Plant chemical defense allocation constrains evolution of tolerance to community change across a range boundary

Because transplant experiments show that performance usually decreases across species range boundaries, some range limits might develop from factors and processes that prevent adaptation to stressful environments. Here, we determined whether an ecological cost of plant defense involving stress associated with changes in the local plant community may contribute to range limit development in the upland mustard species Boechera stricta. In a common garden experiment of 499 B. stricta plants, performance decreased and a multivariate axis of community structure increased across the boundary, indicating increased stress associated with the community change. There was also significant genetic variation (evolutionary potential) among marker‐inferred inbred lines of B. stricta for tolerance to the stress; however, lines with high basal levels of glucosinolate toxins had lower tolerance to the change in community structure. We suggest that defense allocation, which is also needed across the range, may impede adaptation to the stress associated with the community change and thus contribute to range limit development.     

Simultaneous evolution of competitiveness and defense: induced switching in Arabis drummondii

Optimality theory for plant defense against herbivores predicts an evolutionary tradeoff between the abilities to compete and defend. We tested this hypothesis by studying the effects of genetic variation in competitiveness on defense expression. Two closely related and differentially competitive congeners were compared for levels of resistance, tolerance, and secondary metabolite production. In a growth room experiment, plants of Arabis drummondii and A. holboellii were grown in the presence and absence of the common bunch grass Boutelloua gracilis, the specialist herbivore Plutella xylostella, and generalist herbivore Trichoplusia ni. Tolerance to competition, measured as growth next to the grass relative to controls in the absence of grass, was greatest for A. drummondii, the species that occurred in communities with higher densities of inter-specific neighbors. Measures of defense (resistance to herbivores, tolerance to damage, and concentrations of glucosinolates) varied inconsistently between the Arabis, species, depending on type of herbivore, competition level, and type of defense. The better competitor A. drummondii was more resistant to specialist herbivores, as in the field, and exhibited greater herbivore- and competition-induced changes in glucosinolate profiles. Further, when plants of A. drummondii were fed upon in competitive environments, the induced glucosinolate response was reduced while tolerance levels increased in an apparent switching of induced strategies. We suggest that competitiveness and defense responses are sometimes positively correlated because some defensive traits also function as competitive traits. A competitive function for defenses may also explain why defenses were affected by competition. Alternatively, since the induced response did not increase estimates of total glucosinolate content significantly, minimal defense costs might also allow the simultaneous evolution of competitiveness and defense. Finally, when faced with both herbivory and competition, some competitive species, such as A. drummondii, may switch to growth-based rather than toxin-based strategies as recent theoretical models predict.     

Water deficiency induces evolutionary tradeoff between stress tolerance and chemical defense allocation that may help explain range limits in plants

What causes range limits is a central question in evolutionary ecology. Transplant studies indicate that areas just across range boundaries are often stressful. The recent Defense constraint (DC) hypothesis for plants states that the evolution of tolerance to stressful environments across a range boundary is constrained by allocation to chemical defense because of antagonistic crosstalk between abiotic and biotic stress signaling pathways that otherwise could be co‐opted for range expansion. Abscisic acid (ABA) drought stress tolerance and jasmonic acid/ethylene (JA/ET) defense signaling pathways, for example, are known to be antagonistic to one another in Arabidopsis and other species. To test the DC hypothesis, we examined quantitative genetic variation and co‐variation among marker‐inferred inbred lines and sib‐families of Boechera stricta, a close wild relative of Arabidopsis. The dynamics of the defense‐stress tolerance tradeoff was examined across 1) years that differed in precipitation, 2) drought and ABA treatments, and 3) a NPK nutrient supply gradient. In support of the DC hypothesis, we observed the tradeoff a) in the dry year, and b) in response to water deficiency, which c) was affected by ABA treatments, but the interaction between ABA and glucosinolate (GS) toxin levels was not significant. In contrast to the effects of water deficiency, d) the effect of lower NPK supply to cause the tradeoff was only marginally significant. Because an ABA‐mediated stress response is intrinsic to water deficient conditions and because of the known involvement of JA/ET in GS regulation, we suggest that these results provide circumstantial evidence implicating both of these pathways in the tradeoff and thus in the development of range limits.     

