthetic rate an d isoprene emission were highly correlated in undamaged leaves on undamaged plants and damaged leaves on partially defoliated plants. There was no cor relation between photosynthetic rate and isoprene emission in undamaged lea ves on partially defoliated plants. Isoprene emission was also highly corre lated with the number of source leaves on the apical shoot in damage treatm ents. Increased carbon export from source leaves in response to defoliation may have depleted the amount of carbon available for isoprene synthesis, d ecreasing isoprene emission. These results suggest that while isoprene emis sion is controlled at the leaf level in un-damaged plants, emission from le aves on damaged plants is controlled by whole-branch allocation patterns.