Palatability and tolerance to simulated herbivory in native and introduced populations of Alliaria petiolata (Brassicaceae)

The European herb garlic mustard (Alliaria petiolata) is a serious invader of North American deciduous forests. One explanation for its success could be that in the absence of specialized herbivores, selection has favored less defended but more vigorous genotypes. This idea was addressed by comparing offspring from several native and introduced Alliaria populations with respect to their palatability to insect herbivores and their tolerance to simulated herbivory. Feeding rates of a specialist weevil from the native range were significantly greater on American plants, suggesting a loss of resistance in the introduced range. In contrast, there was significant population variation but no continent effect in the feeding rates of a generalist caterpillar. After simulated herbivory, A. petiolata showed a substantial regrowth capacity that involved changes in plant growth, architecture, and allocation. Removal of 75% leaf area or of all bolting stems reduced plant fitness to 81% and 58%, respectively, of the fitness of controls. There was no indication of a difference in tolerance between native and introduced Alliaria populations or of a trade-off between tolerance and resistance.     

Population variation in the cost and benefit of tolerance and resistance against herbivory in Datura stramonium

In this study we examine the hypothesis that divergent natural selection produces genetic differentiation among populations in plant defensive strategies (tolerance and resistance) generating adaptive variation in defensive traits against herbivory. Controlled genetic material (paternal half-sib families) from two populations of the annual Datura stramonium genetically differentiated in tolerance and resistance to herbivory were used. This set of paternal half-sib families was planted at both sites of origin and the pattern of genotypic selection acting on tolerance and resistance was determined, as well as the presence and variation in the magnitude of allocational costs of tolerance. Selection analyses support the adaptive differentiation hypothesis. Tolerance was favored at the site with higher average level of tolerance, and resistance was favored at the site with higher average level of resistance. The presence of significant environmentally dependent costs of tolerance was in agreement with site variation in the adaptive value of tolerance. Our results support the expectation that environmentally dependent costs of plant defensive strategies can generate differences among populations in the evolutionary trajectory of defensive traits and promote the existence of a selection mosaic. The pattern of contrasting selection on tolerance suggests that, in some populations of D. stramonium, tolerance may alter the strength of reciprocal coevolution between plant resistance and natural enemies.     

Effects of generalist herbivory on resistance and resource allocation by the invasive plant,Phytolacca americana

Successful invasions by exotic plants are often attributed to a loss of co‐evolved specialists and a re‐allocation of resources from defense to growth and reproduction. However, invasive plants are rarely completely released from insect herbivory because they are frequently attacked by generalists in their introduced ranges. The novel generalist community may also affect the invasive plant's defensive strategies and resource allocation. Here, we tested this hypothesis using American pokeweed (Phytolacca americana L.), a species that has become invasive in China, which is native to North America. We examined resistance, tolerance, growth and reproduction of plant populations from both China and the USA when plants were exposed to natural generalist herbivores in China. We found that leaf damage was greater for invasive populations than for native populations, indicating that plants from invasive ranges had lower resistance to herbivory than those from native ranges. A regression of the percentage of leaf damage against mass showed that there was no significant difference in tolerance between invasive and native populations, even though the shoot, root, fruit and total mass were larger for invasive populations than for native populations. These results suggest that generalist herbivores are important drivers mediating the defensive strategies and resource allocation of the invasive American pokeweed.     

Effects of simulated herbivory and resources on Chinese tallow tree (Sapium sebiferum, Euphorbiaceae) invasion of native coastal prairie

Trade-offs associated with maintaining herbivory resistance and herbivory tolerance are frequently inferred in plant life histories. Invasive success for many non-native plants is often attributed to novel resistance that repels native herbivores. Chinese tallow tree (Sapium sebiferum, Euphorbiaceae) is a non-native invader that threatens ecosystems throughout the southeastern United States, including imperiled coastal prairie regions. Low herbivore loads due to costly resistance are generally assumed to give Sapium a competitive advantage over native plants. We predicted that if Sapium experienced higher damage levels it would show significant decreases in growth and reduced ability to compete with native prairie vegetation. We conducted full-factorial, paired greenhouse and field experiments designed to assess the effects of simulated leaf herbivory on Sapium growth in the presence of prairie vegetation at different levels of light and nitrogen. Contrary to our expectations, neither low-intensity, chronic defoliation nor high-intensity, acute defoliation negatively affected Sapium seedlings in any resource combination in either experiment. These studies reveal that Sapium possesses considerable phenotypic plasticity, and herbivory tolerance is a newly appreciated trait that likely contributes to its invasive potential.     