Antagonistic selection and pleiotropy constrain the evolution of plant chemical defenses

When pleiotropy is present, genetic correlations may constrain the evolution of ecologically important traits. We used a quantitative genetics approach to investigate constraints on the evolution of secondary metabolites in a wild mustard, Boechera stricta. Much of the genetic variation in chemical composition of glucosinolates in B. stricta is controlled by a single locus, BCMA1/3. In a large‐scale common garden experiment under natural conditions, we quantified fitness and glucosinolate profile in two leaf types and in fruits. We estimated genetic variances and covariances (the G ‐matrix) and selection on chemical profile in each tissue. Chemical composition of defenses was strongly genetically correlated between tissues. We found antagonistic selection between defense composition in leaves and fruits: compounds that were favored in leaves were disadvantageous in fruits. The positive genetic correlations and antagonistic selection led to strong constraints on the evolution of defenses in leaves and fruits. In a hypothetical population with no genetic variation at BCMA1/3, we found no evidence for genetic constraints, indicating that pleiotropy affecting chemical profile in multiple tissues drives constraints on the evolution of secondary metabolites.     

Resistance and tolerance in <Emphasis Type="Italic">Populus tremuloides</Emphasis>: genetic variation,costs, and environmental dependency

Plants defend themselves against herbivores via resistance, which reduces damage, and tolerance, which minimizes the negative effects of damage. Theory predicts the existence of tradeoffs between defense and growth, as well as between resistance and tolerance, that could maintain the genetic variation for resistance and tolerance often observed in plant populations. We examined resistance and tolerance among aspen (Populus tremuloides) trees grown under divergent soil nutrient regimes. This common garden experiment revealed substantial genetic variation for resistance and tolerance under both low- and high-nutrient conditions. Costs of resistance exist, particularly under high-nutrient conditions where allocation to resistance chemicals competes directly with growth for limited carbon resources. We found no significant costs of tolerance, however, under either nutrient condition. Despite genetic variation for both resistance and tolerance, we found no evidence for a tradeoff between these two defense traits suggesting that resistance and tolerance are complementary, rather than mutually exclusive, defenses in aspen.     

Optimal defense in plants: assessment of resource allocation costs

Genetic and environmental variation of functional traits within populations might be maintained by natural selection when resource allocation costs (RACs) balance trait benefits. Despite the intuitive appeal of optimization models, empirical tests have failed to support the importance of RACs for plant traits that confer resistance against pests. To address this discrepancy, we modified an early model by allowing the cost function to vary across a resource gradient as predicted for RACs and by assuming that the benefits depend on variation in the pest population for susceptibility. Instead of the intermediate defense optimum of the original model, defenses were predicted to be either high or absent, depending on resource availability and history. This result is not supported by empirical tests for ecological or evolutionary outcomes, including our own examination of glucosinolate toxins from sib-families of Boechera stricta (Brassicaceae) grown across an NPK fertilizer gradient. Although we detected an apparent cost of defense in the absence of herbivores, the cost did not increase as resources became more limiting. Also defense production did not vary across the resource gradient as predicted by the modified model. Thus, a model based on explicit expectations of RACs produced predictions that are not supported. Instead, other kinds of costs, such as ecological (indirect) costs may be more important. Alternatively, general conflicts in gene expression and antagonistic crosstalk among signaling pathways may underlie apparent costs.     

Plasticity of plant defense and its evolutionary implications in wild populations of Boechera stricta

Phenotypic plasticity is thought to impact evolutionary trajectories by shifting trait values in a direction that is either favored by natural selection (“adaptive” plasticity) or disfavored (“nonadaptive” plasticity). However, it is unclear how commonly each of these types of plasticity occurs in natural populations. To answer this question, we measured glucosinolate defensive chemistry and reproductive fitness in over 1500 individuals of the wild perennial mustard Boechera stricta, planted in four common gardens across central Idaho, United States. Glucosinolate profiles—including total glucosinolate concentration as well as the relative abundances and overall diversity of different compounds—were strongly plastic both among habitats and within habitats. Patterns of glucosinolate plasticity varied greatly among genotypes. Plasticity among sites was predicted to affect fitness in 27.1% of cases; more often than expected by chance, glucosinolate plasticity increased rather than decreased relative fitness. In contrast, we found no evidence for within‐habitat selection on glucosinolate reaction norm slopes (i.e., plasticity along a continuous environmental gradient). Together, our results indicate that glucosinolate plasticity may improve the ability of B. stricta populations to persist after migration to new habitats.     

