Scott McAdam

Purdue University | Purdue · Department of Botany and Plant Pathology

Understanding the relationship between wind speed and gas exchange in plants is a longstanding challenge. Our aim was to investigate the impact of wind speed on maximum rates of gas exchange and the kinetics of stomatal responses. We conducted experiments in different angiosperm and fern species using an infrared gas analyzer equipped with a contro...

Leaf hydraulic traits are considered the key determinants of gas exchange and therefore affect species distributions along environmental gradients, but the patterns of leaf hydraulic traits and their associations with gas exchange across altitudinal gradients remain largely unknown. Here, we measured leaf hydraulic traits, gas exchange, leaf anatom...

Freezing air temperatures kill most leaves, yet the leaves of some species can survive these events. Tracking the temporal and spatial dynamics of freezing remains an impediment to characterizing frost tolerance. Here we deploye time‐lapse imaging and image subtraction analysis, coupled with fine wire thermocouples, to discern the in situ spatial d...

By regulating carbon uptake and water loss by plants, stomata are not only responsible for productivity but also survival during drought. The timing of the onset of stomatal closure is crucial for preventing excessive water loss during drought, but is poorly explained by plant hydraulics alone and what triggers stomatal closure remains disputed. We...

The onset of stomatal closure reduces transpiration during drought. In seed plants, drought causes declines in plant water status which increases leaf endogenous abscisic acid (ABA) levels required for stomatal closure. There are multiple possible points of increased belowground resistance in the soil–plant atmospheric continuum that could decrease...

The vulnerability of plant xylem to embolism can be described as the water potential at which xylem conductivity is lost by 50% (P50). According to the traditional hypothesis of hydraulic vulnerability segmentation, the difference in vulnerability to embolism between branches and roots is positive (P50 root−branch > 0). It is not clear whether this...

The stems of some herbaceous species can undergo basal secondary growth, leading to a continuum in the degree of woodiness along the stem. Whether the formation of secondary growth in the stem base results in differences in embolism resistance between the base and the upper portions of stems is unknown. We assessed the embolism resistance of leaves...

Stomatal regulation is critical for mangroves to survive in the hyper-saline intertidal zone where water stress is severe and water availability is highly fluctuant. However, very little is known about the stomatal sensitivity to vapour pressure deficit (VPD) in mangroves, and its co-ordination with stomatal morphology and leaf hydraulic traits. We...

Stomatal opening in the light, observed in nearly all vascular land plants, is essential for providing access to atmospheric CO2 for photosynthesis. The speed of stomatal opening in the light is critical for maximizing carbon gain in environments in which light intensity changes, yet we have little understanding of how other environmental signals,...

The phytohormone abscisic acid (ABA) plays a major role in closing the stomata of angiosperms. However, recent reports of some angiosperm species having a peaking-type ABA dynamic, in which under extreme drought ABA levels decline to pre-stressed levels, raises the possibility that passive stomatal closure by leaf water status alone can occur in sp...

Understanding the relationship between wind speed and gas exchange in plants is a longstanding challenge. Our aim was to investigate the impact of wind speed on maximum rates of gas exchange and the kinetics of stomatal responses. We conducted experiments using an infrared gas analyzer equipped with a controlled leaf fan, enabling precise control o...

The genus Ajuga is widely distributed in temperate to subtropical regions, and four species are currently recognized in Korea ( A. decumbens , A. multiflora , A. nipponensis , and A. spectabilis ), but epidermal anatomical differences across these species have never been described. A comparative study of the leaf micromorphological characteristics...

The phytohormone abscisic acid (ABA), synthesized as leaf turgor declines, plays a major role in closing stomata in species from this lineage, but recent reports of some angiosperms having a peaking-type ABA dynamic in which under extreme drought ABA levels decline to pre-stressed levels raises the possibility that passive stomatal closure by leaf...

The phytohormone abscisic acid (ABA) is synthesised by plants during drought to close stomata and regulate desiccation tolerance pathways. Conifers and some angiosperms with embolism‐resistant xylem show a peaking‐type (p‐type) response in ABA levels, in which ABA levels increase early in drought then decrease as drought progresses, declining to pr...

