植物防卫的碳-养分平衡假说

碳-养分平衡假说(carbon-nutrient balance hypothesis,CNBH)认为,植物组织中次生代谢物浓度受环境碳-资源有效性控制;植物体内次生代谢物按照化学计量的要求进行分配;资源分配给防卫物的必要条件是资源供应量满足植物生长需求后仍过剩。CNBH自提出以来,其适用范围不断受到限制,解释与预测研究结果的能力逐渐显现不足。期间,对CNBH进行过优化和修改,设置了多种限制条件,以期能使CNBH得到补救,继续成为指导植物-草食动物间相互作用和植物体内资源分配的相关理论。然而,随着研究的逐渐深入,CNBH被证实缺乏逻辑性和内在一致性;CNBH不能满足假说本身的发展要求,缺乏明确可行的量化指标体系,也没有明确地标识出理论预测范围与可测试范围之间的界限。研究表明,CNBH的基本假设本身是错误的;随着人们对植物-草食动物间相互作用的认知能力加强,更深刻地认识到资源在植物体内的分配模式,意识到CNBH假说的严重缺陷。在现有的植物防卫理论中,生长-分化平衡假说(growth-differentiation balance hypothesis)较为成熟,不但具有CNBH的优点,而且更具有植物生理学和进化...     

EFFECTS OF CANOPY POSITION AND IRRADIANCE ON THE LEAF PHYSIOLOGY AND MORPHOLOGY OF PENTACLETHRA MACROLOBA (MIMOSACEAE)

Pentaclethra macroloba (Willd.) Kuntze (Mimosaceae) is a dominant late-successional tree species in the Atlantic lowland forests of Costa Rica. Leaves of P. macroloba from three heights in the forest canopy were compared with leaves of seedlings grown in controlled environment chambers under four different irradiance levels. Changes in leaf characteristics along the canopy gradient paralleled changes resulting from the light gradient under controlled conditions. The effect of light or canopy position on light-saturated photosynthesis was small, with maximum photosynthesis increasing from 5 to 6.5 μmol m^−-2 s^−-1 from understory to canopy. Both chamber grown and field leaves showed large adjustments in photosynthetic efficiency at low light via reductions in dark respiration rates and increases in apparent quantum yields. Light saturation of all leaves occurred at or below 500 μmol m^−-2 s^−-1. Leaf thickness, specific leaf weight, and stomatal density increased to a greater extent than saturated photosynthesis with higher irradiance during growth or height in the canopy. As a result, there was a poor correspondence between leaf thickness and light-saturated photosynthesis on an area basis. It is concluded that Pentaclethra macroloba possesses the characteristics of a typical shade-tolerant species.     

Do the antiherbivore traits of expanding leaves in the Neotropical tree Inga paraensis (Fabaceae) vary with light availability?

Treefall gaps in tropical forests have a profound effect on plants growing in the understory, primarily due to increased light availability. In higher light, mature leaves typically have increased anti-herbivore defenses. However, since the majority of herbivory occurs while leaves are expanding, it is important to determine whether defense expression during the short period of leaf expansion is canalized (invariant) or plastic in response to variation in light. Therefore, we examined young leaves of Inga paraensis (Fabaceae) saplings growing along a light gradient in a terra-firme forest in Central Amazonia. We quantified leaf production and expansion time, dry mass of phenolics, saponins, and nitrogen, ants attracted to extrafloral nectaries, and leaf consumption. Over the entire light gradient, the number of leaves produced per flush increased by 50?% and the mass of phenolic compounds by 20?%, but no other traits changed. On average, 39?% of leaf area was consumed with no difference across the light gradient. Alone, none of the leaf traits was a significant predictor of leaf consumption, except for phenolics, which showed a positive relationship. Multiple regressions showed that leaf consumption was positively related to more leaves per flush and a higher concentration of phenolics in leaves. Unlike studies of mature leaves, young leaves of I. paraensis show low plasticity in defense traits across a light gradient, suggesting that leaf development is canalized.     

