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Abiotic stress limits crop productivity worldwide, making stress-resistant crops a major goal for improving food security. Stress resistance is often assessed by comparing genotypes under a single control and stressed level of an abiotic factor, then ranking the genotypes’ overall performance, using a metric like geometric mean productivity (GMP). However, abiotic stress is rarely a presence/absence phenomenon, but often varies continuously. Here, we tested whether the common practice of assessing performance using a single control and stress level is predictive of performance across a range of stress levels. Twelve sunflower genotypes were grown in the greenhouse under one of eight nutrient levels (from non-limiting to severely limiting) and assessed for performance (shoot dry mass at the budding stage) and related traits at both seedling and budding stages. Contrary to expectations, genotype rankings for GMP were consistent regardless of the two nutrient levels used to calculate GMP. This suggests that, for nutrient stress in sunflower, studies using an arbitrary control and stress level are broadly representative: genotypes with high GMP exhibit high GMP regardless of the nutrient levels used. In addition, GMP values were highly correlated with performance in the respective nutrient levels from which they were calculated; thus, higher GMP for a given genotype indicates that it performed well (relative to other genotypes) under both stressed and non-stressed conditions. Together, these findings strongly support the current paradigm of comparing genotypes at a single control and stress level as a valid strategy for screening for nutrient stress resistance in sunflower. However, few traits were consistently associated with performance (shoot dry mass) at either high- or low-nutrient availability, and seedling traits were rarely predictive of budding-stage traits and performance, suggesting further studies are needed to determine the mechanisms underlying nutrient stress resistance in this important oilseed crop.

Recent work suggests variation in plant growth strategies is governed by a tradeoff in resource acquisition and use, ranging from a rapid resource acquisition strategy to a resource‐conservative strategy. While evidence for this tradeoff has been found in leaves, knowledge of root trait strategies, and whether they reflect adaptive differentiation across environments, is limited. In the greenhouse, we investigated variation in fine root morphology (specific root length and tissue density), chemistry (nitrogen concentration and carbon:nitrogen), and anatomy (root cross‐sectional traits) in populations of 26 Helianthus species and sister Phoebanthus tenuifolius. We also compared root trait variation in this study with leaf trait variation previously reported in a parallel study of these populations. Root traits varied widely and exhibited little phylogenetic signal, suggesting high evolutionary lability. Specific root length and root tissue density were weakly negatively correlated, but neither was associated with root nitrogen, providing little support for a single axis of root trait covariation. Correlations between traits measured in the greenhouse and native site characteristics were generally weak, suggesting a variety of equally viable root trait combinations exist within and across environments. However, high root nitrogen was associated with lower xylem vessel number and cross‐sectional area, suggesting a tradeoff between nutrient investment and water transport capacity. This led to correlations between root and leaf traits that were not always consistent with an acquisition–conservation tradeoff at the whole‐plant level. Given that roots must balance acquisition of water and nutrients with functions like anchorage, exudation, and microbial symbioses, the varied evidence for root trait covariation likely reflects the complexity of interacting selection pressures belowground. Similarly, the lack of evidence for a single acquisition–conservation tradeoff at the whole‐plant level likely reflects the vastly different selection pressures shaping roots and leaves, and the resources they are optimized to obtain.

Leaf defenses are widely recognized as key adaptations and drivers of plant evolution. Across environmentally diverse habitats, the macroevolution of leaf defenses can be predicted by the univariate trade-off model, which predicts that defenses are functionally redundant and thus trade off, and the resource availability hypothesis, which predicts that defense investment is determined by inherent growth rate and that higher defense will evolve in lower resource environments. Here, we examined the evolution of leaf physical and chemical defenses and secondary metabolites in relation to environmental characteristics and leaf economic strategy across 28 species of Helianthus (the sunflowers). Using a phylogenetic comparative approach, we found few evolutionary trade-offs among defenses and no evidence for defense syndromes. We also found that leaf defenses are strongly related to leaf economic strategy, with higher defense in more resource-conservative species, although there is little support for the evolution of higher defense in low-resource habitats. A wide variety of physical and chemical defenses predict resistance to different insect herbivores, fungal pathogens, and a parasitic plant, suggesting that most sunflower defenses are not redundant in function and that wild Helianthus represents a rich source of variation for the improvement of crop sunflower.

The leaf economics spectrum (LES) is a prominent ecophysiological paradigm that describes global variation in leaf physiology across plant ecological strategies using a handful of key traits. Nearly a decade ago, Shipley et al. (2006) used structural equation modelling to explore the causal functional relationships among LES traits that give rise to their strong global covariation. They concluded that an unmeasured trait drives LES covariation, sparking efforts to identify the latent physiological trait underlying the 'origin' of the LES. Here, we use newly developed phylogenetic structural equation modelling approaches to reassess these conclusions using both global LES data as well as data collected across scales in the genus Helianthus. For global LES data, accounting for phylogenetic non-independence indicates that no additional unmeasured traits are required to explain LES covariation. Across datasets in Helianthus, trait relationships are highly variable, indicating that global-scale models may poorly describe LES covariation at non-global scales.