Multilocus Patterns of Nucleotide Diversity, Population Structure and Linkage Disequilibrium in Boechera stricta, a Wild Relative of Arabidopsis

Information about polymorphism, population structure, and linkage disequilibrium (LD) is crucial for association studies of complex trait variation. However, most genomewide studies have focused on model systems, with very few analyses of undisturbed natural populations. Here, we sequenced 86 mapped nuclear loci for a sample of 46 genotypes of Boechera stricta and two individuals of B. holboellii, both wild relatives of Arabidopsis. Isolation by distance was significant across the species range of B. stricta, and three geographic groups were identified by structure analysis, principal coordinates analysis, and distance-based phylogeny analyses. The allele frequency spectrum indicated a genomewide deviation from an equilibrium neutral model, with silent nucleotide diversity averaging 0.004. LD decayed rapidly, declining to background levels in ~10 kb or less. For tightly linked SNPs separated by <1 kb, LD was dependent on the reference population. LD was lower in the specieswide sample than within populations, suggesting that low levels of LD found in inbreeding species such as B. stricta, Arabidopsis thaliana, and barley may result from broad geographic sampling that spans heterogeneous genetic groups. Finally, analyses also showed that inbreeding B. stricta and A. thaliana have ~45% higher recombination per kilobase than outcrossing A. lyrata.     

Tree species diversity alters plant defense investment in an experimental forest plantation in southern Mexico

How plant species diversity affects traits conferring herbivore resistance (e.g., chemical defenses), as well as the mechanisms underlying such effects, has received little attention. One potential mechanism for the effect of diversity on plant defenses is that increased plant growth at high diversity could lead to reduced investment in defenses via growth–defense trade‐offs. We measured tree growth (diameter at breast height) and collected leaves to quantify total phenolics in 2.5‐year‐old plants of six tropical tree species (N = 597 plants) in a young experimental plantation in southern Mexico. Selected plants were classified as monocultures or as polycultures represented by mixtures of four of the six species examined. Tree species diversity had a significant negative effect on total phenolics, where polycultures exhibited a 13 percent lower mean concentration than monocultures. However, there was marked variation in the effects of diversity on defenses among tree species, with some species exhibiting strong reductions in phenolic levels in mixtures, whereas others were unresponsive. In addition, tree species diversity had no effect on growth, nor was the negative effect of diversity on chemical defenses mediated by a growth–defense trade‐off. These results demonstrate that tree diversity can alter investment in chemical defenses in long‐lived tree species but that such effect may not always be under strong control by plant endogenous resource allocation trade‐offs. Regardless of the underlying mechanism, these findings have important implications for predicting effects on consumers and ecosystem function.     

Variation in circadian rhythms is maintained among and within populations in Boechera stricta

Circadian clocks have evolved independently in all three domains of life, and fitness benefits of a functional clock have been demonstrated in experimental genotypes in controlled conditions. Still, little is known about genetic variation in the clock and its fitness consequences in natural populations from heterogeneous environments. Using Wyoming populations of the Arabidopsis relative Boechera stricta as our study system, we demonstrate that genetic variation in the clock can occur at multiple levels: means of circadian period among populations sampled at different elevations differed by less than 1 h, but means among families sampled within populations varied by as much as 3.5 h. Growth traits also varied among and within populations. Within the population with the most circadian variation, we observed evidence for a positive correlation between period and growth and a negative correlation between period and root‐to‐shoot ratio. We then tested whether performance tradeoffs existed among families of this population across simulated seasonal settings. Growth rankings of families were similar across seasonal environments, but for root‐to‐shoot ratio, genotype × environment interactions contributed significantly to total variation. Therefore, further experiments are needed to identify evolutionary mechanisms that preserve substantial quantitative genetic diversity in the clock in this and other species.     

The spitting image of plant defenses: Effects of plant secondary chemistry on the efficacy of caterpillar regurgitant as an anti‐predator defense

