Negative Leaf Water Potential Research Articles

A causal trait model for explaining foliar water uptake capacity

AbstractQuestionsPlants largely vary in their capacity for foliar water uptake (FWU), that is, the capacity to increase leaf water content by directly absorbing water from leaf‐wetting events. Climate change will reduce leaf wetting and increase drought events. Therefore, we need a better understanding of the underlying traits and mechanisms that facilitate FWU.LocationSeasonally dry tropical montane grasslands in Brazil (Campos de Altitude).MethodsWe measured FWU and leaf traits related to wettability, surface conductance, water potential and water storage on up to 55 plant species. By using Direct Acyclic Graph theory and Bayesian modelling, we tested how those leaf traits affect FWU.ResultsWe found that stomatal conductance largely explained interspecific variation in FWU, which was also favoured in species with hydrophilic leaves, high cuticular conductance, more negative leaf water potentials, low dry‐matter content, isohydric behaviour, and more elastic cell walls.ConclusionsDue to the existence of trade‐offs, not all species exhibit an optimal combination of traits that favours FWU. Instead, co‐occurring species have achieved a similar capacity for FWU through distinct trait combinations. Consequently, species engaged in FWU may exhibit differential vulnerabilities to climate change as they can cope with drought using other strategies beside FWU.

Interactions between beech and oak seedlings can modify the effects of hotter droughts and the onset of hydraulic failure.

Mixing species with contrasting resource use strategies could reduce forest vulnerability to extreme events. Yet, how species diversity affects seedling hydraulic responses to heat and drought, including mortality risk, is largely unknown. Using open-top chambers, we assessed how, over several years, species interactions (monocultures vs mixtures) modulate heat and drought impacts on the hydraulic traits of juvenile European beech and pubescent oak. Using modeling, we estimated species interaction effects on timing to drought-induced mortality and the underlying mechanisms driving these impacts. We show that mixtures mitigate adverse heat and drought impacts for oak (less negative leaf water potential, higher stomatal conductance, and delayed stomatal closure) but enhance them for beech (lower water potential and stomatal conductance, narrower leaf safety margins, faster tree mortality). Potential underlying mechanisms include oak's larger canopy and higher transpiration, allowing for quicker exhaustion of soil water in mixtures. Our findings highlight that diversity has the potential to alter the effects of extreme events, which would ensure that some species persist even if others remain sensitive. Among the many processes driving diversity effects, differences in canopy size and transpiration associated with the stomatal regulation strategy seem the primary mechanisms driving mortality vulnerability in mixed seedling plantations.

A comparative analysis of agronomic water-use efficiency and its proxy measures as derived from key morpho-physiological and supportive quantitative genetics attributes of perennial ryegrass under imposed drought.

Water-use efficiency (WUE) is an under-researched but very important drought tolerance trait in forage breeding. This research estimated quantitative genetic parameters of morpho-physiological traits linked to agronomic water-use efficiency (WUEA) and its proxy measures based on δ13C (WUEi) or gas exchange (evapotranspiration, WUEAET, or stomatal conductance WUEASC) of genotypes from half-sib families of Lolium perenne L. (PRG) in a simulated summer drought cycle. Principal component analysis (PCA) of trait data distinguished a group of PRG genotypes where high WUEA and dry matter yield was associated with deep rooting, leaf hydration at more negative leaf osmotic and water potential, and reduced soil moisture depletion. Plants with this trait association sustained net assimilation and postdefoliation regrowth in drought. However, WUEi, WUEASC, and WUEAET were poorly correlated with most traits of interest at p < .05. Another PCA revealed a weak association between WUEA and its proxy measures under conditions tested. Quantitative genetic parameters including high estimates of narrow-sense heritability () of WUEA and related traits emphasized the genetic potential of the key trait combination for selecting PRG for improved drought tolerance. Research findings highlight the relative importance of WUEA and its proxy measures in the broad definition of PRG drought tolerance for breeding purposes.

