Drought-induced fiber water release and xylem embolism susceptibility of intact balsam poplar saplings.

Balsam poplar (Populus balsamifera L.) is a widespread tree species in North America with significant ecological and economic value. However, little is known about the susceptibility of saplings to drought-induced embolism and its link to water release from surrounding xylem fibers. Questions remain regarding localized mechanisms that contribute to the survival of saplings in vivo of this species under drought. Using X-ray micro-computed tomography on intact saplings of genotypes Gillam-5 and Carnduff-9, we found that functional vessels are embedded in a matrix of water-filled fibers under well-watered conditions in both genotypes. However, water-depleted fibers started to appear under moderate drought stress while vessels remained water-filled in both genotypes. Drought-induced xylem embolism susceptibility was comparable between genotypes, and a greater frequency of smaller diameter vessels in GIL-5 did not increase embolism resistance in this genotype. Despite having smaller vessels and a total vessel number that was comparable to CAR-9, stomatal conductance was generally higher in GIL-5 compared to CAR-9. In conclusion, our in vivo data on intact saplings indicate that differences in embolism susceptibility are negligible between GIL-5 and CAR-9, and that fiber water release should be considered as a mechanism that contributes to the maintenance of vessel functional status in saplings of balsam poplar experiencing their first drought event.

Nitrogen isotope discrimination in open-pollinated and hybrid canola suggests indirect selection for enhanced ammonium utilization.

Nitrogen isotope discrimination (Δ15N) may have utility as an indicator of nitrogen use in plants. A simple Δ15N-based isotope mass balance (IMB) model has been proposed to provide estimates of efflux/influx (E/I) ratios across root plasma membranes, the proportion of inorganic nitrogen assimilation in roots (P root) and translocation of inorganic nitrogen to shoots (Ti/Tt) under steady-state conditions. We used the IMB model to investigate whether direct selection for yield in canola (Brassica napus L.) has resulted in indirect selection in traits related to nitrogen use. We selected 23 canola lines developed from 1942 to 2017, including open-pollinated (OP) lines developed prior to 2005 as well as more recent commercial hybrids (CH), and in three separate experiments grew them under hydroponic conditions in a greenhouse with either 0.5 mM ammonium, 0.5 mM nitrate, or 5 mM nitrate. Across all lines, E/I, Proot and Ti/Tt averaged 0.09±0.03, 0.82±0.05 and 0.23±0.06 in the low nitrate experiment, and 0.31±0.06, 0.71±0.07 and 0.42±0.12 in the high nitrate experiment, respectively. In contrast, in the ammonium experiment average E/I was 0.40±0.05 while Ti/Tt averaged 0.07±0.04 and Proot averaged 0.97±0.02. Although there were few consistent differences between OP and CH under nitrate nutrition, commercial hybrids were collectively better able to utilize ammonium as their sole nitrogen source, demonstrating significantly greater overall biomass and a lower Proot and a higher Ti/Tt, suggesting a somewhat greater flux of ammonium to the shoot. Average root and whole-plant Δ15N were also slightly higher in CH lines, suggesting a small increase in E/I. An increased ability to tolerate and/or utilize ammonium in modern canola hybrids may have arisen under intensive mono-cropping.

Unweaving the population structure and genetic diversity of Canadian shrub willow.

Perennial shrub willow are increasingly being promoted in short-rotation coppice systems as biomass feedstocks, for phytoremediation applications, and for the diverse ecosystem services that can accrue. This renewed interest has led to widespread willow cultivation, particularly of non-native varieties. However, Canadian willow species have not been widely adopted and their inherent diversity has not yet been thoroughly investigated. In this study, 324 genotypes of Salix famelica and Salix eriocephala collected from 33 sites of origin were analyzed using 26,016 single nucleotide polymorphisms to reveal patterns of population structure and genetic diversity. Analyses by Bayesian methods and principal component analysis detected five main clusters that appeared to be largely shaped by geoclimatic variables including mean annual precipitation and the number of frost-free days. The overall observed (HO) and expected (HE) heterozygosity were 0.126 and 0.179, respectively. An analysis of molecular variance revealed that the highest genetic variation occurred within genotypes (69%), while 8% of the variation existed among clusters and 23% between genotypes within clusters. These findings provide new insights into the extent of genetic variation that exists within native shrub willow species which could be leveraged in pan-Canadian willow breeding programs.