In the arms race between plants, herbivores, and their natural enemies, specialized herbivores may use plant defenses for their own benefit, and variation in plant traits may affect the benefits that herbivores derive from these defenses. Pieris brassicae is a specialist herbivore of plants containing glucosinolates, a specific class of defensive secondary metabolites. Caterpillars of P. brassicae are known to actively spit on attacking natural enemies, including their main parasitoid, the braconid wasp Cotesia glomerata. Here, we tested the hypothesis that variation in the secondary metabolites of host plants affects the efficacy of caterpillar regurgitant as an anti‐predator defense. Using a total of 10 host plants with different glucosinolate profiles, we first studied natural regurgitation events of caterpillars on parasitoids. We then studied manual applications of water or regurgitant on parasitoids during parasitization events. Results from natural regurgitation events revealed that parasitoids spent more time grooming after attack when foraging on radish and nasturtium than on Brassica spp., and when the regurgitant came in contact with the wings rather than any other body part. Results from manual applications of regurgitant showed that all parameters of parasitoid behavior (initial attack duration, attack interruption, grooming time, and likelihood of a second attack) were more affected when regurgitant was applied rather than water. The proportion of parasitoids re‐attacking a caterpillar within 15 min was the lowest when regurgitant originated from radish‐fed caterpillars. However, we found no correlation between glucosinolate content and regurgitant effects, and parasitoid behavior was equally affected when regurgitant originated from a glucosinolate‐deficient Arabidopsis thaliana mutant line. In conclusion, host plant affects to a certain extent the efficacy of spit from P. brassicae caterpillars as a defense against parasitoids, but this is not due to glucosinolate content. The nature of the defensive compounds present in the spit remains to be determined, and the ecological relevance of this anti‐predator defense needs to be further evaluated in the field.     

Investment in defense and cost of predator-induced defense along a resource gradient

An organism’s investment in different traits to reduce predation is determined by the fitness benefit of the defense relative to the fitness costs associated with the allocation of time and resources to the defense. Inherent tradeoffs in time and resource allocation should result in differential investment in defense along a resource gradient, but competing models predict different patterns of investment. There are currently insufficient empirical data on changes in investment in defensive traits or their costs along resource gradients to differentiate between the competing allocation models. In this study, I exposed tadpoles to caged predators along a resource gradient in order to estimate investment in defense and costs of defense by assessing predator-induced plasticity. Induced defenses included increased tail depth, reduced feeding, and reduced swimming activity; costs associated with these defenses were reduced developmental rate, reduced growth, and reduced survival. At low resource availability, these costs predominately resulted in reduced survival, while at high resource availability the costs yielded a reduced developmental rate. Defensive traits responded strongly to predation risk, but did not respond to resource availability (with the exception of feeding activity), whereas traits construed as costs of defenses showed the opposite pattern. Therefore, defensive traits were highly sensitive to predation risk, while traits construed as costs of defense were highly sensitive to resource allocation tradeoffs. This difference in sensitivity between the two groups of traits may explain why the correlation between the expression of defensive traits and the expression of the associated defense costs was weak. Furthermore, my results indicate that genetic linkages and mechanistic integration of multiple defensive traits and their associated costs may constrain time and resource allocation in ways that are not addressed in existing models. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of plant growth rate and leaf lifetime on the amount and type of anti-herbivore defense

Summary Growth, herbivory and defenses were studied for 41 common tree species in a lowland rainforest in Panama. Species represented a range of shade tolerance, but all individuals were measured in light gaps to control for environmental conditions and the availability of herbivores. Species growth rates and leaf lifetimes differed by almost 50-fold and were related to the degree of shade tolerance. Various measures of plant growth were significantly negatively correlated with an estimate of defense investment, and significantly positively correlated with rates of herbivory. Species with long-lived leaves had significantly higher concentrations of immobile defenses such as tannins and lignins. These data support current hypotheses that the intrinsic growth rate of a species evolutionarily determines the optimal amount and type of defense.     

Increasing substrate heterogeneity as a bet‐hedging strategy for restoring wetland vegetation

Mimicking the natural heterogeneity of wetland substrates, e.g. by roughening surface soil or constructing hummocks, has been shown to facilitate wetland plant establishment. We asked if incorporating substrate heterogeneity could also help plants withstand variation in moisture levels. In a wetland with Carex stricta (tussock sedge) as the main restoration target, we manipulated substrates to create different soil moisture environments for planted C. stricta plugs. Our artificial mounds mimicked tussocks formed by C. stricta in natural meadows (circa 10–40 cm in height); we also varied mound compositions and created shallow depressions. Monitoring demonstrated variation in soil moisture among our treatments and natural differences in soil moisture between experimental blocks. Additionally, rainfall varied from severe drought in year 1 to extreme rainfall in year 2. Plug survival, flowering, cover, biomass, leaf length, and growth rate all varied with treatment, block, and/or year. Interactions among those factors were common. Planting plugs in shallow depressions exacerbated stress in a wet block during a wet year, causing low survival. Planting plugs in moisture‐retaining peat pots allowed them to survive and sustain growth even in a dry block during a dry year. We conclude that heterogeneous substrates can be used to hedge against environmental variability by widening the range of microsites available within a restoration site and thereby moderating stressful conditions in some areas.     