Variation in Hydric Response of Two Industrial Hemp Varieties (Cannabis sativa) to Induced Water Stress

Information on industrial hemp (Cannabis sativa) water use and water stress is sparse. We studied water stress impact in two essential-oil hemp cultivars (‘Wife’ and ‘Cherry’) prompted by anecdotal differences in growth and water use. In a greenhouse setting, we measured water relations, water use, growth, and essential oil (CBD-cannabidiol and THC-delta-9 tetrahydrocannabinol) concentrations. Water stress did not significantly affect THC and CBD concentrations, but both cultivars responded to water stress by reducing transpiration through notably different mechanisms. ‘Cherry’ had more anisohydric behavior, maintaining high stomatal conductance (Gs) and more negative leaf water potential until root zone water depletion triggered partial afternoon stomatal closure to moderate stress, resulting in lower flower and CBD yield. By contrast, water-stressed ‘Wife’ rapidly defoliated half its leaf area in balance with less applied water and so maintained high Gs and flower yield on par with well-watered plants, suggesting potential for deficit irrigation to conserve water and reduce post-harvest vegetation management. Differences in water use translated to provisionally suggested crop coefficients of 1 for ‘Cherry’ and 1.3–1.5 for ‘Wife’, but further research is needed. Because hemp is genetically diverse, and cultivar naming conventions are currently lax, further germplasm screening and research are needed to determine the extent to which either conservative ‘Cherry’ or the water-stress defoliation response of ‘Wife’ is found in the larger population of hemp cultivars.

Selenium Mediated Alterations in Physiology of Wheat under Different Soil Moisture Levels

Soil moisture stress is one of the most serious aspects of climate change. Selenium (Se) is regarded as an essential element for animal health and has been demonstrated to protect plants from a number of abiotic challenges; however, our knowledge of Se-regulated mechanisms for enhancing crop yield is limited. We investigated the effects of exogenous Se supplementation on physiological processes that may impact wheat productivity during soil moisture stress. The plants were grown in plastic containers under screen-house conditions. The experiment was laid out in CRD consisting of three soil moisture regimes, i.e., control (soil moisture content of 12.5 ± 0.05%), moderate (soil moisture content of 8.5 ± 0.05%), and severe moisture stress (soil moisture content of 4.5 ± 0.05%). Selenium was supplied using sodium selenite (Na2SeO3) through soil application before sowing (10 ppm) and foliar application (20 ppm and 40 ppm) at two different growth stages. The foliar spray of Se was applied at the vegetative stage (70 days after planting) and was repeated 3 weeks later, whereas the control consisted of a water spray. The water status, photosynthetic efficiency, and yield were significantly decreased due to the soil’s moisture stress. The exogenous Se application of 40 ppm resulted in decreased negative leaf water potential and improved relative water contents, photosynthetic rate, transpiration rate, and stomatal conductance in comparison to the control (without selenium) under water shortage conditions except the plants treated with soil application of selenium under severe moisture stress at 70 DAS. Subsequently, Se-regulated mechanisms improved 100 seed weight, biological yield, and seed yield per plant. We suggest that Se foliar spray (40 ppm) is a practical and affordable strategy to increase wheat output in arid and semi-arid regions of the world that are experiencing severe water shortages.

Soil-plant hydraulics explain stomatal efficiency-safety tradeoff.

The efficiency-safety tradeoff has been thoroughly investigated in plants, especially concerning their capacity to transport water and avoid embolism. Stomatal regulation is a vital plant behaviour to respond to soil and atmospheric water limitation. Recently, a stomatal efficiency-safety tradeoff was reported where plants with higher maximum stomatal conductance (gmax ) exhibited greater sensitivity to stomatal closure during soil drying, that is, less negative leaf water potential at 50% gmax (ψgs50 ). However, the underlying mechanism of this gmax -ψgs50 tradeoff remains unknown. Here, we utilized a soil-plant hydraulic model, in which stomatal closure is triggered by nonlinearity in soil-plant hydraulics, to investigate such tradeoff. Our simulations show that increasing gmax is aligned with less negative ψgs50 . Plants with higher gmax (also higher transpiration) require larger quantities of water to be moved across the rhizosphere, which results in a precipitous decrease in water potential at the soil-root interface, and therefore in the leaves. We demonstrated that the gmax -ψgs50 tradeoff can be predicted based on soil-plant hydraulics, and is impacted by plant hydraulic properties, such as plant hydraulic conductance, active root lengthand embolism resistance. We conclude that plants may therefore adjust their growth and/or their hydraulic properties to adapt to contrasting habitats and climate conditions.

