Drought and Root Herbivory Interact to Alter the Response of Above-Ground Parasitoids to Aphid Infested Plants and Associated Plant Volatile Signals

Multitrophic interactions are likely to be altered by climate change but there is little empirical evidence relating the responses of herbivores and parasitoids to abiotic factors. Here we investigated the effects of drought on an above/below-ground system comprising a generalist and a specialist aphid species (foliar herbivores), their parasitoids, and a dipteran species (root herbivore).We tested the hypotheses that: (1) high levels of drought stress and below-ground herbivory interact to reduce the performance of parasitoids developing in aphids; (2) drought stress and root herbivory change the profile of volatile organic chemicals (VOCs) emitted by the host plant; (3) parasitoids avoid ovipositing in aphids feeding on plants under drought stress and root herbivory. We examined the effect of drought, with and without root herbivory, on the olfactory response of parasitoids (preference), plant volatile emissions, parasitism success (performance), and the effect of drought on root herbivory. Under drought, percentage parasitism of aphids was reduced by about 40–55% compared with well watered plants. There was a significant interaction between drought and root herbivory on the efficacy of the two parasitoid species, drought stress partially reversing the negative effect of root herbivory on percent parasitism. In the absence of drought, root herbivory significantly reduced the performance (e.g. fecundity) of both parasitoid species developing in foliar herbivores. Plant emissions of VOCs were reduced by drought and root herbivores, and in olfactometer experiments parasitoids preferred the odour from well-watered plants compared with other treatments. The present work demonstrates that drought stress can change the outcome of interactions between herbivores feeding above- and below-ground and their parasitoids, mediated by changes in the chemical signals from plants to parasitoids. This provides a new insight into how the structure of terrestrial communities may be affected by drought.     

Differential Accumulation of Volatile Organic Compounds by Leaves and Roots of Two Guianese Philodendron Species,P. fragrantissimum Kunth and P. melinonii Brongn.

Leaf and root essential oils of two closely related but ecologically distant Philodendron species were extracted in natural conditions in French Guiana and analysed by GC/MS to i) describe the blends of volatile organic compounds (VOCs) produced by those species and ii) analyse species and environment‐based variations in extracts composition. A total of 135 VOCs were detected with a majority of aliphatic sesquiterpenes. P. fragrantissimum produced mainly β‐bisabolene (on average 29.12% of the extract) as well as α‐ and β‐selinene (14.52% and 17.50%, respectively) while in P. melinonii, four aliphatic sesquiterpenes could alternatively be the main component: (E)‐β‐farnesene (up to 91.42% of the extract), germacrene‐D (73.74%), β‐caryophyllene (51.63%) and trans‐α‐bergamotene (41.26%). A significant effect of species and organs on extracts composition was observed while the environment (sun exposure) only affected the relative proportions of monoterpenes and sesquiterpenes in roots of P. melinonii. These results are discussed in the light of the potential role of leaf and root terpenes in Philodendron species.     

Abscisic Acid Elicits the Water-Stress Response in Root Hairs of Arabidopsis thaliana

Water stress has been shown to cause root hairs to become short and bulbous. Because abscisic acid (ABA) mediates a variety of water-stress responses, we investigated the response of Arabidopsis thaliana root hairs to ABA. When wild-type root hairs were treated with ABA, they exhibited the water-stress response. The Arabidopsis mutants abi1 and abi2, which are insensitive to ABA at the seedling stage, did not display the root hair response. These data suggest that ABA may mediate the response of root hairs to water stress. The drought response of root hairs resulting in an inhibition of tip growth will provide an easy screen to select mutations that are insensitive to ABA and/or involved in tip growth.     