Growth and chemical defense in willow seedlings: trade-offs are transient

Many studies have failed to detect costs of defense and some have even found a positive correlation between growth and the concentrations of chemical defenses. These studies contradict the theoretical assumption that anti-herbivore defenses are costly—produced at the expense of growth and/or reproduction. Costs, however, may be transient and therefore difficult to detect. Here we tested the hypothesis that costs of defense would be pronounced early in development when root growth is prioritized (high percent root allocation), but not later in development. To test this hypothesis, we grew F₂ hybrid willow seedlings from five different families, and harvested cohorts of even-aged seedlings after 6, 7, 8 and 9 weeks of growth. Seedlings were divided into root and shoot tissue and shoots were analyzed for phenolics (condensed tannins and phenolic glycosides). We found evidence for transient costs of defense. The concentrations of phenolics were negatively correlated with total biomass, shoot biomass, and the proportion of biomass allocated to roots in week 6. After week 6, however, the concentrations of phenolics were positively correlated with shoot biomass and total biomass, while phenolics were uncorrelated with the proportion of biomass allocated to roots. These results, the first ever, to our knowledge, with woody plants, suggest that costs of defense were transient; specifically, costs were found in early development, when root establishment was a priority. Our findings suggest that studies should focus more on trade-offs early in plant development.     

Resource availability and the trichome defenses of tomato plants

We conducted two experiments to determine how resource availability influenced allocation by tomato (Lycopersicon esculentum) to trichomes, and how different patterns of trichome allocation by plants grown in different resource environments might then influence the behavior of tobacco hornworm (Manduca sexta) caterpillars. In the first experiment we used high and low levels of light and water, and then, using scanning electron microscopy, determined trichome densities on the leaves and stems. We sampled leaves and stems at several places throughout the plant to determine whether there were within-plant differences in allocation to trichomes. The results of the first experiment showed that resource availability influenced allocation to trichome growth. Patterns in high and low-light supported both the growth-differentiation balance hypothesis (GDBH) and the carbon-nutrient balance hypothesis (CNBH). However, the GDBH was not supported by differences among water treatments. Contrary, to predictions of the GDBH, plants with intermediate growth did not have the highest trichome densities, and plants with similar growth differed in trichome density. Possible biological and artifactual explanations are discussed. The first experiment also showed that there was within-plant variation in allocation to trichomes, and that plant resource availability may influence within-plant variation in allocation to trichomes. In the second experiment, we grew plants in high and low-light, and then monitored the behavior of tobacco hornworms on the stems of these plants in the laboratory. This experiment demonstrated that the light environment that tomato plants were grown in influenced the resting behavior of caterpillars. Furthermore, it demonstrated that both glandular and non-glandular trichomes impeded caterpillars from searching for food. Overall, this study indicated that plant resource availability can influence allocation to trichome defenses, and that these differences may affect insect herbivores.     

Defense strategies used by two sympatric vineyard moth pests

Natural enemies including parasitoids are the major biological cause of mortality among phytophagous insects. In response to parasitism, these insects have evolved a set of defenses to protect themselves, including behavioral, morphological, physiological and immunological barriers. According to life history theory, resources are partitioned to various functions including defense, implying trade-offs among defense mechanisms. In this study we characterized the relative investment in behavioral, physical and immunological defense systems in two sympatric species of Tortricidae (Eupoecilia ambiguella, Lobesia botrana) which are important grapevine moth pests. We also estimated the parasitism by parasitoids in natural populations of both species, to infer the relative success of the investment strategies used by each moth. We demonstrated that larvae invest differently in defense systems according to the species. Relative to L. botrana, E. ambiguella larvae invested more into morphological defenses and less into behavioral defenses, and exhibited lower basal levels of immune defense but strongly responded to immune challenge. L. botrana larvae in a natural population were more heavily parasitized by various parasitoid species than E. ambiguella, suggesting that the efficacy of defense strategies against parasitoids is not equal among species. These results have implications for understanding of regulation in communities, and in the development of biological control strategies for these two grapevine pests.     