Enlightening the Pathway of Phytoremediation: Ecophysiology and X-ray Fluorescence Visualization of Two Chilean Hardwoods Exposed to Excess Copper.

In the present climate emergency due to global warming, we are urged to move away from fossil fuels and pursue a speedy conversion to renewable energy systems. Consequently, copper (Cu) will remain in high demand because it is a highly efficient conductor used in clean energy systems to generate power from solar, hydro, thermal and wind energy across the world. Chile is the global leader in copper production, but this position has resulted in Chile having several hundred tailing deposits. We grew two Chilean native hardwood species, quillay (Quillaja saponaria Molina) and espino (Vachellia caven (Molina) Seigler & Ebinger, under three increasing Cu levels (0, 50, and 100 µM) for 6 months in a greenhouse setting. We measured growth, photosynthetic performance and elemental contents of leaves and roots to further evaluate their potential for phytoremediation. Growth of quillay was unaffected by Cu treatment but growth of espino was enhanced, as was its photosynthetic performance, indicating that espino may have an unusually high requirement for copper. Excess Cu was mostly restricted to the roots of both species, where X-ray fluorescence (XRF) mapping indicated some tendency for Cu to accumulate in tissues outside the periderm. Calcium oxalate crystals were prominently visible in XRF images of both species. Nickel (but not Cu) showed a concurrent distribution pattern with these crystals.

Genotypic variation in C and N isotope discrimination suggests local adaptation of heart-leaved willow.

Plants acquire multiple resources from the environment and may need to adjust and/or balance their respective resource-use efficiencies to maximize grow and survival, in a locally adaptive manner. In this study, tissue and whole-plant carbon (C) isotopic composition (δ13C) and carbon/nitrogen (C/N) ratios provided long-term measures of use efficiencies for water (WUE) and nitrogen (NUE), and a nitrogen (N) isotopic composition (δ15N)-based mass balance model was used to estimate traits related to N uptake and assimilation in heart-leaved willow (Salix eriocephala Michx.). In an initial common garden experiment consisting of 34 populations, we found population-level variation in δ13C, C/N ratio and δ15N, indicating different patterns in WUE, NUE and N uptake and assimilation. Although there was no relationship between foliar δ13C and C/N ratios among populations, there was a significant negative correlation between these measures across all individuals, implying a genetic and/or plastic trade-off between WUE and NUE not associated with local adaptation. To eliminate any environmental effect, we grew a subset of 21 genotypes hydroponically with nitrate as the sole N source and detected significant variation in δ13C, δ15N and C/N ratios. Variation in δ15N was mainly due to genotypic differences in the nitrate efflux/influx ratio (E/I) at the root. Both experiments suggested clinal variation in δ15N (and thus N uptake efficiency) with latitude of origin, which may relate to water availability and could contribute to global patterns in ecosystem δ15N. There was a tendency for genotypes with higher WUE to come from more water-replete sites with shorter and cooler growing seasons. We found that δ13C, C/N ratio and E/I were not inter-correlated, suggesting that the selection of growth, WUE, NUE and N uptake efficiency can occur without trade-off.

Establishment of willows using the novel DeValix technique: ecological restoration mats designed for phytotechnologies.

Successful willow (Salix spp., hybrids and cultivars) establishment is a major determinant of their effectiveness when grown for phytotechnologies. Vertically-planted hardwood cuttings have been shown to produce adequate willow growth and survival, although site conditions at phytoremediation installations can make vertical planting methods unsuitable. The DeValix willow mat restoration technique was designed and tested as an alternative horizontal planting method that can be installed by hand in a variety of environmental applications. The DeValix technique was evaluated by testing five willow clones ("Millbrook"; "Sherburne"; "SX61"; "SX67"; "Tully Champion") grown at two phytoremediation sites (Ontonagon, MI; Manitowoc, WI) for the 2019 growing season. Differences in survival and growth were tested among sites, genotypes, and their interactions. Stem height, diameter, and number of stems per mat were compared to identify clones with greater establishment success and higher phytoremediation potential. Results demonstrated significant effects of site (p < 0.0001) and clone (p < 0.0001) on shoot number. Additionally, the site × clone interaction significantly affected stem height (p = 0.0045) and diameter (p = 0.0166). Stem density ranged from 95,000 to 212,000 stems per hectare, indicating the DeValix technique is a viable establishment method for environmental applications, including phytoremediation and shoreline stabilization.

