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Isoprene protects the photosynthetic apparatus of isoprene-emitting plants from oxidative stress. The role of isoprene in the response of plants to drought is less clear., Water was withheld from transgenic isoprene-emitting and non-emitting tobacco ( Nicotiana tabacum) plants, to examine: the response of isoprene emission to plant water deficit; a possible relationship between concentrations of the drought-induced phytohormone abscisic acid ( ABA) and isoprene; and whether isoprene affected foliar reactive oxygen species ( ROS) and lipid peroxidation levels., Isoprene emission did not affect whole-plant water use, foliar ABA concentration or leaf water potential under water deficit. Compared with well-watered controls, droughted non-emitting plants significantly increased ROS content (31-46%) and lipid peroxidation (30-47%), concomitant with decreased operating and maximum efficiencies of photosystem II photochemistry and lower leaf and whole-plant water use efficiency ( WUE). Droughted isoprene-emitting plants showed no increase in ROS content or lipid peroxidation relative to well-watered controls, despite isoprene emission decreasing before leaf wilting., Although isoprene emission protected the photosynthetic apparatus and enhanced leaf and whole-plant WUE, non-emitting plants had 8-24% more biomass under drought, implying that isoprene emission incurred a yield penalty. [ABSTRACT FROM AUTHOR]