Demographic and growth patterns of Pentaclethra macroloba (Willd.) Kuntze,a hyperdominant tree in the Amazon River estuary

Little is known about the life history and environmental factors that regulate the growth rate of hyperdominant trees in flooded Amazonian forests. Pentaclethra macroloba is a hyperdominant tree, and it is widely explored in the Amazon, because its seed oil is a powerful herbal medicine. We evaluated the demographic structure and growth patterns of P. macroloba and tested the effect of the Amazon River flood pulse on its growth. We modeled the growth and determined the age of P. macroloba by analyzing the growth rings of 30 monitored trees in relation to hydroclimatic variables. We also inventoried 240 juvenile and 2072 adult trees arranged in a clustered pattern. The diametric distribution pattern of the juvenile and adult trees was exponential and log-normal, respectively. The trees were found to be up to 102 years old, and 47% of them grew freely toward the canopy. Peak growth in height and diameter occurred at 24 (61.7 cm year^-1) and 46 (9.38 mm year^-1) years, respectively. Pentaclethra macroloba showed cambial dormancy during the seasonal peak of rainfall (R² = 0.41; t = −2.62; p < 0.01) and flooding of the Amazon River (R² = 0.47; t = −3.01; p < 0.01). Increases in rainfall and flood level of the river in the rainy season control the growth rate of P. macroloba, making it a seasonal process. The demographic and growth patterns of P. macroloba respond to the environmental heterogeneity of the estuarine floodplain forest and also reflect its life history over time.     

飞机草入侵种群和原产地种群生长和数量型化学防御物质含量差异的比较研究

增强竞争能力的进化假说认为,在入侵地外来植物逃离了原产地天敌的控制,把原来用于防御的资源分配到生长、生殖等,从而提高竞争力。为探讨进化在恶性外来入侵植物飞机草（Chromolaena odorata）入侵中的作用,在同质种植园中的两个养分条件下比较研究了飞机草原产地和入侵地各8个种群叶片单宁含量,茎和叶片总酚、半纤维素和纤维素含量以及总生物量的差异。结果表明,在两个养分条件下,飞机草入侵种群和原产地种群总生物量差异均不显著,入侵种群茎和叶片半纤维素含量均低于原产地种群;在高养分条件下,飞机草入侵种群叶片纤维素含量低于原产地种群;在低养分条件下,入侵种群茎和叶片总酚含量高于原产地种群。由此,我们得出结论：在入侵地,飞机草未发生加快生长的进化,但数量型化学防御物质发生了遗传变化;降低的半纤维素和纤维素含量可能是对入侵地专性天敌缺乏做出进化响应的结果,提高的总酚含量有利于飞机草防御入侵地的广谱天敌。     

Sesiid Moths Reduce Germination,Seedling Growth,and Survivorship in Pentaclethra macroloba (Mimosoideae), a Locally Dominant Lowland Neotropical Tree1

Monodominant forests are a widespread feature of the humid and wet lowland tropics, but little is known about their origins or factors mediating their persistence. Nonetheless, escape from significant vertebrate and invertebrate seed predation plays a prominent role in most hypotheses. The seeds of Pentaclethra macroloba (Fabaceae: Mimosoideae) have long been thought to be virtually immune to predation, contributing to its local dominance in the canopy of some Mesoamerican forests. Here, we describe herbivory by the larvae of Carmenta surinamensis (Lepidoptera: Sesiidae) on the seeds of P. macroloba, and report the results of studies designed to clarify how this interaction influences germination, seedling growth, and mortality. To this end, we collected P. macroloba seeds at 30‐d intervals for 5 mo along a rain forest transect in Costa Rica. The seeds were monitored in a shade house for 30 d. Adult moths were reared from 43.6 percent of seeds, and significantly affected germination and mortality, and all measures of growth (number of leaves, seedling height, seed and seedling mass at 30 d, and 30‐d change in seed and seedling mass). Based on these observations, we conclude that seed boring by C. surinamensis is a potentially important factor influencing population dynamics in P. macroloba, and warrants further investigation for its prospective role in regulating local abundance in this locally dominant and ecologically significant tree.     

