Abstract
Changes in insect herbivore performance under elevated atmosphere carbon dioxide concentrations e[CO2] are often driven by changes in the nutritional and defensive chemistry of their host plants. Studies addressing how the prolific pest cotton bollworm (Helicoverpa armigera) responds to e[CO2] show that performance usually declines, often associated with lower nutritional (e.g. nitrogen (N) concentrations) quality of host plants under e[CO2]. We investigated the impacts of e[CO2] on nutritional quality and anti-herbivore (jasmonate) defensive signalling in lucerne (Medicago sativa) when challenged by H. armigera. While foliar N decreased under e[CO2], other aspects of nutritional quality (soluble protein, amino acids, foliar C:N) were largely unaffected, potentially due to increased root nodulation under e[CO2]. In contrast, e[CO2] greatly reduced jasmonate signalling in M. sativa following H. armigera attack; jasmonic acid concentrations were ca. 56% lower in attacked plants grown under e[CO2]. Concurrent with this, relative growth rates of H. armigera were ca. 66% higher when feeding on e[CO2]-grown plants. In contrast with previous reports, which we meta-analytically summarise, we provide the first evidence that H. armigera performance can increase under e[CO2]. This may occur in plants, such as M. sativa, where e[CO2] has limited impacts on nutritional quality yet reduces jasmonate defence signalling.
Highlights
Changes in insect herbivore performance under elevated atmosphere carbon dioxide concentrations e[CO2] are often driven by changes in the nutritional and defensive chemistry of their host plants
In accordance with there being no change in root mass, but an increase in nodule numbers, there was an increase in nodule density on the roots of plants grown under e [CO2] compared to a [CO2] (58.9 and 41.0 nodules g -1 dry root mass, respectively) (F1,4 = 14.72, P = 0.019)
Both non-essential (Fig. 2B) and essential (Fig. 2C) amino acids were unaffected by C O2 (F1,4 = 0.28, P = 0.624 and F1,4 = 3.41, P = 0.139, respectively), there was a small decline in concentrations of tyrosine, arginine, leucine and phenylalanine under e [CO2] (Fig. 2C and Table S2)
Summary
Changes in insect herbivore performance under elevated atmosphere carbon dioxide concentrations e[CO2] are often driven by changes in the nutritional and defensive chemistry of their host plants. Studies addressing how the prolific pest cotton bollworm (Helicoverpa armigera) responds to e [CO2] show that performance usually declines, often associated with lower nutritional (e.g. nitrogen (N) concentrations) quality of host plants under e[CO2]. In contrast with previous reports, which we meta-analytically summarise, we provide the first evidence that H. armigera performance can increase under e [CO2] This may occur in plants, such as M. sativa, where e [CO2] has limited impacts on nutritional quality yet reduces jasmonate defence signalling. A growing number of studies, reviewed by Zavala, et al.[5] and Ode, et al.[6], suggest that e[CO2] downregulates constitutive and herbivore-induced activity of the JA signalling pathway Some pests, such as the cotton bollworm (Helicoverpa armigera Hübner), have reached critical pest status because of their economic damage and rapid ability to invade new regions 18. Helicoverpa armigera attacks several legumes, including lucerne (alfalfa) (Medicago sativa L.), but to our knowledge no studies have yet addressed how e [CO2] affects H. armigera when feeding on this important forage legume
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