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Circadian resonance, whereby a plant's endogenous rhythms are tuned to match environmental cues, has been repeatedly shown to be adaptive, although the underlying mechanisms remain elusive. Concomitantly, the adaptive value of nocturnal transpiration in C3 plants remains unknown because it occurs without carbon assimilation. These seemingly unrelated processes are interconnected because circadian regulation drives temporal patterns in nocturnal stomatal conductance, with maximum values occurring immediately before dawn for many species. We grew individuals of six Eucalyptus camaldulensis genotypes in naturally lit glasshouses and measured sunset, predawn and midday leaf gas exchange and whole-plant biomass production. We tested whether sunrise anticipation by the circadian clock and subsequent increases in genotype predawn stomatal conductance led to rapid stomatal opening upon illumination, ultimately affecting genotype differences in carbon assimilation and growth. We observed faster stomatal responses to light inputs at sunrise in genotypes with higher predawn stomatal conductance. Moreover, early morning and midday stomatal conductance and carbon assimilation, leaf area and total plant biomass were all positively correlated with predawn stomatal conductance across genotypes. Our results lead to the novel hypothesis that genotypic variation in the circadian-regulated capacity to anticipate sunrise could be an important factor underlying intraspecific variation in tree growth.

Establishing the temperature dependence of respiration is critical for accurate predictions of the global carbon cycle under climate change. Diurnal temperature fluctuations, or changes in substrate availability, lead to variations in leaf respiration. Additionally, recent studies hint that the thermal sensitivity of respiration could be time‐dependent. However, the role for endogenous processes, independent from substrate availability, as drivers of temporal changes in the sensitivity of respiration to temperature across phylogenies has not yet been addressed. Here, we examined the diurnal variation in the response of respiration to temperatures (R–T relationship) for different lycophyte, fern, gymnosperm and angiosperm species. We tested whether time‐dependent changes in the R–T relationship would impact leaf level respiration modelling. We hypothesized that interactions between endogenous processes, like the circadian clock, and leaf respiration would be independent from changes in substrate availability. Overall, we observed a time‐dependent sensitivity in the R–T relationship across phylogenies, independent of temperature, that affected modelling parameters. These results are compatible with circadian gating of respiration, but further studies should analyse the possible involvement of the clock. Our results indicate time‐dependent regulation of respiration might be widespread across phylogenies, and that endogenous regulation of respiration is likely affecting leaf‐level respiration fluxes. Summary statement: We observed that the sensitivity of respiration to temperature varied temporally across phylogenies, independent of temperature. Our findings highlight that time‐dependent regulation of respiration might be widespread across lycophytes, ferns, gymnosperms and angiosperms. [ABSTRACT FROM AUTHOR]

Vapour pressure deficit is a major driver of seasonal changes in transpiration, but photoperiod also modulates leaf responses. Climate warming might enhance transpiration by increasing atmospheric water demand and the length of the growing season, but photoperiod‐sensitive species could show dampened responses. Here, we document that day length is a significant driver of the seasonal variation in stomatal conductance. We performed weekly gas exchange measurements across a common garden experiment with 12 oak species from contrasting geographical origins, and we observed that the influence of day length was of similar strength to that of vapour pressure deficit in driving the seasonal pattern. We then examined the generality of our findings by incorporating day‐length regulation into well‐known stomatal models. For both angiosperm and gymnosperm species, the models improved significantly when adding day‐length dependences. Photoperiod control over stomatal conductance could play a large yet underexplored role on the plant and ecosystem water balances. We demonstrate that photoperiod exerts a major control on the seasonal pattern of stomatal conductance, being as important as the effects of varying VPD. Inclusion of photoperiod regulation in stomatal models led to significant improvements in model fit across angiosperm and gymnosperm species. [ABSTRACT FROM AUTHOR]