Quantitative and qualitative shifts in defensive metabolites define chemical defense investment during leaf development in Inga,a genus of tropical trees

Selective pressures imposed by herbivores are often positively correlated with investments that plants make in defense. Research based on the framework of an evolutionary arms race has improved our understanding of why the amount and types of defenses differ between plant species. However, plant species are exposed to different selective pressures during the life of a leaf, such that expanding leaves suffer more damage from herbivores and pathogens than mature leaves. We hypothesize that this differential selective pressure may result in contrasting quantitative and qualitative defense investment in plants exposed to natural selective pressures in the field. To characterize shifts in chemical defenses, we chose six species of Inga, a speciose Neotropical tree genus. Focal species represent diverse chemical, morphological, and developmental defense traits and were collected from a single site in the Amazonian rainforest. Chemical defenses were measured gravimetrically and by characterizing the metabolome of expanding and mature leaves. Quantitative investment in phenolics plus saponins, the major classes of chemical defenses identified in Inga, was greater for expanding than mature leaves (46% and 24% of dry weight, respectively). This supports the theory that, because expanding leaves are under greater selective pressure from herbivores, they rely more upon chemical defense as an antiherbivore strategy than do mature leaves. Qualitatively, mature and expanding leaves were distinct and mature leaves contained more total and unique metabolites. Intraspecific variation was greater for mature leaves than expanding leaves, suggesting that leaf development is canalized. This study provides a snapshot of chemical defense investment in a speciose genus of tropical trees during the short, few‐week period of leaf development. Exploring the metabolome through quantitative and qualitative profiling enables a more comprehensive examination of foliar chemical defense investment.     

PHLOROTANNIN ALLOCATION AMONG TISSUES OF NORTHEASTERN PACIFIC KELPS AND ROCKWEEDS

Optimal defense theory (ODT) predicts antiherbivore defensive compounds will be allocated so that the most valuable or most susceptible tissues will be best defended. The growth–differentiation balance hypothesis (GDBH) predicts that defense allocation will be a result of trade-offs between growth and defense. Thus, these two theories predict opposite allocation patterns with respect to “valuable,” actively growing meristematic and reproductive tissues. ODT predicts that meristems and reproductive tissues should have higher defense levels than nonmeristematic vegetative tissues; the GDBH predicts the defense levels of meristems and reproductive tissues will be lower than vegetative tissues. We examined allocation patterns of phlorotannins in 21 species of kelps (Order Laminariales) and rockweeds (Order Fucales) from nine sites on the west coast of the United States to determine if allocation patterns better matched the predictions of ODT or the GDBH and to look for differences in allocation patterns among sites. Within-species differences in phlorotannin levels occurred in 10 of the 21 species examined. Meristems of both kelps and rockweeds had higher phlorotannin levels than nonmeristematic vegetative tissues, consistent with ODT. Phlorotannin levels in reproductive tissues of kelps were higher than vegetative tissues, but levels in reproductive tissues of rockweeds were lower than vegetative tissues, indicating that allocation strategies may follow taxonomic lines. Allocation patterns differed among sites in four of the 16 species collected from more than one site. Differences in allocation patterns among sites were usually changes in the ratios of phlorotannins in well-defended compared to poorly defended tissues, rather than changes in which tissues were well defended or poorly defended. We concluded that environmental variability can have large effects on the concentration of phlorotannins in algae but has limited effects on allocation patterns among tissues.     

Developmental Changes in Direct and Indirect Defenses in the Young Leaves of the Neotropical Tree Genus Inga (Fabaceae)

