Co-variation of declining photosystem II efficiency with methyl salicylate and cis-β-ocimene emissions in Holcus lanatus under high light and temperature

Leaves must sustain high rates of photosynthesis to support growth while simultaneously avoiding over-reduction of the photosynthetic electron transport chain and the formation of damaging reactive oxygen species during daily exposure to high light and temperature conditions. This challenge is especially acute for C₃ species like Holcus lanatus, which exhibit high rates of photorespiration under elevated temperatures and high light often experienced in grassland ecosystems. In leaves of H. lanatus at the Point Reyes Field Station in California, USA, we observed a pronounced midday decline (~ 10-fold) in the quantum efficiency of photosystem II (ΦPSII) despite elevated electron transport rates (ETR), temperature, and incident light, with full recovery in the evening. Light- and temperature-response experiments at the leaf level revealed that net CO₂ photoassimilation, ETR, and the emission of specific volatile organic compounds (VOCs) showed coordinated variation during ΦPSII suppression, suggesting that biosynthetic activity remained high even as photochemical efficiency declined. Among the emitted VOCs, methyl salicylate (MeSA) and cis-β-ocimene, derived from the shikimate and isoprenoid pathways connected with the Calvin–Benson cycle for carbon skeletons, showed strong light- and temperature-responsiveness. In contrast, α-pinene and sabinene emissions showed weak or absent light dependence, declined with increasing leaf temperature, and peaked at night in the field. Given their known roles as signaling compounds, these patterns raise the possibility that, as products of metabolism closely linked to photosynthesis, MeSA and cis-β-ocimene may function as internal feedback signals associated with adjustment of the photosynthetic light reactions. Although trans-β-ocimene is widely regarded as the predominant isomer in plant emissions, especially in biotic stress responses, our sequence analysis predicts that 18% of β-ocimene-producing species possess a cis-β-ocimene synthase, including a validated example in Cannabis sativa, suggesting the independent evolution of plastid-localized enzymes that preferentially produce the cis isomer. By comparison, UniProt lists cis in only ~ 7% of species (one verified cis-β-ocimene synthase from Cannabis sativa and two terpene synthases from the social amoeba Dictyostelium discoideum). Together, these findings support three broader hypotheses: first, that photosynthesis-linked metabolites such as MeSA may participate in feedback signaling associated with the light reactions of photosynthesis; second, that MeSA and β-ocimene production, particularly cis-β-ocimene in H. lanatus, may be coordinately linked during high light and temperature stress; and third, that cis-β-ocimene may be more widespread in nature than is currently recognized, with a substantial proportion of species potentially harboring dedicated cis synthases. This apparent decoupling of ΦPSII from electron transport, net CO₂ assimilation, and biosynthesis under thermal and light stress has important implications for photosynthesis modeling and solar-induced fluorescence interpretation, both of which often assume a constant or uniformly scaled ΦPSII.

Metaphorical illustration of the regulation of ΦPSII efficiency, water oxidation, and electron transport during the light reactions of photosynthesis, coupled to CO₂ assimilation and volatile metabolite biosynthesis. Yellow light, representing solar energy, enters the photosynthetic cell through a lens, with some rays refracted away—symbolizing proposed methyl salicylate (MeSA)-mediated regulation of ΦPSII. At the core, Photosystem II (PSII) is embedded within a Yin-Yang motif, depicting the dynamic balance between water oxidation and carbon assimilation. In the yellow region, absorbed light drives PSII activity, initiating the photolysis of water (H₂O → O₂ + e⁻) and electron transport (blue arrows) through the photosynthetic electron transport chain. This energy powers CO₂ fixation into CH₂O (formaldehyde equivalents), supporting downstream biosynthesis of volatile aromatic and isoprenoid metabolites such as MeSA and cis-β-ocimene (highlighted in green), via the shikimate and isoprenoid pathways fed by Calvin-Benson cycle intermediates. The visual metaphor emphasizes the energetic and metabolic coordination that enables plants to adaptively regulate photosynthesis in response to fluctuating light environments.