Leaf-level Physiological Responses Research Articles

Night-time warming in the field reduces nocturnal stomatal conductance and grain yield but does not alter daytime physiological responses.

Global nocturnal temperatures are rising more rapidly than daytime temperatures and have a large effect on crop productivity. In particular, stomatal conductance at night (gsn ) is surprisingly poorly understood and has not been investigated despite constituting a significant proportion of overall canopy water loss. Here, we present the results of 3 yr of field data using 12 spring Triticum aestivum genotypes which were grown in NW Mexico and subjected to an artificial increase in night-time temperatures of 2°C. Under nocturnal heating, grain yields decreased (1.9% per 1°C) without significant changes in daytime leaf-level physiological responses. Under warmer nights, there were significant differences in the magnitude and decrease in gsn , values of which were between 9 and 33% of daytime rates while respiration appeared to acclimate to higher temperatures. Decreases in grain yield were genotype-specific; genotypes categorised as heat tolerant demonstrated some of the greatest declines in yield in response to warmer nights. We conclude the essential components of nocturnal heat tolerance in wheat are uncoupled from resilience to daytime temperatures, raising fundamental questions for physiological breeding. Furthermore, this study discusses key physiological traits such as pollen viability, root depth and irrigation type may also play a role in genotype-specific nocturnal heat tolerance.

Impacts of climate and tree morphology on tree-ring stable isotopes in central Mongolia.

Recent climate extremes in Mongolia have ignited a renewed interest in understanding past climate variability over centennial and longer time scales across north-central Asia. Tree-ring width records have been extensively studied in Mongolia as proxies for climate reconstruction, however, the climate and environmental signals of tree-ring stable isotopes from this region need to be further explored. Here, we evaluated a 182-year record of tree-ring δ13C and δ18O from Siberian Pine (Pinus sibirica Du Tour) from a xeric site in central Mongolia (Khorgo Lava) to elucidate the environmental factors modulating these parameters. First, we analyzed the climate sensitivity of tree-ring δ13C and δ18O at Khorgo Lava for comparison with ring-width records, which have been instrumental in reconstructing hydroclimate in central Mongolia over two millennia. We also compared stable isotope records of trees with partial cambial dieback ('strip-bark morphology'), a feature of long-lived conifers growing on resource-limited sites, and trees with a full cambium ('whole-bark morphology'), to assess the inferred leaf-level physiological behavior of these trees. We found that interannual variability in tree-ring δ13C and δ18O reflected summer hydroclimatic variability, and captured recent, extreme drought conditions, thereby complementing ring-width records. The tree-ring δ18O records also had a spring temperature signal and thus expanded the window of climate information recorded by these trees. Over longer time scales, strip-bark trees had an increasing trend in ring-widths, δ13C (and intrinsic water-use efficiency, iWUE) and δ18O, relative to whole-bark trees. Our results suggest that increases in iWUE at this site might be related to a combination of leaf-level physiological responses to increasing atmospheric CO2, recent drought, and stem morphological changes. Our study underscores the potential of stable isotopes for broadening our understanding of past climate in north-central Asia. However, further studies are needed to understand how stem morphological changes might impact stable isotopic trends.

Climate variability supersedes grazing to determine the anatomy and physiology of a dominant grassland species

Grassland ecosystems are historically shaped by climate, fire, and grazing which are essential ecological drivers. These grassland drivers influence morphology and productivity of grasses via physiological processes, resulting in unique water and carbon-use strategies among species and populations. Leaf-level physiological responses in plants are constrained by the underlying anatomy, previously shown to reflect patterns of carbon assimilation and water-use in leaf tissues. However, the magnitude to which anatomy and physiology are impacted by grassland drivers remains unstudied. To address this knowledge gap, we sampled from three locations along a latitudinal gradient in the mesic grassland region of the central Great Plains, USA during the 2018 (drier) and 2019 (wetter) growing seasons. We measured annual biomass and forage quality at the plot level, while collecting physiological and anatomical traits at the leaf-level in cattle grazed and ungrazed locations at each site. Effects of ambient drought conditions superseded local grazing treatments and reduced carbon assimilation and total productivity in A. gerardii. Leaf-level anatomical traits, particularly those associated with water-use, varied within and across locations and between years. Specifically, xylem area increased when water was more available (2019), while xylem resistance to cavitation was observed to increase in the drier growing season (2018). Our results highlight the importance of multi-year studies in natural systems and how trait plasticity can serve as vital tool and offer insight to understanding future grassland responses from climate change as climate played a stronger role than grazing in shaping leaf physiology and anatomy.