AM fungi root colonization increases the production of essential isoprenoids vs. nonessential isoprenoids especially under drought stress conditions or after jasmonic acid application

Previous studies have shown that root colonization by arbuscular mycorrhiza (AM) fungi enhances plant resistance to abiotic and biotic stressors and finally plant growth. However, little is known about the effect of AM on isoprenoid foliar and root content. In this study we tested whether the AM symbiosis affects carbon resource allocation to different classes of isoprenoids such as the volatile nonessential isoprenoids (monoterpenes and sesquiterpenes) and the non-volatile essential isoprenoids (abscisic acid, chlorophylls and carotenoids). By subjecting the plants to stressors such as drought and to exogenous application of JA, we wanted to test their interaction with AM symbiosis in conditions where isoprenoids usually play a role in resistance to stress and in plant defence. Root colonization by AM fungi favoured the leaf production of essential isoprenoids rather than nonessential ones, especially under drought stress conditions or after JA application. The increased carbon demand brought on by AM fungi might thus influence not only the amount of carbon allocated to isoprenoids, but also the carbon partitioning between the different classes of isoprenoids, thus explaining the not previously shown decrease of root volatile isoprenoids in AM plants. We propose that since AM fungi are a nutrient source for the plant, other carbon sinks normally necessary to increase nutrient uptake can be avoided and therefore the plant can devote more resources to synthesize essential isoprenoids for plant growth.     

Drought effects on volatile organic compound emissions from Scots pine stems

Tree stems have been identified as sources of volatile organic compounds (VOCs) that play important roles in tree defence and atmospheric chemistry. Yet, we lack understanding on the magnitude and environmental drivers of stem VOC emissions in various forest ecosystems. Due to the increasing importance of extreme drought, we studied drought effects on the VOC emissions from mature Scots pine (Pinus sylvestris L.) stems. We measured monoterpenes, acetone, acetaldehyde and methanol emissions with custom-made stem chambers, online PTR-MS and adsorbent sampling in a drought-prone forest over the hot-dry summer of 2018 and compared the emission rates and dynamics between trees in naturally dry conditions and under long-term irrigation (drought release). The pine stems were significant monoterpene sources. The stem monoterpene emissions potentially originated from resin, based on their similar monoterpene spectra. The emission dynamics of all VOCs followed temperature at a daily scale, but monoterpene and acetaldehyde emission rates decreased nonlinearly with drought over the summer. Despite the dry conditions, large peaks of monoterpene, acetaldehyde and acetone emissions occurred in late summer potentially due to abiotic or biotic stressors. Our results highlight the potential importance of stem emissions in the ecosystem VOC budget, encouraging further studies in diverse environments.     

Hylobius abietis L. feeding on the novel host Pinus brutia Ten. increases emission of volatile organic compounds

Plants respond to feeding by herbivorous insects by producing volatile organic chemicals, which mediate interactions between herbivores and plants. Yet, few studies investigated whether such plant responses to herbivory differ between historical host and novel plants. Here, we investigated whether herbivory by the pine weevil Hylobius abietis causes a release of volatile organic chemicals from a novel tree Pinus brutia and compared the relative amounts of volatiles released from herbivore's historical hosts and P. brutia. We collected volatiles emitted from P. brutia seedlings that were either subjected to feeding by H. abietis or no feeding. Our results indicated that feeding increased emission of volatile compounds, composed of monoterpenes and sesquiterpenes, and that the emission was several fold higher in the damaged seedlings than in undamaged seedlings. In particular, emission of monoterpenes and sesquiterpenes increased by 4.4‐and 10‐fold in the damaged plants, respectively. Strikingly, individual monoterpenes and sesquiterpenes showed much greater dissimilarity between damaged and undamaged seedlings. Furthermore, several minor monoterpenes showed negative relationships with the weevil gnawed area. We discussed these results with the results of previous studies focused on historical host plants of H. abietis and hypothesized the ecological relevance and importance of our results pertaining relevance to the plant–herbivory interactions.     