Sexual differences in defensive and protective mechanisms of Populus cathayana exposed to high UV‐B radiation and low soil nutrient status

Previous studies have indicated that Populus cathayana Rehder females are more sensitive and less tolerant to stressful environments than males, but it is still unknown whether there are sexual differences in defensive and protective traits under high UV‐B (HUVB) radiation and low soil nutrient status. In this study, P. cathayana was employed as a model species to investigate sex‐related physiological and biochemical responses to UV‐B radiation under different soil nutrient conditions. Cuttings were exposed to two UV‐B radiation regimes (ambient UV‐B radiation and decreased UV‐B radiation) under two soil nutrient status (topsoil and deep soil) for 100 days over one growing season. Both HUVB radiation and low soil nutrient status induced greater decreases in plant growth, dry mass accumulation, gas exchange and leaf nitrogen use efficiency in females than in males, and greater increases in lipid peroxide and antioxidant enzyme activities, and secondary defense capacities in males than in females. Moreover, sexually different responses happened also in organelle ultrastructure. Our results showed that: (1) females suffered greater negative effects and exhibited lower defense capacities than did males under HUVB radiation, low soil nutrient status and their combination; (2) low soil nutrient status reduced plant's sensitivity to HUVB radiation by increasing allocation to defense and decreasing allocation to growth compared with high soil nutrient status. These results provide evidence that sexually different tradeoffs happen between growth and defense in P. cathayana under HUVB radiation and low soil nutrient status.     

Costs of defense: correlated responses to divergent selection for foliar glucosinolate content in <Emphasis Type="Italic">Brassica rapa</Emphasis>

The evolutionary response of plant populations to herbivore imposed selection for defense may theoretically be constrained by the costs of defense, yet few studies convincingly demonstrate such costs. We investigated possible constraints on the evolution of defense in rapid cycling Brassica rapa by divergently selecting lines for investment in foliar glucosinolate content, a chemical defense in this species. Costs would then result in a significant correlated response to artificially imposed selection in the direction opposite to the direct response of foliar glucosinolate production. Correlated responses of date of first flowering, total flower number, number of seeds per fruit, and mean seed mass were examined. After three generations of selection, there was a significant direct response in glucosinolate content of the leaves of B. rapa. Furthermore, we found significant correlated responses in both total flower production and number of seeds produced per fruit, but not date of first flowering or mean seed mass. Lines selected for high glucosinolates produced fewer flowers and seeds per fruit compared to those selected for low glucosinolates while lines selected for low glucosinolates showed the opposite response. Thus, costs of defense were demonstrated and may constrain the evolution of foliar glucosinolate production in this plant species.     

Vertebrate defense against parasites: Interactions between avoidance,resistance, and tolerance

Hosts can utilize different types of defense against the effects of parasitism, including avoidance, resistance, and tolerance. Typically, there is tremendous heterogeneity among hosts in these defense mechanisms that may be rooted in the costs associated with defense and lead to trade‐offs with other life‐history traits. Trade‐offs may also exist between the defense mechanisms, but the relationships between avoidance, resistance, and tolerance have rarely been studied. Here, we assessed these three defense traits under common garden conditions in a natural host–parasite system, the trematode eye‐fluke Diplostomum pseudospathaceum and its second intermediate fish host. We looked at host individuals originating from four genetically distinct populations of two closely related salmonid species (Atlantic salmon, Salmo salar and sea trout, Salmo trutta trutta) to estimate the magnitude of variation in these defense traits and the relationships among them. We show species‐specific variation in resistance and tolerance and population‐specific variation in resistance. Further, we demonstrate evidence for a trade‐off between resistance and tolerance. Our results suggest that the variation in host defense can at least partly result from a compromise between different interacting defense traits, the relative importance of which is likely to be shaped by environmental components. Overall, this study emphasizes the importance of considering different components of the host defense system when making predictions on the outcome of host–parasite interactions.     

