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Abstract Rising atmospheric CO₂levels place terrestrial ecosystems under novel environmental conditions, and research in field settings is key to understanding how real plant communities will respond. Despite decades of progress in elevated CO₂(eCO₂) experiments, major gaps persist in our knowledge of plant responses to interacting influences of climate change, especially in areas outside North America and Western Europe.With a goal to expand access to field‐based eCO₂research, we designed, built, and tested TinyCO₂, a low‐cost field experiment for climate change research on plants. TinyCO₂features sixteen 0.62‐m²plot areas, half with ambient and half with elevated (+200 ppm) CO₂concentrations, and is suitable for short‐stature plants (≤0.5 m in height).Using a proportional‐integral control algorithm and constant sampling of air within the plots, TinyCO₂achieves consistent elevation of [CO₂] averaging +196.9 ppm. During testing, 95.1% of measured CO₂concentrations fell within 20% of the setpoint (ambient CO₂ + 200 ppm). A streamlined design and efficient use of instrumentation reduced the cost of the system to roughly one‐fifth of the cost of similar experiments from the past 30 years ($13.68 vs. $64.65 ppm⁻¹ m⁻², adjusted to 2024 USD).Our results demonstrate a system capable of precise and accurate field‐based CO₂elevation for significantly reduced cost. We envision the TinyCO₂design being implemented in a multitude of field‐based eCO₂studies, perhaps as part of a globally distributed collaborative network experiment. 

ABSTRACT Identifying the physiological mechanisms by which plants are adapted to drought is critical to predict species responses to climate change. We measured the responses of leaf hydraulic and stomatal conductances (K_leafandg_s, respectively) to dehydration, and their association with anatomy, in seven species of CaliforniaCeanothusgrown in a common garden, including some of the most drought‐tolerant species in the semi‐arid flora. We tested for matching of maximum hydraulic supply and demand and quantified the role of decline ofK_leafin driving stomatal closure. AcrossCeanothusspecies, maximumK_leafandg_swere negatively correlated, and bothK_leafandg_sshowed steep declines with decreasing leaf water potential (i.e., a high sensitivity to dehydration). The leaf water potential at 50% decline ing_swas linked with a low ratio of maximum hydraulic supply to demand (i.e., maximumK_leaf:g_s). This sensitivity ofg_s, combined with low minimum epidermal conductance and water storage, could contribute to prolonged leaf survival under drought. The specialized anatomy of subg.Cerastesincludes trichomous stomatal crypts and pronounced hypodermis, and was associated with higher water use efficiency and water storage. Combining our data with comparative literature of other California species, species of subg. Cerastesshow traits associated with greater drought tolerance and reliance on leaf water storage relative to other California species. In addition to drought resistance mechanisms such as mechanical protection and resistance to embolism, drought avoidance mechanisms such as sensitive stomatal closure could contribute importantly to drought tolerance in dry‐climate adapted species. 

Abstract Plants differ widely in how soil drying affects stomatal conductance (g_s) and leaf water potential (ψ_leaf), and in the underlying physiological controls. Efforts to breed crops for drought resilience would benefit from a better understanding of these mechanisms and their diversity. We grew 12 diverse genotypes of common bean (Phaseolus vulgarisL.) and four of tepary bean (P. acutifolius;a highly drought resilient species) in the field under irrigation and post‐flowering drought, and quantified responses ofg_sandψ_leaf, and their controls (soil water potential [ψ_soil], evaporative demand [Δw] and plant hydraulic conductance [K]). We hypothesised that (i) common beans would be more “isohydric” (i.e., exhibit strong stomatal closure in drought, minimisingψ_leafdecline) than tepary beans, and that genotypes with largerψ_leafdecline (more “anisohydric”) would exhibit (ii) smaller increases in Δw, due to less suppression of evaporative cooling by stomatal closure and hence less canopy warming, but (iii) largerKdeclines due toψ_leafdecline. Contrary to our hypotheses, we found that half of the common bean genotypes were similarly anisohydric to most tepary beans; canopy temperature was cooler in isohydric genotypes leading to smaller increases in Δwin drought; and that stomatal closure andKdecline were similar in isohydric and anisohydric genotypes.g_sandψ_leafwere virtually insensitive to drought in one tepary genotype (G40068). Our results highlight the potential importance of non‐stomatal mechanisms for leaf cooling, and the variability in drought resilience traits among closely related crop legumes. 

