Alterations in the spectral composition of the incident irradiation modulates isoprene emission and the environmental stress response of velvet bean (Mucuna pruriens)

Background/Question/Methods

Isoprene emission from the vegetation is an important biosphere-atmosphere interaction and a key constraint to the chemical properties of the troposphere.  Isoprene reactions can contribute to ozone formation and indirectly affect greenhouse gas accumulation, reinforcing global warming. Isoprene emission is affected by the physical environment and altered by exposure to environmental stress however, the underlying mechanisms are not yet fully understood. It has been hypothesized that changes in climate may also trigger alterations in the amount of light and the composition of the solar spectrum reaching the surface of the Earth. To investigate the interactive effects of increasing light intensities and changes in the spectral composition of the incident irradiance on isoprene emission and plant eco-physiology, we exposed 4 weeks old isoprene emitter velvet bean plants (Mucuna pruriens) to high irradiances of either white light or high light with an increased proportion of the red spectral component, for two weeks, with a 14 h photoperiod, at 30°C. We followed variations in photosynthesis, isoprene emission, pigment composition and the accumulation of photoinhibition-induced hydrogen peroxide and membrane degradation markers during the treatment.

Results/Conclusions

Plants were grown in a greenhouse from seeds at 14 h photoperiod and 400 µmolm^-2s^-1 photosynthetically active radiation (PAR), 30C day/20C night temperatures, for four weeks, than exposed to 1200 µmolm^-2s^-1 PAR or 1200 µmolm^-2s^-1 intensity irradiation with a 25% increment in the proportion of the red spectral component. Controls were kept at growth intensities. Mature leaves were sampled for photosynthesis, isoprene emission, pigment content, H₂O₂ and thiobarbituric acid reactive substances (TBARS, as stress markers) accumulation prior to, and after 7 and 14 days of high light exposure. Preliminary results indicate that leaves of plants exposed to high light with an increased proportion of the red spectral component emitted less isoprene (37-30 nmolm^-2s^-1) and accumulated higher amounts of H₂O₂ and TBARS (900 nmolcm^-2 and 40–45 µmolg^-1fw) than plants exposed to high light mimicking the natural solar irradiance spectrum (52-58 nmolm^-2s^-1 isoprene; 300 nmolcm^-2 H₂O₂ and 22–26 µmolg^-1fw TBARS). We hypothesize, that isoprene emission is sensitive to alterations in the spectral composition of the incident irradiation, and changes in emission rates may alter plant response to environmental stresses. Our results confirm that leaves emitting less isoprene are more sensitive to photoinhibition induced damages.