Differential effects of type and quantity of leaf damage on growth,reproduction and defence of lima bean (Phaseolus lunatus L.)

Folivores are major plant antagonists in most terrestrial ecosystems. However, the quantitative effects of leaf area loss on multiple interacting plant traits are still little understood. We sought to contribute to filling this lack of understanding by applying different types of leaf area removal (complete leaflets versus leaflet parts) and degrees of leaf damage (0, 33 and 66%) to lima bean (Phaseolus lunatus) plants. We quantified various growth and fitness parameters including above‐ and belowground biomass as well as the production of reproductive structures (fruits, seeds). In addition, we measured plant cyanogenic potential (HCNp; direct chemical defence) and production of extrafloral nectar (EFN; indirect defence). Leaf damage reduced above‐ and belowground biomass production in general, but neither variation in quantity nor type of damage resulted in different biomass. Similarly, the number of fruits and seeds was significantly reduced in all damaged plants without significant differences between treatment groups. Seed mass, however, was affected by both type and quantity of leaf damage. Leaf area loss had no impact on HCNp, whereas production of EFN decreased with increasing damage. While EFN production was quantitatively affected by leaf area removal, the type of damage had no effect. Our study provides a thorough analysis of the quantitative and qualitative effects of defoliation on multiple productivity‐related and defensive plant traits and shows strong differences in plant response depending on trait. Quantifying such plant responses is vital to our understanding of the impact of herbivory on plant fitness and productivity in natural and agricultural ecosystems.     

Volatile-mediated signalling in barley induces metabolic reprogramming and resistance against the biotrophic fungus Blumeria hordei

• Plants have evolved diverse secondary metabolites to counteract biotic stress. Volatile organic compounds (VOCs) are released upon herbivore attack or pathogen infection. Recent studies suggest that VOCs can act as signalling molecules in plant defence and induce resistance in distant organs and neighbouring plants. However, knowledge is lacking on the function of VOCs in biotrophic fungal infection on cereal plants.
• We analysed VOCs emitted by 13 ± 1-day-old barley plants (Hordeum vulgare L.) after mechanical wounding using passive absorbers and TD-GC/MS. We investigated the effect of pure VOC and complex VOC mixtures released from wounded plants on the barley–powdery mildew interaction by pre-exposure in a dynamic headspace connected to a powdery mildew susceptibility assay. Untargeted metabolomics and lipidomics were applied to investigate metabolic changes in sender and receiver barley plants.
• Green leaf volatiles (GLVs) dominated the volatile profile of wounded barley plants, with (Z)-3-hexenyl acetate (Z3HAC) as the most abundant compound. Barley volatiles emitted after mechanical wounding enhanced resistance in receiver plants towards fungal infection. We found volatile-mediated modifications of the plant–pathogen interaction in a concentration-dependent manner. Pre-exposure with physiologically relevant concentrations of Z3HAC resulted in induced resistance, suggesting that this GLV is a key player in barley anti-pathogen defence.
• The complex VOC mixture released from wounded barley and Z3HAC induced e.g. accumulation of chlorophyll, linolenic acid and linolenate-conjugated lipids, as well as defence-related secondary metabolites, such as hordatines in receiving plants. Barley VOCs hence induce a complex physiological response and disease resistance in receiver plants.

     

Genetic variation in circadian regulation of nocturnal stomatal conductance enhances carbon assimilation and growth

