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Premise of the study Patterns of plant stem traits are expected to align with a "fast-slow" plant economic spectrum across taxa. Although broad patterns support such tradeoffs in field studies, tests of hypothesized correlated trait evolution and adaptive differentiation are more robust when taxa relatedness and environment are taken into consideration. Here we test for correlated evolution of stem and leaf traits and their adaptive differentiation across environments in the herbaceous genus, Helianthus. Methods Stem and leaf traits of 14 species of Helianthus (28 populations) were assessed in a common garden greenhouse study. Phylogenetically independent contrasts were used to test for evidence of correlated evolution of stem hydraulic and biomechanical properties, correlated evolution of stem and leaf traits, and adaptive differentiation associated with source habitat environments. Key results Among stem traits, there was evidence for correlated evolution of some hydraulic and biomechanical properties, supporting an expected tradeoff between stem theoretical hydraulic efficiency and resistance to bending stress. Population differentiation for suites of stem and leaf traits was found to be consistent with a "fast-slow" resource-use axis for traits related to water transport and use. Associations of population traits with source habitat characteristics supported repeated evolution of a resource-acquisitive "drought-escape" strategy in arid environments. Conclusions This study provides evidence of correlated evolution of stem and leaf traits consistent with the fast-slow spectrum of trait combinations related to water transport and use along the stem-to-leaf pathway. Correlations of traits with source habitat characteristics further indicate that the correlated evolution is associated, at least in part, with adaptive differentiation of Helianthus populations among native habitats differing in climate.

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.

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.