This research analyzes the establishment success of the DeValix technique, a novel horizontal planting method for willow cultivars, and evaluates the DeValix technique as an alternative to other horizontal techniques and traditional vertical planting methods currently used in environmental applications. Results from this study add to the current knowledge of planting techniques and assesses the use of the DeValix technique for planting willow cultivars that are currently being tested in several phytotechnology systems.

Genome, Transcriptome, and Germplasm Sequencing Uncovers Functional Variation in the Warm-Season Grain Legume Horsegram Macrotyloma uniflorum (Lam.) Verdc.

Horsegram is a grain legume with excellent nutritional and remedial properties and good climate resilience, able to adapt to harsh environmental conditions. Here, we used a combination of short- and long-read sequencing technologies to generate a genome sequence of 279.12Mb, covering 83.53% of the estimated total size of the horsegram genome, and we annotated 24,521 genes. De novo prediction of DNA repeats showed that approximately 25.04% of the horsegram genome was made up of repetitive sequences, the lowest among the legume genomes sequenced so far. The major transcription factors identified in the horsegram genome were bHLH, ERF, C2H2, WRKY, NAC, MYB, and bZIP, suggesting that horsegram is resistant to drought. Interestingly, the genome is abundant in Bowman-Birk protease inhibitors (BBIs), which can be used as a functional food ingredient. The results of maximum likelihood phylogenetic and estimated synonymous substitution analyses suggested that horsegram is closely related to the common bean and diverged approximately 10.17 million years ago. The double-digested restriction associated DNA (ddRAD) sequencing of 40 germplasms allowed us to identify 3,942 high-quality SNPs in the horsegram genome. A genome-wide association study with powdery mildew identified 10 significant associations similar to the MLO and RPW8.2 genes. The reference genome and other genomic information presented in this study will be of great value to horsegram breeding programs. In addition, keeping the increasing demand for food with nutraceutical values in view, these genomic data provide opportunities to explore the possibility of horsegram for use as a source of food and nutraceuticals.

Natural climate solutions for Canada.

Alongside the steep reductions needed in fossil fuel emissions, natural climate solutions (NCS) represent readily deployable options that can contribute to Canada's goals for emission reductions. We estimate the mitigation potential of 24 NCS related to the protection, management, and restoration of natural systems that can also deliver numerous co-benefits, such as enhanced soil productivity, clean air and water, and biodiversity conservation. NCS can provide up to 78.2 (41.0 to 115.1) Tg CO2e/year (95% CI) of mitigation annually in 2030 and 394.4 (173.2 to 612.4) Tg CO2e cumulatively between 2021 and 2030, with 34% available at ≤CAD 50/Mg CO2e. Avoided conversion of grassland, avoided peatland disturbance, cover crops, and improved forest management offer the largest mitigation opportunities. The mitigation identified here represents an important potential contribution to the Paris Agreement, such that NCS combined with existing mitigation plans could help Canada to meet or exceed its climate goals.

Experimental support for genomic prediction of climate maladaptation using the machine learning approach Gradient Forests.

Gradient Forests (GF) is a machine learning algorithm that is gaining in popularity for studying the environmental drivers of genomic variation and for incorporating genomic information into climate change impact assessments. Here we (i) provide the first experimental evaluation of the ability of "genomic offsets" - a metric of climate maladaptation derived from Gradient Forests - to predict organismal responses to environmental change, and (ii) explore the use of GF for identifying candidate SNPs. We used high-throughput sequencing, genome scans, and several methods, including GF, to identify candidate loci associated with climate adaptation in balsam poplar (Populus balsamifera L.). Individuals collected throughout balsam poplar's range also were planted in two common garden experiments. We used GF to relate candidate loci to environmental gradients and predict the expected magnitude of the response (i.e., the genetic offset metric of maladaptation) of populations when transplanted from their "home" environment to the common gardens. We then compared the predicted genetic offsets from different sets of candidate and randomly selected SNPs to measurements of population performance in the common gardens. We found the expected inverse relationship between genetic offset and performance: populations with larger predicted genetic offsets performed worse in the common gardens than populations with smaller offsets. Also, genetic offset better predicted performance than did "naive" climate transfer distances. However, sets of randomly selected SNPs predicted performance slightly better than did candidate SNPs. Our study provides evidence that genetic offsets represent a first order estimate of the degree of expected maladaptation of populations exposed to rapid environmental change and suggests GF may have some promise as a method for identifying candidate SNPs.