The leaf economics spectrum (LES) describes a major axis of plant functional trait variation worldwide, defining suites of leaf traits aligned with resource-acquisitive to resource-conservative ecological strategies. The LES has been interpreted to arise from leaf-level trade-offs among ecophysiological traits common to all plants. However, it has been suggested that the defining leaf-level trade-offs of the LES may not hold within specific functional groups (e.g., herbs) nor within many groups of closely related species, which challenges the usefulness of the LES paradigm across evolutionary scales. Here, we examine the evolution of the LES across 28 species of the diverse herbaceous genus Helianthus (the sunflowers), which occupies a wide range of habitats and climate variation across North America. Using a phylogenetic comparative approach, we find repeated evolution of more resource-acquisitive LES strategies in cooler, drier, and more fertile environments. We also find macroevolutionary correlations among LES traits that recapitulate aspects of the global LES, but with one major difference: leaf mass per area is uncorrelated with leaf lifespan. This indicates that whole-plant processes likely drive variation in leaf lifespan across Helianthus, rather than leaf-level trade-offs. These results suggest that LES patterns do not reflect universal physiological trade-offs at small evolutionary scales.

Plant root systems mediate ecological processes in the rhizosphere through the exudation of organic compounds. Although exudate composition is thought to depend strongly on plant nutrient status, little is known about the influence of multi-nutrient stresses. In this study, we examined responses to short-term (3 d) nutrient limitation in Helianthus annuus (common sunflower), and root exudates were collected for 2, 4, or 6 h with the trap-solution method. Root exudates, analyzed by means of gas chromatography-mass spectrometry, consisted of over 60 sugars, sugar alcohols, amino acids, organic acids, and phosphates, with sugars and organic acids generally detected in the highest quantities. Twenty-five of the detected metabolites, including half of the organic acids, sugars, and sugar alcohols, differed in relative abundance among the three sampling intervals, exhibiting higher abundance in sampling intervals greater than 2 h. Similarly, 24 of the detected metabolites, including half of the amino acids, phosphates, and sugar alcohols, were affected by nutrient supply, with 20 exhibiting higher abundance in the high-nutrient treatment. Fumaric acid, quinic acid, and glucose were detected at significantly higher levels in the low-nutrient treatment, potentially representing an adaptive response to nutrient limitation in sunflower. However, as sampling interval exerted a strong influence on the apparent effects of nutrient supply, future studies should consider the potential impacts of sampling-interval length in comparative analyses of genotypes or treatments.

Summary 1. Efforts to understand the effects of plant traits on carbon and nutrient cycling have recently focused on species variation and the potential for species data to improve predictions of past, present and future variation in ecosystems. However, the evolutionary lability of relevant traits among closely related species and the extent of intraspecific variation warrant further consideration. 2. Here, we examine interspecific and intraspecific variation in leaf N, LMA, root N and SRL at multiple scales, using Helianthus as a representative study system. 3. Substantial evolutionary lability of traits is demonstrated by interspecific variation in phylogenetically explicit analyses of closely related Helianthus species, and population differentiation of wild H. anomalus and cultivated H. annuus. 4. Intraspecific variation in leaf N and LMA, including genetic, environmental and ontogenetic responses, demonstrates that trait values for a single species can encompass a surprisingly large portion of the range encompassed by species in the leaf economics spectrum. 5. Synthesis. We recommend using data from selected natural populations to model effects of leaf N and LMA on decomposition, while using data from common garden experiments to determine evolutionary lability and thus inform potential for evolutionary change. If the high evolutionary lability of traits demonstrated for Helianthus is found for other important genera, this would suggest that these key ecophysiological traits are likely to respond to the selective pressures of global climate and land-use change.

Most C3 plant species have partially open stomata during the night especially in the 3-5 h before dawn. This pre-dawn stomatal opening has been hypothesized to enhance early-morning photosynthesis (A) by reducing diffusion limitations to CO2 at dawn. We tested this hypothesis in cultivated Helianthus annuus using whole-shoot gas exchange, leaf level gas exchange and modelling approaches. One hour pre-dawn low-humidity treatments were used to reduce pre-dawn stomatal conductance (g). At the whole-shoot level, a difference of pre-dawn g (0.40 versus 0.17 mol m(-2) s(-1)) did not significantly affect A during the first hour after dawn. Shorter term effects were investigated with leaf level gas exchange measurements and a difference of pre-dawn g (0.10 versus 0.04 mol m(-2) s(-1)) affected g and A for only 5 min after dawn. The potential effects of a wider range of stomatal apertures were explored with an empirical model of the relationship between A and intercellular CO2 concentration during the half-hour after dawn. Modelling results demonstrated that even extremely low pre-dawn stomatal conductance values have only a minimal effect on early-morning A for a few minutes after dawn. Thus, we found no evidence that pre-dawn stomatal opening enhances A.

• Plant resource-use traits are generally hypothesized to be adaptively differentiated for populations distributed along resource gradients. Although nutrient limitations are expected to select for resource-conservative strategies, water limitations may select for either resource-conservative or -acquisitive strategies. We test whether population differentiation reflects local adaptation for traits associated with resource-use strategies in a desert annual (Helianthus anomalus) distributed along a gradient of positively covarying water and nutrient availability. • We compared quantitative trait variation (Q(ST)) with neutral genetic differentiation (F(ST)), in a common garden glasshouse study, for leaf economics spectrum (LES) and related traits: photosynthesis (A(mass), A(area)), leaf nitrogen (N(mass), N(area)), leaf lifetime (LL), leaf mass per area (LMA), leaf water content (LWC), water-use efficiency (WUE, estimated as δ(13)C) and days to first flower (DFF). • Q(ST)-F(ST) differences support adaptive differentiation for Amass , N(mass), N(area), LWC and DFF. The trait combinations associated with drier and lower fertility sites represent correlated trait evolution consistent with the more resource-acquisitive end of the LES. There was no evidence for adaptive differentiation for A(area), LMA and WUE. • These results demonstrate that hot dry environments can selectively favor correlated evolution of traits contributing to a resource-acquisitive and earlier reproduction 'escape' strategy, despite lower fertility.

