The concept of iso‐ vs. anisohydry has been used to describe the stringency of stomatal regulation of plant water potential (ψ). However, metrics that accurately and consistently quantify species’ ...operating ranges along a continuum of iso‐ to anisohydry have been elusive. Additionally, most approaches to quantifying iso/anisohydry require labour‐intensive measurements during prolonged drought. We evaluated new and previously developed metrics of stringency of stomatal regulation of ψ during soil drying in eight woody species and determined whether easily‐determined leaf pressure–volume traits could serve as proxies for their degree of iso‐ vs. anisohydry. Two metrics of stringency of stomatal control of ψ, (1) a ‘hydroscape’ incorporating the landscape of ψ over which stomata control ψ, and (2) the slope of the daily range of ψ as pre‐dawn ψ declined, were strongly correlated with each other and with the leaf osmotic potential at full and zero turgor derived from pressure–volume curves.
Species' differences in the stringency of stomatal control of plant water potential represent a continuum of isohydric to anisohydric behaviours. However, little is known about how quasi‐steady‐state ...stomatal regulation of water potential may relate to dynamic behaviour of stomata and photosynthetic gas exchange in species operating at different positions along this continuum. Here, we evaluated kinetics of light‐induced stomatal opening, activation of photosynthesis and features of quasi‐steady‐state photosynthetic gas exchange in 10 woody species selected to represent different degrees of anisohydry. Based on a previously developed proxy for the degree of anisohydry, species' leaf water potentials at turgor loss, we found consistent trends in photosynthetic gas exchange traits across a spectrum of isohydry to anisohydry. More anisohydric species had faster kinetics of stomatal opening and activation of photosynthesis, and these kinetics were closely coordinated within species. Quasi‐steady‐state stomatal conductance and measures of photosynthetic capacity and performance were also greater in more anisohydric species. Intrinsic water‐use efficiency estimated from leaf gas exchange and stable carbon isotope ratios was lowest in the most anisohydric species. In comparisons between gas exchange traits, species rankings were highly consistent, leading to species‐independent scaling relationships over the range of isohydry to anisohydry observed.
Little is known about how quasi‐steady‐state stomatal regulation of water potential along a continuum of isohydry to anisohydry may relate to dynamic behaviour of stomata and photosynthetic gas exchange in species operating at different positions along this continuum. We found pronounced and consistent trends in dynamic and quasi‐steady‐state regulation of photosynthetic gas exchange in 10 diverse woody species identified as occupying different positions along a continuum of isohydry to anisohydry based on their leaf water potentials at turgor loss (ΨTLP). Rates of light‐induced stomatal opening and activation of photosynthesis increased with increasing anisohydry as did quasi‐steady‐state operating stomatal conductance and maximum photosynthetic rate. Our results have important implications for understanding multiple functional trade‐offs that cause species to operate at different points along a whole‐plant economics spectrum of fast to slow strategies in terms of rates of resource acquisition and use. Use of ΨTLP as a proxy for an array of other plant functional traits may simplify efforts to model species' responses to different drought scenarios.
The degree of plant iso/anisohydry, a widely used framework for classifying species‐specific hydraulic strategies, integrates multiple components of the whole‐plant hydraulic pathway. However, little ...is known about how it associates with coordination of functional and structural traits within and across different organs. We examined stem and leaf hydraulic capacitance and conductivity/conductance, stem xylem anatomical features, stomatal regulation of daily minimum leaf and stem water potential (Ψ), and the kinetics of stomatal responses to vapour pressure deficit (VPD) in six diverse woody species differing markedly in their degree of iso/anisohydry. At the stem level, more anisohydric species had higher wood density and lower native capacitance and conductivity. Like stems, leaves of more anisohydric species had lower hydraulic conductance; however, unlike stems, their leaves had higher native capacitance at their daily minimum values of leaf Ψ. Moreover, rates of VPD‐induced stomatal closure were related to intrinsic rather than native leaf capacitance and were not associated with species' degree of iso/anisohydry. Our results suggest a trade‐off between hydraulic storage and efficiency in the leaf, but a coordination between hydraulic storage and efficiency in the stem along a spectrum of plant iso/anisohydry.