Studies on Ectomycorrhizae of Pine II. Growth Inhibition of Mycorrhizal Fungi by Volatile Organic Constituents of Pinus silvestris (Scots Pine) Roots

The effects of several monoterpenes and the sesquiterpene longifolene on the vegetative growth of two mycorrhizal fungi, Boletus variegatus and Rhizopogon roseolus, were studied in petri plate experiments on an agar medium. The test substances are natural constituents of Pinus silvestris (Scots pine) roots. Vapors from 20 μl of all the test substances, upon a 5 day exposure inhibited the growth of both the mycorrhizal fungi 55% to 86%. The two fungi, however, differed in their tolerance to individual compounds. It is suggested that the terpenes and the sesquiterpenes play an important role in the development of the symbiotic condition in ectomycorrhizae by restricting the growth of the mycorrhizal fungi within the host root.     

Long-term effects of exogenous methyl jasmonate application on Scots pine (Pinus sylvestris) needle chemical defence and diprionid sawfly performance

Scots pine [ Pinus sylvestris L. (Pinaceae)] trees with four different seed origins were exposed to exogenous applications of the elicitor, methyl jasmonate (MeJA), for three consecutive years. We studied the effects of MeJA on needle chemistry (including monoterpenes, sesquiterpenes, and tricyclic resin acids), plant growth, and the performance of two diprionid sawflies, the European pine sawfly ( Neodiprion sertifer Geoffr.) and the common pine sawfly ( Diprion pini L.) (both Hymenoptera: Diprionidae). In general, foliar MeJA application affected the whole range of needle secondary chemistry with significantly higher concentrations of two monoterpenes, β-pinene and limonene, in particular. Furthermore, for some seed origins the growth rates of N. sertifer and D. pini larvae were lower on needles of MeJA-treated plants with either high total terpene or high resin acid concentrations. However, inconsistencies in diprionid sawfly performance within each studied Scots pine origin suggest genetic variance in needle secondary chemistry. The differences between selected seed origins and notably variable responses to MeJA application imply that adaptation of the seed to new conditions may have had an impact on secondary chemistry and, thus, on insect performance. Finally, our results suggest that modification of Scots pine defence by a low-concentration exogenous elicitor affects the production of terpenoids in the newly growing needles, leading to poorer pine sawfly performance in origins with high terpenoid content, while not harming the growth of Scots pine trees.     

环境因子对植物释放挥发性化合物的影响

对近年来有关环境因子与植物释放挥发性化合物关系的研究进展进行了综合和概括。本文主要包括3类挥发性化合物。⑴异戊二烯是由叶绿体产生并且直接释放到大气中的C5化合物。⑵单萜类化合物是一类环状或非环状的C10化合物,它在植物体内合成后首先贮存于体内的特殊结构中（如树脂道、油腺）,然后由此通过气孔向大气中释放。⑶含氧挥发性化合物以各种形式释放大大气中。它包括醇、醛、酮、酯和有机酸。本文的重点是前两者,主要阐述了二方面内容：⑴植物军发性化合物的生物合成和释放机理。⑵环境因子（如温度、光照、水分胁迫、营养、CO2浓度、空气湿度）及植物的发育阶段、机械损伤和昆虫取食等对植物挥发性化合物合成与释放的影响机制。     

Wood borer performance and wood characteristics of drought‐stressed Scots pine seedlings

Four‐year‐old Scots pine [Pinus sylvestris L. (Pinaceae)] seedlings were exposed to medium and severe drought stress for two consecutive years. The anatomical properties of drought‐stressed Scots pine wood and their impact on the performance of destructive wood boring early instars of Hylotrupes bajulus L. (Coleoptera: Cerambycidae) were studied. Drought stress significantly decreased diameter of earlywood tracheids in both growing years and diameter of latewood tracheids after the second growing season only. Cell lumen area was significantly decreased by both medium and severe drought stress compared to well‐watered controls. In addition, area of cell lumen was significantly smaller in severe drought than in medium drought treatment. The drought stress marginally increased the number of resin canals in the wood, but did not affect the size of resin canals either in wood or bark. The relative growth rate of xylophagous H. bajulus neonatal larvae was not significantly affected by drought stress during the 106‐day feeding period on Scots pine wood blocks. The results show that although water availability was an important factor affecting the development and anatomy of wood cells, observed changes in wood characteristics did not affect the performance of early instars feeding on wood processed from drought‐stressed young Scots pine seedlings.     