Decreased resistance and increased tolerance to native herbivores of the invasive plant Sapium sebiferum

Release from natural enemies may favor invasive plants evolving traits associated with reduced herbivore‐resistance and faster‐growth in introduced ranges. Given a genetic trade‐off between resistance and tolerance, invasive plants could also become more tolerant to herbivory than conspecifics in the native range. We conducted a field common garden study in the native range of Sapium sebiferum using seeds from native Chinese populations and invasive North American populations to compare their growth and herbivory resistance. We also performed a cage‐pot experiment to compare their resistance and tolerance to Bikasha collaris beetles that are specialist feeders on S. sebiferum trees in China. Results of the common garden study showed that Sapium seedlings of invasive populations relative to native populations were more frequently attacked by native herbivores. Growth and leaf damage were significantly higher for invasive populations than for native populations. Growth of invasive populations was not significantly affected by insecticide spray, but insecticide spray benefited that of native populations. In the bioassay trial, beetles preferentially consumed leaf tissue of invasive populations compared to native populations when beetles had a choice between them. Regression of percent leaf damage on biomass showed that invasive populations tolerated herbivory more effectively than native populations. Our results suggest that S. sebiferum from the introduced range had lower resistance but higher tolerance to specialist herbivores. Both defense strategies could have evolved as a response to the escape from natural enemies in the introduced range.     

No evidence for evolutionarily decreased tolerance and increased fitness in invasive Chromolaena odorata: implications for invasiveness and biological control

Tolerance, the degree to which plant fitness is affected by herbivory, is associated with invasiveness and biological control of introduced plant species. It is important to know the evolutionary changes in tolerance of invasive species after introduction in order to understand the mechanisms of biological invasions and assess the feasibility of biological control. While many studies have explored the evolutionary changes in resistance of invasive species, little has been done to address tolerance. We hypothesized that compared with plants from native populations, plants from invasive populations may increase growth and decrease tolerance to herbivory in response to enemy release in introduced ranges. To test this hypothesis, we compared the differences in growth and tolerance to simulated herbivory between plants from invasive and native populations of Chromolaena odorata, a noxious invader of the tropics and subtropics, at two nutrient levels. Surprisingly, flower number, total biomass (except at high nutrient), and relative increase in height were not significantly different between ranges. Also, plants from invasive populations did not decrease tolerance to herbivory at both nutrient levels. The invader from both ranges compensated fully in reproduction after 50?% of total leaf area had been damaged, and achieved substantial regrowth after complete shoot damage. This strong tolerance to damage was associated with increased resource allocation to reproductive structures and with mobilization of storage reserves in roots. The innately strong tolerance may facilitate invasion success of C. odorata and decrease the efficacy of leaf-feeding biocontrol agents. Our study highlights the need for further research on biogeographical differences in tolerance and their role in the invasiveness of exotic plants and biological control.     

Altered patterns of growth, physiology and reproduction in invasive genotypes of Solidago gigantea (Asteraceae)

