Isoprene emission from the vegetation is an important biosphere-atmosphere interaction and a key constraint to the chemical properties of the troposphere, affecting its oxidative capacity. Isoprene reactions may allow for ozone formation and indirectly affect greenhouse gas accumulation, reinforcing global warming. Isoprene emission is controlled by the physical environment and affected by exposure to environmental stress. A warming climate has been hypothesized to prolong plant growth seasons, with yet unknown consequences on plant life-cycle, onset of senescence, and seasonal variations in isoprene emission from the vegetation. To gain a better understanding of the relationship between isoprene emission capacity, senescence progression and leaf abscission at the end of the growth season, in a changing climate, the current study investigated seasonal changes in the emission capacity and plant oxidant-antioxidant balance, in correlation with changes in micro-climate in pin oaks (Quercus palustris) in Missouri in samples collected between June – October, 2015. In addition, we also identified of a 200 base pair segment of the gene encoding for the pin oak isoprene synthase, and developed an expression assay, that we used to assess variations in the expression of the gene encoding for isoprene synthase (Isps), the enzyme controlling the emission of this volatile.
Results/Conclusions
Isoprene emission of south-facing leaves at 2-4 m canopy height was measured biweekly. Isoprene emission rates were sampled from a cuvette enclosure outflow, concentrating the air onto adsorbents and analysed by GC-MS. Mature leaf samples were collected and analysed for pigment composition, antioxidant capacities, the accumulation of hydrogen peroxide (H2O2) and membrane degradation markers (thiobarbituric acid reactive substances, TBARS) of senescence. Isps gene expression was also assessed. All the above measured data were correlated with measured local climate parameters. Our data demonstrate that oaks maintaining high isoprene emission capacities of 30 – 35 µgC g-1DW h-1 and high Isps expression levels as late as mid-October also exhibit a delay in the onset of senescence (indicated by the absence of detectable levels of TBARS), as compared to oaks whose emission capacity decreased earlier in the season. We attribute this delay to sustained low reactive oxygen species accumulation rate in these plants (with H2O2 concentrations ranging between 15 – 37 µmolg-1FW throughout the season), potentially controlled by the strong antioxidant action of isoprene. We also speculate that the delay in senescence may also be responsible for the observed late abscission of leaves from strong isoprene emitter species, such as pin oaks.