Plant fitness is affected by herbivory, and in moist tropical forests, 70 percent of herbivore damage occurs on young leaves. Thus, to understand the effects of herbivory on tropical plant fitness, it is necessary to understand how tropical young leaves survive the brief, but critical, period of susceptibility. In this study, we surveyed three species of Inga during young leaf expansion. Three classes of toxic secondary metabolites (phenolics, saponins, and tyrosine), extrafloral nectar production, leaf area, and extrafloral nectary area were measured at randomly assigned young leaf sizes. In addition, all defenses were compared for potential trade‐offs during leaf expansion. No trade‐offs among defenses were found, and the concentration of all defenses, except tyrosine, decreased during leaf expansion. We suggest that plants continued to increase phenolic and saponin content, but at a rate that resulted in decreasing concentrations. In contrast, tyrosine content per leaf steadily increased such that a constant concentration was maintained regardless of young leaf size. Nectar production remained constant during leaf expansion, but, because young leaf area increased by tenfold, the investment in extrafloral nectar per leaf area significantly decreased. In addition, nectary area did not change during leaf expansion and therefore the relative size of the nectary significantly decreased during young leaf expansion. These results support the predictions of the optimal defense hypothesis and demonstrate that the youngest leaves have the highest investment in multiple defenses, most likely because they have the highest nitrogen content and are most susceptible to a diversity of herbivores.     

Effects of soil nitrogen levels on growth and defense of the native and introduced genotypes of alligator weed

土壤氮水平对喜旱莲子草原产地和引入地基因型生长和防御的影响 同种植物生长在资源丰富生境中的个体,其防御水平被认为低于生长在资源匮乏生境中的个体。然而,生境的养分水平如何影响植物的诱导抗性和耐受性,以及这种影响在入侵植物的原产地和引入 地种群间是否存在差异,目前均知之甚少。本研究以入侵植物喜旱莲子草(Alternanthera philoxeroides)的原产地阿根廷和引入地美国的基因型为研究对象设计同质园实验,以探究土壤氮水平对植物的生长、组成和诱导性[莲草直胸跳甲(Agasicles hygrophila)取食诱导]化学防御以及耐受性的影响。实验中,我们测定了植物总生物量、伸长速率(生长速率的表征)以及叶片和根系中总碳、总氮和三萜皂苷(化学防御物质)的含量。研究结果显示,植物在低土壤氮水平下表现出较高的组成抗性(植物在低土壤氮水平下的叶片三萜皂苷含量高于其在高土壤氮水平的33%)和耐受性[植物被取食后总生物量下降的程度更低(植物在高土壤氮水平和低土壤氮水平下被取食后总生物量分别下降了24%和15%)],而在高土壤氮水平下表现出较高的诱导抗性(在高土壤氮水平下的植物被取食后叶片三萜皂苷含量与空白对照的植物相比升高了24%)。植物的组成抗性和耐受性与生长速率存在权衡,但诱导抗性与生长速率存在显著的正相关性。此外,引入地基因型在低土壤氮水平下叶片碳含量显著低于原产地基因型(-6%),但这种差异在高土壤氮水平下消失。这些结果表明,土壤氮水平 影响植物对不同防御策略的选择偏好,并且在决定引入地基因型的表现时与植食作用存在交互作用。     

Floral-induced and constitutive defense against florivory: a comparison of chemical traits in 12 herb species

Little is known about how plants protect flowers—their reproductive organs—against florivory. Additionally, the induced floral defense system has been examined in only a few species. We tested the inducibility of putative floral defenses and investigated the relationship between natural florivory and the floral defenses of 12 naturally growing plant species. The relationships between florivory and four chemical traits (nitrogen, phosphorus, total phenolics, and condensed tannins) were investigated in 12 plant species. We also studied whether flowers induce changes in chemical defenses in response to artificial damage in 10 plant species. A higher concentration of floral nitrogen was associated with a decreasing frequency of florivore attacks. Among the four traits of the 10 plant species studied, no trait changed in response to the artificial damage. We suggest that induced defense systems may not be advantageous for flowers, although it is also possible that these species simply do not use induced defense in any of their plant parts.     