迷迭香对干旱胁迫的生理响应及其诱导挥发性有机化合物的释放

为探讨干旱胁迫对迷迭香(Rosmarinus officinalis)生理生化特性及挥发性有机化合物(VOC)释放规律的影响, 该文采用盆栽称重控水法研究了轻度(LD)、中度(MD)和重度(SD)干旱胁迫对迷迭香二年生实生苗叶片细胞膜透性、可溶性糖、可溶性蛋白质和丙二醛(MDA)含量以及脂氧合酶和抗氧化保护酶活性的影响, 并采用热脱附/气相色谱/质谱联用技术对不同干旱胁迫下迷迭香释放的挥发性有机化合物成分进行了分析。结果表明: 干旱胁迫对迷迭香叶片可溶性糖和可溶性蛋白质含量有明显的影响, MD和SD处理12天时其含量极显著地增加(p < 0.01), 与对照相比可溶性糖分别增加了51.5%和87.4%, 可溶性蛋白质含量分别增加了0.82和1.40倍。在MD和SD胁迫下, 超氧化物歧化酶、过氧化物酶和过氧化氢酶对干旱胁迫的响应存在一定差异, 表现为相互协调的作用。随着干旱胁迫时间的延长, 迷迭香体内MDA含量极显著地增加(p < 0.01), 细胞膜损伤率显著增加。分析显示, 迷迭香释放的VOC主要是萜烯类化合物, 占总量的46.0%以上; 随着干旱胁迫增强, 迷迭香释放的VOCs总量减少, 种类增多; LD、MD和SD胁迫处理萜烯类化合物相对含量与对照相比分别增加了14.4%、17.0%和23.7%; 干旱胁迫还明显诱导绿叶挥发物(green leaf volatiles)和醛类化合物的释放, 诱导产生了2-己烯醛、叶醇、山梨醛和癸醛4种新组分。研究表明: 干旱胁迫条件下, 迷迭香能够通过调节保护酶活性、渗透调节物质含量和释放VOCs来提高抗旱性。     

Physiological response of tomatoes at drought,heat and their combination followed by recovery

In nature, crops encounter a combination of abiotic stresses that severely limit yield. Our aim was to dynamically expose the changes of tomatoes' physiological parameters to drought, heat and their combination and thereby clarify the relationship between the responses to single and combined stress. We studied the effect of single and combined drought and heat stresses on the shoot and root of two tomato cultivars (Sufen No.14 as CV1; Jinlingmeiyu as CV2). After being exposed to combined stress for 6 days, the dry weight of shoot and root significantly decreased. The F_q′/F_m′ (quantum yield of photosystem II) was significantly lower in CV1 upon drought and combined stress and in CV2 subjected to combined stress (between days 4 and 6) compared to control. The relative water content during combined stress was significantly lower than control from day 4 to recovery day 2. On days 3 and 6, the water loss rate significantly increased under heat stress and decreased at drought and combined stress, respectively. The combined stress caused severe damages on photosystem II and chloroplast ultrastructure. The root activity after stress recovered even though drought significantly increased the activity from day 2 to day 6. Combined stress result in complex responses during tomato growth. The CV1 was more heat tolerant than CV2, but there was no varietal difference at drought and combined stress. This study contributes to the understanding of the underlying physiological response mechanism of plant to combined stress and crop improvement by providing valuable information for abiotic stress‐tolerant tomato breeding.     

