Interspecific variation of plant traits associated with resistance to herbivory among four species of Ficus (moraceae)

Trees harbouring ants are better defended than con-generic and sympatric ant-free trees

Plant strategies against herbivores are classically divided into chemical, physical, biotic defences. However, little is known about the relative importance of each type of plant defence, especially in the same species. Using the myrmecophyte Triplaris americana (both with and without ants), and the congeneric non-myrmecophyte T. gardneriana, we tested whether ant defence is more effective than other defences of naturally ant-free myrmecophytes and the non-myrmecophyte congeneric species, all spatially co-occurring. In addition, we investigated how plant traits vary among plant groups, and how these traits modulate herbivory. We sampled data on leaf area loss and plant traits from these tree groups in the Brazilian Pantanal floodplain, and found that herbivory is sixfold lower in plants with ants than in ant-free plants, supporting a major role of biotic defences against herbivory. Whereas ant-free plants had more physical defences (sclerophylly and trichomes), they had little effect on herbivory-only sclerophylly modulated herbivory, but with opposite effects depending on ants' presence and species identity. Despite little variation in the chemicals among plant groups, tannin concentrations and δ¹³C signatures negatively affected herbivory in T. americana plants with ants and in T. gardneriana, respectively. We showed that ant defence in myrmecophytic systems is the most effective against herbivory, as the studied plants could not fully compensate the lack of this biotic defence. We highlight the importance of positive insect-plant interactions in limiting herbivory, and therefore potentially plant fitness.

Xanthotoxin: an aphicidal coumarin from Ficus petiolaris against Melanaphis sacchari Zehntner (Hemiptera: Aphididae)

In this research, we evaluated the aphicidal effect of the ethanolic extract of stems and bark of Ficus petiolaris Kunth (Moraceae), in laboratory bioassays in an artificial diet against apterous adult females of Melanaphis sacchari Zehntner (Hemiptera: Aphididae). The extract was evaluated at different concentrations (500,1,000,1,500,2,000,and2,500ppm), and the highest percentage of mortality (82%) was found at 2,500 ppm after 72 h. The positive control imidacloprid (Confial®) at 1% eliminated 100% of the aphids, and the negative control (artificial diet) only presented mortality of 4%. The chemical fractionation of the stem and bark extract of F. petiolaris yielded five fractions of FpR1-5, which were each evaluated at 250, 500, 750, and 1,000 ppm. FpR2 had the strongest aphicidal effect, with 89% mortality at 72 hours at 1,000 ppm. The pure xanthotoxin compound extracted from this fraction was even more effective, with 91% aphid mortality after 72 h at 100 ppm. The lethal concentration (LC₅₀) of xanthotoxin was 58.7 ppm (72h). Our results indicate that the extract of F. petiolaris showed toxic activity against this aphid, and its xanthotoxin compound showed strong aphicidal activity at low concentrations.

Hemiepiphytic figs kill their host trees: acquiring phosphorus is a driving factor

Hemiepiphytic figs killing their host trees is an ecological process unique to the tropics. Yet the benefits and adaptive strategies of their special life history remain poorly understood. We compared leaf phosphorus (P) content data of figs and palms worldwide, and functional traits and substrate P content of hemiepiphytic figs (Ficus tinctoria), their host palm and nonhemiepiphytic conspecifics at different growth stages in a common garden. We found that leaf P content of hemiepiphytic figs and their host palms significantly decreased when they were competing for soil resources, but that of hemiepiphytic figs recovered after host death. P availability in the canopy humus and soil decreased significantly with the growth of hemiepiphytic figs. Functional trait trade-offs of hemiepiphytic figs enabled them to adapt to the P shortage while competing with their hosts. From the common garden to a global scale, the P competition caused by high P demand of figs may be a general phenomenon. Our results suggest that P competition is an important factor causing host death, except for mechanically damaging and shading hosts. Killing hosts benefits hemiepiphytic figs by reducing interspecific P competition and better acquiring P resources in the P-deficient tropics, thereby linking the life history strategy of hemiepiphytic figs to the widespread P shortage in tropical soils.