Introduced plants may leave their specialized herbivores behind when they invade new ranges. The Evolution of Increased Competitive Ability (EICA) Hypothesis holds that this escape from herbivory could lead to reduced investment in defenses, thereby freeing resources for growth and reproduction. We tested the prediction that introduced genotypes of Solidago gigantea would outperform native genotypes when grown in the absence of herbivores, and examined whether tolerance to insect herbivory has changed in introduced genotypes. S. gigantea is native to North America and an exotic invasive in Europe. Insect damage reduced plant growth and biomass for both native and exotic genotypes. While there was no evidence that continent of origin influenced the degree to which plants compensated for herbivory, the mechanisms contributing to recovery differed for native and exotic plants. Damaged US plants showed enhanced photosynthetic rates to a greater extent than damaged European plants, while damaged European plants carried more leaves than damaged US plants. At the end of the season, leaf mass of European plants was significantly greater than that of US plants. Contrary to the predictions of the EICA hypothesis, US plants were more likely to flower than European plants. European plants invested significantly more of their total reproductive biomass into rhizomes rather than flowers than US plants. While other work with S. gigantea has supported some aspects of the EICA hypothesis, the results reported here generally do not. We conclude that multiple factors influence the success of introduced plants.     

Invasive and native populations of common ragweed exhibit strong tolerance to foliar damage

Tolerance and resistance are defence strategies evolved by plants to cope with damage due to herbivores. The introduction of exotic species to a new biogeographical range may alter the plant–herbivore interactions and induce selection pressures for new plant defence strategies with a modified resource allocation. To detect evolution in tolerance to herbivory in common ragweed, we compared 3 native (North America) and 3 introduced (France) populations, grown in a common garden environment. We explored the effect of leaf herbivory on plant vegetative and reproductive traits. Plants were defoliated by hand, simulating different degrees of insect grazing by removing 0%, 50% or 90% of each leaf blade. Total and shoot dry biomasses were not affected by increasing defoliation, whereas root dry biomass and root:shoot ratio decreased significantly for native and introduced populations. Furthermore, defoliation treatments did not affect any of the plant reproductive traits measured. Hence, common ragweed displayed an efficient reallocation of resources in shoot biomass at the expense of roots following defoliation, which allows the species to tolerate herbivory without obvious costs for fitness. We did not detect any difference in herbivory tolerance between introduced and native populations, but significant differences were found in reproduction with invasive populations producing more seeds than native populations. As a result, tolerance to herbivory has been maintained in the introduced plant populations. We discuss some implications of these preliminary results for biological control strategies dedicated to common ragweed.     

COSTS OF INDUCED RESPONSES AND TOLERANCE TO HERBIVORY IN MALE AND FEMALE FITNESS COMPONENTS OF WILD RADISH

Theory predicts that plant defensive traits are costly due to trade-offs between allocation to defense and growth and reproduction. Most previous studies of costs of plant defense focused on female fitness costs of constitutively expressed defenses. Consideration of alternative plant strategies, such as induced defenses and tolerance to herbivory, and multiple types of costs, including allocation to male reproductive function, may increase our ability to detect costs of plant defense against herbivores. In this study we measured male and female reproductive costs associated with induced responses and tolerance to herbivory in annual wild radish plants (Raphanus raphanistrum). We induced resistance in the plants by subjecting them to herbivory by Pieris rapae caterpillars. We also induced resistance in plants without leaf tissue removal using a natural chemical elicitor, jasmonic acid; in addition, we removed leaf tissue without inducing plant responses using manual clipping. Induced responses included increased concentrations of indole glucosinolates, which are putative defense compounds. Induced responses, in the absence of leaf tissue removal, reduced plant fitness when five fitness components were considered together; costs of induction were individually detected for time to first flower and number of pollen grains produced per flower. In this system, induced responses appear to impose a cost, although this cost may not have been detected had we only quantified the traditionally measured fitness components, growth and seed production. In the absence of induced responses, 50% leaf tissue removal, reduced plant fitness in three out of the five fitness components measured. Induced responses to herbivory and leaf tissue removal had additive effects on plant fitness. Although plant sibships varied greatly (49–136%) in their level of tolerance to herbivory, costs of tolerance were not detected, as we did not find a negative association between the ability to compensate for damage and plant fitness in the absence of damage. We suggest that consideration of alternative plant defense strategies and multiple costs will result in a broader understanding of the evolutionary ecology of plant defense.     

