Consequences of inhibition of isoprene synthesis in Phragmites australis leaves exposed to elevated temperatures

Isoprene is emitted by a variety of plant species, but its role remains elusive. Several experiments indicated that isoprene may have a protective role under stressful conditions. We investigated if the inhibition of endogenous isoprene synthesized by Phragmites australis leaves provokes oxidative stress, i.e. induces the accumulation of reactive oxygen species and products of membrane degradation, and reduces the physiological performances of leaves exposed to elevated temperatures. By increasing temperature from 30 to 48 °C, photosynthesis decreased both in isoprene-emitting leaves and in leaves in which isoprene was inhibited by feeding 20 μM fosmidomycin. However, isoprene inhibition was incomplete and the residual emission showed the same temperature-dependence of the emission from isoprene-emitting leaves, peaking at 44 °C. At this temperature a significantly higher electron transport rate was observed in isoprene-emitting leaves than in isoprene-inhibited leaves suggesting that isoprene may have facilitated electron flow through the photosynthetic/photorespiratory cycles. More H2O2 and malonyldialdehyde contents and higher catalase and peroxidase activities were observed in isoprene-inhibited than in isoprene-emitting leaves. These changes were less evident after exposure to the temperature ramp up to 48 °C than in leaves maintained at 30 °C. This suggests that isoprene, independent of the temperature stress, effectively reduces the accumulation of reactive oxygen species and protects membrane from denaturation.