Stomatal decoupling: New insights into environmental drivers and underlying physiological mechanisms during simulated heatwaves in temperate and tropical tree species

By 2024, anthropogenic greenhouse gas emissions have increased global average surface temperatures 1.55 °C above pre-industrial levels. This has led to an increase in both the intensity and frequency of heat waves. In recent years, it has been shown that the strong relationship (e.g., the coupling) between net photosynthetic CO₂ assimilation (A_net) and stomatal conductance (g_s) is decreased or even lost at high temperatures (Diao et al., 2024; Marchin et al., 2023). However, the isolated effect of environmental drivers (e.g. air temperature, vapour pressure deficit; VPD_air) and the underlying plant physiological mechanisms are not yet fully understood (Mills et al., 2024).

To improve our understanding why at high temperatures g_s continues to increase while A_net decreases, we conducted a climate chamber experiment with 3 temperate (Alnus cordata, Acer platanoides, Phillyrea angustifolia) and 3 tropical (Terminalia microcarpa, Terma tomentosa, Syzygium jambos) tree species. In one chamber, we increased the air temperature (T_air) from 20 to 40 °C in 5 °C steps (2 days at every temperature) while keeping the VPD_air at 1.2 kPa. In the second chamber, we increased T_air the same way, but simultaneously increased VPD_air every step from 1.2 to 6 kPa. One subset per chamber was kept well-watered (e.g. at capacity; ~35 vol-%), while in the other subset the trees were exposed to soil drought (~8 vol-%). Every second day, we conducted leaf gas exchange measurements.

Across all species, we observed g_s to continue to increase while A_net decreased (but never reached 0 or negative values) at high temperatures above 35 °C under constant VPD, while increasing VPD maintained the coupling between the two by decreasing g_s. However, the transpiration rate (E) showed the same pattern of decoupling under both VPD regimes. Since E is directly driven by g_s and VPD, plants need to upregulate g_s in order to upregulate E if VPD is too low. While E is important for the regulation of leaf temperature, it is also crucial for other plant physiological processes. We speculate that another reason for increasing E may be that E drives sap flow, which is important for the internal transport and distribution of nutrients, O₂ and CO₂ in plants. Thus, an increased sap flow might be crucial to sustain tree functioning during high-temperature driven periods of accelerated metabolic activity. Future specifically designed experiments are needed to simultaneously investigate plant physiological responses in different tissues as well as at the whole plant level.

 

Diao, H., Cernusak, L.A., Saurer, M., Gessler, A., Siegwolf, R.T.W., Lehmann, M.M., 2024. Uncoupling of stomatal conductance and photosynthesis at high temperatures: mechanistic insights from online stable isotope techniques. New Phytologist 241, 2366–2378. https://doi.org/10.1111/nph.19558

Marchin, R.M., Medlyn, B.E., Tjoelker, M.G., Ellsworth, D.S., 2023. Decoupling between stomatal conductance and photosynthesis occurs under extreme heat in broadleaf tree species regardless of water access. Global Change Biology gcb.16929. https://doi.org/10.1111/gcb.16929

Mills, C., Bartlett, M.K., Buckley, T.N., 2024. The poorly‐explored stomatal response to temperature at constant evaporative.pdf. Plant, Cell & Environment.