Vapor pressure difference between the leaf and atmosphere (VPD) is the most important regulator of daytime transpiration, yet the mechanism driving stomatal responses to an increase in VPD in angiosperms remains unresolved. Here, we sought to characterize the mechanism driving stomatal closure at high VPD in an angiosperm species, particularly test...

Stomatal closure limits transpiration during drought, restricting water potential decline and delaying the onset of embolism. While critical for ensuring survival during drought, the mechanisms driving stomatal closure during drought remain equivocal. The hormone abscisic acid (ABA) will close stomata in seed plants and is synthesized as leaf turgo...

Drought resistance is essential for plant production under water-limiting environments. Abscisic acid (ABA) plays a critical role in stomata but its impact on hydraulic function beyond the stomata is far less studied. We selected genotypes differing in their ability to accumulate ABA to investigate its role in drought-induced dysfunction. All genot...

Stomata are small epidermal pores responsible for the strict control of the amount of CO2 that diffuses into the leaves while controlling the amount of water vapor lost to the atmosphere. The time required for the stomatal valve opening and closing is coordinated with an optimized hydraulic supply and strongly responds to the surrounding environmen...

By regulating carbon uptake and water loss by plants, stomata are not only responsible for productivity but also survival during drought. The timing of stomatal closure is crucial for preventing excessive water loss during drought, yet has high ecological variability between species. An aspect of stomatal response that remains disputed is the mecha...

Carbon-water trade-offs in plants are adjusted through stomatal regulation. Stomatal opening enables carbon uptake and plant growth, whereas plants circumvent drought by closing stomata. The specific effects of leaf position and age on stomatal behavior remain largely unknown, especially under edaphic and atmospheric drought. Here, we compared stom...

This scientific commentary refers to ‘Ethylene constrains stomatal reopening in Fraxinus chinensis post moderate drought’ by Bi et al. (doi: 10.1093/treephys/tpac144). Drought is an existential threat to terrestrial plant life (Brodribb et al. 2020). The severing of the liquid continuum between the soil and the evaporating surfaces of the leaf by x...

Senescence vividly marks the onset of the final stages of the life of a leaf, yet the triggers and drivers of this process are still not fully understood. The hormone abscisic acid (ABA) is an important regulator of leaf senescence in model herbs, but the function of this hormone has not been widely tested in deciduous trees. Here we investigate th...

Stomatal regulation is critical for mangroves to survive water deficits and highly fluctuating ambient water availability in the hyper-saline intertidal zone. Despite the importance of stomatal regulation in mangroves very little is known about stomatal sensitivity to vapour pressure deficit (VPD), and the co-ordination of this trait with stomatal...

The Marsileaceae is a small family of semi-aquatic ferns displaying numerous traits commonly observed in angiosperms, including heterospory, sophisticated hydraulic architecture, and high rates of atmospheric gas exchange. Despite these similar traits, Marsileaceae is comparatively ecologically limited. Most species are found in Marsilea which is s...

The cause of reduced leaf-level transpiration under elevated CO2 remains largely elusive. Here, we assessed stomatal, hydraulic and morphological adjustments in a long-term experiment on Aleppo pine (Pinus halepensis) seedlings germinated and grown for 22-40 months under elevated (eCO2; c. 860 ppm) or ambient (aCO2; c. 410 ppm) CO2. We assessed if...

The resistance of xylem conduits to embolism is a major factor defining drought tolerance and can set the distributional limits of species across rainfall gradients. Recent work suggests that the proximity of vessels to neighbors increases the vulnerability of a conduit. We therefore investigated whether the relative vessel area of xylem correlates...

Maintaining water transport in the xylem is critical for vascular plants to grow and survive. The drought-induced accumulation of embolism, when gas enters xylem conduits, causes declines in hydraulic conductance (K) and is ultimately lethal. Several methods can be used to estimate the degree of embolism in xylem, from measuring K in tissues to dir...

Leaf water potential (ψ leaf), typically measured using the pressure chamber, is the most important metric of plant water status, providing high theoretical value and information content for multiple applications in quantifying critical physiological processes including drought responses. Pressure chamber measurements of ψ leaf (ψ leafPC) are most...