Intracanopy CO2 and light interactions on Humulus lupulus L. net canopy carbon gain under current and future atmospheric CO2 concentrations

Photosynthetic light response curves were measured in four cultivars of Humulus lupulus L. (hop) at CO2 concentrations of 415, 500, 600, and 700 μmol CO2 mol−1. Short-term elevated CO2 increased net photosynthesis (An), light-saturated rates of photosynthesis, and quantum yield and decreased the light compensation point and leaf respiration. To illustrate the effects on canopy carbon gain, we used a rigorously validated physiologically-explicit canopy process model (MAESTRA). The cultivar-specific leaf-level physiological response parameters at 415, 500, 600, and 700 μmol CO2 mol−1 were extrapolated throughout the canopy to reproduce the cultivar-specific carbon gain under current and future atmospheric CO2 conditions. Under sub optimal light conditions, the initial CO2 increase of 415 to 500 μmol CO2 mol−1 resulted in the highest relative carbon gain increase (∼18%), whereas enrichment from 500 to 600 and 600 to 700 μmol CO2 mol−1 resulted in an additional carbon uptake of ∼16% and ∼12%. Ratios of An to increased CO2 indicated that shaded leaves benefit more from elevated CO2 than light saturated leaves. Estimated daily canopy carbon uptake increased ∼24% from 415 to 500 µmol mol−1 CO2. Daily carbon uptake increased by ∼17% from 500 to 600 µmol mol−1 CO2 and ∼18% from 600 to 700 µmol mol−1. Thus, in comparison to leaves exposed to direct sunlight, hop canopy carbon gain in shaded diffuse light lower canopy layers may contribute a greater proportion of total canopy carbon in future atmospheric CO2 conditions.

A triple tree-ring constraint for tree growth and physiology in a global land surface model

Abstract. Annually resolved tree-ring records extending back to pre-industrial conditions have the potential to constrain the responses of global land surface models at interannual to centennial timescales. Here, we demonstrate a framework to simultaneously constrain the representation of tree growth and physiology in the ORCHIDEE global land surface model using the simulated variability of tree-ring width and carbon (Δ13C) and oxygen (δ18O) stable isotopes in six sites in boreal and temperate Europe. We exploit the resulting tree-ring triplet to derive integrative constraints for leaf physiology and growth from well-known mechanistic relationships among the variables. ORCHIDEE simulates Δ13C (r=0.31–0.80) and δ18O (r=0.36–0.74) better than tree-ring width (r<0.55), with an overall skill similar to that of a tree-ring model (MAIDENiso) and another isotope-enabled global vegetation model (LPX-Bern). The comparison with tree-ring data showed that growth variability is not well represented in ORCHIDEE and that the parameterization of leaf-level physiological responses (stomatal control) to drought stress in the temperate region can be constrained using the interannual variability of tree-ring stable isotopes. The representation of carbon storage and remobilization dynamics emerged as a critical process to improve the realism of simulated growth variability, temporal carryover, and recovery of forest ecosystems after climate extremes. Simulated forest gross primary productivity (GPP) correlates with simulated tree-ring Δ13C and δ18O variability, but the origin of the correlations with tree-ring δ18O is not entirely physiological. The integration of tree-ring data and land surface models as demonstrated here should guide model improvements and contribute towards reducing current uncertainties in forest carbon and water cycling.