Going underground: soil hydraulic properties impacting maize responsiveness to water deficit

PurposeAlthough the coordination between stomatal closure and aboveground hydraulics has extensively been studied, our understanding of the impact of belowground hydraulics on stomatal regulation remains incomplete. Here, we investigated whether and how the water use of maize (Zea mays L.) varied under hydraulically contrasting soil textures. Our hypothesis is that a textural-specific drop in the hydraulic conductivity is associated with a distinct decrease in transpiration during soil drying.MethodsMaize plants were grown in contrasting soil textures (sand, sandy loam, loam) and exposed to soil drying. We measured the relationships between transpiration rate, soil water content as well as soil and leaf water potential. We used a soil-plant hydraulic model to reproduce the experimental observations and infer the hydraulic conductance of the soil-plant system during soil drying.ResultsWe observed the impact of soil texture on plant response to soil drying in various relationships. The soil water potentials at which transpiration decreased were more than one order of magnitude more negative in loam than in sand. The soil-plant conductance decreased not only abruptly but also at less negative soil water potentials in sand than in sandy loam or loam. Stomata closed at less negative leaf water potentials in sand than in loam. The model predictions matched well the experimental observations.ConclusionThe results elucidated that the critical soil water content and potential at which plants close stomata depends on the soil texture. These findings support our plea to include soil properties for understanding and predicting stomatal regulation during soil drying.

Spatial and seasonal patterns of water use in Mediterranean coastal dune vegetation

AimsThis paper examines the water dynamics of a coastal dune plant community, addressing spatial and seasonal variations. We aimed to detect the patterns of water use by plants at the community level according to their distribution across a coastal dune gradient from beach to inland.MethodsFive sites were established: upper beach, embryo-dune, slack, foredune, and inland. Eight perennial species were collected seasonally to analyse the isotopic composition and water potential. Soil water samples at 3 depths, groundwater, and atmosphere, were obtained to determine plant water sources.ResultsThe species from Inland and foredune plant communities, Retama, Juniperus, and Helichrysum, showed the most stable isotopic signal throughout the year. On the contrary, the species most abundant on the upper beach, embryo-dune, and slack (Ammophila, Achillea, and Polygonum) showed the highest variability. Water deficit decreased the dependence on shallow and mid-soil layers along the beach-inland gradient. Beach and embryo-dune sites showed less negative leaf water potential values than the other positions in the dune gradient.ConclusionsThree factors mark the proportion of water sources used for vegetation in coastal vegetation: community composition, distance to the sea, and seasonality. Coastal dune vegetation exhibited a species-specific response in water uptake that was modified by its location on the gradient. From upper beach to inland, the plant communities showed a slight progressive increase in the use of water from deeper layers. This pattern was similar and overlapped with the wet to dry seasonal pattern.

Using global databases to disentangle trait-specific and environmental influences on forest drought sensitivity.

Recent extreme climate events, such as droughts, have led to unprecedented forest dieback worldwide. Serra-Maluquer et al. (2022) used open access, global scale databases to link woody species' drought tolerance capabilities to their specific set of life-history traits. They found that species that produce denser woody tissues and can tolerate more negative leaf water potentials are more resilient to extreme drought events, regardless of local conditions. This is an invited commentary on Serra-Maluquer et al., https://doi.org/10.1111/gcb.16123.

Significance of root hairs for plant performance under contrasting field conditions and water deficit.

Previous laboratory studies have suggested selection for root hair traits in future crop breeding to improve resource use efficiency and stress tolerance. However, data on the interplay between root hairs and open-field systems, under contrasting soils and climate conditions, are limited. As such, this study aims to experimentally elucidate some of the impacts that root hairs have on plant performance on a field scale. A field experiment was set up in Scotland for two consecutive years, under contrasting climate conditions and different soil textures (i.e. clay loam vs. sandy loam). Five barley (Hordeum vulgare) genotypes exhibiting variation in root hair length and density were used in the study. Root hair length, density and rhizosheath weight were measured at several growth stages, as well as shoot biomass, plant water status, shoot phosphorus (P) accumulation and grain yield. Measurements of root hair density, length and its correlation with rhizosheath weight highlighted trait robustness in the field under variable environmental conditions, although significant variations were found between soil textures as the growing season progressed. Root hairs did not confer a notable advantage to barley under optimal conditions, but under soil water deficit root hairs enhanced plant water status and stress tolerance resulting in a less negative leaf water potential and lower leaf abscisic acid concentration, while promoting shoot P accumulation. Furthermore, the presence of root hairs did not decrease yield under optimal conditions, while root hairs enhanced yield stability under drought. Selecting for beneficial root hair traits can enhance yield stability without diminishing yield potential, overcoming the breeder's dilemma of trying to simultaneously enhance both productivity and resilience. Therefore, the maintenance or enhancement of root hairs can represent a key trait for breeding the next generation of crops for improved drought tolerance in relation to climate change.