喜旱莲子草在青藏高原对模拟增温的可塑性: 引入地和原产地种群的比较

气候变暖背景下植物可通过关键性状的表型可塑性来适应环境温度的增加。表型可塑性增强进化假说预测定植到新环境中的入侵植物种群具有演化出更强表型可塑性的潜力。此前对可塑性进化的研究涵盖了外来植物性状对水分条件、光照变化、土壤养分、邻体根系以及天敌防御等的响应, 而较少有研究关注增温条件下植物重要性状的可塑性进化。已有的部分研究多集中在温带和热带地区, 而较少关注入侵植物在高寒地区对增温的响应; 且研究多集中在植物生长相关性状, 较少关注功能性状和防御性状。本研究采用同质园实验比较了喜旱莲子草6个引入地(中国)种群和6个原产地(阿根廷)种群, 在西藏拉萨模拟全天增温2℃处理下的适合度性状、功能性状和防御性状的响应差异。结果表明: (1)高寒地区模拟全天增温显著提高了喜旱莲子草总生物量(+36.4%)、地上生物量(+34.5%)、贮藏根生物量(+51.4%)和毛根生物量(+33.6%), 降低了分枝强度(-19.8%)和比茎长(-30.2%); (2)模拟全天增温使引入地种群的比叶面积和黄酮含量增加, 而原产地种群则相反。这些结果表明高寒地区全天增温2℃对喜旱莲子草可能是一种有利条件。引入地种群的适合度性状对模拟全天增温2℃的响应比原产地种群更强, 而其光能利用相关性状和防御性状的响应可能提升了其在高寒地区的适合度。因此, 在未来全球气候变暖的背景下, 高寒地区温度升高可能更有利于喜旱莲子草引入地种群的定植和扩散。     

Specificity of constitutive and induced resistance: pigment glands influence mites and caterpillars on cotton plants

Cotton plants contain suites of phytochemicals thought to be important in defense against herbivores, some of which are localized in pigment glands which contain gossypol and other terpenoid aldehydes. The simple genetic basis for the expression of these glands has led to the development of near-isogenic glanded and glandless genotypes. Glands may also be phenotypically induced by herbivory. We determined the consequences of constitutive and induced gland expression on two types of herbivores, spider mites (cell content feeders) and noctuid caterpillars (leaf chewers).Induction of glands was strongly dependent on the density of attackers. Spider mite herbivory on cotyledons (1) increased the density (but not total number) of glands on cotyledons linearly, (2) increased the density and total number of glands on the first true leaf linearly, and (3) affected the density and total number of glands on the second true leaf non-linearly, compared to controls. Neither constitutive nor induced expression of glands affected mite population growth. An equal reduction of mite population size on induced glanded and glandless plants (50%) relative to uninduced controls indicated that factors other than glands were associated with induced resistance to mites. Constitutive gland expression had a strong negative impact on caterpillar performance, reducing growth by 45%. Induced resistance to caterpillars was three times stronger in glanded genotypes than in glandless genotypes, indicating that factors associated with induced resistance to caterpillars are strongly associated with glands. Three cotton varieties were highly variable in their constitutive and induced resistance to mites and caterpillars.Thus, defense of cotton plants against herbivores can be roughly categorized as constitutive and inducible factors associated with terpenoid aldehyde containing pigment glands that are effective against caterpillars, and factors not associated with glands that are effective against mites.     

Compensation and resistance to herbivory in seagrasses: induced responses to simulated consumption by fish

