Rethinking Stomatal Temperature Responses Through a Hydraulic Lens

Stomata act as the primary regulators of plant gas exchange, explicitly mediating the trade‐off between CO₂ assimilation and transpirational water loss. This balance depends on how stomata integrate multiple environmental signals, a long‐standing question in plant physiology. Under climate warming, this issue becomes more pressing because rising temperature simultaneously affects carbon fixation and water loss. In practice, however, temperature rarely varies independently. It often covaries with vapour pressure deficit (VPD), making it difficult to isolate the direct effects of warming on stomatal behaviour. VPD strongly regulates both stomatal acclimation and instantaneous opening strategies (Marchin et al. 2016), so its influence can mask temperature‐driven responses, leading to biased estimates of how trees adjust to warming (Slot et al. 2024). For this reason, separating VPD effects (expressed as the leaf–air water vapour gradient, Δw) from temperature effects becomes necessary. Only by controlling Δw can researchers test how temperature alone regulates stomata, which remains a key but unresolved mechanism, known as the direct response of stomata to temperature (DRST).

Mills et al. (2026) make an important contribution to this field by proposing and testing an elegant physiological hypothesis for the hydraulic mechanism underlying DRST. They propose that warming decreases water viscosity, thereby enhancing plant hydraulic conductance (K). Hence, under steady‐state transpiration, this increase in K leads to a less negative (i.e., higher) leaf water potential, which may induce an increase in stomatal conductance (g _s) (Figure 1). Moreover, because the sensitivity of leaf water potential to changes in K is positively related to transpiration rate, DRST is predicted to be stronger under conditions of high Δw. To test this hypothesis, the authors manipulated leaf temperature while holding Δw constant across six angiosperm species spanning diverse growth forms and functional types. The results show that g _s increased in almost all species when the temperature rose by approximately 9°C–10°C. Importantly, the temperature response of g _s was consistently stronger at high Δw than at low Δw. These responses were directionally consistent with model predictions and provide qualitative support for a hydraulically mediated DRST.

Figure 1.

A diagram of the direct stomatal response to temperature. By maintaining a constant leaf–air water vapour gradient (Δw), model predictions indicate that warming reduces water viscosity, thereby increasing plant hydraulic conductance (K) and ultimately enhancing stomatal conductance (g_s ). This figure contains major findings from Mills et al. (2026) and future research directions. b, an empirical coefficient determined mainly by light intensity; T, temperature; V _cmax, maximum carboxylation rate; J _max, maximum electron transport rate; A _sat, net photosynthetic rate.

Mills's study also relied on a technical refinement that was central to its conclusions. The authors used a dual environmental control system to manipulate leaf temperature while keeping Δw constant. An inner leaf chamber controlled leaf temperature and measured gas exchange, while an outer climate chamber kept the surrounding air close to leaf temperature. Similar chamber‐based approaches have been used previously (Urban et al. 2017; Diao et al. 2024). Here, the system was used specifically to prevent condensation in the gas–exchange lines and to separate temperature effects from humidity effects during measurements of photosynthesis, stomatal conductance and transpiration. As a result, the authors were able to separate temperature effects from humidity effects in an active and controlled manner. The authors also tightly integrated experiments with theory. They used a hydraulic–stomatal model originally proposed by Buckley et al. (2012) and linked it with published estimates of stomatal sensitivity to Δw (K/b; b is an empirical coefficient determined mainly by light intensity) and the temperature response coefficient (Q ₁₀) of hydraulic conductance. This framework generated explicit predictions of DRST at high and low Δw, thereby facilitating the comparison of empirical results with model predictions to provide a clear physiological mechanism.

The authors interpreted their experimental results within a process‐based modelling framework. The model suggests that enhancement of DRST by Δw should remain limited if only changes in water viscosity and published organ‐level Q ₁₀ values are considered. However, the Q ₁₀ values used in the model were derived from studies that differ substantially in measurement conditions (e.g., temperature range, methodology and experimental material), which limits direct comparability. Priority should therefore be given to Q ₁₀ values obtained under similar temperature ranges (23°C–32°C) and comparable tissues or treatments, to those used in the experiment. Where such data are lacking, future studies should directly measure hydraulic conductance as a function of temperature, K(T), along leaf‐to‐stomata water transport pathways, including mesophyll tissues and the guard cells to reduce uncertainty. Mills et al. (2026) found a clear and consistent pattern, such that when leaf temperature increased from approximately 23°C to 32°C, while keeping VPD constant, steady‐state g _s rose in all species, including herbaceous and woody plants. Hence, warming alone consistently promoted stomatal opening, indicating that stomatal temperature responses were not simply a passive consequence of changes in VPD. On average, g _s increased by approximately 50% per 10°C rise in temperature, although the response varied widely among species. Across all six species, stomatal conductance was always higher at high Δw than at low Δw. Although the interaction between Δw and temperature was not statistically significant (overall mixed‐model p = 0.175; n = 3–6 per species), the direction of the effect was consistent across species. Although higher Δw was associated with a stronger stomatal response to temperature, it did not provide conclusive quantitative evidence of causality. Confirming this relationship will require larger sample sizes or additional mechanistic measurements.