Changes in the properties of calcium-carbon inclusions during leaf development and their possible relationship with leaf functional maturation in three inclusion-bearing species

In many plant species, carbon-calcium inclusion (calcium oxalate crystals or cystoliths containing calcium carbonate) formation is a fundamental part of their physiology even necessary for normal growth and development. Despite the long-standing studies on carbon-calcium inclusions, the alterations in their properties during leaf development and their possible association with the maturation of the photosynthetic machinery have not been previously examined. In order to acquire more insights into this subject, we examined three of the most common species bearing abundant inclusions of different types, i.e., Amaranthus hybridus, Vitis vinifera, and Parietaria judaica. Results of our study showed that, irrespective of species and type of inclusion, similar patterns in the alterations of their properties are observed during leaf maturation, except for some differences in cell differentiation and distribution between raphides and druses in Vitis vinifera. As expected, inclusion formation has taken place at very early developmental stages and maximum density was observed in very young leaves. Inclusion properties are changing in a coordinated way with leaf area and these modifications are compatible with the concept that each idioblast or lithocyst "services" a finite number/area of adjacent cells. This tight coordination is also evident at the whole leaf level. Moreover, we observed an association of the properties of carbon-calcium inclusions and gas exchange, suggesting a possible implication of these structures in photosynthesis.

Defence mechanisms of Ficus: pyramiding strategies to cope with pests and pathogens

Ficus species have adapted to diverse environments and pests by developing physical or chemical protection strategies. Physical defences are based on the accumulation of minerals such as calcium oxalate crystals, amorphous calcium carbonates and silica that lead to tougher plants. Additional cellular structures such as non-glandular trichomes or laticifer cells make the leaves rougher or sticky upon injury. Ficus have also established structures that are able to produce specialized metabolites (alkaloids, terpenoids, and phenolics) or proteins (proteases, protease inhibitors, oxidases, and chitinases) that are toxic to predators. All these defence mechanisms are distributed throughout the plant and can differ depending on the genotype, the stage of development or the environment. In this review, we present an overview of these strategies and discuss how these complementary mechanisms enable effective and flexible adaptation to numerous hostile environments.

Population Explosions of Tiger Moth Lead to Lepidopterism Mimicking Infectious Fever Outbreaks

Lepidopterism is a disease caused by the urticating scales and toxic fluids of adult moths, butterflies or its caterpillars. The resulting cutaneous eruptions and systemic problems progress to clinical complications sometimes leading to death. High incidence of fever epidemics were associated with massive outbreaks of tiger moth Asota caricae adult populations during monsoon in Kerala, India. A significant number of monsoon related fever characteristic to lepidopterism was erroneously treated as infectious fevers due to lookalike symptoms. To diagnose tiger moth lepidopterism, we conducted immunoblots for tiger moth specific IgE in fever patients' sera. We selected a cohort of patients (n = 155) with hallmark symptoms of infectious fevers but were tested negative to infectious fevers. In these cases, the total IgE was elevated and was detected positive (78.6%) for tiger moth specific IgE allergens. Chemical characterization of caterpillar and adult moth fluids was performed by HPLC and GC-MS analysis and structural identification of moth scales was performed by SEM analysis. The body fluids and chitinous scales were found to be highly toxic and inflammatory in nature. To replicate the disease in experimental model, wistar rats were exposed to live tiger moths in a dose dependant manner and observed similar clinico-pathological complications reported during the fever epidemics. Further, to link larval abundance and fever epidemics we conducted cointegration test for the period 2009 to 2012 and physical presence of the tiger moths were found to be cointegrated with fever epidemics. In conclusion, our experiments demonstrate that inhalation of aerosols containing tiger moth fluids, scales and hairs cause systemic reactions that can be fatal to human. All these evidences points to the possible involvement of tiger moth disease as a major cause to the massive and fatal fever epidemics observed in Kerala.