Circadian resonance, whereby a plant's endogenous rhythms are tuned to match environmental cues, has been repeatedly shown to be adaptive, although the underlying mechanisms remain elusive. Concomitantly, the adaptive value of nocturnal transpiration in C₃ plants remains unknown because it occurs without carbon assimilation. These seemingly unrelated processes are interconnected because circadian regulation drives temporal patterns in nocturnal stomatal conductance, with maximum values occurring immediately before dawn for many species. We grew individuals of six Eucalyptus camaldulensis genotypes in naturally lit glasshouses and measured sunset, predawn and midday leaf gas exchange and whole‐plant biomass production. We tested whether sunrise anticipation by the circadian clock and subsequent increases in genotype predawn stomatal conductance led to rapid stomatal opening upon illumination, ultimately affecting genotype differences in carbon assimilation and growth. We observed faster stomatal responses to light inputs at sunrise in genotypes with higher predawn stomatal conductance. Moreover, early morning and midday stomatal conductance and carbon assimilation, leaf area and total plant biomass were all positively correlated with predawn stomatal conductance across genotypes. Our results lead to the novel hypothesis that genotypic variation in the circadian‐regulated capacity to anticipate sunrise could be an important factor underlying intraspecific variation in tree growth.     

Ecological significance of light quality in optimizing plant defence

Plants balance the allocation of resources between growth and defence to optimize fitness in a competitive environment. Perception of neighbour‐detection cues, such as a low ratio of red to far‐red (R:FR) radiation, activates a suite of shade‐avoidance responses that include stem elongation and upward leaf movement, whilst simultaneously downregulating defence. This downregulation is hypothesized to benefit the plant either by mediating the growth‐defence balance in favour of growth in high plant densities or, alternatively, by mediating defence of individual leaves such that those most photosynthetically productive are best protected. To test these hypotheses, we used a 3D functional–structural plant model of Brassica nigra that mechanistically simulates the interactions between plant architecture, herbivory, and the light environment. Our results show that plant‐level defence expression is a strong determinant of plant fitness and that leaf‐level defence mediation by R:FR can provide a fitness benefit in high densities. However, optimal plant‐level defence expression does not decrease monotonically with plant density, indicating that R:FR mediation of defence alone is not enough to optimize defence between densities. Therefore, assessing the ecological significance of R:FR‐mediated defence is paramount to better understand the evolution of this physiological linkage and its implications for crop breeding.     

Changing water use and adaptive strategies along rainfall gradients in Mediterranean lupins

• There is growing interest in harnessing the genetic and adaptive diversity of crop wild relatives to improve drought resilience of elite cultivars. Rainfall gradients exert strong selection pressure on both natural and agricultural ecosystems. Understanding plant responses to these facilitates crop improvement.
• Wild and domesticated narrow‐leafed lupin (NLL) collected along Mediterranean terminal drought stress gradients was evaluated under contrasting reproductive phase water supply in controlled field, glasshouse and cabinet studies. Plant phenology, growth and productivity, water use and stress responses were measured over time.
• There is an integrated suite of adaptive changes along rainfall gradients in NLL. Low rainfall ecotypes flower earlier, accumulate lower seed numbers, biomass and leaf area, and have larger root:shoot ratios than high rainfall ecotypes. Water‐use is lower and stress onset slower in low compared to high rainfall ecotypes. Water‐use rates and ecotypic differences in stress response (Ψleaf decline, leaf loss) are lower in NLL than yellow lupin (YL). To mitigate the effects of profligate water use, high rainfall YL ecotypes maintain higher leaf water content over declining leaf water potential than low rainfall ecotypes. There is no evidence for such specific adaptation in NLL.
• The data suggests that appropriate phenology is the key adaptive trait to rainfall gradients in NLL because of the flow‐on effects on biomass production, fitness, transpiration and stress onset, and the lack of physiological adaptations as in YL. Accordingly, it is essential to match phenology with target environment in order to minimize risk and maximize yield potential.

     

Plant volatiles induced by herbivore eggs prime defences and mediate shifts in the reproductive strategy of receiving plants

Plants can detect cues associated with the risk of future herbivory and modify defence phenotypes accordingly; however, our current understanding is limited both with respect to the range of early warning cues to which plants respond and the nature of the responses. Here we report that exposure to volatile emissions from plant tissues infested with herbivore eggs promotes stronger defence responses to subsequent herbivory in two Brassica species. Furthermore, exposure to these volatile cues elicited an apparent shift from growth to reproduction in Brassica nigra, with exposed plants exhibiting increased flower and seed production, but reduced leaf production, relative to unexposed controls. Our results thus document plant defence priming in response to a novel environmental cue, oviposition‐induced plant volatiles, while also showing that plant responses to early warning cues can include changes in both defence and life‐history traits.     