Temporal Changes in Ozone Concentrations and Their Impact on Vegetation

Tropospheric concentrations of phytotoxic ozone (O3) have undergone a great increase from preindustrial 10–15 ppbv to a present-day concentration of 35–40 ppbv in large parts of the industrialised world due to increased emissions of O3 precursors including NOx, CO, CH4 and volatile organic compounds. The rate of increase in O3 concentration ranges between 1 ppbv per decade in remote locations of the Southern hemisphere and 5 ppbv per decade in the Northern hemisphere, where largest sources of O3 precursors are located. Molecules of O3 penetrating into the leaves through the stomatal apertures trigger the formation of reactive oxygen species, leading thus to the damage of the photosynthetic apparatus. Accordingly, it is assumed, that O3 increase reduces the terrestrial carbon uptake relative to the preindustrial era. Here we summarise the results of previous manipulative experiments in laboratory growth cabinets, field open-top chambers and free-air systems together with O3 flux measurements under natural growth conditions. In particular, we focus on leaf-level physiological responses in trees, variability in stomatal O3 flux and changes in carbon fluxes and biomass production in forest stands. As the results reported in the literature are highly variable, ranging from negligible to severe declines in photosynthetic carbon uptake, we also discuss the possible interactions of O3 with other environmental factors including solar radiation, drought, temperature and nitrogen deposition. Those factors were found to have great potential to modulate stomata openness and O3 fluxes.

Leaf antioxidant machinery stimulation by Meloidogyne javanica infestation: A case study on Cucumis melo seedlings

Following nematode infestation, leaf-localized responses are vital not only as early infection signs but also as an indication of actions required to preserve crop productivity. In this context, the leaf-level physiological responses of cantaloupe seedlings were evaluated after 40 consecutive days of cultivation under different Meloidogyne javanica juveniles (J2s) inoculation regimes (0, 100, and 850 J2s plant−1). Leaf growth parameters and photosynthetic pigments (chlorophyll, carotenoids) content were not affected by J2s infestation, while foliar Potassium concentration significantly declined. The RuBIsCo activase gene expression was negatively associated with the J2s inoculation level. Total flavonoids, total phenolics, lipid peroxidation indexes, phenylalanine ammonia-lyase activity, as well as, ascorbate peroxidase gene transcription, were higher in the inoculated plants (regardless of the J2s inoculant level). Taken together, present data indicate that J2s infestation impacts primarily the Potassium levels in leaves. Additionally, a substantial stimulation of the plant antioxidant machinery (independent of the nematode infestation intensity) is triggered. Finally, it was established that the transcriptional regulation of the RuBIsCo activase 1 gene under biotic stress has the capacity to be employed as a potential stress indicator marker.

Tortoise or hare: Will resprouting oaks or reseeding pines dominate following severe wildfire?

Abstract Post-fire shifts in species composition and growth are driven in part by alterations in resource availability. However, less is known about the extent to which differences in fire severity will affect leaf-level physiological responses in co-occurring species. Therefore, this study tested loblolly pine (Pinus taeda L.) saplings’ and resprouting oaks’ (Quercus stellata Wangenh. (post oak) and Q. marilandica Muenchh. (blackjack oak)) responses to various stresses following a catastrophic wildfire in 2011 that occurred in the Lost Pines region in Texas, USA. General plant responses (height, stem number, and plant density) and leaf-level stress indicators [gas exchange (photosynthesis (A), transpiration (E) and stomatal conductance (gs), pressure-volume parameters, and leaf biochemistry] were monitored along resource availability gradients (two burn severities and three soil types) in the summer of 2016. Nearly 5 years after the fire, there were 180% more pines in moderate severity transects than high severity transects. However, pines in high severity areas were similar in height to those growing in moderate severity areas. Pines consistently had low A, E, and gs rates when compared to co-occurring oaks. Pine osmotic potential at full turgor (ΠFT), turgor loss point (ΠTLP), and bulk modulus of elasticity (e) were 33, 30, and 58% greater, respectively, than those of oaks. Loblolly pine leaf-level gas exchange and pressure-volume results suggest this species adopts a drought-avoidance strategy for survival. Low gas exchange rates in pine saplings may reflect greater stomatal control, which could reduce cavitation risks, or may have arisen from reduced access to water resources by saplings’ shallow roots. Gas exchange and pressure-volume parameters of the oaks reflect greater drought tolerance than pine, with post oak being the more tolerant of the two oak species. Leaf-level gas exchange and pressure-volume measurements were coordinated, and results suggest that post oak utilized osmotic adjustment to maintain gas exchange in the more exposed, high severity areas. Additionally, differences in foliar nutrient and phenolic contents between oaks and pines may reflect differences in their leaf-level gas exchange capabilities. Results suggest loblolly pine regeneration may be slower in high burn-severity areas, and that differences in physiological strategies to cope with water stress among the three species may shift stand composition to oak dominance in parts of this region following wildfire.