The phytohormone abscisic acid (ABA) is synthesized by plants during drought to close stomata and regulate desiccation tolerance pathways. In conifers and a few angiosperms with embolism resistant xylem a peaking-type (p-type) response in ABA levels has been observed, in which ABA levels increase early in drought then decrease as drought progresses...

Forest mortality during drought has been attributed to hydraulic failure, which can be challenging to measure. A limited number of alternative proxies for incipient leaf death exist. Here we investigate whether a terminal increase in abscisic acid (ABA) levels in leaves occurs across vascular land plants and is an indicator of imminent leaf death....

Plant specialized 1,4-naphthoquinones present a remarkable case of convergent evolution. Species across multiple discrete orders of vascular plants produce diverse 1,4-naphthoquinones via one of several pathways using different metabolic precursors. Evolution of these pathways was preceded by events of metabolic innovation and many appear to share...

• Drought events may increase the likelihood that the plant water transport system becomes interrupted by embolism. Yet our knowledge about the temporal frequency of xylem embolism in the field is frequently lacking, as it requires detailed, long-term measurements. • We measured xylem embolism resistance and midday xylem water potentials during the...

Xylem embolism resistance varies across species influencing drought tolerance, yet little is known about the determinants of the embolism resistance of an individual conduit. Here we conducted an experiment using the optical vulnerability method to test whether individual conduits have a specific water potential threshold for embolism formation and...

Significance Plants need to constantly adapt to a changing environment. Adaptation includes responses to biotic and abiotic stress. Key elements determining the response to abiotic stress are the cell walls surrounding all plant cells and the phytohormone abscisic acid, which influence turgor pressure in plants. Turgor pressure in plant cells is mu...

C4 plants frequently experience high light and high temperature conditions in the field, which reduce growth and yield. However, the mechanisms underlying these stress responses in C4 plants have been under-explored, especially the coordination between mesophyll (M) and bundle sheath (BS) cells. We investigated how the C4 model plant Setaria viridi...

Xylem embolism resistance varies across species influencing drought tolerance, yet little is known about the determinants of the embolism resistance of an individual conduit. Here we conducted an experiment using the optical vulnerability method to test whether individual conduits have a specific water potential threshold for embolism formation and...

Plant cells can be distinguished from animal cells by their cell walls and high turgor pressure. Although changes in turgor and stiffness of cell walls seem coordinated, we know little about the mechanism responsible for coordination. Evidence has accumulated that plants, like yeast, have a dedicated cell wall integrity maintenance mechanism. This...

Global warming is expected to dramatically accelerate forest mortality as temperature and drought intensity increase. Predicting the magnitude of this impact urgently requires an understanding of the process connecting atmospheric drying to plant tissue damage. Recent episodes of forest mortality worldwide have been widely attributed to dry conditi...

Plant specialized 1,4-naphthoquinones present a remarkable case of convergent evolution. Species across multiple discrete orders of vascular plants produce diverse 1,4-naphthoquinones via one of several pathways using different metabolic precursors. Evolution of these pathways was preceded by events of metabolic innovation and many appear to share...

The alternation of generations in land plants occurs between the sporophyte phase and the gametophyte phase. The sporophytes of seed plants develop self-maintained, multicellular meristems, and these meristems determine plant architecture. The gametophytes of seed plants lack meristems and are heterotrophic. In contrast, the gametophytes of seed-fr...

The alternation of generations in land plants occurs between the sporophyte phase and the gametophyte phase. The sporophytes of seed plants develop self-maintained, multicellular meristems, and these meristems determine plant architecture. The gametophytes of seed plants lack meristems and are heterotrophic. In contrast, the gametophytes of seed-fr...

Xylem resistance to embolism is a key metric determining plant survival during drought. Yet, we have a limited understanding of the degree of plasticity in vulnerability to embolism. Here, we tested whether light availability influences embolism resistance in leaves and stems. The optical vulnerability method was used to assess stem and leaf resist...