Increased nutrient availability decreases insect resistance in cranberry

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Anthropogenic changes to leaf litter input affect the fitness of a larval amphibian

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Effects of amount and recurrence of leaf herbivory on the induction of direct and indirect defences in wild cotton

• The induction of defences in response to herbivory is a key mechanism of plant resistance. While a number of studies have investigated the time course and magnitude of plant induction in response to a single event of herbivory, few have looked at the effects of recurrent herbivory. Furthermore, studies measuring the effects of the total amount and recurrence of herbivory on both direct and indirect plant defences are lacking. To address this gap, here we asked whether insect leaf herbivory induced changes in the amount and concentration of extrafloral nectar (an indirect defence) and concentration of leaf phenolic compounds (a direct defence) in wild cotton (Gossypium hirsutum).
• We conducted a greenhouse experiment where we tested single event or recurrent herbivory effects on defence induction by applying mechanical leaf damage and caterpillar (Spodoptera frugiperda) regurgitant.
• Single events of 25% and 50% leaf damage did not significantly influence extrafloral nectar production or concentration. Extrafloral nectar traits did, however, increase significantly relative to controls when plants were exposed to recurrent herbivory (two episodes of 25% damage). In contrast, phenolic compounds increased significantly in response to single events of  leaf damage but not to recurrent damage. In addition, we found. that local induction of extrafloral nectar production was stronger than systemic induction, whereas the reverse pattern was observed for phenolics.
• Together, these results reveal seemingly inverse patterns of induction of direct and indirect defences in response to herbivory in wild cotton.

     

Consequences of early chilling stress in two Triticum species: plastic responses and adaptive significance

Phenotypic plasticity of two primitive wheat species (Triticum monococcum L. and Triticum dicoccum S.) was studied in response to early chilling stress. Selection pressure differentials, gradients and plasticity costs on plant morphogenesis, growth and reserve carbohydrate consumption were estimated. Regression analysis was applied to investigate differential developmental changes and patterns between treatments. Four‐day‐old seedlings of T. monococcum and T. dicoccum, differing in plant stature and reserve carbohydrates, were given an early chilling temperature (4 °C for 42 day) and compared with control plants grown at 23 °C. Early chilling stress resulted in a significant increase in leaf mass ratio (LMR) and relative growth rate (RGR), a reduction in flag leaf size, total biomass, specific leaf area (SLA) and reserve carbohydrate storage at flowering, together with advanced onset of flowering. Selection pressure within the early chilling environment favoured early flowering, smaller SLA, higher LMR and lower reserve carbohydrates, suggesting the observed responses were adaptive. Furthermore, a regression of daily cumulative plant biomass derived from a crop growth simulation model (CERES‐Wheat) on crop vegetation period revealed a divergent developmental pattern in early‐chilled plants. Using selection pressure gradient analysis, we found similar responses among these traits, except for SLA and sucrose, indicating that these two traits have indirect effects on fitness. Thus, the total effects of SLA and reserve sucrose on relative fitness seem to be buffered via the rapid growth rate in chilled plants. While lower SLA may reduce early chilling stress effects at an individual leaf level, a higher LMR and use of reserve carbohydrates indicated that compensatory growth of chilled plants during the recovery period relied on the concerted action of altered resource allocation and reserve carbohydrate consumption. However, a significant cost of plasticity was evident only for flowering time, LMR and fructan levels in the early chilling environment. Our results demonstrate that morphological and intrinsic developmental (ontogenetic) patterns in two Triticum species respond to early chilling stress.     