Warming alters the positive impact of elevated CO2 concentration on cotton growth and physiology during soil water deficit.

Alterations in climate factors such as rising CO2 concentration ([CO2]), warming and reduced precipitation may have significant impacts on plant physiology and growth. This research investigated the interactive effects of elevated [CO2], warming and soil water deficit on biomass production, leaf-level physiological responses and whole-plant water use efficiency (WUEP) in cotton (Gossypium hirsutum L.). Cotton was grown in the glasshouse under two [CO2] treatments (CA, 400µLL-1; CE, 640µLL-1) and two temperature treatments (TA, 28°C:17°C day:night; TE, 32°C:21°C day:night). Plants were subjected to two progressive water deficit cycles, with a 5-day recovery period between the water deficit periods. CE increased vegetative biomass and photosynthetic rates, and decreased stomatal conductance in TA; however, these responses to CE were not evident under TE. CE increased whole-plant water loss under TA, but increased WUEp, whereas increased whole-plant water loss in TE decreased WUEp regardless of atmospheric [CO2]. CE may provide some positive growth and physiological benefits to cotton at TA if sufficient water is available but CE will not mitigate the negative effects of rising temperature on cotton growth and physiology in future environments.

Ecophysiological performance of the rare terrestrial orchid Platanthera integrilabia across contrasting habitats

Platanthera integrilabia is a rare terrestrial orchid species generally associated with semi-open forested wetlands in the southeastern US. It has been suggested that P. integrilabia has restrictive abiotic resource requirements; however, these requirements have not been implicitly studied despite their potential application to species conservation. We investigated the influence of light and soil moisture on P. integrilabia at landscape and local scales and population and organismal levels across and within four occurrences with contrasting canopy openness. We also evaluated the potential for leaf-level physiological responses to light and soil moisture to reflect habitat suitability and influence performance. At landscape scales, light and soil moisture were not associated with P. integrilabia density or individual size. Across sites, measures of photosynthetic light response and water-use efficiency indicated that P. integrilabia can maximize photosynthetic efficiency and energy gain in contrasting light and soil moisture environments. Minimal associations of these measures with abiotic variations within sites suggested that the capacity for adjustments across small spatial and/or temporal scales may be limited. Preservation of existing habitats associated with viable P. integrilabia occurrences is warranted, but the leaf-level ecophysiology of this species indicates that its habitat suitability also could include more open and drier sites. For populations of P. integrilabia experiencing declines in habitats with recent disturbance, we suggest the possibility that well-managed modifications to canopy cover may be beneficial.

Biomass and physiological responses of Quercus robur (L.) young trees during 2 years of treatments with different levels of ozone and nitrogen wet deposition

The root biomass of oak young trees significantly decreased after 2 years of exposure to high levels of ozone, but increased nitrogen wet deposition tended to partly contrast this effect. A 2-year Open-Top Chamber (OTC) experiment with young Quercus robur trees that were exposed to different levels of ozone (O3) and nitrogen deposition was performed in Curno (Northern Italy) for the FP7 Project ECLAIRE. The plants were exposed to four levels of ozone (−40 % of ambient ozone in charcoal-filtered OTCs, −5 % in non-filtered OTCs, and +30 and +75 % in O3-enriched OTCs) and two levels of nitrogen wet deposition (tap water and tap water +70 kg N ha−1 year−1). The stomatal conductance and A/Ci response curves were measured during the two experimental seasons, and in October, the plant dry biomass partition between the roots and stem was assessed. Oak plants were moderately sensitive to O3. After the second year of treatments, the dose–response relationships based on the O3 stomatal flux indicated a 4.6 % of root biomass loss and a 12.1 % of reduction of the number of leaves per 10 mmol O3 m−2 absorbed by plants grown with no nitrogen addition. Ozone also decreased both the stomatal conductance and the maximum carboxylation rate allowed by Rubisco (V cmax) during the first year of treatments. However, the effect on V cmax was lost during the second year, and the plants showed an uncoupling between leaf-level physiological responses and plant-level biomass responses. Increased nitrogen deposition enhanced the growth of plants and partially mitigated the O3 impact on biomass and physiology, but no significant effect of the interaction between the two factors was found. The data that were collected could contribute to the definition of the O3 dose–response relationships based on biomass losses for deciduous trees in Southern Europe climatic conditions and could improve the O3 risk assessment models by providing new information about the effect of increased nitrogen deposition on the ozone impact.

