Plants have to cope with a plethora of biotic stresses such as herbivory and pathogen attacks throughout their life cycle. The biotic stresses typically trigger rapid emissions of volatile products of lipoxygenase pathway (LOX products, various C6 aldehydes, alcohols and derivatives, also called green leaf volatiles) associated with oxidative burst. Further a variety of defense pathways is activated, leading to induction of synthesis and emission of a complex blend of volatiles, often including methyl salicylate, indole, mono-, homo- and sesquiterpenes. The airborne volatiles are involved in systemic responses leading to elicitation of emissions from non-damaged plant parts. For several abiotic stresses, it has been demonstrated that volatile emissions are quantitatively related to the stress dose. The biotic impacts under natural conditions vary in severity from mild to severe, but it is unclear whether volatile emissions also scale with the severity of biotic stresses in a dose-dependent manner. Furthermore, biotic impacts are typically recurrent, but it is poorly understood how direct stress-triggered and systemic emission responses are silenced during periods intervening sequential stress events. Here we review the information on induced emissions elicited in response to biotic attacks, and argue that biotic stress severity vs. emission rate relationships should follow principally the same dose-response relationships as previously demonstrated for several abiotic stresses. Analysis of several case studies investigating the elicitation of emissions in response to chewing herbivores, aphids, rust fungi, powdery mildew and Botrytis, suggests that induced emissions do respond to stress severity in dose-dependent manner. Bi-phasic emission kinetics of several induced volatiles have been demonstrated in these experiments, suggesting that next to immediate stress-triggered emissions, biotic stress elicited emissions typically have a secondary induction response, possib

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Quantitative patterns between plant volatile emissions induced by biotic stresses and the degree of damage.

1 Plantation Forests.- 2 Biology of Plantation Growth.- 3 Growth Rates and Wood Quality.- 4 Choosing the Species and Site.- 5 Establishment.- 6 Nutrient Management.-7 Stand Density and Initial Spacing.- 8 Thinning.- 9 Pruning.- 10 Pests.- 11 Diseases.- 12 Tree Breeding.- 13 Mixed-species Plantations.- 14 Conclusion.- References.

Growing Plantation Forests

Plants are constantly exposed to various stresses, which can degrade their health. The stresses can be alleviated by the application of methyl jasmonate (MeJA), which is a hormone involved in plant signalling. MeJA induces synthesis of defensive compounds and initiates the expression of pathogenesis-related genes involved in systemic acquired resistance and local resistance. Thus, MeJA may be used against pathogens, salt stress, drought stress, low temperature, heavy metal stress and toxicities of other elements. The application of MeJA improves growth, induces the accumulation of active compounds, and affects endogenous hormones levels, and other physiological and biochemical characteristics in stressed plants. Furthermore, MeJA antagonises the adverse effects of osmotic stress by regulating inorganic penetrating ions or organic penetrants to suppress the absorption of toxic ions. MeJA also mitigates oxidative stress by activating antioxidant systems to scavenge reactive oxygen species (ROS) in stressed plants. For these reasons, we reviewed the use of exogenous MeJA in alleviating biotic (pathogens and insects) and abiotic stresses in plants.

The roles of methyl jasmonate to stress in plants.

Living trees are the main source of biogenic volatile organic compounds (BVOCs) in forest ecosystems, but substantial emissions originate from leaf and wood litter, the rhizosphere and from microorganisms. This review focuses on temperate and boreal forest ecosystems and the roles of BVOCs in ecosystem function, from the leaf to the forest canopy and from the forest soil to the atmosphere level. Moreover, emphasis is given to the question of how BVOCs will help forests adapt to environmental stress, particularly biotic stress related to climate change. Trees use their vascular system and emissions of BVOCs in internal communication, but emitted BVOCs have extended the communication to tree population and whole community levels and beyond. Future forestry practices should consider the importance of BVOCs in attraction and repulsion of attacking bark beetles, but also take an advantage of herbivore-induced BVOCs to improve the efficiency of natural enemies of herbivores. BVOCs are extensively involved in ecosystem services provided by forests including the positive effects on human health. BVOCs have a key role in ozone formation but also in ozone quenching. Oxidation products form secondary organic aerosols that disperse sunlight deeper into the forest canopy, and affect cloud formation and ultimately the climate. We also discuss the technical side of reliable BVOC sampling of forest trees for future interdisciplinary studies that should bridge the gaps between the forest sciences, health sciences, chemical ecology, conservation biology, tree physiology and atmospheric science.

https://link.springer.com/content/pdf/10.1007/s10342-019-01213-2.pdf

Unravelling the functions of biogenic volatiles in boreal and temperate forest ecosystems

Phloem sap quality can differ between and within plants, and affect the performance of aphids. In turn, aphid infestation may change the chemical composition and nutritional value of phloem sap. However, the effects of different aphid species on the overall phloem sap composition of distinct parts within plant individuals in relation to aphid performance remain unclear. To test the specificity of plant responses to aphids, we used two chemotypes of Tanacetum vulgare plants and placed the monophagous aphids Macrosiphoniella tanacetaria and Uroleucon tanaceti on different plant parts (stems close to the inflorescence, young and old leaves). Aphid population growth was determined and sugars, organic acids, amino acids and metabolic fingerprints of phloem exudates were analysed. Macrosiphoniella tanacetaria performed best on stems, whereas U. tanaceti performed best on old leaves, indicating differences in niche conformance. Aphid infestation led to distinct changes in the phloem exudate composition of distinct metabolite classes, differing particularly between plant parts but less between chemotypes. In summary, plant responses to aphids are highly specific for the chemotype, plant part, metabolite class and aphid species. These changes may indicate that aphids construct their own niche, optimizing the food quality on the plant parts they prefer.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.15335