Ozone-induced responses in Croton floribundus Spreng. (Euphorbiaceae): metabolic cross-talk between volatile organic compounds and calcium oxalate crystal formation

Here, we proposed that volatile organic compounds (VOC), specifically methyl salicylate (MeSA), mediate the formation of calcium oxalate crystals (COC) in the defence against ozone (O3) oxidative damage. We performed experiments using Croton floribundus, a pioneer tree species that is tolerant to O3 and widely distributed in the Brazilian forest. This species constitutively produces COC. We exposed plants to a controlled fumigation experiment and assessed biochemical, physiological, and morphological parameters. O3 induced a significant increase in the concentrations of constitutive oxygenated compounds, MeSA and terpenoids as well as in COC number. Our analysis supported the hypothesis that ozone-induced VOC (mainly MeSA) regulate ROS formation in a way that promotes the opening of calcium channels and the subsequent formation of COC in a fast and stable manner to stop the consequences of the reactive oxygen species in the tissue, indeed immobilising the excess calcium (caused by acute exposition to O3) that can be dangerous to the plant. To test this hypothesis, we performed an independent experiment spraying MeSA over C. floribundus plants and observed an increase in the number of COC, indicating that this compound has a potential to directly induce their formation. Thus, the tolerance of C. floribundus to O3 oxidative stress could be a consequence of a higher capacity for the production of VOC and COC rather than the modulation of antioxidant balance. We also present some insights into constitutive morphological features that may be related to the tolerance that this species exhibits to O3.

Failure under stress: the effect of the exotic herbivore Adelges tsugae on biomechanics of Tsuga canadensis

BACKGROUND AND AIMS

Exotic herbivores that lack a coevolutionary history with their host plants can benefit from poorly adapted host defences, potentially leading to rapid population growth of the herbivore and severe damage to its plant hosts. The hemlock woolly adelgid (Adelges tsugae) is an exotic hemipteran that feeds on the long-lived conifer eastern hemlock (Tsuga canadensis), causing rapid mortality of infested trees. While the mechanism of this mortality is unknown, evidence indicates that A. tsugae feeding causes a hypersensitive response and alters wood anatomy. This study investigated the effect of A. tsugae feeding on biomechanical properties at different spatial scales: needles, twigs and branches.

METHODS

Uninfested and A. tsugae-infested samples were collected from a common garden experiment as well as from naturally infested urban and rural field sites. Tension and flexure mechanical tests were used to quantify biomechanical properties of the different tissues. In tissues that showed a significant effect of herbivory, the potential contributions of lignin and tissue density on the results were quantified.

KEY RESULTS

Adelges tsugae infestation decreased the abscission strength, but not flexibility, of needles. A. tsugae feeding also decreased mechanical strength and flexibility in currently attacked twigs, but this effect disappeared in older, previously attacked branches. Lignin and twig tissue density contributed to differences in mechanical strength but were not affected by insect treatment.

CONCLUSIONS

Decreased strength and flexibility in twigs, along with decreased needle strength, suggest that infested trees experience resource stress. Altered growth patterns and cell wall chemistry probably contribute to these mechanical effects. Consistent site effects emphasize the role of environmental variation in mechanical traits. The mechanical changes measured here may increase susceptibility to abiotic physical stressors in hemlocks colonized by A. tsugae. Thus, the interaction between herbivore and physical stresses is probably accelerating the decline of eastern hemlock, as HWA continues to expand its range.

Interspecific variation in compensatory regrowth to herbivory associated with soil nutrients in three Ficus (Moraceae) saplings