Species-specific adaptations explain resilience of herbaceous understorey to increased precipitation variability in a Mediterranean oak woodland.

To date, the implications of the predicted greater intra-annual variability and extremes in precipitation on ecosystem functioning have received little attention. This study presents results on leaf-level physiological responses of five species covering the functional groups grasses, forbs, and legumes in the understorey of a Mediterranean oak woodland, with increasing precipitation variability, without altering total annual precipitation inputs. Although extending the dry period between precipitation events from 3 to 6weeks led to increased soil moisture deficit, overall treatment effects on photosynthetic performance were not observed in the studied species. This resilience to prolonged water stress was explained by different physiological and morphological strategies to withstand periods below the wilting point, that is, isohydric behavior in Agrostis, Rumex, and Tuberaria, leaf succulence in Rumex, and taproots in Tolpis. In addition, quick recovery upon irrigation events and species-specific adaptations of water-use efficiency with longer dry periods and larger precipitation events contributed to the observed resilience in productivity of the annual plant community. Although none of the species exhibited a change in cover with increasing precipitation variability, leaf physiology of the legume Ornithopus exhibited signs of sensitivity to moisture deficit, which may have implications for the agricultural practice of seeding legume-rich mixtures in Mediterranean grassland-type systems. This highlights the need for long-term precipitation manipulation experiments to capture possible directional changes in species composition and seed bank development, which can subsequently affect ecosystem state and functioning.

Remote sensing used to detect moisture status of pecan orchards grown in a desert environment

Remote-sensing techniques can detect and up-scale leaf-level physiological responses to large areas, and provide significant and reliable information on water use and irrigation management. The objectives of this study were to screen leaf-level physiological changes that occur during the cyclic irrigation of pecan orchards to determine which responses best represent changes in moisture status of plants and link plant physiological changes to remotely sensed surface reflectance data derived from the Landsat Thematic Mapper and Enhanced Thematic Mapper Plus (ETM+). The study was conducted simultaneously on two southern New Mexico mature pecan orchards. For both orchards, plant physiological responses and remotely sensed surface reflectance data were collected from trees that were either well watered or in water deficit. Remotely sensed variables included reflectance in band 1, the ratio between shortwave infrared (SWIR) bands (B5:B7), the normalized difference vegetation index, and SWIR moisture indices. Midday stem water potential (Ψsmd) was the best performing leaf-level physiological response variable for detecting moisture status in pecans. The B5:B7 ratio positively and significantly correlated with Ψsmd in five of six irrigation cycles while multiple linear regression weighted with six remotely sensed surface reflectance variables revealed a significant relationship with moisture status in all cycles in both orchards (R2 > 0.73). Because changes in the B5:B7 band ratio and multiple regression of spectral variables correlate with the moisture status of pecan orchards, we conclude that remotely sensed data hold promise for detecting the moisture status of pecans.