A meta-analysis of mesophyll conductance to CO2 in relation to major abiotic stresses in poplar species.

Mesophyll conductance (gm) determines the diffusion of CO2 from the substomatal cavities to the site of carboxylation in the chloroplasts and represents a critical component of the diffusive limitation of photosynthesis. In this study, we evaluated the average effect sizes of different environmental constraints on gm in Populus spp., a forest tree model. We collected raw data of 815 A-Ci response curves from 26 datasets to estimate gm, using a single curve-fitting method to alleviate method-related bias. We performed a meta-analysis to assess the effects of different abiotic stresses on gm. We found a significant increase in gm from the bottom to the top of the canopy that was concomitant with the increase of maximum rate of carboxylation and light-saturated photosynthetic rate (Amax). gm was positively associated with increases in soil moisture and nutrient availability, but was insensitive to increasing soil copper concentration and did not vary with atmospheric CO2 concentration. Our results showed that gm was strongly related to Amax and to a lesser extent to stomatal conductance (gs). Moreover, a negative exponential relationship was obtained between gm and specific leaf area, which may be used to scale-up gm within the canopy.

Physiological Response of Populus balsamifera and Salix eriocephala to Salinity and Hydraulic Fracturing Wastewater: Potential for Phytoremediation Applications.

Natural and anthropogenic soil degradation is resulting in a substantial rise in the extension of saline and industrially-polluted soils. Phytoremediation offers an environmentally and economically advantageous solution to soil contamination. Three growth trials were conducted to assess the stress tolerance of native Canadian genotypes of Populus balsamifera L., Salix eriocephala Michx., and one hybrid willow (S. discolor × S. dasyclados) to salinity and hydraulic fracturing (fracking) wastewater. Thirty-three genotypes were grown in NaCl or fracking wastewater solutions between 0 and 7 mS-1 over a period of 3-4 months. P. balsamifera was observed to be relatively salt-intolerant compared to S. eriocephala and hybrid willow, which is likely caused by an inability of P. balsamifera to restrict Na+ translocation. Photosynthesis and transpiration decreased with salinity treatments, and severe reductions occurred with exposure to fracking solutions. Raffinose and stachyose content was tripled in leaf and root tissues. In willows, Na+ was primarily confined to root tissues, Cl- accumulated up to 5% dry weight in leaves, and K+ was translocated from roots to leaves. Willow genotypes CAM-2 and STL-2 displayed the greatest maintenance of growth and resistance to necrotic symptoms in all trials, suggesting that these genotypes may be useful for practical application and further field study.

Differences in growth and physiological and metabolic responses among Canadian native and hybrid willows (Salix spp.) under salinity stress.

Globally, soil salinization is becoming increasingly prevalent, due to local hydrogeologic phenomena, climate change and anthropogenic activities. This has significantly curtailed current world food production and limits future production potential. In the prairie region of North America, sulfate salts, rather than sodium chloride, are often the predominant cause of soil degradation. In order to amend soil quality, revegetate salt-affected sites and recover economic loss associated with soil salinization, the establishment of short-rotation coppice plantations with willows (Salix spp.) has been suggested as a possible solution. To screen for the best candidates for such an application, 20 hybrid and 16 native willow genotypes were treated with three different salt conditions for 3 months. The treatments were designed to reflect the salt composition and concentrations on North American prairies. Under moderate salinity treatment (7 dS m-1), hybrid willows had better growth, as they established quickly while managing salt transport and mineral nutrition balance. However, native willows showed higher potential for long-term survival under severe salinity treatment (14 dS m-1), showing a lower sodium:potassium ratio in roots and better photosynthetic performance. Two native willow genotypes with high osmotic and salinity tolerance indices, specifically LAR-10 and MJW-9, are expected to show superior potential for remediating salt-affected sites. In addition, we observed significantly higher sulfate/sulfur concentrations in both leaf and root tissues in response to the severe salinity treatment, shedding light on the effect of sulfate salinity on sulfate uptake, and potentially sulfur metabolism in plants.

Potential effects of a high CO2 future on leguminous species.