C(3) plants dominate many landscapes and are critically important for ecosystem water cycling. At night, plant water losses can include transpiration (E(night)) from the canopy and hydraulic redistribution (HR) from roots. We tested whether E(night) limits the magnitude of HR in a greenhouse study using Artemisia tridentata, Helianthus anomalus and Quercus laevis. Plants were grown with their roots split between two compartments. HR was initiated by briefly withholding all water, followed by watering only one rooting compartment. Under study conditions, all species showed substantial E(night) and HR (highest minus lowest soil water potential [Psi(s)] during a specified diel period). Suppressing E(night) by canopy bagging increased HR during the nightly bagging period (HR(N)) for A. tridentata and H. anomalus by 73 and 33% respectively, but did not affect HR(N) by Q. laevis. Total daily HR (HR(T)) was positively correlated with the Psi(s) gradient between the rooting compartments, which was correlated with light and/or atmospheric vapour pressure deficit (VPDa) the prior day. For A. tridentata, HR(T) was negatively correlated with night-time VPDa. Ecological implications of the impact of E(night) on HR may include decreased plant productivity during dry seasons, altered ecosystem water flux patterns and reduced nutrient cycling in drying soils.

Habitats that differ in soil resource availability are expected to differ for selection on resource-related plant traits. Here, we examined spatial and temporal variation in phenotypic selection on leaf ecophysiological traits for 10 Helianthus populations, including two species of hybrid origin, Helianthus anomalus and Helianthus deserticola, and artificial hybrids of their ancestral parents. Leaf traits assessed were leaf size, succulence, nitrogen (N) concentration and water-use efficiency (WUE). Biomass and leaf traits of artificial hybrids indicate that the actively moving dune habitat of H. anomalus was more growth limiting, with lower N availability but higher relative water availability than the stabilized dune habitat of H. deserticola. Habitats differed for direct selection on leaf N and WUE, but not size or succulence, for the artificial hybrids. However, within the H. anomalus habitat, direct selection on WUE also differed among populations. Across years, direct selection on leaf traits did not differ. Leaf N was the only trait for which direct selection differed between habitats but not within the H. anomalus habitat, suggesting that nutrient limitation is an important selective force driving adaptation of H. anomalus to the active dune habitat.

Significant water loss occurs throughout the night via partially open stomata in many C(3) and C(4) plant species. Although apparently wasteful in terms of water use, nighttime transpiration (E(night)) is hypothesized to benefit plants by enhancing nutrient supply. We tested the hypothesis that plants with greater E(night) would have improved plant nutrient status and greater fitness, estimated as pre-bolting biomass, for Arabidopsis thaliana. Two very different levels of E(night) were generated in plants by exposing them to high vs low nighttime leaf-to-air vapor pressure deficits (VPD(leaf)) in controlled environment chambers. An assessment of responses of nighttime leaf conductance (g(night)) to VPD(leaf) indicated that E(night) differed by at least 80% between the treatments. This large difference in E(night), imposed over the entire vegetative growth phase of Arabidopsis, had no effect on leaf nutrient content (N, Ca, K) or pre-bolting rosette biomass. The lack of response to differences in E(night) held true for both a high and a low nitrogen (N) treatment, even though the low N treatment decreased leaf N and biomass by 40-60%. The N treatment had no effect on g(night). Thus, higher E(night) did not provide a nutrient or growth benefit to Arabidopsis, even when the plants were N-limited.

Night-time leaf conductance (g(night)) and transpiration may have several adaptive benefits related to plant water, nutrient and carbon relations. Little is known, however, about genetic variation in g(night) and whether this variation correlates with other gas exchange traits related to water use and/or native habitat climate. We investigated g(night) in 12 natural accessions and three near isogenic lines (NILs) of Arabidopsis thaliana. Genetic variation in g(night) was found for the natural accessions, and g(night) was negatively correlated with native habitat atmospheric vapour pressure deficit (VPD(air)), suggesting lower g(night) may be favoured by natural selection in drier habitats. However, there were also significant genetic correlations of g(night) with daytime gas exchange traits expected to affect plant fitness [i.e. daytime leaf conductance, photosynthesis and intrinsic water-use efficiency (WUE(i))], indicating that selection on daytime gas exchange traits may result in indirect selection on g(night). The comparison of three NILs to their parental genotypes identified one quantitative trait locus (QTL) contributing to variation in g(night). Further characterization of genetic variation in g(night) within and among populations and species, and of associations with other traits and native habitats will be needed to understand g(night) as a putatively adaptive trait.

