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Summary Plants integrate environmental stimuli to optimize photosynthesis vs water loss by controlling stomatal apertures. However, stomatal responses to temperature elevation and the underlying molecular genetic mechanisms remain less studied.We developed an approach for clamping leaf‐to‐air vapor pressure difference (VPD_leaf) to fixed values, and recorded robust reversible warming‐induced stomatal opening in intact plants. We analyzed stomatal temperature responses of mutants impaired in guard cell signaling pathways for blue light, abscisic acid (ABA), CO₂, and the temperature‐sensitive proteins, Phytochrome B (phyB) and EARLY‐FLOWERING‐3 (ELF3).We confirmed thatphot1‐5/phot2‐1leaves lacking blue‐light photoreceptors showed partially reduced warming‐induced stomatal opening. Furthermore, ABA‐biosynthesis, phyB, and ELF3 were not essential for the stomatal warming response. Strikingly,Arabidopsis(dicot) andBrachypodium distachyon(monocot) mutants lacking guard cell CO₂sensors and signaling mechanisms, includinght1,mpk12/mpk4‐gc, andcbc1/cbc2abolished the stomatal warming response, suggesting a conserved mechanism across diverse plant lineages. Moreover, warming rapidly stimulated photosynthesis, resulting in a reduction in intercellular (CO₂). Interestingly, further enhancing heat stress caused stomatal opening uncoupled from photosynthesis.We provide genetic and physiological evidence that the stomatal warming response is triggered by increased CO₂assimilation and stomatal CO₂sensing. Additionally, increasing heat stress functions via a distinct photosynthesis‐uncoupled stomatal opening pathway. 

A variety of proxies have been developed to reconstruct paleo-CO2 from fossil leaves. These proxies rely on some combination of stomatal morphology, leaf δ13C, and leaf gas exchange. A common conceptual framework for evaluating these proxies is lacking, which has hampered efforts for inter-comparison. Here we develop such a framework, based on the underlying physics and biochemistry. From this conceptual framework, we find that the more extensively parameterised proxies, such as the optimisation model, are likely to be the most robust. The simpler proxies, such as the stomatal ratio model, tend to under-predict CO2, especially in warm (>15
C) and moist (>50% humidity) environments. This identification of a structural underprediction may help to explain the common observation that the simpler proxies often produce estimates of paleo-CO2 that are lower than those from the more complex proxies and other, non-leaf-based CO2 proxies. The use of extensively parameterised models is not always possible, depending on the preservation state of the fossils and the state of knowledge about the fossil's nearest living relative. With this caveat in mind, our analysis highlights the value of using the most complex leafbased model as possible.