Legumes provide an important source of food and feed due to their high protein levels and many health benefits, and also impart environmental and agronomic advantages as a consequence of their ability to fix nitrogen through their symbiotic relationship with rhizobia. As a result of our growing population, the demand for products derived from legumes will likely expand considerably in coming years. Since there is little scope for increasing production area, improving the productivity of such crops in the face of climate change will be essential. While a growing number of studies have assessed the effects of climate change on legume yield, there is a paucity of information regarding the direct impact of elevated CO2 concentration (e[CO2]) itself, which is a main driver of climate change and has a substantial physiological effect on plants. In this review, we discuss current knowledge regarding the influence of e[CO2] on the photosynthetic process, as well as biomass production, seed yield, quality, and stress tolerance in legumes, and examine how these responses differ from those observed in non-nodulating plants. Although these relationships are proving to be extremely complex, mounting evidence suggests that under limiting conditions, overall declines in many of these parameters could ensue. While further research will be required to unravel precise mechanisms underlying e[CO2] responses of legumes, it is clear that integrating such knowledge into legume breeding programs will be indispensable for achieving yield gains by harnessing the potential positive effects, and minimizing the detrimental impacts, of CO2 in the future.

Learning from methylomes: epigenomic correlates of Populus balsamifera traits based on deep learning models of natural DNA methylation.

Epigenomes have remarkable potential for the estimation of plant traits. This study tested the hypothesis that natural variation in DNA methylation can be used to estimate industrially important traits in a genetically diverse population of Populus balsamifera L. (balsam poplar) trees grown at two common garden sites. Statistical learning experiments enabled by deep learning models revealed that plant traits in novel genotypes can be modelled transparently using small numbers of methylated DNA predictors. Using this approach, tissue type, a nonheritable attribute, from which DNA methylomes were derived was assigned, and provenance, a purely heritable trait and an element of population structure, was determined. Significant proportions of phenotypic variance in quantitative wood traits, including total biomass (57.5%), wood density (40.9%), soluble lignin (25.3%) and cell wall carbohydrate (mannose: 44.8%) contents, were also explained from natural variation in DNA methylation. Modelling plant traits using DNA methylation can capture tissue-specific epigenetic mechanisms underlying plant phenotypes in natural environments. DNA methylation-based models offer new insight into natural epigenetic influence on plants and can be used as a strategy to validate the identity, provenance or quality of agroforestry products.

Discerning the effects of phosphate status on the metabolism of hybrid poplar.

Accumulation of phosphate in leaves as external environmental phosphate concentrations increase has been observed across the plant kingdom. The excess storage of anions, such as phosphate, has various metabolic trade-offs, including a corresponding influx of counter-ions to maintain charge balance and/or the reduction in organic acid content to maintain internal pH. The leaves and roots of four hybrid poplar genotypes were tested for differences in metabolic response to increasing external phosphate and further effects on patterns of anion resorption among hybrid poplar and willow were explored. Organic acid concentrations increased or remained constant across treatments, suggesting that metabolic adjustments were made in response to greater influxes of inorganic cations rather than a response to increasing phosphate. During senescence, the hybrid poplar Tristis had higher sulfate and organic acid resorption, while hybrid willow, AAFC-5, had higher phosphate resorption proficiencies, suggesting differing anion remobilization mechanisms. Furthermore, phosphate accumulation was shown to continue well after bud-set in poplar hybrids, which may contribute to the low phosphorus resorption efficiency. This indicates that closely related species, with similar growth strategies, show preferential resorption toward different nutrients.

Biotechnological strategies for improved photosynthesis in a future of elevated atmospheric CO2.

MAIN CONCLUSION: The improvement of photosynthesis using biotechnological approaches has been the focus of much research. It is now vital that these strategies be assessed under future atmospheric conditions. The demand for crop products is expanding at an alarming rate due to population growth, enhanced affluence, increased per capita calorie consumption, and an escalating need for plant-based bioproducts. While solving this issue will undoubtedly involve a multifaceted approach, improving crop productivity will almost certainly provide one piece of the puzzle. The improvement of photosynthetic efficiency has been a long-standing goal of plant biotechnologists as possibly one of the last remaining means of achieving higher yielding crops. However, the vast majority of these studies have not taken into consideration possible outcomes when these plants are grown long-term under the elevated CO2 concentrations (e[CO2]) that will be evident in the not too distant future. Due to the considerable effect that CO2 levels have on the photosynthetic process, these assessments should become commonplace as a means of ensuring that research in this field focuses on the most effective approaches for our future climate scenarios. In this review, we discuss the main biotechnological research strategies that are currently underway with the aim of improving photosynthetic efficiency and biomass production/yields in the context of a future of e[CO2], as well as alternative approaches that may provide further photosynthetic benefits under these conditions.

