Abstract
Isoprene emissions importantly protect plants from heat stress, but the emissions become inhibited by instantaneous increase of [CO2], and it is currently unclear how isoprene-emitting plants cope with future more frequent and severe heat episodes under high [CO2]. Hybrid aspen (Populus tremula x Populus tremuloides) saplings grown under ambient [CO2] of 380 μmol mol−1 and elevated [CO2] of 780 μmol mol−1 were used to test the hypothesis that acclimation to elevated [CO2] reduces the inhibitory effect of high [CO2] on emissions. Elevated-[CO2]-grown plants had greater isoprene emission capacity and a stronger increase of isoprene emissions with increasing temperature. High temperatures abolished the instantaneous [CO2] sensitivity of isoprene emission, possibly due to removing the substrate limitation resulting from curbed cycling of inorganic phosphate. As a result, isoprene emissions were highest in elevated-[CO2]-grown plants under high measurement [CO2]. Overall, elevated growth [CO2] improved heat resistance of photosynthesis, in particular, when assessed under high ambient [CO2] and the improved heat resistance was associated with greater cellular sugar and isoprene concentrations. Thus, contrary to expectations, these results suggest that isoprene emissions might increase in the future.
Highlights
Isoprene is the most abundant reactive volatile hydrocarbon emitted from a wide range of plant species (Fineschi et al, 2013; Monson et al, 2013; Sharkey et al, 2013)
In Sun et al (2012) this enhanced emission capacity became evident by increased isoprene emission rate at 30 °C under high light intensity of 2000 μmol m−2 s−1 and our study further demonstrates that this enhancement is maintained over the entire temperature response (Fig. 2d)
Apart from these general observations, our study demonstrates several important novel aspects of environmental responses of isoprene emission under high temperature and in plants developed in different atmospheric [CO2]: (i) the CO2 sensitivity of isoprene emission was lost at temperatures higher than 35–40 °C (Fig. 2c, d); (ii) as a result of the loss of [CO2] sensitivity of emissions, isoprene emission rates in elevated-[CO2]grown plants exceeded the emissions in ambient-[CO2]-grown plants at higher temperatures of 40–50 °C; and (iii) high-temperature emission enhancement was maintained at high and at moderate light intensity (Fig. 2c, d, Fig. 4c, d)
Summary
Isoprene is the most abundant reactive volatile hydrocarbon emitted from a wide range of plant species (Fineschi et al, 2013; Monson et al, 2013; Sharkey et al, 2013). Isoprene can directly stabilize biomembranes avoiding excessive fluidity at high temperatures (Sharkey et al, 2001; Singsaas et al, 1997; Siwko et al, 2007), but isoprene can quench reactive oxygen species formed under heat stress (Affek and Yakir, 2002; Loreto et al, 2001; Vickers et al, 2009a, 2009b). Rapid synthesis of volatile isoprene is especially advantageous in environments with intermittent heat periods such as those occurring during sunflecks when elicitation of other protective mechanisms is too slow (Behnke et al, 2007, 2013; Niinemets and Monson, 2013; Singsaas et al, 1999; Singsaas and Sharkey, 1998), but leaves may rapidly heat up to temperatures 45–50 °C (Singsaas et al, 1999; Singsaas and Sharkey, 1998; Valladares and Niinemets, 2007)
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