Herbivory can induce changes in plant traits that may involve both tolerance mechanisms that compensate for biomass loss and resistance traits that reduce herbivore preference. Seagrasses are marine vascular plants that possess many attributes that may favour tolerance and compensatory growth, and they are also defended with mechanisms of resistance such as toughness and secondary metabolites. We quantified phenotypic changes induced by herbivore damage on the temperate seagrass Posidonia oceanica in order to identify specific compensatory and resistance mechanisms in this plant, and to assess any potential trade-offs between these two strategies of defence. We simulated three natural levels of fish herbivory by repeatedly clipping seagrass leaves during the summer period of maximum herbivory. Compensatory responses were determined by measuring shoot-specific growth, photosynthetic rate, and the concentration of nitrogen and carbon resources in leaves and rhizomes. Induced resistance was determined by measuring the concentration of phenolic secondary metabolites and by assessing the long-term effects of continued clipping on herbivore feeding preferences using bioassays. Plants showed a significant ability to compensate for low and moderate losses of leaf biomass by increasing aboveground growth of damaged shoots, but this was not supported by an increase in photosynthetic capacity. Low levels of herbivory induced compensatory growth without any measurable effects on stored resources. In contrast, nitrogen reserves in the rhizomes played a crucial role in the plant’s ability to compensate and survive herbivore damage under moderate and high levels of herbivory, respectively. We found no evidence of inducibility of long-term resistance traits in response to herbivory. The concentration of phenolics decreased with increasing compensatory growth despite all treatments having similar carbon leaf content, suggesting reallocation of these compounds towards primary functions such as cell-wall construction.     

Latitudinal variation in resistance and tolerance to herbivory in the perennial herb Lythrum salicaria is related to intensity of herbivory and plant phenology

Both the length of the growing season and the intensity of herbivory often vary along climatic gradients, which may result in divergent selection on plant phenology, and on resistance and tolerance to herbivory. In Sweden, the length of the growing season and the number of insect herbivore species feeding on the perennial herb Lythrum salicaria decrease from south to north. Previous common‐garden experiments have shown that northern L. salicaria populations develop aboveground shoots earlier in the summer and finish growth before southern populations do. We tested the hypotheses that resistance and tolerance to damage vary with latitude in L. salicaria and are positively related to the intensity of herbivory in natural populations. We quantified resistance and tolerance of populations sampled along a latitudinal gradient by scoring damage from natural herbivores and fitness in a common‐garden experiment in the field and by documenting oviposition and feeding preference by specialist leaf beetles in a glasshouse experiment. Plant resistance decreased with latitude of origin, whereas plant tolerance increased. Oviposition and feeding preference in the glasshouse and leaf damage in the common‐garden experiment were negatively related to damage in the source populations. The latitudinal variation in resistance was thus consistent with reduced selection from herbivores towards the northern range margin of L. salicaria. Variation in tolerance may be related to differences in the timing of damage in relation to the seasonal pattern of plant growth, as northern genotypes have developed further than southern have when herbivores emerge in early summer.     

Wild strawberry shows genetic variation in tolerance but not resistance to a generalist herbivore

Plants’ defenses against herbivores usually include both resistance and tolerance mechanisms. Their deployment has predominantly been studied in either single‐plant genotypes or multiple genotypes exposed to single herbivores. In natural situations, however, most plants are attacked by multiple herbivores. Therefore, aims of this study were to assess and compare the effects of single and multiple herbivores on plant resistance and tolerance traits, and the consequences for overall plant performance. For this, we exposed multiple genotypes of wild woodland strawberry (Fragaria vesca) to jasmonic acid (JA), to mimic chewing herbivory and induce the plants’ defense responses, and then introduced the generalist herbivore Spodoptera littoralis to feed on them. We found that woodland strawberry consistently showed resistance to S. littoralis herbivory, with no significant genetic variation between the genotypes. By contrast, the studied genotypes showed high variation in tolerance, suggesting evolutionary potential in this trait. Prior JA application did not alter these patterns, although it induced an even higher level of resistance in all tested genotypes. The study provides novel information that may be useful for breeders seeking to exploit tolerance and resistance mechanisms to improve strawberry crops’ viability and yields, particularly when multiple herbivores pose significant threats.     