The inhibition of polyamine biosynthesis weakens the drought tolerance in white clover (<Emphasis Type="Italic">Trifolium repens</Emphasis>) associated with the alteration of extensive proteins

Changes of endogenous polyamine (PA) levels could be a key adaptive response to drought in plants. White clover pretreated with or without dicyclohexylamine (DCHA), an inhibitor of PA biosynthesis, was subjected to drought stress induced by 18% polyethylene glycol 6000 for 8 days in controlled growth chambers. Results showed that drought stress significantly increased endogenous PA content, whereas DCHA significantly decreased PA accumulation under drought stress. The attenuate PA biosynthesis was unfavorable for plant growth and drought tolerance, as reflected by significantly lower relative water content, relative growth rate, instantaneous water use efficiency, and cell membrane stability in leaves in response to drought. On the basis of proteomic analysis, the inhibition of PA synthesis decreased the accumulation of many key differentially expressed proteins including (1) ribosomal structure and biogenesis: elongation factor, ribosomal protein S10E, and 30S ribosomal protein; (2) amino acid transport and metabolism: cysteine synthase, delta-1-pyrroline-5-carboxylate synthetase, and glutamate decarboxylase; (3) carbohydrate metabolism and energy production: photosystem apoprotein, sucrose-phosphate synthase, phosphogluconate dehydrogenase, sucrose-phosphatase, NADH oxidoreductase, and ATP synthase; (4) antioxidant metabolism: catalase, peroxidase I, ascorbate peroxidase, and glutathione S-transferase; and (5) other biological processes: heat shock protein 70, heat shock protein 90, and calcium-dependent protein kinase associated with the decreased drought tolerance in white clover. These findings indicate that PAs play a critical role in the regulation of growth, ribosome, amino acid and energy metabolism, and antioxidant reactions in white clover under drought stress. Drought-induced increases in endogenous PAs could be one of key adaptive responses against drought stress in white clover.     

干旱复水对柑橘幼苗叶片光合、叶绿素荧光和根系构型的影响

干旱严重影响柑橘的生长和发育.为探索柑橘对干旱胁迫的响应机制,本试验以抗旱性不同的三湖红橘和三湖化红为材料,通过盆栽控水进行干旱胁迫和复水处理,研究处理后植株叶片光合、叶绿素荧光和根系构型的变化.结果表明: 干旱显著降低了两种柑橘幼苗的净光合作用速率、气孔导度、蒸腾速率和胞间CO₂浓度,而三湖红橘的下降幅度更小;复水后,光合参数均有所恢复,但仍低于对照.三湖红橘水分利用率在干旱15 d后开始显著高于对照,而三湖化红除干旱15 d外的其他处理时间均低于对照.干旱提高了两种幼苗的PSII最大光合效率,但抑制了三湖化红的PSII实际光合效率.干旱到一定程度后,两种幼苗的PSII电子传递速率和光化学淬灭均下降,干旱和复水后非光化学淬灭在三湖红橘中下降,但在三湖化红中上升.根系构型分析表明,干旱导致两种幼苗的根表面积和根体积下降,同时抑制了三湖化红的总根长,但能够提高三湖红橘的总根长和总根尖数.进一步分析不同直径的侧根长度发现,三湖红橘的一类侧根长度在干旱胁迫10 d后开始增加,而三湖化红的一类侧根长度在干旱前期没有变化,干旱20 d时显著下降;除三湖红橘的三类侧根外,两种幼苗其余直径等级侧根的生长均受干旱抑制.除总根尖数外,复水后根系生长各参数均没有恢复.干旱对三湖红橘光合性能的影响小于三湖化红,并且前者能够维持更高的水分和光能利用率.干旱后三湖红橘根尖数和细根长度增加,可能有助于提高其对水分的吸收能力.     

HS-SPME-GC/MS analysis of the volatile compounds of <Emphasis Type="Italic">Achillea collina</Emphasis>: Evaluation of the emissions fingerprint induced by <Emphasis Type="Italic">Myzus persicae</Emphasis> infestation

A Headspace Solid-phase Microextraction (HS-SPME) method combined with Gas Chromatography-Mass Spectrometry (GC/MS) was developed and optimized to extrat and analyze the volatile compounds of aerial parts of Achillea collina Becker ex Rchb. and to investigate the effect of the phlem feeding aphid Myzus persicae Sulzer on the Volatile Organic Compounds (VOCs) emitted by the infested plants. The extraction of 1 g of powdered freeze dried plant samples for 120 min at 30°C using divinylbenzene-carbowax-polydimethylsiloxane (DVB/CAR/PDMS) fiber showed the highest area counts for the majority of the volatile compounds. Overall, 62 and 80 volatile compounds were detected in control and infested plant samples respectively. In A. collina infested plants, we observed a great increase in both monoterpenes and sesquiterpenes fractions. Several changes among alcohols also occurred, particularly regarding Z-3-hexen-1-ol, E-3-hexen-1-ol and E-2-hexen-1-ol proposing these compounds as herbivore-induces plant volatiles (HIPVs). New perspective for agricultural practice may derive from the opportunity to identify novel herbivores-induced plant VOCs active as plant protection agents.     