Several models of hybrid zone evolution predict the same spatial patterns of genotypic distribution whether or not structuring is due to environment-dependent or -independent selection. In this study, we tested for evidence of environment-dependent selection in an Iris fulva x Iris brevicaulis hybrid population by examining the distribution of genotypes in relation to environmental gradients. We selected 201 Louisiana Iris plants from within a known hybrid population (80 m x 80 m) and placed them in four different genotypic classes (I. fulva, I. fulva-like hybrid, I. brevicaulis-like hybrid and I. brevicaulis) based on seven species-specific random amplified polymorphic DNA (RAPD) markers and two chloroplast DNA haplotypes. Environmental variables were then measured. These variables included percentage cover by tree canopy, elevation from the high water mark, soil pH and percentage soil organic matter. Each variable was sampled for all 201 plants. Canonical discriminant analysis (CDA) was used to infer the environmental factors most strongly associated with the different genotypic groups. Slight differences in elevation (-0.5 m to +0.4 m) were important for distinguishing habitat distributions described by CDA, even though there were no statistical differences between mean elevations alone. I. brevicaulis occurred in a broad range of habitats, while I. fulva had a narrower distribution. Of all the possible combinations, I. fulva-like hybrids and I. brevicaulis-like hybrids occurred in the most distinct habitat types relative to one another. Each hybrid class was not significantly different from its closest parent with regard to habitat occupied, but was statistically unique from its more distant parental species. Within the hybrid genotypes, most, but not all, RAPD loci were individually correlated with environmental variables. This study suggests that, at a very fine spatial scale, environment-dependent selection contributed to the genetic structuring of this hybrid zone.

Natural hybridization accompanied by a shift in niche preference by hybrid genotypes can lead to hybrid speciation. Natural selection may cause the fixation of advantageous alleles in the ecologically diverged hybrids, and the loci experiencing selection should exhibit a reduction in allelic diversity relative to neutral loci. Here, we analyzed patterns of genetic diversity at 59 microsatellite loci associated with expressed sequence tags (ESTs) in a homoploid hybrid sunflower species, Helianthus anomalus. We used two indices, ln RV and ln RH, to compare variation and heterozygosity (respectively) at each locus between the hybrid species and its two parental species, H. annuus and H. petiolaris. Mean values of ln RV and ln RH were significantly lower than zero, which implies that H. anomalus experienced a population bottleneck during its recent evolutionary history. After correcting for the apparent bottleneck, we found six loci with a significant reduction in variation or with heterozygosity in the hybrid species, compared to one or both of the parental species. These loci should be viewed as a ranked list of candidate loci, pending further sequencing and functional analyses. Sequence data were generated for two of the candidate loci, but population genetics tests failed to detect deviations from neutral evolution at either locus. Nonetheless, a greater than eight-fold excess of nonsynonymous substitutions was found near a putative N-myristoylation motif at the second locus (HT998), and likelihood-based models indicated that the protein has been under selection in H. anomalus in the past and, perhaps, in one or both parental species. Finally, our data suggest that selective sweeps may have united populations of H. anomalus isolated by a mountain range, indicating that even low gene-flow species may be held together by the spread of advantageous alleles.

We investigated relationships between whole-tree hydrau- lic architecture and stomatal conductance in Pinus palus- tris Mill. (longleaf pine) across habitats that differed in soil properties and habitat structure. Trees occupying a xeric habitat (characterized by sandy, well-drained soils, higher nitrogen availability and lower overstory tree density) were shorter in stature and had lower sapwood-to-leaf area ratio ( A S : A L ) than trees in a mesic habitat. The soil-leaf water potential gradient ( Ψ Ψ S − Ψ L ) and leaf-specific hydraulic conductance ( k L ) were similar between sites, as was tissue- specific hydraulic conductivity ( K S ) of roots. Leaf and can- opy stomatal conductance ( g S and G S , respectively) were also similar between sites, and they tended to be somewhat higher at the xeric site during morning hours when vapour pressure deficit ( D ) was low. A hydraulic model incorpo- rating tree height, A S : A L and Ψ S − Ψ L accurately described the observed variation in individual tree G Sref ( G S at D = 1 kPa) across sites and indicated that tree height was an important determinant of G Sref across sites. This, com- bined with a 42% higher root-to-leaf area ratio ( A R : A L ) at the xeric site, suggests that xeric site trees are hydraulically well equipped to realize equal - and sometimes higher - potential for conductance compared with trees on mesic sites. However, a slightly more sensitive stomatal closure response to increasing D observed in xeric site trees sug- gests that this potential for higher conductance may only be reached when D is low and when the capacity of the hydraulic system to supply water to foliage is not greatly challenged.

Summary 1 Recent studies of hybridization have shown that extreme (e.g. transgressive) phenotypes can be generated during massive genetic recombination events such as interspecific hybridization. Extreme hybrid phenotypes appear to have the potential to create new lineages and lead to evolutionary divergence. 2 Hybrid genotypes of some salt marsh species have proven capable of reengineering their ecosystems, while in other cases, hybridization has transformed plant communities by facilitating species invasion. 3 Using the Louisiana irises, a naturally hybridizing group of species, we assessed the potential for hybrids of Iris brevicaulis and I. fulva to possess extreme physiological and growth traits. Our glasshouse experiment used two soil water environments (wet and flood) to measure hybrid trait expression at both the genotypic class and individual genotype level. 4 At the genotypic class level, two of the three hybrid classes (F1s, and backcrosses towards each parental species) were transgressive for at least one physiological trait (F1s for root mass proportion, specific leaf area and final leaf area; backcrosses to I. brevicaulis for leaf area ratio). Three fitness components (total biomass, ramet production and flower production) were measured on each plant in the experiment. All three hybrid classes had greater clonal fitness (total biomass) but not greater sexual fitness (flower number) than parent species. 5 At the individual genotype level, two F1 hybrids showed extreme physiological trait expression: one for specific leaf area, and the other for shallow root allocation. Flower production was the only fitness component that showed some degree of environment-dependence at the individual genotype level. 6 Although early generation hybrid classes can contain rare genotypes with extreme phenotypic trait expression, and hybrid fitness components were equal or superior to both parental species in most cases, there was not a strong association between transgressive traits and elevated hybrid fitness.

