Plants invest a significant amount of their photosynthetically fixed carbon into the synthesis and emission of a volatile hydrocarbon, isoprene. Isoprene reactions can contribute to ozone formation and indirectly affect greenhouse gas accumulation, with positive feedbacks on global warming. Isoprene has also been hypothesized to provide enhanced environmental stress resistance to plants. Extended growth seasons, as predicted consequences of a warming climate are expected to also impact volatile production and emission from the vegetation as well as their roles in plant-environment interactions. To gain a better understanding on how extended growth seasons may affect volatile synthesis, emission, oxidant-antioxidant balance and the timing of senescence in isoprene emitter species, the current study investigated the responses of the isoprene emitter legume, velvet bean (Mucuna pruriens) throughout its growth cycle, to conditions simulating longer and warmer summers as compared to controls, in a greenhouse study. We followed variations in photosynthesis, isoprene emission, pigment composition and the accumulation of hydrogen peroxide (H2O2) and membrane degradation markers (thiobarbituric acid reactive substances, TBARS), as early molecular indicators of the onset of senescence at regular intervals throughout the growth cycle of the plants.
Results/Conclusions
Plants were grown from seeds at high (39°C day/30°C night) and low (27°C day/22°C night), control temperatures. In addition, plants growing at both temperatures were exposed during the day to either 1100 or 400 (low µmolm-2s-1 irradiances of photosynthetically active radiation. Our results show that the onset if senescence was delayed by over two weeks in plants grown under high temperature conditions, as compared to controls. These plants emitted the highest amounts of isoprene (52-58 nmol m -2s-1) and accumulated early, and maintained high concentrations of H2O2 (2900-3000 µmolg-1FW) throughout their life-cycle. Interestingly, this was not associated with extensive of lipid peroxidation (26–32 µmolg-1FW TBARS), and not observed in control plants. These preliminary results indicate that plants grown at high temperatures, in particular, if also exposed to high light irradiances, exhibit a strengthened antioxidant defense capacity, presumably primed by the enhanced production of the cellular signal molecule, H2O2. These plants also exhibited the highest isoprene emission capacities. We speculate that the strong control of isoprene emission over the leaf H2O2 – antioxidant balance is also responsible for “priming” stress resistance and delaying the onset of senescence in velvet bean.