Infection with an asymptomatic virus in rice results in a delayed drought response.

Infection of viruses in plants often modifies plant responses to biotic and abiotic stresses. In the present study we examined the effects of Rice tungro spherical virus (RTSV) infection on drought response in rice. RTSV infection delayed the onset of leaf rolling by 1-2 days. During the delay in drought response, plants infected with RTSV showed higher stomatal conductance and less negative leaf water potential under drought than those of uninfected plants, indicating that RTSV-infected leaves were more hydrated. Other growth and physiological traits of plants under drought were not altered by infection with RTSV. An expression analysis of genes for drought response-related transcription factors showed that the expression of OsNAC6 and OsDREB2a was less activated by drought in RTSV-infected plants than in uninfected plants, further suggesting improved water status of the plants due to RTSV infection. RTSV accumulated more in plants under drought than in well-watered plants, indicating the increased susceptibility of rice plants to RTSV infection by drought. Collectively, these results indicated that infection with RTSV can transiently mitigate the influence of drought stress on rice plants by increasing leaf hydration, while drought increased the susceptibility of rice plants to RTSV.

Functional relationships between hydraulic traits and the timing of diurnal depression of photosynthesis.

The hydraulic coordination along the water transport pathway helps trees provide adequate water supply to the canopy, ensuring that water deficits are minimized and that stomata remain open for CO2 uptake. We evaluated the stem and leaf hydraulic coordination and the linkages between hydraulic traits and the timing of diurnal depression of photosynthesis across seven evergreen tree species in the southern Andes. There was a positive correlation between stem hydraulic conductivity (ks ) and leaf hydraulic conductance (KLeaf ) across species. All species had similar maximum photosynthetic rates (Amax ). The species with higher ks and KLeaf attained Amax in the morning, whereas the species with lower ks and KLeaf exhibited their Amax in the early afternoon concurrently with turgor loss. These latter species had very negative leaf water potentials, but far from the pressure at which the 88% of leaf hydraulic conductance is lost. Our results suggest that diurnal gas exchange dynamics may be determined by leaf hydraulic vulnerability such that a species more vulnerable to drought restrict water loss and carbon assimilation earlier than species less vulnerable. However, under stronger drought, species with earlier CO2 uptake depression may increase the risk of hydraulic failure, as their safety margins are relatively narrow.

Dry‐season decline in tree sapflux is correlated with leaf turgor loss point in a tropical rainforest

Abstract Water availability is a key determinant of forest ecosystem function and tree species distributions. While droughts are increasing in frequency in many ecosystems, including in the tropics, plant responses to water supply vary with species and drought intensity and are therefore difficult to model. Based on physiological first principles, we hypothesized that trees with a lower turgor loss point (πtlp), that is, a more negative leaf water potential at wilting, would maintain water transport for longer into a dry season. We measured sapflux density of 22 mature trees of 10 species during a dry season in an Amazonian rainforest, quantified sapflux decline as soil water content decreased and tested its relationship to tree πtlp, size and leaf predawn and midday water potentials measured after the onset of the dry season. The measured trees varied strongly in the response of water use to the seasonal drought, with sapflux at the end of the dry season ranging from 37 to 117% (on average 83 ± 5 %) of that at the beginning of the dry season. The decline of water transport as soil dried was correlated with tree πtlp (Spearman’s ρ ≥ 0.63), but not with tree size or predawn and midday water potentials. Thus, trees with more drought‐tolerant leaves better maintained water transport during the seasonal drought. Our study provides an explicit correlation between a trait, measurable at the leaf level, and whole‐plant performance under drying conditions. Physiological traits such as πtlp can be used to assess and model higher scale processes in response to drying conditions. A plain language summary is available for this article.

Gender dimorphism in Corema album across its biogeographical area and implications under a scenario of extreme drought events