Plant compensatory regrowth is an induced process that enhances plant tolerance to herbivory. Plant behavior against herbivores differs between species and depends on resource availability, thus making general predictions related to plant compensatory regrowth difficult. To understand how soil nutrients determine the degree of compensatory regrowth for different plant species, we selected saplings of three Ficus species and treated with herbivore insects and artificial injury in both glasshouse conditions and in the field at two soil nutrient levels. Compensatory regrowth was calculated by biomass, relative growth rate and photosynthetic characteristics. A similar pattern was found in both the glasshouse and in the field for species F. hispida, where overcompensatory regrowth was triggered only under fertile conditions, and full compensatory regrowth occurred under infertile conditions. For F. auriculata, overcompensatory regrowth was stimulated only under infertile conditions and full compensatory regrowth occurred under fertile conditions. Ficus racemosa displayed full compensatory regrowth in both soil nutrient levels, but without overcompensatory regrowth following any of the treatments. The three Ficus species differed in biomass allocation following herbivore damage and artificial injury. The root/shoot ratio of F. hispida decreased largely following herbivore damage and artificial injury, while the root/shoot ratio for F. auriculata increased against damage treatments. The increase of shoot and root size for F. hispida and F. auriculata, respectively, appeared to be caused by a significant increase in photosynthesis. The results indicated that shifts in biomass allocation and increased photosynthesis are two of the mechanisms underlying compensatory regrowth. Contrasting patterns among the three Ficus species suggest that further theoretical and empirical work is necessary to better understand the complexity of the plant responses to herbivore damage.

Within-plant distribution of induced resistance in apple seedlings: rapid acropetal and delayed basipetal responses

Induction of plant resistance by herbivory is a complex process, which follows a temporal dynamic and varies spatially at the within-plant scale. This study aimed at improving the understanding of the induction process in terms of time scale and within-plant allocation, using apple tree seedlings (Malus × domestica) as plant model. Feeding preferences of a leaf-chewing insect (Spodoptera littoralis) for previously damaged and undamaged plants were assessed for six different time intervals with respect to the herbivore damage treatment and for three leaf positions. In addition, main secondary defense compounds were quantified and linked to herbivore feeding preferences. Significant herbivore preference for undamaged plants (induced resistance) was first observed 3 days after herbivore damage in the most apical leaf. Responses were delayed in the other leaf positions, and induced resistance decreased within 10 days after herbivore damage simultaneously in all tested leaf positions. Chemical analysis revealed higher concentrations of the flavonoid phloridzin in damaged plants as compared to undamaged plants. This indicates that herbivore preference for undamaged apple plants may be linked to phloridzin, which is the main secondary metabolite of apple leaves. The observed time course and distribution of resistance responses within plants contribute to the understanding of induction processes and patterns, and support the optimal defense theory stating young tissue to be prioritized. Moreover, induced resistance responses occurred also basipetally in leaves below the damage site, which suggests that signaling pathways involved in resistance responses are not unidirectional.

In tropical lowland rain forests monocots have tougher leaves than dicots, and include a new kind of tough leaf

BACKGROUND AND AIMS

There has been little previous work on the toughness of the laminae of monocots in tropical lowland rain forest (TLRF) despite the potential importance of greater toughness in inhibiting herbivory by invertebrates. Of 15 monocot families with >100 species in TLRF, eight have notably high densities of fibres in the lamina so that high values for toughness are expected.

METHODS

In north-eastern Australia punch strength was determined with a penetrometer for both immature leaves (approx. 30 % final area on average) and fully expanded, fully toughened leaves. In Singapore and Panama, fracture toughness was determined with an automated scissors apparatus using fully toughened leaves only.

KEY RESULTS

In Australia punch strength was, on average, 7x greater in shade-tolerant monocots than in neighbouring dicots at the immature stage, and 3x greater at the mature stage. In Singapore, shade-tolerant monocots had, on average, 1.3x higher values for fracture toughness than neighbouring dicots. In Panama, both shade-tolerant and gap-demanding monocots were tested; they did not differ in fracture toughness. The monocots had markedly higher values than the dicots whether shade-tolerant or gap-demanding species were considered.

CONCLUSIONS

It is predicted that monocots will be found to experience lower rates of herbivory by invertebrates than dicots. The tough monocot leaves include both stiff leaves containing relatively little water at saturation (e.g. palms), and leaves which lack stiffness, are rich in water at saturation and roll readily during dry weather or even in bright sun around midday (e.g. gingers, heliconias and marants). Monocot leaves also show that it is possible for leaves to be notably tough throughout the expansion phase of development, something never recorded for dicots. The need to broaden the botanist's mental picture of a 'tough leaf' is emphasized.

Systemic induced resistance: a risk-spreading strategy in clonal plant networks?