Testing the optimal defense theory and the growth-differentiation balance hypothesis in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Two prominent theories proposed to explain patterns of chemical defense expression in plants are the optimal defense theory (ODT) and the growth-differentiation balance hypothesis (GDBH). The ODT predicts that plant parts with high fitness value will be highly defended, and the GDBH predicts that slow growing plant parts will have more resources available for defense and thus will have higher defense levels than faster growing tissues. We examined growth rate, fitness value, and defense protein levels in leaves of a wild and lab ecotype of Arabidopsis thaliana to address whether patterns of defense protein expression in this plant conform to predictions of either the ODT or the GDBH. We divided leaves of A. thaliana into six leaf classes based on three developmental stages: vegetative, bolting, and flowering; with two leaf ages at each stage: young and old. We assessed the fitness value of leaves by determining the impact of the removal of each leaf class on total seed production and germination rates. Although A. thaliana was highly tolerant to defoliation, young leaves were more valuable than old in general, and young leaves on bolting plants were the most valuable leaf class in particular. Young leaves on vegetative plants grew fastest in both ecotypes, while old leaves on bolting and flowering plants grew slowest. Finally, defense levels were assessed in each leaf class by quantifying the constitutive and inducible expression of four defense-related proteins. Expression of guaiacol peroxidase and chitinase activity conformed largely to GDBH predictions. Expression of trypsin inhibitor and polyphenoloxidase activity varied by leaf class and treatment, but conformed to neither GDBH nor ODT predictions.     

Jasmonic acid signalling and herbivore resistance traits constrain regrowth after herbivore attack in Nicotiana attenuata

Because traits conferring resistance on herbivores can reduce fitness-associated traits, trade-offs may occur between tolerance and resistance responses. We examined these trade-offs in genotypes of Nicotiana attenuata that were transformed to silence trypsin proteinase inhibitor (TPI) production (AS-Natpi), an antiherbivore defense associated with (14%) reductions in seed production, and the jasmonate signal cascade that elicits these defenses (AS-Nalox3), by measuring stalk and axillary branch growth and seed production after two defoliation regimes and Manduca sexta larval attack to bottom or middle and top stalk leaves. Larval attack and defoliation at middle and top leaves depressed seed production and increased axillary branching more than at bottom leaves. AS-Nalox3 and AS-Natpi plants produced significantly longer (two- to fourfold) branches than did wild-type (WT) plants, results that are consistent with resource-based trade-offs between resistance and regrowth. Methyl jasmonate (MeJA) treatment of AS-Nalox3 plants restored WT branch growth, suggesting that jasmonic acid (JA) signalling suppresses regrowth and contributes to apical dominance. These results are consistent with the existence of JA- and resource-mediated trade-offs between regrowth and herbivore resistance traits.     

Changes in Formation and Localization of Phenolic Compounds in the Tissues of European and Canadian Yew during Dedifferentiation <Emphasis Type="Italic">In Vitro</Emphasis>

Changes in formation and localizations of phenolic compounds, including flavans, were investigated in the tissues of European and Canadian yew (Taxus baccata L. and T. canadensis Marsh.) during dedifferentiation in vitro. Annual shoots of European yew had the highest capacity for synthesizing these compounds. During the summer growth period, the content of total soluble phenolic compounds and flavans in these shoots was 30–40% higher than in the winter. Cell dedifferentiation and growth in vitro was accompanied by enhanced synthesis of phenolic compounds, including flavans, the change in tissue localization of these compounds, and an increase in the number of cells containing phenolics. Significant accumulation of phenolic compounds in callus cells resulted in necroses following two subcultures in the European and Canadian yew cultures initiated from summer explants, and following seven subcultures of the European yew calli initiated from winter explants. These data allow us to suggest that a high level of phenolic compounds in yew calli could be the reason for their necrosis.     

THE BEGINNINGS OF ANTARCTIC MACROALGAL CHEMICAL ECOLOGY: DEFENSES AGAINST HERBIVORES IN A NITROGEN REPLETE,CARBON LIMITED OCEAN