Leaf-level physiological responses of Tamarix ramosissima to increasing salinity

Over the past century, the invasive halophytic shrub Tamarix ramosissima Ledeb. has increased in abundance and distribution in riparian ecosystems of western North America. These increases coincide with anthropogenic modification of river systems, which decrease the rate of periodic overbank flooding, leading to an increase in soil salinity. Increased soil salinity negatively impacts the physiology of native riparian tree species, but the impact of increased soil salinity on T. ramosissima physiology is incompletely known. To measure the impacts of soil salinity on T. ramosissima, we measured leaf-level responses across a broad range of surface-soil salinity concentrations at two sites in western Kansas. Photosynthesis at 2000 μmol m −2 s −1 ( A 2000 ), stomatal conductance to water ( g s ), intercellular CO 2 concentration ( C i ), and leaf δ 13C showed little change over surface-soil salinities from 0.5 to 17.65 mmhos/cm. The small variation in leaf-level physiological responses suggests robust functioning of T. ramosissima across a broad range of surface-soil salinities. Leaf-level physiology and δ 13C responses were assessed by canopy position, but responses were not significantly different. These results are among the first to show broad acclimation and robust physiological functioning for many leaf-level processes measured on mature trees grown across a wide surface-soil salinity gradient in the field.

Physiological Responses of <i>Tamarix ramosissima</i> to Extreme NaCl Concentrations

Hydrologic alterations of river systems in western North America over the past century have increased soil salinity, contributing to the establishment and spread of an introduced halophytic species, Tamarix ramosissima (Ledeb.). The physiological responses of Tamarix ramosissima to salinity stress are incompletely known. To assess the salinity tolerance of this species, we measured several whole plant and leaf-level physiological responses of Tamarix ramosissima cuttings grown in a controlled environment over three NaCl concentrations (0, 15 and 40 g·l –1 ). Tamarix ramosissima photosynthesis (A2000), stomatal conductance to water (gs), water potential (Ψw), and the maximum quantum yield of photosystem II (Fv/Fm) decreased at 15 and 40 g·l –1 NaCl compared to control treatments. However, after approximately 35 days, Tamarix ramosissima had increased photosynthetic rates, maximum quantum yield of photosystem II, and stomatal conductance to water. These data suggests that physiological functioning of Tamarix ramosissima acclimated to extremely high NaCl concentrations over a relatively short period of time. Additionally, we present preliminary evidence that suggests proline synthesis may be the mechanism by which this species adjusts osmotically to increasing salinity.

Foliar 13C and 18O reveal differential physiological responses of canopy foliage to pre-planting weed control in a young spotted gum (Corymbia citriodora subsp. Variegata) plantation

Weed control may improve the growth of forest plantations by influencing soil water and nutrient availability, but our knowledge of leaf-level physiological responses to weed control at different within-canopy positions is limited for tropical and subtropical plantations. Foliar carbon (delta(13)C) and oxygen (delta(18)O) isotope compositions, gas exchange, and nitrogen (N(mass)) and phosphorus (P(mass)) concentrations at four canopy positions were assessed in a young spotted gum (Corymbia citriodora subsp. Variegata (F. Muell.) A.R. Bean & M.W. McDonald) plantation subjected to either weed control or no weed control treatment, to test if leaves at different positions within the tree canopy had the same physiological responses to the weed control treatment. Weed control increased foliar delta(13)C but lowered delta(18)O in the upper-outer and upper-inner canopy, indicating that weed control resulted in a higher foliar photosynthetic capacity at upper-canopy positions, a conclusion confirmed by gas exchange measurements. The increased photosynthetic capacity resulting from weed control can be explained by an increase in foliar N(mass). In the lower-outer canopy, weed control reduced foliar delta(13)C while lowering delta(18)O even more than in the upper-canopy, suggesting strong enhancement of the partial pressure of CO(2) in the leaf intercellular spaces and of foliar stomatal conductance in lower-canopy foliage. This conclusion was supported by gas exchange measurements. Foliar photosynthesis in the lower-inner canopy showed no significant response to weed control. The finding that leaves at different canopy positions differ in their physiological responses to weed control highlights the need to consider the canopy position effect when examining competition for soil nutrient and water resources between weeds and trees.