Author Correction: Sexual homomorphism in dioecious trees: extensive tests fail to detect sexual dimorphism in Populus.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Sexual homomorphism in dioecious trees: extensive tests fail to detect sexual dimorphism in Populus †.

The evolution of sexual dimorphism and expansion of sex chromosomes are both driven through sexual conflict, arising from differing fitness optima between males and females. Here, we pair work in poplar (Populus) describing one of the smallest sex-determining regions known thus far in complex eukaryotes (~100 kbp) with comprehensive tests for sexual dimorphism using >1300 individuals from two Populus species and assessing 96 non-reproductive functional traits. Against expectation, we found sexual homomorphism (no non-reproductive trait differences between the sexes), suggesting that gender is functionally neutral with respect to non-reproductive features that affect plant survival and fitness. Combined with a small sex-determining region, we infer that sexual conflict may be effectively stymied or non-existent within these taxa. Both sexual homomorphism and the small sex-determining region occur against a background of strong environmental selection and local adaptation in Populus. This presents a powerful hypothesis for the evolution of dioecious species. Here, we suggest that environmental selection may be sufficient to suppress and stymy sexual conflict if it acts orthogonal to sexual selection, thereby placing limitations on the evolution of sexual dimorphism and genomic expansion of sex chromosomes.

Comparative physiology of allopatric Populus species: geographic clines in photosynthesis, height growth, and carbon isotope discrimination in common gardens.

Populus species with wide geographic ranges display strong adaptation to local environments. We studied the clinal patterns in phenology and ecophysiology in allopatric Populus species adapted to similar environments on different continents under common garden settings. As a result of climatic adaptation, both Populus tremula L. and Populus balsamifera L. display latitudinal clines in photosynthetic rates (A), whereby high-latitude trees of P. tremula had higher A compared to low-latitude trees and nearly so in P. balsamifera (p = 0.06). Stomatal conductance (g s) and chlorophyll content index (CCI) follow similar latitudinal trends. However, foliar nitrogen was positively correlated with latitude in P. balsamifera and negatively correlated in P. tremula. No significant trends in carbon isotope composition of the leaf tissue (δ(13)C) were observed for both species; but, intrinsic water-use efficiency (WUEi) was negatively correlated with the latitude of origin in P. balsamifera. In spite of intrinsically higher A, high-latitude trees in both common gardens accomplished less height gain as a result of early bud set. Thus, shoot biomass was determined by height elongation duration (HED), which was well approximated by the number of days available for free growth between bud flush and bud set. We highlight the shortcoming of unreplicated outdoor common gardens for tree improvement and the crucial role of photoperiod in limiting height growth, further complicating interpretation of other secondary effects.

Investigating the drought-stress response of hybrid poplar genotypes by metabolite profiling.

Drought stress is perhaps the most commonly encountered abiotic stress plants experience in the natural environment, and it is one of the most important factors limiting plant productivity. Here, we employed untargeted metabolite profiling to examine four drought-stressed hybrid poplar (Populus spp.) genotypes for their metabolite content, using gas chromatography coupled to mass spectrometry. The primary objective of these analyses was to characterize the metabolite profile of poplar trees to assess relative drought resistance and to investigate the underlying biochemical mechanisms employed by the genotypes to combat drought. Metabolite profiling identified key metabolites that increased or decreased in relative abundance upon exposure to drought stress. Overall, amino acids, the antioxidant phenolic compounds catechin and kaempferol, as well as the osmolytes raffinose and galactinol exhibited increased abundance under drought stress, whereas metabolites involved in photorespiration, redox regulation and carbon fixation showed decreased abundance under drought stress. One clone in particular, Okanese, displayed unique responses to the imposed drought conditions. This clone was found to have higher leaf water potential, but lower growth rate relative to the other clones tested. Okanese also had lower accumulation of osmolytes such as raffinose, galactinol and proline, but higher overall levels of antioxidants such as catechin and dehydroascorbic acid. As such, it was proposed that osmotic adjustment as a mechanism for drought avoidance in this clone is not as well developed in comparison with the other clones investigated in this study, and that a possible alternative mechanism for the enhanced drought avoidance displayed by Okanese may be due to differential allocation of resources or better retention of water.