In dioecious species, traits may have evolved because of significantly different resource demands associated with male versus female sexual reproduction. It is generally assumed that females have higher reproductive costs, thereafter in long-lived species, males often exceed females in vigour and suffer lower physiological limitations under stress. However, large-scale studies of gender dimorphism and the link between survival and physiological responses and compensation mechanisms are still lacking. In this study, we compared canopy performance, photochemical efficiency, leaf water potential, Δ13C, leaf nitrogen content and δ15N in both genders of the dioecious species Corema album (Ericaceae) across all the species’ biogeographical distribution. This area corresponds to a wide climatic gradient, from temperate and humid to Mediterranean, across the Atlantic coast of the Iberian Peninsula.Our results evidenced that the species responds to the bioclimatic gradient, and most variables were significantly correlated with annual rainfall and bioclimatic indices, although gender dimorphism was only manifested in Δ13C and in leaf water potential at the peak of the drought season. Female plants have significantly higher values of Δ13C, which could be attributed to lower water use efficiency; however, male plants presented more negative leaf water potentials than females, especially in the populations with the warmest and most arid climate.We concluded that C. album populations adjust to the climatic gradient at different hierarchical scales, from physiological instantaneous to time-integrated traits, with gender dimorphism only occurring in sub-optimal environments The absence of morphological differences between genders and the maintenance of a less negative water potential in females in the dry period in southern more arid populations, suggests the existence of compensatory mechanisms at underground level.

The Effect of Plant Spacing on the Water Status of Soil and Grapevines

The effect of plant spacing on soil water content and plant water status is described. The higher root densities of narrower plantings resulted in a more rapid depletion of soil water content. This resulted in a more negative leaf water potential which, in turn, resulted in earlier stomatal closure, affecting transpiration rate negatively. Consequently grapes from narrower spaced vines ripened under higher water stress conditions than those from wider spaced vines.

Ecophysiological Responses of Two Olive Tree Cultivars (Olea europaea L. CV Koroneiki and Chemlali) Under Three Water Treatments

In this paper, our objective is to assess and compare the effects of three levels of irrigation application on the ecophysiological behavior and plant dry matter accumulation of two young olive tree cultivars Koroneiki, a promising greek cultivar, and Chemlali, the best local cultivar. According to this assessment, we can determine the most efficient water treatment that could be suitable for Mediterranean environments subjected to water shortage conditions. Measurements were made in the green house of the Tunisian Olive tree Institute under normal day-light conditions from March, 16th to April, 21st 2015. Three water treatments were applied which are T100% (control treatment: Daily irrigation at 100% of Available Water Content (AWC)), T50% (Daily irrigation at 50% of AWC) and T0% (Without watering). The results showed that the two olive tree cultivars possess important mechanisms to overtake limited water resources. However, some striking variations existed between the two cultivars studied. Indeed, as water stress increased (T0%), Chemlali maintained longer a high midday leaf water potential compared (- 4.54 MPa) to Koroneiki (- 5.8 MPa). T50% treatment seems to be sufficient for both cultivars. Measurements of total Osmotic Adjustment (OA) showed that olive trees use this mechanism to create very negative leaf water potentials in order to be able to extract water from a dry root environment. The root/shoot ratio of Chemlali plants at T50% treatment was the highest (1.08) comparatively with the other two treatments (0.70 and 0.79 for T100% and T0% AWC water treatments, respectively). This result shows that Chemlali plants valorize low quantities of water (T50%) rather than high quantities (T100%) and Koroneiki plants behave better when it is irrigated at 100% AWC. To conclude, Chemlali plants irrigated at 50% AWC, compared to Koroneiki plants, are the most suitable to tolerate water restriction conditions.

Selenium Supplementation Affects Physiological and Biochemical Processes to Improve Fodder Yield and Quality of Maize (Zea mays L.) under Water Deficit Conditions.

Climate change is one of the most complex challenges that pose serious threats to livelihoods of poor people who rely heavily on agriculture and livestock particularly in climate-sensitive developing countries of the world. The negative effects of water scarcity, due to climate change, are not limited to productivity food crops but have far-reaching consequences on livestock feed production systems. Selenium (Se) is considered essential for animal health and has also been reported to counteract various abiotic stresses in plants, however, understanding of Se regulated mechanisms for improving nutritional status of fodder crops remains elusive. We report the effects of exogenous selenium supply on physiological and biochemical processes that may influence green fodder yield and quality of maize (Zea mays L.) under drought stress conditions. The plants were grown in lysimeter tanks under natural conditions and were subjected to normal (100% field capacity) and water stress (60% field capacity) conditions. Foliar spray of Se was carried out before the start of tasseling stage (65 days after sowing) and was repeated after 1 week, whereas, water spray was used as a control. Drought stress markedly reduced the water status, pigments and green fodder yield and resulted in low forage quality in water stressed maize plants. Nevertheless, exogenous Se application at 40 mg L-1 resulted in less negative leaf water potential (41%) and enhanced relative water contents (30%), total chlorophyll (53%), carotenoid contents (60%), accumulation of total free amino acids (40%) and activities of superoxide dismutase (53%), catalase (30%), peroxidase (27%), and ascorbate peroxidase (27%) with respect to control under water deficit conditions. Consequently, Se regulated processes improved fodder yield (15%) and increased crude protein (47%), fiber (10%), nitrogen free extract (10%) and Se content (36%) but did not affect crude ash content in water stressed maize plants. We propose that Se foliar spray (40 mg L-1) is a handy, feasible and cost-effective approach to improve maize fodder yield and quality in arid and semi-arid regions of the world facing acute shortage of water.