In salt marsh habitats, noted for their extreme environments, a widely held assumption is that a few large clones dominate plant populations. Using a large number of polymorphic genetic markers, we were able to test this assumption for two salt marsh plants known to span extreme salinity gradients. For both species, clonal diversity was surprisingly high across populations: Simpson's diversity indices were 0.96 and 0.99. Although clonal diversity was high, there was no pattern of association between specific clones or alleles with salt microhabitat. Our findings suggest that sexual reproduction and recruitment from seeds may have been generally underappreciated as an important ecological force in the salt marsh. Furthermore, clonal diversity has implications for conservation and restoration of these critical coastal habitats, particularly with regard to buffering environmental change or disease. Recent studies also suggest that high levels of intraspecific diversity can affect a variety of community and ecosystem processes.

Summary 1 Relative fitness of hybrid genotypes will determine the role that hybrids can play in the evolution of a plant species complex. To realistically evaluate hybrid fitness and how environmental variation affects fitness in a long-lived species, all life stages must be considered. 2 We evaluated germination, seedling survival and growth of two Louisiana iris species and their early generation hybrids in several experimental environments created by manipulating light and water levels. 3 Species and hybrids all required similar moist conditions for germination. The proportion of germinated seeds was highest in shade and seedling survival highest in sun. 4 Iris brevicaulis exhibited the lowest germination and seedling survival overall, yet those individuals that survived grew vigorously. Iris fulva had high levels of germination and seedling survival, but yielded the smallest plants at the end of one season of growth. 5 Germination rates, seedling survival and seedling growth of hybrids equalled or exceeded one or both parent species, indicating that hybrids in this system have high relative fitness at seed and seedling stages in several environmental conditions. 6 We conclude that Iris fulva and I. brevicaulis share a common regeneration niche and similar early stage relative fitness with their hybrids. Thus, environment-dependent fitness in this system is due primarily to selection at adult stages.

Predawn plant water potential (Ψw, measured with leaf psychrometers) and surrogate measurements made with the pressure chamber (termed Ψpc here) are used to infer comparative ecological performance, based on the expectation that these plant potentials reflect the wettest soil Ψw accessed by roots. There is growing evidence, however, that some species exhibit substantial predawn disequilibrium (PDD), defined as plant Ψw or Ψpc at predawn substantially more negative than the Ψw of soil accessed by roots. In the western Great Basin desert, the magnitude of PDD calculated as soil Ψw minus predawn leaf Ψw was as large as 1.4 and 2.7 MPa for two codominant shrub species, Chrysothamnus nauseosus and Sarcobatus vermiculatus, respectively. The magnitude of PDD calculated as soil Ψw minus predawn Ψpc was smaller, up to 0.6 and 2.1 MPa for Chrysothamnus and Sarcobatus, respectively. For both species, mechanisms contributing to PDD included nighttime transpiration and putative leaf apoplastic solutes, but not hydraul...

Summary 1. Aristida stricta and Schizachyrium scoparium are C 4 perennial bunchgrass species native to Pinus palustris savannas in the south-eastern USA. Species differences in growth rate, tissue nutrient content and distribution suggest that A. stricta is more stress-tolerant ( sensu Grime) than S. scoparium . In addition, A. stricta retains leaves all year, whereas S. scoparium is winter deciduous. Based on these observations, we wanted to determine (1) whether root longevity was higher for A. stricta as compared to S. scoparium ; and (2) whether the leaf habits of these species had consequences for seasonal patterns of root production and death. 2. Using in situ rhizotrons, we recorded root number and length of these species over the course of 2·5 years on flat, transparent 50 〈 50 cm windows. Kaplan‐Meier (productlimit) analysis was used to produce survival functions and to estimate median lifespans for the right-censored root longevity data. These survival functions were compared using log-rank χ 2 statistics, and a proportional hazards model was used to examine the effects of season and species on the survival of each cohort. Final demographic variables were analysed using anova . 3. The median lifespan of A. stricta was 777 days whereas that of S. scoparium was 374 days. This significant difference in median lifespan was consistent between species for all cohorts in both years. Also, coincident with winter leaf senescence, root production nearly stopped for S. scoparium whereas A. stricta produced roots all year. This is probably due to the winter photosynthetic capacity of A. stricta . Root death did not exhibit any seasonal pattern in either species. A pulse of roots produced in apparent response to the 2000 drought was evident for S. scoparium but not for A. stricta . This cohort, however, rapidly died during the dormant season, probably due to the lack of winter photosynthesis in that species. 4. Aristida stricta and S. scoparium exhibited root production, death and longevity patterns that were predicted on the basis of ecological and leaf habit differences between them. These findings suggest species-specific differences in performance and element cycling in these nutrient-poor soils.