A central contribution of the study was the quantification of the systematic divergence between herbaceous and woody species, when empirical results were compared with model predictions. In the herbaceous species—barley, maize, and common bean—the temperature‐induced increase in stomatal conductance exceeded the model's 95% confidence range, whereas in woody species—lianas and eucalypt—it remained close to model expectations. This contrast suggests that models based only on bulk, whole‐leaf hydraulic parameters may not fully capture the more localised water‐status signals that are relevant to stomatal regulation. It also highlighted differences in herbaceous and woody plants, where experimental and synthesis studies have shown that herbaceous species generally exhibit higher stomatal sensitivity, faster hydraulic responses, and weaker buffering of leaf water status than woody plants (Bourbia et al. 2021; Huang et al. 2024). These results suggest that stomata in herbaceous plants may experience rapid and localised hydraulic signals that are poorly represented by bulk leaf properties. Gas exchange responses further clarified the nature of this deviation. The observed increase in intercellular CO₂ concentration with warming suggests that photosynthetic demand alone was an unlikely primary driver of stomatal opening, although indirect photosynthetic influences cannot be excluded. More direct physiological metrics (e.g., net photosynthetic rate, maximum carboxylation rate (V _cmax), maximum electron transport rate (J _max) and mesophyll conductance (g _m)) are required for the confirmation. Instead, this pattern points to water‐related control. Taken together, these findings indicate that the water status sensed by stomata, particularly in herbaceous species, is not fully captured by whole‐leaf hydraulic metrics. The authors therefore proposed that stomatal temperature responses may depend on finer‐scale processes, such as local water potential gradients around guard cells or temperature‐sensitive vapour‐phase water transport within the leaf, consistent with recent theoretical and experimental work highlighting microscale hydraulic heterogeneity within leaves (Buckley and Sack 2024; Jain et al. 2025). By revealing where whole‐leaf models fall short, the observed divergence reframes stomatal temperature responses as a multiscale hydraulic issue and represents a key advancement in the theoretical framework describing DRST (Mills et al. 2024).

Although photosynthesis was not the primary focus of this study, the DRST provides important mechanistic insights for understanding and predicting photosynthetic performance under warming. The observed DRST indicates that even under fixed VPD, increases in temperature can independently drive changes in g _s via hydraulic pathways, demonstrating that stomatal responses may not rely solely on VPD variation. However, studies of thermal acclimation have often attributed observed stomatal responses primarily to biochemical adjustments of the photosynthetic apparatus (e.g., changes in V _cmax and J _max) or to stomatal responses driven by VPD (e.g., empirical functions relating g _s to VPD) (Dusenge et al. 2021; Slot et al. 2024). In general, they have overlooked the direct pathway defined by ‘temperature → water viscosity → hydraulic conductance → stomatal aperture → CO₂ supply’ (i.e., DRST; Figure 1). Future studies require more targeted experimental designs. First, experiments should explicitly decouple temperature from VPD by manipulating leaf temperature while keeping Δw constant, simultaneously measuring gas exchange and photosynthetic capacity on the same leaves. Second, hydraulic conductance should be quantified as a function of temperature, K(T). Third, these hydraulic measurements should be paired with estimates of V _cmax and J _max to distinguish intrinsic biochemical acclimation from changes in CO₂ supply driven by hydraulic–stomatal responses. This distinction becomes particularly challenging under hydraulic limitation, where stomatal regulation and photosynthetic capacity may interact strongly (Fang et al. 2022). Finally, incorporating empirically constrained DRST and K(T) relationships into photosynthetic thermal acclimation models will be essential to evaluate potential biases when the DRST pathway is ignored, especially in predictions of tree performance under future warming (Figure 1).

In summary, Mills et al. (2026) demonstrated that stomatal conductance exhibited a consistent positive response to rising temperature when the evaporative demand was controlled, and this response strengthened with an increase in Δw. They further demonstrated that organ‐scale hydraulic traits alone cannot fully explain this behaviour. Instead, their findings highlighted the role of internal leaf water transport as a key process in stomatal regulation. Together, these results advance our understanding of hydraulic control of stomata and provide a new perspective for reassessing plant transpiration and water regulation strategies under climate warming.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.