Isoprenoid emissions, photosynthesis and mesophyll diffusion conductance in response to blue light

• The negative effect of blue light on photosynthesis was particularly strong in species with higher photosynthetic rates.
• The inhibition of photosynthesis involved coordinated reductions in stomatal and mesophyll conductance.
• Blue light negatively affected isoprene and monoterpene emissions in concert with photosynthetic inhibition.
• Both isoprene and monoterpene emissions were shown to be inversely dependent upon intercellular [CO2].

The effects of blue light (BL) on leaf gas exchange of Populus × canadensis, a strong isoprene emitter, and Quercus ilex and Citrus reticulata, two monoterpene emitters with respectively small and large storage pools for monoterpenes, were studied. Leaves were initially exposed to a saturating photosynthetic photon flux density (PPFD) of white light (WL), which was then progressively reduced to perform WL-response curves. Leaves acclimated to saturating WL were then quickly exposed to equivalent BL levels to perform BL-response curves. Blue light did not significantly affect photosynthetic parameters in the light-limited portion of the PPFD-response curves in both P. × canadensis and Q. ilex. Whereas photosynthesis (A), stomatal conductance (gs), and mesophyll conductance (gm) were significantly decreased at high PPFDs of BL. A was similarly inhibited by BL in C. reticulata, but there was no significant effect of light quality on gs. Overall these results show that the negative effect of BL on photosynthesis is widespread in tree species with different leaf characteristics, and that this involves coordinated reductions in gs and gm. BL negatively affected isoprene emission and, to a lesser extent monoterpene emissions, in concert with photosynthetic inhibition. Interesting, both isoprene and monoterpene emissions were shown to be inversely dependent upon intercellular [CO2]. These results indicate that a change in light spectral quality, which can vary during the day, between days and within seasons, can alter photosynthesis and isoprenoid emissions, depending on the PPFD intensity. Such effects should be strongly considered in photosynthesis and volatile isoprenoid emission models.