Amsler, C. D.¹, Iken, K. B.¹, McClintock, J. B.¹, Furrow, F. B.², & Baker, B. J.^{2 1}Department of Biology, University of Alabama at Birmingham, Birmingham AL 35294-1170 USA; ²Department of Chemistry, Florida Institute of Technology, Melbourne, FL 32901 USA We examined palatability of Antarctic Peninsula macroalgae in feeding bioassays with three common, sympatric macroalgal-consuming omnivores (amphipod, fish, sea star). Antarctic macroalgae have low C:N ratios, high nitrogen contents, and are usually growth limited by carbon (light). The Carbon Nutrient Balance Hypothesis (CNBH) predicts that macroalgae would produce nitrogenous secondary metabolites for defense rather than high levels of non-nitrogenous defenses under these conditions. To date, feeding bioassays have been performed on fragments of thallus from 26 macroalgal species and 21 (81%) were rejected by at least one omnivore. Organic extracts from 13 macroalgal species rejected as thallus were used in feeding bioassays. At least one extract from 12 species (92%) was rejected by at least one omnivore, suggesting that chemical defenses against herbivores probably are present in at least some of the macroalgal species. We have identified a number of specific, non-nitrogenous secondary metabolites in these extracts and previous workers have also reported non-nitrogenous secondary metabolites from antarctic macroalgae. Additional extracts targeting nitrogenous metabolites from 25 species were subjected to thin layer chromatography and visualized by stains specific for nitrogenous compounds. No nitrogenous secondary metabolites were identified by this or other methods. Nitrogenous secondary metabolites are also extremely rare in macroalgae from other areas of the world. Consequently, although our bioassays suggest that chemical defenses probably do occur, our data cast doubt on the applicability of the CNBH for predicting the chemical composition of macroalgal defenses under carbon limited conditions. (NSF OPP9814538, OPP9901076)     

Tradeoff between physical and chemical defense in plant seeds is mediated by seed mass

Plants have evolved both physical and chemical defenses to make the nutrients of attacked organs difficult to access or more toxic to resist animal consumption or/and pathogen attack. Although it is intuitive that a tradeoff could exist between physical and chemical defenses because of finite defense resources, many studies have failed to detect this tradeoff. We hypothesized that tradeoff between physical and chemical defenses in individual organs was mediated by the total resource allocation to those organs. In this study, we tested whether a tradeoff between physical (i.e. fiber content, which has proved to be a good indicator of investment into seed coat) and chemical defenses (i.e. total phenolics, which are abundant chemical defenses in plant seeds) existed in plant seeds by using 163 common species collected from Xishuangbanna tropical forest, southwest China. Then we tested whether this tradeoff was mediated by seed mass which could be a potential proxy of total resource investment per seed. Among the 163 species, there was large interspecific variation in both total phenolics (from 0.01 to 20.52%) and fiber content (from 4.47 to 81.49%). Our results supported our hypothesis: negative relationships between physical and chemical defenses were much stronger among small seeds than among large seeds. Our study suggests that total resource acquisition must be considered when evaluating defense tradeoffs. However, it is usually extremely difficult to measure this resource acquisition variation, thus we suggest utilizing easily measured proxies of acquisition variation to quantify tradeoffs.     

Growth,plant quality and leaf damage patterns in a dioecious tree species: is gender important?

Frequently, female plants allocate more resources to reproductive structures and defense-related secondary compounds in comparison with male plants that invest more resources to growth, reflecting trade-offs between reproduction, growth and defense. Therefore, differences in herbivory can be expected between genders. In this study, over two years, we analyzed the differences in plant chemical defense, nutritional quality, plant size and herbivory between genders in the dioecious tree, Spondias purpurea in a Mexican tropical dry forest. We estimated the total leaf area and the area consumed by folivory using a digital image of each leaf. The nutritional quality was estimated as water content, and the concentration of chlorophyll and total nonstructural carbohydrates. The secondary metabolites analyzed were total content of soluble phenolics, flavonoids, protein precipitation capacity of tannins, gallotannins, soluble proanthocyanidins, hydrolyzable tannins and ellagitannins. Our results differ from most of studies that analyze the differential herbivory patterns in dioecious plants. We found that female trees had higher levels of herbivory than male trees of S. purpurea. In the same way, female trees showed higher size and nutritional quality than males, while chemical defense was higher in male trees. The higher percentage of folivory in female trees of S. purpurea is associated with greater nutritional quality and lower chemical defenses. Our results show that male-biased herbivory might not be universal in dioecious species. Therefore, studies of fitness components affected by herbivory are necessary to understand the evolution of dioecy and the importance of herbivores as selective agents on breeding system features.