The concept of isohydry versus anisohydry describes a continuum of stringency of regulation of water status in plants exposed to soil and atmospheric drought. However, very little is known about the complex coordination and integration of traits and physiological responses underlying this whole‐plant behaviour. Here, we characterize stomatal regulation of leaf and stem water potential in relation to leaf and stem hydraulic traits as well as the kinetics of stomatal responses to increasing leaf‐to‐air vapour pressure deficit in six woody species previously shown by us to span a wide spectrum of isohydric to anisohydric behaviour. Our results provide new insights concerning biophysical and physiological trade‐offs and constraints associated with where species operate along a continuum of isohydry to anisohydry.
We investigated the response of Helianthus species nighttime conductance (gnight) and transpiration (Enight) to soil nutrient and water limitations in nine greenhouse studies. The studies primarily ...used wild Helianthus annuus, but also included a commercial and early domesticate of H. annuus and three additional wild species (Helianthus petiolaris Nutt., Helianthus deserticola Heiser, and Helianthus anomalus Blake). Well-watered plants of all species showed substantial gnight (0.023-0.225 mol m⁻² s⁻¹) and Enight (0.29-2.46 mmol m⁻² s⁻¹) measured as instantaneous gas exchange. Based on the potential for transpiration to increase mass flow of mobile nutrients to roots, we hypothesized that gnight and Enight would increase under limiting soil nutrients but found no evidence of responses in all six studies testing this. Based on known daytime responses to water limitation, we hypothesized that gnight and Enight would decrease when soil water availability was limited, and results from all four studies testing this supported our hypothesis. We also established that stomatal conductance at night was on average 5 times greater than cuticular conductance. Additionally, gnight and Enight varied nocturnally and across plant reproductive stages while remaining relatively constant as leaves aged. Our results further the ability to predict conditions under which nighttime water loss will be biologically significant and demonstrate that for Helianthus, gnight can be regulated.
C₃ plants dominate many landscapes and are critically important for ecosystem water cycling. At night, plant water losses can include transpiration (Enight) from the canopy and hydraulic ...redistribution (HR) from roots. We tested whether Enight limits the magnitude of HR in a greenhouse study using Artemisia tridentata, Helianthus anomalus and Quercus laevis. Plants were grown with their roots split between two compartments. HR was initiated by briefly withholding all water, followed by watering only one rooting compartment. Under study conditions, all species showed substantial Enight and HR (highest minus lowest soil water potential Ψs during a specified diel period). Suppressing Enight by canopy bagging increased HR during the nightly bagging period (HRN) for A. tridentata and H. anomalus by 73 and 33% respectively, but did not affect HRN by Q. laevis. Total daily HR (HRT) was positively correlated with the Ψs gradient between the rooting compartments, which was correlated with light and/or atmospheric vapour pressure deficit (VPDa) the prior day. For A. tridentata, HRT was negatively correlated with night-time VPDa. Ecological implications of the impact of Enight on HR may include decreased plant productivity during dry seasons, altered ecosystem water flux patterns and reduced nutrient cycling in drying soils.
Use of real specimens brings the study of biology to life. This activity brings easily acquired plant specimens into the classroom to tackle common alternative conceptions regarding life, size, ...complexity, the nature of science, and plants as multicellular organisms. The activity occurs after a discussion of the characteristics of life and engages students in application of course content and utilization of scientific thinking. It is appropriate for any class in which the nature of life and its structural complexities are addressed and in which teachers want to help students gain familiarity with plants as multicellular organisms.
Nighttime transpirational water loss from C3 trees occurs without carbon gain and is both common and substantial. However, the magnitude of this water loss varies and a better understanding of the ...environmental factors driving this variation is needed. We investigated the response of nighttime conductance (g(night)) and transpiration (E(night)) to soil nitrogen limitation. We used instantaneous gas exchange measurements in greenhouse studies of Populus angustifolia James (narrowleaf cottonwood) and Populus balsamifera L. spp. trichocarpa (Torr. & A. Gray ex Hook.) Brayshaw (black cottonwood). g(night) for sufficiently watered plants ranged from 0.045 to 0.308 mol m–2 s–1 for P. balsamifera and 0.037 to 0.188 mol m–2 s–1 for P. angustifolia, which was much larger than minimum leaf conductance (g(min); up to 0.005 mol m–2 s–1 in the dark). Long-term nitrogen limitation sufficient to substantially reduce biomass did not affect g(night) or E(night) when potentially confounding water stress effects were eliminated. We conclude that nighttime water loss from two Populus species is large and although it is under stomatal control is not regulated at night in response to soil nitrogen availability.