Summary 1 Optimality theory suggests that roots should be kept alive until the efficiency of resource acquisition is maximized (i.e. a maximum ratio of benefits to costs). Because root efficiency may vary with environmental conditions, ecological distributions of plant species may be linked to different patterns of root demography. 2 In a greenhouse study, we investigated fine root turnover (growth and death) for three woody species from the fall-line sandhills of the south-eastern USA. Pinus palustris Mill. is a generalist in this habitat, whereas Quercus marilandica Muenchh. occurs in more fertile, mesic habitats relative to Quercus laevis Walt. Seedlings were grown under four resource treatments (water + nutrients) for 7 months: high resources, low resources, and short-term exposure (last 2 months of the study) to resource enrichment or depletion. 3 Increasing fine root longevity may be optimal in resource-poor sites because root efficiency may be maximized by less root turnover and resource loss. As expected, fine root death and growth was less in species from xeric habitats (Q. laevis), but greater in species from mesic habitats (Q. marilandica). The generalist species P. palustris exhibited high growth but little death of fine roots. 4 When soil resources decrease, less root turnover may reduce resource loss. Fine root growth of all three species decreased at low resources. Fine root death decreased at low resources in Q. marilandica, the only species with significant root death. 5 Demographic responses differed between fine roots and leaves. Although leaf and fine root death were greater in Q. marilandica than in Q. laevis, leaf death, unlike root death, did not change with resource availability. Short-term resource enrichment or depletion affected leaf production but not fine root demography in the Quercus species. 6 Given that fine root morphology affects root maintenance and construction costs, we expected greater fine root growth and death in species with thinner roots of high specific root length (SRL) and low density. However, the species with the greatest root turnover, Q. marilandica, had thick and dense roots of low SRL. 7 Results from our study with woody species indicate that fine root demography, but not fine root morphology, was linked to ecological distribution in a narrow geographical range. Differences in fine root turnover under different resource availability were consistent with the optimality theory, and may reflect trade-offs between tolerance and competitive ability below ground: less root turnover of xeric species under low fertility may conserve resources, whereas faster root growth of mesic species potentially maximizes resource uptake in more fertile soils.

We tested the relative fitness of two Louisiana Iris species (Iris brevicaulis and I. fulva) and their first-generation backcross hybrids in three experimental watering treatments: dry, field capacity, and flooded. Leaf area expansion rate, gas exchange (A(max), g(s), c(i)), and biomass at final harvest were measured for each species and hybrid class in all three environmental treatments. Fitness (based on total biomass) of the four genotypic classes differed significantly with environment. All genotypic classes performed most poorly in the dry treatment. The fitness ranking of genotypic class also changed across environments (significant genotypic class by treatment interaction) with hybrid genotype fitness shifting relative to parental genotypes. Integrating over all treatments, backcrosses to I. fulva showed the lowest fitness, whereas backcrosses to I. brevicaulis outperformed I. fulva. The differences in fitness were apparently achieved by a combination of differences in photosynthesis and allocation. In this system, hybrids are not necessarily less fit than their parents, and the relationship between hybrid and parental fitness is influenced by environmental conditions, lending support to the Hybrid Novelty model of hybrid zone evolution.

Diploid hybrid lineages often are ecologically distinct from their parental species. However, it is unclear whether this niche divergence is typically achieved via hybrid intermediacy, a mixture of parental traits, and/or the evolution of extreme (transgressive) morphological and ecophysiological features. Here we compare an extensively studied hybrid sunflower species, Helianthus anomalus, with its putative parents, H. annuus and H. petiolaris, for 41 morphological and 12 ecophysiological traits. Helianthus anomalus was morphologically intermediate for one trait (2.4%), parental-like for 23 traits (56.1%), and transgressive for 17 traits (41.5%). For ecophysiological traits, H. anomalus was not significantly different from one or both parents for nine traits (75%), and was transgressive for the remaining three (25%). Thus, H. anomalus appears to be a mosaic of parental-like and transgressive phenotypes. Although the fitness effects of the transgressive characters are not yet known, many of these characters are consistent with adaptations reported for other sand dune plants. Genetic studies are currently underway to ascertain whether these extreme characters arose as a direct byproduct of hybridization or whether they evolved via mutational divergence.

Pueraria lobata (kudzu), a clonal, leguminous vine, is invading the southeastern United States at a rate of 50 000 ha per year. Genetic variability and clonal diversity were measured in 20 southeastern U.S. populations using 14 allozyme loci. Within its U.S. range, 92.9% of the loci were polymorphic and overall genetic diversity was 0.290. Such high levels of genetic diversity are consistent with its history of multiple introductions over an extended period of time. The average proportions of polymorphic loci and genetic diversity within populations were 55.7% (range 5 28.6‐85.7%) and 0.213 (range 5 0.114‐0.317), respectively. The proportion of total genetic diversity found among populations was similar to species with equivalent life history characters ( GST 5 0.199). No regional patterns of variation were seen. The number of putative genotypes in each population ranged from 2 to 26. Mean genotypic diversity was 0.694, ranging from 0.223 to 0.955. Such high levels of genotypic diversity indicate that local sites are often colonized by several propagules (most likely seeds) and/or that sexual reproduction occurs within populations after establishment. An excess of heterozygosity was observed in populations with few unique genets, implying that selection for highly heterozygous individuals may occur in populations of P. lobata.