The genus Disporum Salisb. is widely distributed in East Asia, yet phylogenetically relevant morphological traits useful for differentiating many of the small, perennial, herbaceous species remain poorly described. To address this, leaf, floral, pollen, and orbicule micromorphology of four Korean Disporum species was investigated using light and sc...

As the closest extant sister group to seed plants, ferns are an important reference point to study the origin and evolution of plant genes and traits. One bottleneck to the use of ferns in phylogenetic and genetic studies is the fact that genome-level sequence information of this group is limited, due to the extreme genome sizes of most ferns. Cera...

C4 plants frequently experience damaging high light (HL) and high temperature (HT) conditions in native environments, which reduce growth and yield. However, the mechanisms underlying these stress responses in C4 plants have been under-explored, especially the coordination between mesophyll (M) and bundle sheath (BS) cells. We investigated how the...

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Embolism spreading in dehydrating angiosperm xylem is driven by gas movement between embolised and sap‐filled conduits. Here, we examine how the proximity to pre‐existing embolism and hydraulic segmentation affect embolism propagation. Based on the optical method, we compared xylem embolism resistance between detached leaves and leaves attached to...

The plant cuticle is the final barrier for volatile organic compounds (VOCs) to cross for release to the atmosphere, yet its role in the emission process is poorly understood. Here, using a combination of reverse-genetic and chemical approaches, we demonstrate that the cuticle imposes substantial resistance to VOC mass transfer, acting as a sink/co...

Embolism spreading in dehydrating angiosperm xylem is driven by gas movement between embolised and sap-filled conduits. Here, we examine how proximity to pre-existing embolism and hydraulic segmentation affect embolism propagation. Based on the optical method, we compared xylem embolism resistance between detached leaves and leaves attached to bran...

Angiosperm dominance in terrestrial landscapes is partially attributable to high photosynthetic capacities. Angiosperms benefit from diverse anatomical and physiological adaptations, making it difficult to determine which factors may have been prerequisites for the evolution of enhanced photosynthetic rates in this group. We employed a novel approa...

Stomata respond to changes in light environments through multiple mechanisms that jointly regulate the tradeoff between carbon assimilation and water loss. The stomatal response to blue light is highly sensitive, rapid, not driven by photosynthesis, and present in most vascular plant groups but is believed to have been lost in the ancestor of lepto...

High rates of water loss in young, expanding leaves have previously been attributed to open stomata that only develop a capacity to close once exposed to low humidity and high abscisic acid (ABA) levels. To test this model, we quantified water loss through stomata and cuticle in expanding leaves of Quercus rubra. Stomatal anatomy and density were o...

Dynamic variation of the stomatal pore in response to changes in leaf-air vapour pressure difference (VPD) constitutes a critical regulation of daytime gas exchange. The stomatal response to VPD has been associated with both foliage abscisic acid (ABA) and leaf water potential (Ψ l ); however, causation remains a matter of debate. Here, we seek to...

Lycophytes are the earliest diverging extant lineage of vascular plants, sister to all other vascular plants. Given that most species are adapted to ever‐wet environments, it has been hypothesized that lycophytes, and by extension the common ancestor of all vascular plants, have few adaptations to drought. We investigated the responses to drought o...

Abscisic acid (ABA) is found in a wide diversity of organisms, yet we know most about the hormonal action of this compound in the ecologically dominant and economically important angiosperms. In angiosperms, ABA regulates a suite of critical responses from desiccation tolerance through to seed dormancy and stomatal closure. Work exploring the funct...

The perception pathway for endogenous auxin has been well described, yet the mode of action of synthetic auxin herbicides, used for over 70 years, remains uncharacterized. We utilized transcriptomics and targeted physiological studies to investigate the unknown rapid response to synthetic auxin herbicides in the globally problematic weed species Er...

The evolution of terrestrial plants capable of growing upwards into the dry atmosphere profoundly transformed the Earth. A transition from small, ‘non-vascular’ bryophytes to arborescent vascular plants during the Devonian period is partially attributed to the evolutionary innovation of an internal vascular system capable of functioning under the s...