Urbanization transforms environments in ways that alter biological evolution. We examined whether urban environmental change drives parallel evolution by sampling 110,019 white clover plants from ...6169 populations in 160 cities globally. Plants were assayed for a Mendelian antiherbivore defense that also affects tolerance to abiotic stressors. Urban-rural gradients were associated with the evolution of clines in defense in 47% of cities throughout the world. Variation in the strength of clines was explained by environmental changes in drought stress and vegetation cover that varied among cities. Sequencing 2074 genomes from 26 cities revealed that the evolution of urban-rural clines was best explained by adaptive evolution, but the degree of parallel adaptation varied among cities. Our results demonstrate that urbanization leads to adaptation at a global scale.
We investigated the response of Helianthus species nighttime conductance (g(night)) and transpiration (E(night)) to soil nutrient and water limitations in nine greenhouse studies. The studies ...primarily used wild Helianthus annuus, but also included a commercial and early domesticate of H. annuus and three additional wild species (Helianthus petiolaris Nutt., Helianthus deserticola Heiser, and Helianthus anomalus Blake). Well-watered plants of all species showed substantial g(night) (0.023-0.225 mol m(-2) s(-1)) and E(night) (0.29-2.46 mmol m(-2) s(-1)) measured as instantaneous gas exchange. Based on the potential for transpiration to increase mass flow of mobile nutrients to roots, we hypothesized that g(night) and E(night) would increase under limiting soil nutrients but found no evidence of responses in all six studies testing this. Based on known daytime responses to water limitation, we hypothesized that g(night) and E(night) would decrease when soil water availability was limited, and results from all four studies testing this supported our hypothesis. We also established that stomatal conductance at night was on average 5 times greater than cuticular conductance. Additionally, g(night) and E(night) varied nocturnally and across plant reproductive stages while remaining relatively constant as leaves aged. Our results further the ability to predict conditions under which nighttime water loss will be biologically significant and demonstrate that for Helianthus, g(night) can be regulated.
Summary Background The inconsistent effect of hypothermia treatment on severe brain injury in previous trials might be because hypothermia was induced too late after injury. We aimed to assess ...whether very early induction of hypothermia improves outcome in patients with severe brain injury. Methods The National Acute Brain Injury Study: Hypothermia II (NABIS: H II) was a randomised, multicentre clinical trial of patients with severe brain injury who were enrolled within 2·5 h of injury at six sites in the USA and Canada. Patients with non-penetrating brain injury who were 16–45 years old and were not responsive to instructions were randomly assigned (1:1) by a random number generator to hypothermia or normothermia. Patients randomly assigned to hypothermia were cooled to 35°C until their trauma assessment was completed. Patients who had none of a second set of exclusion criteria were either cooled to 33°C for 48 h and then gradually rewarmed or treated at normothermia, depending upon their initial treatment assignment. Investigators who assessed the outcome measures were masked to treatment allocation. The primary outcome was the Glasgow outcome scale score at 6 months. Analysis was by modified intention to treat. This trial is registered with ClinicalTrials.gov , NCT00178711. Findings Enrolment occurred from December, 2005, to June, 2009, when the trial was terminated for futility. Follow-up was from June, 2006, to December, 2009. 232 patients were initially randomised a mean of 1·6 h (SD 0·5) after injury: 119 to hypothermia and 113 to normothermia. 97 patients (52 in the hypothermia group and 45 in the normothermia group) did not meet any of the second set of exclusion criteria. The mean time to 35°C for the 52 patients in the hypothermia group was 2·6 h (SD 1·2) and to 33°C was 4·4 h (1·5). Outcome was poor (severe disability, vegetative state, or death) in 31 of 52 patients in the hypothermia group and 25 of 56 in the normothermia group (relative risk RR 1·08, 95% CI 0·76–1·53; p=0·67). 12 patients in the hypothermia group died compared with eight in the normothermia group (RR 1·30, 95% CI 0·58–2·52; p=0·52). Interpretation This trial did not confirm the utility of hypothermia as a primary neuroprotective strategy in patients with severe traumatic brain injury. Funding National Institute of Neurological Disorders and Stroke.
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