Summary 1 We investigated experimentally differences in abiotic stress tolerance and the effects of plant–plant interactions for two desert shrubs, Chrysothamnus nauseosus and Sarcobatus vermiculatus, along a soil salinity (NaCl) and boron (B) gradient at Mono Lake, California, USA. Based on differences in natural distribution, and the classical expectation of a trade-off between competitive ability and stress tolerance, we hypothesized that (i) Chrysothamnus would have greater competitive ability than Sarcobatus at the low salinity end of the gradient, and that (ii) Sarcobatus would be more stress tolerant than Chrysothamnus. 2 Juvenile target plants of Chrysothamnus and Sarcobatus were planted into four sites along the gradient. Biomass was determined by destructive harvests over two growing seasons. At each site, interspecific relative competitive ability was assessed as the effect of Sarcobatus neighbours on Chrysothamnus targets compared to the effect of Chrysothamnus neighbours on Sarcobatus targets. Stress tolerance was assessed as the ability of each species to survive and grow, in the absence of neighbours, at different sites along the gradient. 3 The two species did not differ in the relative strength of plant–plant interactions, providing no support for the expectation that Chrysothamnus had greater competitive ability than Sarcobatus. Furthermore, there was no evidence for competition or facilitation, either interspecific or intraspecific, at any site in either year of the study. However, fertilization treatments demonstrated nutrient limitations, soil water reached limiting levels and root systems of targets and neighbours overlapped substantially. It is therefore surprising that plant–plant interactions among juveniles apparently play little role in the growth and survival of shrubs in this saline desert habitat. 4 Sarcobatus was more stress tolerant than Chrysothamnus and the two species performed optimally at different sites along the gradient. Sarcobatus juveniles grew best at the two most saline sites and survived at all sites, whereas Chrysothamnus juveniles grew best at a low-salinity site and did not survive at the most saline site. The difference in site of optimal performance may be due to differences in nutrient limitations or to interactions between nutrient availability and sodium (Na) and B tolerance.

We tested expectations that two desert shrubs would differ in germination and seedling relative growth rate (RGR) responses to Na and Cs stress. The study species, Chrysothamnus nauseosusssp. consimilis and Sarcobatus vermiculatus (hereafter referred to by genus), differ in their distribution along salinity gradients, with Chrysothamnusinhabiting only less saline areas. In growth chamber studies, declining Cs (20.82 to 22.71 MPa) inhibited germination of both species, and Chrysothamnus was less tolerant of Cs stress than Sarcobatus. Germination fell below 10% for Chrysothamnus at 21.64 MPa (NaCl and PEG), and for Sarcobatus at 22.4 MPa PEG. Neither species exhibited ion toxicity. There was substantial ion enhancement for Sarcobatus in lower Cs, allowing for 40% germination in 22.71 MPa NaCl. For seedling RGR, species were not different at 20.29 or 20.82 MPa (0 and 100 mmol/L NaCl, respectively), but Chrysothamnus RGR declined substantially at 21.3 MPa (200 mmol/L NaCl). The greater stress tolerance of Sarcobatus was not associated with a lower RGR under nonsaline conditions. Species differences in seed and seedling Cs stress tolerance probably contribute to the restricted distribution of Chrysothamnus to less saline areas. The Na uptake of Sarcobatus seedlings enhances its ability to deal with declining Cs and establish in more saline areas.

At Mono Lake, California, we investigated field water relations, leaf and xylem chemistry, and gas exchange for two shrub species that commonly co-occur on marginally saline soils, and have similar life histories and rooting patterns. Both species had highest root length densities close to the surface and have large tap roots that probably reach ground water at 3.4-5.0 m on the study site. The species differed greatly in leaf water relations and leaf chemistry. Sarcobatus vermiculatus had a seasonal minimum predawn xylem pressure potential (Wpd) of -2.7 MPa and a midday potential (Wind) of -4.1 MPa. These were significantly lower than for Chrysothamnus nauseosus, which had a minimum Wpd of -1.0 MPa and Wind of -2.2 MPa. Sarcobatus had leaf Na of up to 9.1 % and K up to 2.7 % of dry mass, and these were significantly higher than for Chrysothamnus which had seasonal maxima of 0.4% leaf Na and 2.4 % leaf K. The molar ratios of leaf K/Na, Ca/Na, and Mg/Na were substantially lower for Sarcobatus than for Chrysothamnus. Xylem ionic contents indicated that both species excluded some Na at the root, but that Chrysothamnus was excluding much more than Sarcobatus. The higher Na content of Sarcobatus leaves was associated with greater leaf succulence, lower calculated osmotic potential, and lower xylem pressure potentials. Despite large differences in water relations and leaf chemistry, these species maintained similar diurnal patterns and rates of photosynthesis and stomatal conductance to water vapor diffusion. Sarcobatus Wpd may not reflect soil moisture availability due to root osmotic and hydraulic properties.

Water-use efficiency is thought to be related to plant performance and natural selection for plants in arid habitats, based on a general expectation that increased water-use efficiency is associated with decreased carbon gain and biomass accumulation. Using leaf carbon isotope discrimination (A) to determine integrated water-use efficiency, we estimated genetic variance for, and examined the relationships among A, biomass, and gas exchange characters for full-sibling families of the woody shrub, Chrysothamnus nauseosus, grown from seed collected at Tintic, Utah. In both well-watered greenhouse and common garden experiments, and water-limited common garden experiments, there were significant family differences for A, biomass, and morphological characters, indicating a potential for genetic change in response to selection. However, estimates of broadsense heritabilities for A were low, indicating that the rate of change in response to selection would be relatively slow. This was consistent with the large amount of phenotypic plasticity observed for A as it differed with water treatment and year in the garden experiment. Phenotypically, aboveground biomass and A were negatively correlated within the well-watered treatments (i.e., more water-use efficient plants were larger), not correlated within the water-limited treatment, and positively correlated for combined well-watered and water-limited garden treatments, suggesting that variation in both photosynthetic capacity and stomatal limitation contribute to the variation in A. In contrast to the phenotypic correlations, genetic correlations for biomass and A were consistently negative within each treatment, and selection for higher water-use efficiency through low A for C. nauseosus plants in this population would tend to shift populations toward larger plants. For C. nauseosus, increased water-use efficiency is not necessarily associated with decreased carbon gain.