喜旱莲子草对同基因型邻体根系的表型可塑性: 入侵地和原产地的比较

植物对邻体根系的表型可塑性是指与无邻体对照相比, 即使个体平均可获取土壤资源相同, 在有邻体根系存在时植物也会改变根系生物量分配, 并影响其他功能性状和适合度。表型可塑性进化假说(evolution of plasticity hypothesis)认为外来植物在入侵地进化出了更强的表型可塑性。对该假说的验证多集中于外来植物对光照、水分、养分以及天敌等的可塑性进化, 但对邻体根系的可塑性在入侵植物中是否发生进化尚未见报道。我们采用同质园实验比较了喜旱莲子草(Alternanthera philoxeroides)入侵地(美国)和原产地(阿根廷)各5个基因型的适合度与功能性状对同基因型邻体根系的可塑性。结果表明: 喜旱莲子草的根冠比(P = 0.088)和比叶面积(P = 0.007)对同基因型邻体根系的可塑性在入侵地和原产地基因型间存在差异: 入侵地基因型在有邻体根系时根冠比和比叶面积增加, 而原产地基因型则相反。但是, 总生物量、贮藏根生物量、比茎长和分枝强度对邻体根系的可塑性在入侵地和原产地间没有显著差异。此外, 与分隔邻体根系相比, 同基因型邻体根系存在时总生物量(+9.9%)和贮藏根生物量(+13.9%)显著增加, 比茎长(-9.5%)显著降低。最后, 与原产地基因型相比, 总体上入侵地基因型的总生物量(+62.0%)和贮藏根生物量(+58.9%)增加, 比茎长(-28.5%)和分枝强度(-42.8%)降低。这些结果表明喜旱莲子草入侵地基因型与资源利用相关功能性状(如根冠比和比叶面积)对邻体根系的可塑性方向与原产地基因型相反; 但适合度和株型相关性状(如比茎长和分枝强度)对同基因型邻体根系的可塑性与原产地没有差异。     

Enhanced fitness and greater herbivore resistance: implications for dandelion invasion in an alpine habitat

Several hypotheses have been proposed to explain the defense strategies of invasive plants in new ranges. In the absence of specialist herbivores, it is believed that invasive plants may allocate fewer resources to resistance and more to growth and reproduction, thus increasing tolerance to damage in the invasive genotypes. In order to test these predictions, we compared both performance (growth and reproduction) and defense strategies (tolerance and resistance) of two populations of Taraxacum officinale, one from the native range in the French Alps, and one from the introduced range in the Chilean Andes. Individuals from the introduced population demonstrated improved reproductive traits relative to those from the native population, although there was no discernible difference in biomass accumulation. Additionally, reduced tolerance was evident in the case of the former; whereas fitness traits of native plants were unaffected by damage, invasive plants reduced growth and seed output by 25 and 30% respectively following damage treatments. Increases in levels of phenols and anthocyanins, produced as a defense response to herbivory, were observed in introduced plants. Our results suggest that reallocation of resources to reproduction may be an important factor favouring invasive success of T. officinale in Chile, and that a higher investment in chemical resistance traits in this population may also be a factor in this regard.     

Effects of plant traits and the relative abundance of common woody species on seedling herbivory in the Thousand Island Lake region

Aims Plant-herbivore interaction is a hot topic in the study of biodiversity and ecosystem functions. Herbivores can negatively affect seedling growth and therefore can alter the dynamics of plant recruitment. However, previous studies do not fully reveal the relative importance of different plant functional traits on herbivory intensity and rarely link herbivory to the relative abundance of plant species.Methods Here, we measured 11 plant functional traits and the relative abundance of seedlings of 16 common woody species in the subtropical forests on 29 islands in Thousand Island Lake, East China. We then used multivariate regression and variance partitioning to test the contribution of functional traits and the relative abundance to interspecific differences of insect herbivory intensity.Important findings Our study found that both plant functional traits (e.g. carbon nitrogen ratio, leaf thickness) and the relative abundance of woody species played important roles in herbivory intensity, and they jointly contributed 54% of the variance of the interspecific differences. Among these factors, species with higher defensive ability, lower nutrient content and higher relative abundance had lower herbivory intensity. We suggest to consider both individual level traits (functional traits) and community level attributes (the relative abundance) in future herbivory studies.     

Tolerance to herbivory in lupin genotypes with different alkaloid concentration: Interspecific differences between Lupinus albus L. and L. angustifolius L.