Abstract PremiseThe adaptive significance of amphistomy (stomata on both upper and lower leaf surfaces) is unresolved. A widespread association between amphistomy and open, sunny habitats suggests the adaptive benefit of amphistomy may be greatest in these contexts, but this hypothesis has not been tested experimentally. Understanding amphistomy informs its potential as a target for crop improvement and paleoenvironment reconstruction. MethodsWe developed a method to quantify “amphistomy advantage” () as the log‐ratio of photosynthesis in an amphistomatous leaf to that of the same leaf but with gas exchange blocked through the upper surface (pseudohypostomy). Humidity modulated stomatal conductance and thus enabled comparing photosynthesis at the same total stomatal conductance. We estimated and leaf traits in six coastal (open, sunny) and six montane (closed, shaded) populations of the indigenous Hawaiian species ʻilima (Sida fallax). ResultsCoastal ʻilima leaves benefit 4.04 times more from amphistomy than montane leaves. Evidence was equivocal with respect to two hypotheses: (1) that coastal leaves benefit more because they are thicker and have lower CO₂conductance through the internal airspace and (2) that they benefit more because they have similar conductance on each surface, as opposed to most conductance being through the lower surface. ConclusionsThis is the first direct experimental evidence that amphistomy increases photosynthesis, consistent with the hypothesis that parallel pathways through upper and lower mesophyll increase CO₂supply to chloroplasts. The prevalence of amphistomatous leaves in open, sunny habitats can partially be explained by the increased benefit of amphistomy in “sun” leaves, but the mechanistic basis remains uncertain. 

Abstract Leaf surface conductance to water vapor and CO2 across the epidermis (gleaf) strongly determines the rates of gas exchange. Thus, clarifying the drivers of gleaf has important implications for resolving the mechanisms of photosynthetic productivity and leaf and plant responses and tolerance to drought. It is well recognized that gleaf is a function of the conductances of the stomata (gs) and of the epidermis + cuticle (gec). Yet, controversies have arisen around the relative roles of stomatal density (d) and size (s), fractional stomatal opening (α; aperture relative to maximum), and gec in determining gleaf. Resolving the importance of these drivers is critical across the range of leaf surface conductances, from strong stomatal closure under drought (gleaf,min), to typical opening for photosynthesis (gleaf,op), to maximum achievable opening (gleaf,max). We derived equations and analyzed a compiled database of published and measured data for approximately 200 species and genotypes. On average, within and across species, higher gleaf,min was determined 10 times more strongly by α and gec than by d and negligibly by s; higher gleaf,op was determined approximately equally by α (47%) and by stomatal anatomy (45% by d and 8% by s), and negligibly by gec; and higher gleaf,max was determined entirely by d. These findings clarify how diversity in stomatal functioning arises from multiple structural and physiological causes with importance shifting with context. The rising importance of d relative to α, from gleaf,min to gleaf,op, enables even species with low gleaf,min, which can retain leaves through drought, to possess high d and thereby achieve rapid gas exchange in periods of high water availability. 

Abstract Photosynthesis is co-limited by multiple factors depending on the plant and its environment. These include biochemical rate limitations, internal and external water potentials, temperature, irradiance and carbon dioxide ( CO2). Amphistomatous leaves have stomata on both abaxial and adaxial leaf surfaces. This feature is considered an adaptation to alleviate CO2 diffusion limitations in productive environments as the diffusion path length from stomate to chloroplast is effectively halved in amphistomatous leaves. Plants may also reduce CO2 limitations through other aspects of optimal stomatal anatomy: stomatal density, distribution, patterning and size. Some studies have demonstrated that stomata are overdispersed compared to a random distribution on a single leaf surface; however, despite their prevalence in nature and near ubiquity among crop species, much less is known about stomatal anatomy in amphistomatous leaves, especially the coordination between leaf surfaces. Here, we use novel spatial statistics based on simulations and photosynthesis modelling to test hypotheses about how amphistomatous plants may optimize CO2 diffusion in the model angiosperm Arabidopsis thaliana grown in different light environments. We find that (i) stomata are overdispersed, but not ideally dispersed, on both leaf surfaces across all light treatments; (ii) the patterning of stomata on abaxial and adaxial leaf surfaces is independent and (iii) the theoretical improvements to photosynthesis from abaxial–adaxial stomatal coordination are miniscule (≪1%) across the range of feasible parameter space. However, we also find that (iv) stomatal size is correlated with the mesophyll volume that it supplies with CO2, suggesting that plants may optimize CO2 diffusion limitations through alternative pathways other than ideal, uniform stomatal spacing. We discuss the developmental, physical and evolutionary constraints that may prohibit plants from reaching this theoretical adaptive peak of uniform stomatal spacing and inter-surface stomatal coordination. These findings contribute to our understanding of variation in the anatomy of amphistomatous leaves. 