Viable bacterial aerosol particle size distributions in the midsummer atmosphere at an isolated location in the high desert chaparral

The viable bacterial particle size distribution in the atmosphere at the Hanford Nuclear Reservation, Richland, WA during two, 1-week periods in June 1992, was observed at three intervals during the day (morning, midday and evening) and at three heights (2, 4, and 8 m) above ground level. The distributions were significantly different (P=0.01) between the two, 1-week sampling periods and between morning, midday, and evening observations, but not between the three heights. Approximately 30 to 50% fell into the largest particle size category; 7.0µm aerodynamic diameter. All particle size categories were at their minimum bacterial concentration at around noon, with the lowest concentrations in the smaller size categories (<2.1µm aerodynamic diameter). This suggests, that at this high desert location, solar radiation (SR) damage to airborne bacteria is particle size discriminatory. There is a relatively greater effect on the smaller size categories at midday and a relatively lesser effect in the morning and evening.     

Plasticity in seedling morphology,biomass allocation and physiology among ten temperate tree species in response to shade is related to shade tolerance and not leaf habit

• Mechanisms of shade tolerance in tree seedlings, and thus growth in shade, may differ by leaf habit and vary with ontogeny following seed germination. To examine early responses of seedlings to shade in relation to morphological, physiological and biomass allocation traits, we compared seedlings of 10 temperate species, varying in their leaf habit (broadleaved versus needle‐leaved) and observed tolerance to shade, when growing in two contrasting light treatments – open (about 20% of full sunlight) and shade (about 5% of full sunlight).
• We analyzed biomass allocation and its response to shade using allometric relationships. We also measured leaf gas exchange rates and leaf N in the two light treatments.
• Compared to the open treatment, shading significantly increased traits typically associated with high relative growth rate (RGR) – leaf area ratio (LAR), specific leaf area (SLA), and allocation of biomass into leaves, and reduced seedling mass and allocation to roots, and net assimilation rate (NAR). Interestingly, RGR was not affected by light treatment, likely because of morphological and physiological adjustments in shaded plants that offset reductions of in situ net assimilation of carbon in shade. Leaf area‐based rates of light‐saturated leaf gas exchange differed among species groups, but not between light treatments, as leaf N concentration increased in concert with increased SLA in shade.
• We found little evidence to support the hypothesis of a increased plasticity of broadleaved species compared to needle‐leaved conifers in response to shade. However, an expectation of higher plasticity in shade‐intolerant species than in shade‐tolerant ones, and in leaf and plant morphology than in biomass allocation was supported across species of contrasting leaf habit.

     

Timing of growth determines fitness and performance of a galling insect on willow

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Growing larger with domestication: a matter of physiology,morphology or allocation?

• Domestication might affect plant size. We investigated whether herbaceous crops are larger than their wild progenitors, and the traits that influence size variation.
• We grew six crop plants and their wild progenitors under common garden conditions. We measured the aboveground biomass gain by individual plants during the vegetative stage. We then tested whether photosynthesis rate, biomass allocation to leaves, leaf size and specific leaf area (SLA) accounted for variations in whole‐plant photosynthesis, and ultimately in aboveground biomass.
• Despite variations among crops, domestication generally increased the aboveground biomass (average effect +1.38, Cohen's d effect size). Domesticated plants invested less in leaves and more in stems than their wild progenitors. Photosynthesis rates remained similar after domestication. Variations in whole‐plant C gains could not be explained by changes in leaf photosynthesis. Leaves were larger after domestication, which provided the main contribution to increases in leaf area per plant and plant‐level C gain, and ultimately to larger aboveground biomass.
• In general, cultivated plants have become larger since domestication. In our six crops, this occurred despite lower investment in leaves, comparable leaf‐level photosynthesis and similar biomass costs of leaf area (i.e. SLA) than their wild progenitors. Increased leaf size was the main driver of increases in aboveground size. Thus, we suggest that large seeds, which are also typical of crops, might produce individuals with larger organs (i.e. leaves) via cascading effects throughout ontogeny. Larger leaves would then scale into larger whole plants, which might partly explain the increases in size that accompanied domestication.