The driver of leaf mortality during drought stress is a critical unknown. We utilized the commercially important tree Persea americana, in which there is large variation in the degree of drought-induced leaf death across the canopy, to test whether embolism formation in the xylem during drought drives this leaf mortality. A large range in the numbe...

The best predictor of leaf level photosynthetic rate is the porosity of the leaf surface, as determined by the number and aperture of stomata on the leaf (Wong et al. 1979; Franks and Beerling 2009; Boyer 1976). This remarkable correlation between stomatal porosity (or diffusive conductance to water vapour‐ gs) and CO2 assimilation rate (A) applies...

Stomatal responses to changes in leaf water status are critical for minimizing excessive water loss during soil drought. A major debate has surrounded the evolution of stomatal responses to water status and this debate has particularly focused on the evolution of the regulatory role of the drought hormone abscisic acid (ABA). Studies relying on the...

(Key message) Highly resistant xylem has evolved multiple times over the past 400 million years. (Context) Water is transported under tension in xylem and consequently is vulnerable to invasion by air and the formation of embolism. A debate has raged over whether embolism formation is non-reversible occurring at low water potentials or a regular di...

Stomatal responses to changes in leaf water status are important for the diurnal regulation of gas exchange and the survival of plants during drought. These stomatal responses in angiosperm species are well characterized, yet an ongoing debate surrounds the role of metabolism, particularly the role of the hormone abscisic acid (ABA), in functionall...

PREMISE OF THE STUDY: The densities of veins and stomata govern leaf water supply and gas exchange. They are coordinated to avoid overproduction of either veins or stomata. In many species, where leaf area is greater at low light, this coordination is primarily achieved through differential cell expansion, resulting in lower stomatal and vein densi...

The hormone abscisic acid (ABA) plays a critical role in enhancing plant survival during water deficit. Recent molecular evidence suggests that ABA is synthesized in the phloem companion cells and guard cells. However, the nature of cell turgor and water status in these two cell types cannot easily account for the rapid, water status-triggered ABA...

Recent studies have revealed that some responses of fern stomata to environmental signals differ from those of their relatives in seed plants. However, it is unknown whether the biophysical properties of guard cells differ fundamentally between species of both clades. Intracellular micro‐electrodes and the fluorescent Ca ²⁺ reporter FURA 2 were use...

Rapid biosynthesis of abscisic acid (ABA) in the leaf, triggered by a decrease in cell volume, is essential for a functional stomatal response to vapour pressure deficit (VPD) in angiosperms. However, it is not known whether rapid biosynthesis of ABA is triggered in other plant tissues as well. Through the application of external pressure to flower...

Angiosperms are able to respond rapidly to the first sign of dry conditions, a decrease in air humidity, more accurately described as an increase in the vapor pressure deficit between the leaf and the atmosphere (VPD), by abscisic acid (ABA)-mediated stomatal closure. The genes underlying this response offer valuable candidates for targeted selecti...

Species are often classified along a continuum from isohydric to anisohydric, with isohydric species exhibiting tighter regulation of leaf water potential through stomatal closure in response to drought. We investigated plasticity in stomatal regulation in an isohydric (Eucalyptus camaldulensis) and an anisohydric (Acacia aptaneura) angiosperm spec...

Land plants lose vast quantities of water to the atmosphere during photosynthetic gas exchange. In angiosperms a complex network of veins irrigates the leaf, and it is widely held that the density and placement of these veins determines maximum leaf hydraulic capacity and thus maximum photosynthetic rate. This theory is largely based on interspecif...

Homologs of the Arabidopsis core abscisic acid (ABA) signalling component OPEN STOMATA1 (OST1) are best known for their role in closing stomata in angiosperm species. We recently characterised a fern OST1 homolog, GAMETOPHYTES ABA INSENSITIVE ON ANTHERDIOGEN 1 (GAIA1), which is not required for stomatal closure in ferns, consistent with physiologic...

Background/Question/Methods Drought can cause major damage to plant communities, but species damage thresholds and post-drought recovery of forest productivity are not yet predictable. We asked the question how should forest net primary productivity recover following exposure to severe drought? We used an El Niño drought event as a natural experime...