Abstract Stomata are the gatekeepers of plant water use and must quickly respond to changes in plant water status to ensure plant survival under fluctuating environmental conditions. The mechanism for their closure is highly sensitive to disturbances in leaf water status, which makes isolating their response to declining water content difficult to characterise and to compare responses among species. Using a small‐scale non‐destructive nuclear magnetic resonance spectrometer as a leaf water content sensor, we measure the stomatal response to rapid induction of water deficit in the leaves of nine species of eucalypt from contrasting climates. We found a strong linear correlation between relative water content at 50% stomatal conductance (RWC_gs50) and mean annual temperature at the climate of origin of each species. We also show evidence for stomata to maintain control over water loss well below turgor loss point in species adapted to warmer climates and secondary increases in stomatal conductance despite declining water content. We propose that RWC_gs50is a promising trait to guide future investigations comparing stomatal responses to water deficit. It may provide a useful phenotyping trait to delineate tolerance and adaption to hot temperatures and high leaf‐to‐air vapour pressure deficits. 

Abstract PremisePrevious studies have suggested a trade‐off between trichome density (D_t) and stomatal density (D_s) due to shared cell precursors. We clarified how, when, and why this developmental trade‐off may be overcome across species. MethodsWe derived equations to determine the developmental basis forD_tandD_sin trichome and stomatal indices (i_tandi_s) and the sizes of epidermal pavement cells (e), trichome bases (t), and stomata (s) and quantified the importance of these determinants ofD_tandD_sfor 78 California species. We compiled 17 previous studies ofD_t–D_srelationships to determine the commonness ofD_t–D_sassociations. We modeled the consequences of differentD_t–D_sassociations for plant carbon balance. ResultsOur analyses showed that higherD_twas determined by higheri_tand lowere, and higherD_sby higheri_sand lowere. Across California species, positiveD_t–D_scoordination arose due toi_t–i_scoordination and impacts of the variation ine. AD_t–D_strade‐off was found in only 30% of studies. Heuristic modeling showed that species sets would have the highest carbon balance with a positive or negative relationship or decoupling ofD_tandD_s, depending on environmental conditions. ConclusionsShared precursor cells of trichomes and stomata do not limit higher numbers of both cell types or drive a generalD_t–D_strade‐off across species. This developmental flexibility across diverse species enables differentD_t–D_sassociations according to environmental pressures. Developmental trait analysis can clarify how contrasting trait associations would arise within and across species. 

Abstract Background Remote sensing instruments enable high-throughput phenotyping of plant traits and stress resilience across scale. Spatial (handheld devices, towers, drones, airborne, and satellites) and temporal (continuous or intermittent) tradeoffs can enable or constrain plant science applications. Here, we describe the technical details of TSWIFT (Tower Spectrometer on Wheels for Investigating Frequent Timeseries), a mobile tower-based hyperspectral remote sensing system for continuous monitoring of spectral reflectance across visible-near infrared regions with the capacity to resolve solar-induced fluorescence (SIF). Results We demonstrate potential applications for monitoring short-term (diurnal) and long-term (seasonal) variation of vegetation for high-throughput phenotyping applications. We deployed TSWIFT in a field experiment of 300 common bean genotypes in two treatments: control (irrigated) and drought (terminal drought). We evaluated the normalized difference vegetation index (NDVI), photochemical reflectance index (PRI), and SIF, as well as the coefficient of variation (CV) across the visible-near infrared spectral range (400 to 900 nm). NDVI tracked structural variation early in the growing season, following initial plant growth and development. PRI and SIF were more dynamic, exhibiting variation diurnally and seasonally, enabling quantification of genotypic variation in physiological response to drought conditions. Beyond vegetation indices, CV of hyperspectral reflectance showed the most variability across genotypes, treatment, and time in the visible and red-edge spectral regions. Conclusions TSWIFT enables continuous and automated monitoring of hyperspectral reflectance for assessing variation in plant structure and function at high spatial and temporal resolutions for high-throughput phenotyping. Mobile, tower-based systems like this can provide short- and long-term datasets to assess genotypic and/or management responses to the environment, and ultimately enable the spectral prediction of resource-use efficiency, stress resilience, productivity and yield.