     

The efficient physiological strategy of a novel tomato genotype to adapt to chronic combined water and heat stress

• Climate change is increasing the frequency of high temperature shocks and water shortages, pointing to the need to develop novel tolerant varieties and to understand the mechanisms employed to withstand combined abiotic stresses.
• Two tomato genotypes, a heat-tolerant Solanum lycopersicum accession (LA3120) and a novel genotype (E42), previously selected as a stable yielding genotype under high temperatures, were exposed to single and combined water and heat stress. Plant functional traits, pollen viability and physiological (leaf gas exchange and chlorophyll a fluorescence emission measurements) and biochemical (antioxidant content and antioxidant enzyme activity) measurements were carried out. A Reduced Representation Sequencing approach allowed exploration of the genetic variability of both genotypes to identify candidate genes that could regulate stress responses.
• Both abiotic stresses had a severe impact on plant growth parameters and on the reproductive phase of development. Growth parameters and leaf gas exchange measurements revealed that the two genotypes used different physiological strategies to overcome individual and combined stresses, with E42 having a more efficient capacity to utilize the limiting water resources. Activation of antioxidant defence mechanisms seemed to be critical for both genotypes to counteract combined abiotic stresses. Candidate genes were identified that could explain the different physiological responses to stress observed in E42 compared with LA3120.
• Results here obtained have shown how new tomato genetic resources can be a valuable source of traits for adaptation to combined abiotic stresses and should be used in breeding programmes to improve stress tolerance in commercial varieties.

     

A fast method to detect the occurrence of nonhomogeneous distribution of stomatal aperture in heterobaric plant leaves

Summary Pressure infiltration of water into a leaf via the stomatal pores can be used to quickly determine whether all stomata are open, or as recently described for several mesophytic and xerophytic species, whether there is a non-homogeneous distribution of stomatal opening (stomatal patchiness) on the leaf surface. Information about this phenomenon is important since the commonly used algorithms for calculation of leaf conductance from water vapor exchange measurements imply homogeneously open stomata, which in the occurrence of stomatal patchiness will lead to erroneous results. Infiltration experiments in a growth chamber with leaves of the Mediterranean evergreen shrub Arbutus unedo, carried out under simulated Mediterranean summer day conditions, where the species typically exhibits a strong midday stomatal closure, revealed a temporary occurrence of stomatal patchiness during the phase of stomatal closure in the late morning and during the stomatal reopening in the afternoon. Leaves were, however, found to be fully (i.e. homogeneously) infiltratable in the morning and in the evening. At midday during maximum stomatal closure, leaves were almost non-infiltratable. During the day, the infiltrated amount of water was found to be linearly correlated with porometer measurements of leaf conductance of the same leaves, carried out with the attached leaves immediately before infiltration.     

Diurnal variation in the temperature response of leaf extension of two bunchgrass species in the field

The effect of temperature on short-term leaf extension rates was studied for two cool-season tussok grasses, Agropyron desertorum and Pseudoroegneria spicata, growing in the field under a variety of water stress and defoliation conditions. Leaf extension rates and air temperatures were monitored every half hour during numerous 12- to 65-h periods in three growing seasons using auxanometers constructed of precision resistors. For both species, a three-phase relationship between leaf extension rate and temperature was observed during diurnal cycles. Leaf extension rate increased linearly with temperature from dawn until midday (phase 1). Leaf extension then increased rapidly, reaching maximum rates in the early evening (approximately 1900h), despite decreasing temperatures during this period (phase 2). Finally, leaf extension rate declined with temperature from evening until dawn (phase 3). This diurnal cycle was described by linear (phase 1) and quadratic (phases 2 and 3 combined) regression models. Although the rate of leaf extension and daily integrals were affected by the water stress and defoliation treatments, the diurnal pattern was consistently observed. Temperature was probably a major factor governing leaf extension rates at night (phase 3), but it appeared unimportant in controlling leaf extension between dawn and midday. The relative importance of physiological and environmental factors controlling leaf extension rate appears to shift during the day in these species under field conditions.