(141) Leaf Abscisic Acid Accumulation in Response to Substrate Water Content: Variation Among Acer rubrum L. Genotypes

Quantitative differences in leaf abscisic acid (ABAL) among four cultivars of red (Acer rubrum L.) and one Freeman maple (Acer × freemanii E. Murray) were investigated. This study tested the hypothesis that ABAL concentration can be used to compare the effects of water stress on the gas exchange response of five different maple cultivars, including four red maple genotypes, `Summer Red', `October Glory', `Autumn Flame', and `Franksred' (Red Sunset), as well as one hybridized Freeman maple genotype, `Jeffersred' (Autumn Blaze). Cloned genotypes of red and Freeman maple were subjected to two treatments: 1) irrigated daily to container capacity or 2) irrigation withheld for one drought and recovery cycle. Leaf abscisic acid concentration, gas exchange, and whole-tree sap flow measurements were conducted under both conditions. Over the course of the drought stress and recovery phase, net photosynthesis (Anet), stomatal conductance (gs), and transpiration (E) declined as ABAL and instantaneous water use efficiency (A/gs) increased. This study found that ABAL tracked gs and that stomatal responsiveness to substrate moisture deficit is likely mediated by ABA accumulation in leaf tissue. This research demonstrates a leaf-level physiological response to substrate volumetric water content that appears to depend on ABAL concentration. In addition, the evidence in this study indicates that ABAL may be used as a potential surrogate for the gs response to substrate water stress and could become part of a cultivar drought tolerance selection strategy for red and Freeman maple.

Leaf Abscisic Acid Accumulation in Response To Substrate Water Content: Linking Leaf Gas Exchange Regulation with Leaf Abscisic Acid Concentration

Quantitative differences in leaf abscisic acid (ABAL) among four cultivars of red maple (Acer rubrum L.) and one freeman maple (Acer ×freemanii E. Murray) cultivar were investigated. This study tested the hypothesis that ABAL concentration can be used to compare the effects of water stress on the gas exchange response of five different maple genotypes, including four red maple cultivars [`Summer Red', `October Glory', `Autumn Flame', and `Franksred' ('Red Sunset')] and one hybridized freeman maple cultivar ['Jeffersred' ('Autumn Blaze')]. Two-year-old cloned genotypes of red maple and freeman maple were subjected to two treatments: irrigated daily to container capacity or irrigation withheld for one drought and recovery cycle. Leaf abscisic acid concentration, gas exchange, and wholetree sap flow measurements were conducted under well-watered and drought stress conditions. Over the course of the drought stress and recovery phase, net photosynthesis (Anet), stomatal conductance (gs), and transpiration (E) declined as ABAL and instantaneous water use efficiency (A/gs) increased. Until severe water stress conditions were prominent, water use was higher in `Summer Red' as compared to `October Glory'. This study found that ABAL tracked gs and that stomatal responsiveness to substrate moisture deficit is likely mediated by ABA accumulation in leaf tissue. This research demonstrates a leaf level physiological response to substrate volumetric water content that appears to depend on ABAL concentration. In addition, the evidence in this study indicates that ABAL may be used as a potential surrogate for the gs response to substrate water stress and could become part of a cultivar drought tolerance selection strategy for red maple and freeman maple.

Limitations and perspectives about scaling ozone impacts in trees

We review the need for scaling effects of ozone (O 3) from juvenile to mature forest trees, identify the knowledge presently available, and discuss limitations in scaling efforts. Recent findings on O 3/soil nutrient and O 3/CO 2 interactions from controlled experiments suggest consistent scaling patterns for physiological responses of individual leaves to whole-plant growth, carbon allocation, and water use efficiency of juvenile trees. These findings on juvenile trees are used to develop hypotheses that are relevant to scaling O 3 effects to mature trees, and these hypotheses are examined with respect to existing research on differences in response to O 3 between juvenile and mature trees. Scaling patterns of leaf-level physiological response to O 3 have not been consistent in previous comparisons between juvenile and mature trees. We review and synthesize current understanding of factors that may cause such inconsistent scaling patterns, including tree-size related changes in environment, stomatal conductance, O 3 uptake and exposure, carbon allocation to defense, repair, and compensation mechanisms, and leaf production phenology. These factors should be considered in efforts to scale O 3 responses during tree ontogeny. Free-air O 3 fumigation experiments of forest canopies allow direct assessments of O 3 impacts on physiological processes of mature trees, and provide the opportunity to test current hypotheses about ontogenetic variation in O 3 sensitivity by comparing O 3 responses across tree-internal scales and ontogeny.