Distinct defense strategies allow different grassland species to cope with root herbivore attack

Root-feeding insect herbivores are of substantial evolutionary, ecological and economical importance. Plants can resist insect herbivores through a variety of tolerance and resistance strategies. To date, few studies have systematically assessed the prevalence and importance of these strategies for root-herbivore interactions across different plant species.Here, we characterize the defense strategies used by three different grassland species to cope with a generalist root herbivore, the larvae of the European cockchafer Melolontha melolontha.Our results reveal that the different plant species rely on distinct sets of defense strategies. The spotted knapweed (Centaurea stoebe) resists attack by dissuading the larvae through the release of repellent chemicals. White clover (Trifolium repens) does not repel the herbivore, but reduces feeding, most likely through structural defenses and low nutritional quality. Finally, the common dandelion (Taraxacum officinale) allows M. melolontha to feed abundantly but compensates for tissue loss through induced regrowth.Synthesis: Three co-occurring plant species have evolved different solutions to defend themselves against attack by a generalist root herbivore. The different root defense strategies may reflect distinct defense syndromes.

Root volatiles in plant-plant interactions I: Characterization of root sesquiterpene emissions from Centaurea stoebe and their effects on other plants

Volatile organic compounds (VOCs) emitted by plant leaves can influence the physiology of neighboring plants. In contrast to interactions above ground, little is known about the role of VOCs in belowground plant-plant interactions. Here, we characterize constitutive root volatile emissions of the spotted knapweed (Centaurea stoebe) and explore the impact of these volatiles on the germination and growth of different sympatric plant species. We show that C. stoebe roots emit high amounts of sesquiterpenes, with estimated release rates of (E)-β-caryophyllene above 3 μg g−1 dw h−1. Sesquiterpene emissions show little variation between different C. stoebe populations, but vary substantially between different Centaurea species. Through root transcriptome sequencing, we identify six root-expressed sesquiterpene synthases (TPSs). Two root-specific TPSs, CsTPS4 and CsTPS5, are sufficient to produce the full blend of emitted root sesquiterpenes. Volatile exposure experiments demonstrate that C. stoebe root volatiles have neutral to positive effects on the germination and growth of different sympatric neighbors. Thus, constitutive root sesquiterpenes produced by two C. stoebe TPSs are associated with facilitation of sympatric neighboring plants. The release of root VOCs may thus influence C. stoebe abundance and plant community structure in nature.

The Economic Cost of Noxious Weeds on Montana Grazing Lands

We distributed a 16-question survey concerning noxious weed abundance, impacts, and management to livestock producers grazing on privately owned or leased grazing lands in Montana. The noxious weeds most commonly reported as being present on respondents’ grazing units were Canada thistle [Cirsium arvense (L.) Scop.] (64% of grazing units) and leafy spurge (Euphorbia esula L.) (45% of grazing units), and these species also reportedly caused the greatest reductions in livestock forage. Houndstongue (Cynoglossum officinale L.) was more prevalent than either spotted knapweed (Centaurea stoebe L.) or diffuse knapweed (Centaurea diffusa Lam.) (39% vs. 32% and 10%, respectively, of grazing units), but collectively C. stoebe and C. diffusa were reported to cause greater forage reductions than C. officinale. The top three strategies used to manage noxious weeds were chemical control, grazing, and biological control. Combining survey responses with forage-loss models derived from field data for C. stoebe and E. esula, we estimated the combined cost of noxious weed management and forage losses on privately owned rangeland to be $3.54 ha−1 yr−1, or $7,243 annually for an average size grazing unit (i.e., 2,046 ha [5,055 ac]). Our estimates of economic losses are lower than many estimates from previous studies, possibly because we focused only on direct costs related to private grazing land, while other studies often consider indirect impacts. Nonetheless, our estimates are substantial; for example, our estimated loss equates to 24% of the average per-hectare lease rate for Montana grazing land.

Comparative Phytotoxicity of Aqueous Extracts of Centaurea maculosa and Melilotus officinalis on Germinability and Growth of Wheat

Weed incursion in cultivated fields is a serious biological problem, which causes considerable yield losses of economically important field crops through allelopathy and competitive interactions. Spotted knapweed (Centaurea maculosa) and yellow melilot (Melilotus officinalis) are among the most prevalent weeds in cultivated fields of Pakistan, particularly in wheat fields. Like many other weeds, they may pose allelopathic and competitive challenges to field crops. The objective of this study was to evaluate allelopathic potentials of the two weeds on germination and seedling growth of wheat under laboratory conditions. Amounts of 15, 20, 25 and 30% concentrated leaf aqueous extracts of C. maculosa and M. officinalis were prepared and tested for their effects on germination percentage, radicle and coleoptile lengths of wheat. Germination, radicle and coleoptile lengths of test crop declined significantly at higher concentration (25-30%) of the extracts of two weeds. However, concentration of M. officinalis up to 20% had either no effect or stimulatory effects on the studied growth characteristics of wheat. Highest germination inhibition and decreased seedling growth were caused by leaf extracts of C. maculosa at 30% extract concentration. Growth inhibition was generally dependent on the extract concentration. The study indicated that both plant exhibited allelopathic activity and growth inhibitory effects on wheat at higher concentration; however, extracts of C. maculosa were more phytoinhibitory than M. officinalis. The weeds could serve as potent allelopathic plants for management of other weeds.