Cassava biology and physiology

Cassava or manioc (Manihot esculenta Crantz), a perennial shrub of the New World, currently is the sixth world food crop for more than 500 million people in tropical and sub-tropical Africa, Asia and Latin America. It is cultivated mainly by resource-limited small farmers for its starchy roots, which are used as human food either fresh when low in cyanogens or in many processed forms and products, mostly starch, flour, and for animal feed. Because of its inherent tolerance to stressful environments, where other food crops would fail, it is often considered a food-security source against famine, requiring minimal care. Under optimal environmental conditions, it compares favorably in production of energy with most other major staple food crops due to its high yield potential. Recent research at the Centro Internacional de Agricultura Tropical (CIAT) in Colombia has demonstrated the ability of cassava to assimilate carbon at very high rates under high levels of humidity, temperature and solar radiation, which correlates with productivity across all environments whether dry or humid. When grown on very poor soils under prolonged drought for more than 6 months, the crop reduce both its leaf canopy and transpiration water loss, but its attached leaves remain photosynthetically active, though at greatly reduced rates. The main physiological mechanism underlying such a remarkable tolerance to drought was rapid stomatal closure under both atmospheric and edaphic water stress, protecting the leaf against dehydration while the plant depletes available soil water slowly during long dry periods. This drought tolerance mechanism leads to high crop water use efficiency values. Although the cassava fine root system is sparse, compared to other crops, it can penetrate below 2 m soil, thus enabling the crop to exploit deep water if available. Leaves of cassava and wildManihotpossess elevated activities of the C₄ enzyme PEP carboxylase but lack the leaf Kranz anatomy typical of C₄ species, pointing to the need for further research on cultivated and wild Manihot to further improve its photosynthetic potential and yield, particularly under stressful environments. Moreover, a wide range in values of K _m (CO₂) for the C₃ photosynthetic enzyme Rubisco was found among cassava cultivars indicating the possibility of selection for higher affinity to CO₂, and consequently higher leaf photosynthesis. Several plant traits that may be of value in crop breeding and improvement have been identified, such as an extensive fine root system, long leaf life, strong root sink and high leaf photosynthesis. Selection of parental materials for tolerance to drought and infertile soils under representative field conditions have resulted in developing improved cultivars that have high yields in favorable environments while producing reasonable and stable yields under stress.     

Climate change shifts natural selection and the adaptive potential of the perennial forb Boechera stricta in the Rocky Mountains

Heritable genetic variation is necessary for populations to evolve in response to anthropogenic climate change. However, antagonistic genetic correlations among traits may constrain the rate of adaptation, even if substantial genetic variation exists. We examine potential genetic responses to selection by comparing multivariate genetic variance–covariances of traits and fitness (multivariate Robertson–Price identities) across different environments in a reciprocal transplant experiment of the forb Boechera stricta in the Rocky Mountains. By transplanting populations into four common gardens arrayed along an elevational gradient, and exposing populations to control and snow removal treatments, we simulated future and current climates and snowmelt regimes. Genetic variation in flowering and germination phenology declined in plants moved downslope to warmer, drier sites, suggesting that these traits may have a limited ability to evolve under future climates. Simulated climate change via snow removal altered the strength of selection on flowering traits, but we found little evidence that genetic correlations among traits are likely to affect the rate of adaptation to climate change. Overall, our results suggest that climate change may alter the evolutionary potential of B. stricta, but reduced expression of genetic variation may be a larger impediment to adaptation than constraints imposed by antagonistic genetic correlations.     

Anti-herbivory defense of two <Emphasis Type="Italic">Vicia</Emphasis> species with and without extrafloral nectaries

The effects of direct and indirect defenses differ among plant species, and the variation in the mode of plant defenses might reflect physiological and/or ecological constraints of each mode of defense related to the growth and reproduction of individual plant species. To evaluate the advantages and disadvantages of indirect ant-mediated defense via extrafloral nectaries (EFNs), we compared the herbivory pressure, leaf chemicals, vegetative growth, and reproduction between two species of vetches, Vicia sativa var. angustifolia (Reichard) Wahlenb (Leguminosae) with EFNs and V. hirsuta (L.) SF Gray without EFNs (or with very small EFNs). Indirect ant defense of V. sativa was not consistently reliable because of the low constancy of ant attraction. In addition, V. sativa was more vulnerable to attack by herbivores than V. hirsuta. The estimated total amount of sugars secreted by EFNs of V. sativa corresponded to 0.5% of total leaf biomass, and 0.07% of total plant biomass, indicating a low investment to the production of extrafloral nectar. Vicia sativa plants grew more rapidly than V. hirsuta plants during the reproductive stage. Therefore, we consider that V. sativa adopts the ant defense via EFNs in spite of its low reliability because the indirect ant defense supported by EFNs requires only low investment, allowing the plants to attain rapid growth in the early spring.