Informing arid region mine-site restoration through comparative ecophysiology of Acacia species under drought

Establishing vegetation on disturbed sites in arid environments is difficult due to decreased water availability caused by altered soil properties: depth, compaction, and hydraulic characteristics. Plants cope with moisture stress through a combination of traits, including physiological strategies such as anisohydry and isohydry. We used a typical mine restoration substrate in a glasshouse pot experiment to investigate drought tolerance of nine Pilbara region Acacia species classified according to habitat preferences defined by preferred soil type: alluvial (fine textured), sandy, rocky, and generalists without a clear soil preference. Seedlings were examined to (1) determine physiological shoot, and morphological shoot and root traits associated with drought tolerance, and (2) identify if these traits were correlated with species’ soil preferences. Species from alluvial, rocky, and one sandy soil species were more anisohydric. These species had higher stomatal conductance at more negative leaf water potentials. Alluvial soil species had greater total biomass allocation to lateral roots, whereas two coarser textured soil species had high allocation to tap roots. Soil preference was a poor predictor of plant water relations, presumably due to complex soil profiles in nature associated with widely differing hydraulic characteristics, and interactions among plant functional traits influencing water uptake, transport and loss.

Leaf Water Potential and Crop Water Stress Index Variation for Full and Deficit Irrigated Cotton in Mediterranean Conditions

Deficit irrigation is a common practice to cope with limited water availability. A two-year field experiment was conducted in order to determine the water status of cotton crop ( Gossypium hirsutum L.) under full and deficit irrigation conditions. The experiment was set up as a split plot design with four replicates, two main plots (irrigation treatments) and two sub plots (cotton cultivars). The lower (non-stressed) and upper (fully stressed) base lines for the determination of Crop Water Stress Index (CWSI) of cotton crop were estimated. Plant water status, expressed in terms of the water potential index (WPI) and CWSI, was measured throughout the growing seasons. The adopted irrigation schemes produced a clear differentiation among cultivars concerning their plant water status. Water shortage resulted in more negative leaf water potential and WPI and greater CWSI values. Leaf water potential, WPI and CWSI values, variation throughout both growing seasons, indicated greater adaptability for “Julia” in water stress conditions, while “Zoi” showed a significant water stress behavior during the phases of yield formation and ripening, even for the full irrigated treatment. In general, use of limited drip irrigation regime with 50% of water requirement had significant benefits in terms of saved irrigation water.

Differences in seedling water-stress response of two co-occurring Banksia species

In the South-west Australian Floristic Region, timing of rainfall is critical for successful seedling establishment, as is surviving the first year’s summer drought for population persistence. Predictions of a warmer, drier future, therefore, threaten the persistence of obligate seeding species. Here, we investigate the drought tolerance of two co-occurring Banksia (Proteaceae) species by withholding water in pots to different extents of soil drying. Seed was collected from high- and low-rainfall populations, to test for niche differentiation in water-use strategies at the species level, as well as population differentiation. On the basis of a more negative leaf water potential at minimal levels of stomatal conductance and quantum yield, B. coccinea was considered to be more drought tolerant than B. baxteri. This was supported at the anatomical level according to xylem-vessel attributes, with a higher estimated collapse pressure suggesting that B. coccinea is less vulnerable to xylem cavitation. Population contrasts were observed mainly for B. baxteri, with a lower leaf-expansion increment rate in the low-rainfall population providing for drought avoidance, which was reflected in a higher rate of survival than with the high-rainfall population in which 87.5% of plants showed complete leaf senescence. The implications of species differences in water-use strategies are that community dynamics may start to shift as the climate changes. Importantly, this shift may be population dependent. A systematic understanding of adaptive capacity will help inform the choice of population for use in revegetation programs, which may lead to increased resilience and persistence in the face of environmental change. The results of the present study suggest that should declines in B. baxteri populations be noted, revegetating with seed collected from the low-rainfall population may help improve the chances of this species surviving into the future.