Lupin genotypes accumulate alkaloids that act as feeding deterrents for several kinds of herbivores. Breeding sweet (low alkaloid) genotypes resulted in a greater dependence on pesticides. Besides the concentration of defensive chemicals, plants possess another way to deal with herbivory, to allocate post-damage resources to growth in order to reach compensation in biomass (tolerance).

These two ways to deal with herbivores were postulated as alternative strategies, as scarce resources allocated to one function (growth or secondary metabolism) would not be available for the other function. Genotypes could differ in the way they respond to herbivory; identifying those genotypes with greater ability to overcome the damage would be useful to decrease the use of pesticides.

The aim of this work was to compare tolerance to herbivory in Lupinus albus and Lupinus angustifolius genotypes with contrasting alkaloid concentration. Tolerance was evaluated by comparing growth and grain yield of field-grown cut and uncut plants. Cutting treatments were performed at flowering by cutting 50% of the upper shoot biomass (including the main apex, stems, flowers and leaves). Differences between species were found in their tolerance to herbivory. While L. angustifolius showed full compensation in growth or grain yield that allowed cut plants to equal controls biomass or yield after damage, simulated herbivory reduced growth and grain yield in L. albus.     

Leaf turgor loss point at full hydration for 41 native and introduced tree and shrub species from Central Europe

过去几年,中欧的森林遭受了由气候变化引起的直接后果—干旱。所有这些森林都有着悠久的经营历史,重新种植受损森林的责 任落到了土地所有者的身上。因此,土地所有者和政府正在寻找适合在树木灭绝地区重新种植的树种。事实上,很好的了解物种的抗旱性 对当前树木的重新种植非常关键。我们在41种原产于或引进于中欧的木本树种中确定了一个被广泛认可的叶片耐旱特性(在充分水合作用 下膨压损失点的叶片水势,即π_tlp)。采用渗透快速评价法测定了暴露于阳光下的叶片在充分水合作用下的渗透势(π_osm),并将其转化为π_tlp。 本地种的平均π_tlp为−2.33 ± 0.33 MPa。引进树种Aesculus hypocastania的π_tlp为−1.70 ± 0.11 MPa。引进树种Pseudotsuga menzesii的π_tlp值 为−3.02 ± 0.14 MPa,耐旱性最强。绝对值较低的负值πtlp表示抗旱性较低,绝对值较高的负值π_tlp表示抗旱性较高。例如,两种本地物种 Illex aquifolium和Alnus glustinosa是潮湿生境的自然伴生物种,其π_tlp值分别为−1.75 ± 0.02 MPa和−1.76 ± 0.03 MPa。     

No evolution of reduced resistance and compensation for psyllid herbivory by the invasive <Emphasis Type="Italic">Genista monspessulana</Emphasis>

The evolution of redirecting resources from plant defense to growth or reproduction may explain why some exotic species are successful invaders in new environments. For example, the evolution of increased competitive ability hypothesis posits that escape from herbivores by invasive plants results in the selection of more vigorous genotypes that reduce their allocation of resources to defense. In addition, understanding the defense strategy of an invasive plant may help forecast the likely impact of herbivory. We tested the prediction of reduced defense (i.e., resistance) in Genista monspessulana, measured indirectly as the performance of a specialist psyllid herbivore, by comparing five native and introduced plant populations. We also examined the ability of G. monspessulana to compensate for herbivory in the presence and the absence of psyllids for a single plant population from the native and introduced regions. Plant origin (native or introduced) did not influence the psyllid’s abundance and population growth rate, suggesting no change in resistance to herbivory for introduced plants. Similarly, we found no overall difference in plant performance between individuals in the presence and the absence of psyllid herbivory, suggesting that G. monspessulana was able to fully compensate for herbivory. Damaged plants compensated by changing the pattern of branching, which also resulted in greater dry leaf biomass. We conclude that evolution of reduced defenses does not explain the success of G. monspessulana as an invader and that compensation for herbivory may limit the efficacy of the psyllid as a biological control agent.