Clonal plant networks consist of interconnected individuals (ramets) of different sizes and ages. They represent heterogeneous ramet assemblages with marked differences in quality and attractiveness for herbivores. Here, feeding preferences of a generalist herbivore (Spodoptera exigua) for differently-aged ramets of Trifolium repens were studied, and changes in herbivore preference in response to systemic defense induction were investigated. Dual-choice tests were used to assess the preference of herbivores for young versus mature ramets of induced and uninduced plants, respectively. Additionally, leaf traits related to nutrition, biomechanics and chemical defense were measured to explain variation in tissue quality and herbivore preference. Young ramets were heavily damaged in control plants. After systemic defense induction, damage on young ramets was greatly reduced, while damage on mature ramets increased slightly. Defense induction increased leaf strength and thickness, decreased leaf soluble carbohydrates and substantially changed phenolic composition of undamaged ramets connected to attacked individuals. Systemic induced resistance led to a more dispersed feeding pattern among ramets of different ages. It is proposed that inducible defense acts as a risk-spreading strategy in clonal plants by equalizing herbivore preference within the clone, thereby avoiding extended selective feeding on valuable plant tissues.

Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought?

Severe droughts have been associated with regional-scale forest mortality worldwide. Climate change is expected to exacerbate regional mortality events; however, prediction remains difficult because the physiological mechanisms underlying drought survival and mortality are poorly understood. We developed a hydraulically based theory considering carbon balance and insect resistance that allowed development and examination of hypotheses regarding survival and mortality. Multiple mechanisms may cause mortality during drought. A common mechanism for plants with isohydric regulation of water status results from avoidance of drought-induced hydraulic failure via stomatal closure, resulting in carbon starvation and a cascade of downstream effects such as reduced resistance to biotic agents. Mortality by hydraulic failure per se may occur for isohydric seedlings or trees near their maximum height. Although anisohydric plants are relatively drought-tolerant, they are predisposed to hydraulic failure because they operate with narrower hydraulic safety margins during drought. Elevated temperatures should exacerbate carbon starvation and hydraulic failure. Biotic agents may amplify and be amplified by drought-induced plant stress. Wet multidecadal climate oscillations may increase plant susceptibility to drought-induced mortality by stimulating shifts in hydraulic architecture, effectively predisposing plants to water stress. Climate warming and increased frequency of extreme events will probably cause increased regional mortality episodes. Isohydric and anisohydric water potential regulation may partition species between survival and mortality, and, as such, incorporating this hydraulic framework may be effective for modeling plant survival and mortality under future climate conditions.

Medicago truncatula mutants demonstrate the role of plant calcium oxalate crystals as an effective defense against chewing insects

Calcium oxalate is the most abundant insoluble mineral found in plants and its crystals have been reported in more than 200 plant families. In the barrel medic Medicago truncatula Gaertn., these crystals accumulate predominantly in a sheath surrounding secondary veins of leaves. Mutants of M. truncatula with decreased levels of calcium oxalate crystals were used to assess the defensive role of this mineral against insects. Caterpillar larvae of the beet armyworm Spodoptera exigua Hübner show a clear feeding preference for tissue from calcium oxalate-defective (cod) mutant lines cod5 and cod6 in choice test comparisons with wild-type M. truncatula. Compared to their performance on mutant lines, larvae feeding on wild-type plants with abundant calcium oxalate crystals suffer significantly reduced growth and increased mortality. Induction of wound-responsive genes appears to be normal in cod5 and cod6, indicating that these lines are not deficient in induced insect defenses. Electron micrographs of insect mouthparts indicate that the prismatic crystals in M. truncatula leaves act as physical abrasives during feeding. Food utilization measurements show that, after consumption, calcium oxalate also interferes with the conversion of plant material into insect biomass during digestion. In contrast to their detrimental effects on a chewing insect, calcium oxalate crystals do not negatively affect the performance of the pea aphid Acyrthosiphon pisum Harris, a sap-feeding insect with piercing-sucking mouthparts. The results confirm a long-held hypothesis for the defensive function of these crystals and point to the potential value of genes controlling crystal formation and localization in crop plants.