Aphid infestation leads to plant part-specific changes in phloem sap chemistry, which may indicate niche construction

Conifer defences can be induced by exogenous chemical elicitors, thereby reducing damage caused by bark-feeding insects. However, the insect behavioural mechanisms involved are poorly understood. Thus, effects of artificially induced plant defences on feeding behaviour of the pine weevil (Hylobius abietis), a serious forest pest, were examined to explore mechanisms involved in decision-making processes connected to feeding. To induce plant defences, we applied methyl jasmonate (MeJA), a naturally occurring plant hormone, to young Norway spruce (Picea abies) plants. The weevils’ feeding behaviour on plants with and without MeJA treatment was studied in both a no-choice and a choice laboratory experiment. MeJA treatment did not affect the initiation of feeding, but it affected the weevils’ subsequent feeding patterns. In the no-choice experiment, the only observed effect of its treatment was that it reduced the size of the initial feeding scars. In the choice experiment, it reduced both the numbers and sizes of the feeding scars and hence the total debarked area. Thus, the MeJA-induced resistance did not deter the pine weevils from attacking the spruce plants, but reduced the amounts they consumed at one place, which would reduce risks of girdling and plant death. This may be the behavioural mechanism behind the previously recorded increases in survival rates of MeJA-treated plants in the field.

Effects of jasmonate-induced resistance in conifer plants on the feeding behaviour of a bark-chewing insect, Hylobius abietis

Induced defenses change the chemical composition of pine seedlings and influence meal properties of the pine weevil Hylobius abietis.

The pine weevil (Hylobius abietis (L.); Coleoptera: Curculionidae) has a high economic impact on forest regeneration in Europe. The general biology of the pine weevil has received considerable attention, but there is insufficient knowledge about its diel behaviour and time budget. Therefore, the feeding and locomotion behaviour of individual adult weevils on Norway spruce (Picea abies (L.) Karst.) seedlings is observed for 24 hour-periods in the laboratory. Both girdled and non-girdled seedlings are used to assess how the weevils’ behaviour is influenced by the plants physiological response to the girdling. The locomotion pattern shows a distinct maximum during the beginning of the dark phase whereas most feeding occurs during the second half of the dark phase and the first hours of the subsequent light phase. The girdling treatment increase the time that weevils spend on the seedlings during the first part of an observation session, but has no effect on their feeding pattern. The weevils’ time budgets on girdled and non-girdled seedlings are similar. On average, they spend 34 % of their time in locomotion and 6 % on feeding. Females spend more time feeding than males (7.1 % vs. 4.2 %), possibly because they have higher food requirements, e.g. for egg production. Females also spend more time in total on the seedlings than males (26.3 % vs. 7.0 %). This study reveals, in high temporal resolution, the diel feeding and locomotion behaviour and time budget of male and female pine weevils.

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Diel behaviour and time budget of the adult pine weevil Hylobius abietis

The pine-tree lappet moth, Dendrolimus pini (L.), belonging to the family La.siocampidae has a wide distribution in Eurasia. In Sweden it is normally found up to 61 degrees N and normally occurs at low densities. In central Europe there have been several reports of outbreaks in historical time. In Sweden the last known outbreak before the one reported here was in 1938-40 in Varmland. In this paper we report a recent severe D. pini outbreak that was discovered in 2012 on the small island Furuskar in the Stockholm archipelago. The distribution of the damage on the island as well as a detailed inventory of larvae and pupae on individual trees is presented. Several photographs are presented to illustrate the damage. In the paper we formulate and try to answer several questions that arose as a natural consequence of this rare event: Why was this particular island hit? What are the causes behind the outbreak? What will happen to the trees and the vegetation on the island? Will the outbreak continue? Will the outbreak spread?

A rare event - an isolated outbreak of the pine-tree lappet moth (Dendrolimus pini) in the Stockholm archipelago

Adult pine weevils (Hylobius abietis (L.)) feed on the tender bark of branches and roots of mature conifer trees and on the stem bark of conifer seedlings. Their feeding on mature trees does not cause any economic damage, but their feeding on planted seedlings is so devastating that the pine weevil is considered one of the most important forest pest insects in Europe. We asked whether the pine weevil prefers seedlings over other regularly utilized food sources. This question is of particular interest because new approaches to seedling protection are based on decreasing any preference for seedlings by using less palatable plants or by enhancing their defence (by genetic selection or by methyl jasmonate treatment). In a laboratory choice experiment we tested pine weevil feeding preferences for seedlings compared with branches and roots from mature trees (separately for Norway spruce and Scots pine). Pine weevils preferred roots, but not branches, of Norway spruce over seedlings of the same species. With Scots pine there were no clear preferences, but the weevils showed a tendency to prefer roots over seedlings. These results provide support for seedling protection approaches that attempt to redirect pine feeding from planted seedlings to other food sources.

Frauke Fedderwitz

Papers

Effects of jasmonate-induced resistance in conifer plants on the feeding behaviour of a bark-chewing insect, Hylobius abietis

Induced defenses change the chemical composition of pine seedlings and influence meal properties of the pine weevil Hylobius abietis.

Diel behaviour and time budget of the adult pine weevil Hylobius abietis

A rare event - an isolated outbreak of the pine-tree lappet moth (Dendrolimus pini) in the Stockholm archipelago

Does the pine weevil (Hylobius abietis) prefer conifer seedlings over other main food sources