The purpose of this study was to determine if and how the two genetically distinct forms, marsh and dune, of Sporobolus virginicus (L.) Kunth. tolerate anaerobic substrates. The treatments in the hydroponic study, conducted in the greenhouse for approximately 6 months, involved growing the marsh and dune forms in aerobic, anaerobic, and alternating aeration treatments. Plants were examined for morphological and physiological responses to the aeration treatments. In response to the continuous anaerobic treatment, the dune form of S. virginicus exhibited increased stolon biomass, but no difference of total biomass or rhizome aerenchyma when compared with the aerobic treatment. In response to alternating aeration, rhizome aerenchyma increased, total biomass decreased, and stolon biomass remained constant. Belowground transport of oxygen enabled the root tissue in all of the aeration treatments to maintain aerobic respiration. The marsh form grown in the alternating aeration treatment had the same total biomass but more rhizome aerenchyma when compared to the aerobic treatment. Growth in the continuous anaerobic treatment resulted in a reduction of total biomass and increased rhizome arenchyma. Marsh form roots did not appear to be respiring anaerobically or producing ethanol or additional malate at the time of harvest; however, root respiration was higher in the anaerobic and alternating treatments. The marsh and dune forms of S. virginicus were able to adjust morphologically or physiologically or to use existing morphological features to tolerate anaerobic substrates. Thus, it appears that the distribution of the two forms of S. virginicus found in coastal sand dunes and in salt marshes is not limited by differences in ability to tolerate waterlogged soils. SINCE Sporobolus virginicus (L.) Kunth., a grass of worldwide distribution in the tropics and sub-tropics, is found in both wetland and nonwetland habitats, its anaerobic substrate tolerance capacity is of interest. A perennial with numerous branching and creeping rhizomes, it typically forms extensive colonies on sandy or muddy seashores and in saline marshes (Hitchcock, 197 1). Sporobolus virginicus has two genetically distinct growth forms which because of their distributions are designated here as marsh and dune forms. The marsh form is listed as the type specimen of S. virginicus by Hitchcock (1971) and is described as var. mil Received for publication 14 January 1984; revision accepted 16 August 1984. This research was supported by U.S. Environmental Protection Agency, Corvallis Environmental Research Laboratory through grant number R-806824010, and the office of Sea Grant, NOAA, through the University of Delaware Sea Grant College Program Project R/B12, grant number NA80AA-D-00106. Manuscript preparation was suported by Contract DE-AC09-76SR00819 between the U.S. Department of Energy and the University of Georgia's Savannah River Ecology Laboratory. 2 Present address: Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29801. nor Bail. by Smith-White (1979) in Australia. It has blades approximately 5 cm in length, culms 10-40 cm in height, and grows at the fringes of saltpans, in the low and high marsh, and along river banks. The dune form is listed as a robust form of S. virginicus by Hitchcock (1971) and is referred to as var. virginicus by Smith-White (1979). It has culms up to 1 m in height, longer and broader leaves, and generally occurs on sandy beaches and coastal dunes. Sapelo Island, Georgia (approx. 31°25'N latitude and 8 1°1 5 'W longitude) is the northern most reported collection site where both forms grow in adjacent habitats in North America (Gallagher, 1979). When raised from seed under identical conditions the two forms retain their divergent morphological characteristics. The occurrence of both the marsh and dune forms on Sapelo Island provided an opportunity to look for intraspecific diffierences in the anaerobic substrate tolerance characteristics of S. virginicus. Breen, Everson and Rodgers (1977) studied the effiects of salinity and inundation on various ages of young marsh form S. virginicus plants growing in pots. They found that the young plants are sensitive to inun

Growth, biomass, and survival of bald cypress (Taxodium distichum (L.) Richard), water tupelo (Nyssa aquatica L.), black willow (Salix nigra Marshall), and button bush (Cephalanthus occidentalis L.) were examined in a 3 {times} 3 factorial experiment varying water temperatures (Ambient, mid, and high ({approximately} 40 C)) and water levels (drained, saturated, and flooded). Stem diameter and height, biomass, and survivorship for water tupelo and bald cypress were all reduced by the high/flooded treatment. Black willow growth had the greatest variability among nonlethal flooding and temperature treatments, and achieved the greatest biomass of the four species. In the high/flooded treatment, however, only 47% of the black willow seedlings survived and stem diameter, height, and biomass of survivors were greatly reduced. Button bush had intermediate variability of growth to the nonlethal treatments as compared to the other study species. Survival of button bush seedlings in the high/flooded treatment was high (87%), but root biomass of the survivors was reduced. Interspecific differences in growth, biomass, survivorship, and morphological characteristics existed among these swamp species to experimental conditions. These responses may help explain vegetation patterns in a thermally impacted swamp.