Fine root production and carbohydrate concentrations of mature longleaf pine (Pinus palustris P. Mill.) as affected by season of prescribed fire and drought

The historical range of longleaf pine (Pinus palustris P. Mill) has been greatly reduced, in part, by lack of fire. Recently, the application of fire has become an accepted practice for the restoration of longleaf pine to former parts of its natural range. This study was designed to evaluate the effects of season of prescribed fire on the root growth and nonstructural carbohydrate concentrations of longleaf pine, and identify the time of year when fire has the least negative effect on longleaf pine root processes. We found that root growth was generally less on July-burned plots than on either March- or May-burned plots and we attribute these responses to the effect of fire on interaction between root processes and the soil environment. Specifically, soil water and temperature conditions may have been less favorable for root growth on the July-burned plots compared to the March- and May-burned plots. With two years of information on the seasonal dynamics of foliage production, root growth, and root carbohydrates, we determined that at our study site, optimal prescribed fire would impact tree growth less in November through March compared to other times of the year. We also observed that severe drought during the 1998 growing season was associated with a 3-month delay in peak root growth, and prolonged drought in 1999 through 2000 coincided with a reduction in root starch storage. We conclude that season of prescribed fire potentially affects root processes, but that severe or prolonged drought may either interact with or override these effects.     

Ectomycorrhizal responses to organic and inorganic nitrogen sources when associating with two host species

While it is established that increasing atmospheric inorganic nitrogen (N) deposition reduces ectomycorrhizal fungal biomass and shifts the relative abundances of fungal species, little is known about effects of organic N deposition. The effects of organic and inorganic N deposition on ectomycorrhizal fungi may differ because responses to inorganic N deposition may reflect C-limitation. To compare the effects of organic and inorganic N additions on ectomycorrhizal fungi, and to assess whether host species may influence the response of ectomycorrhizal fungi to N additions, we conducted an N addition experiment at a field site in the New Jersey pine barrens. Seedlings of two host species, Quercus velutina (black oak) and Pinus rigida (pitch pine), were planted at the base of randomly-selected mature pitch pine trees. Nitrogen was added as glutamic acid, ammonium, or nitrate at a rate equivalent to 227.5 kg ha⁻¹ y⁻¹ for eight weeks, to achieve a total application of 35 kg ha⁻¹ during the 10-week study period. Organic and inorganic N additions differed in their effects on total ectomycorrhizal root tip abundance across hosts, and these effects differed for individual morphotypes between oak and pine seedlings. Mycorrhizal root tip abundance across hosts was 90 % higher on seedlings receiving organic N compared to seedlings in the control treatment, while abundances were similar among seedlings receiving the inorganic N treatments and seedlings in the control. On oak, 33–83 % of the most-common morphotypes exhibited increased root tip abundances in response to the three forms of N, relative to the control. On pine, 33–66 % of the most-common morphotypes exhibited decreased root tip abundance in response to inorganic N, while responses to organic N were mixed. Plant chemistry and regression analyses suggested that, on oak seedlings, mycorrhizal colonization increased in response to N limitation. In contrast, pine root and shoot N and C contents did not vary in response to any form of N added, and mycorrhizal root tip abundance was not associated with seedling N or C status, indicating that pine received sufficient N. These results suggest that in situ organic and inorganic N additions differentially affect ectomycorrhizal root tip abundance and that ectomycorrhizal fungal responses to N addition may be mediated by host tree species.