ARS | Publication request: Cotton Plant, Gossypium hirsutum L., defense in response to nitrogen fertilization

Submitted to: Journal of Chemical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 24, 2008
Publication Date: N/A

Interpretive Summary: Some plants exhibit dynamically induced defense responses following insect attack including the accumulation of toxic biochemicals that inhibit insect growth and also volatiles that serve as attractants for natural enemies of the pests. These responses are known to vary under different growth conditions, yet surprisingly the influence of nitrogen fertilization on crop plant defenses remains largely unknown. In collaboration with the researchers at the University of Georgia, scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, have discovered that increased nitrogen fertilization in commercial cultivar of cotton results in decreased production of antifeedants (terpenoid aldehydes) and volatiles after caterpillar attack. Many studies have demonstrated that caterpillars prefer well fertilized high-nitrogen plants that constitute a more nutritious food source. This study provides new mechanistic insight into additional reasons for this preference. Jasmonic acid is an established key hormone regulating plant defense responses. In low nitrogen plants, caterpillar attack strongly induced JA accumulation, subsequent production of terpenoid aldehydes and volatiles; however; similarly attacked high nitrogen plants produced significantly less JA and no increases in either terpenoid aldehydes or volatiles. The glands of cotton leaves contain both large pre-existing pools of volatiles that are released by caterpillar feeding and newly synthesized volatiles more specifically associated with attack. In short range attraction assays, caterpillars feeding on low and high nitrogen plants were equally parasitized by wasp Cotesia marginiventris. While induced plant volatiles are known to be important signals in the long range attraction of parasitoids, we found that at short ranges the differential volatile production caused by fertilization did not help nor impair the ability of parasitoids to locate their caterpillar hosts on cotton.

Technical Abstract: Plants respond to insect herbivory by producing dynamic changes in an array of defense-related volatile and non-volatile secondary metabolites. A scaled response relative to herbivory levels and nutrient availability would be adaptive, particularly under nutrient-limited conditions, in minimizing the costs of expressed defensive pathways and synthesis. In this study we investigated effects of varying nitrogen (N) fertilization (42, 112, 196 and 280 ppm N) on levels of cotton plant (Gossypium hirsutum) phytohormones [jasmonic acid (JA) and salicylic acid (SA)], terpenoid aldehydes (hemigossypolone, heliocides H1, H2, H3, and H4), and volatile production in response to beet armyworm (Spodoptera exigua) herbivory. Additional bioassays assessed parasitoid (Cotesia marginiventris) host-searching behaviors in response to cotton plants grown under various N fertilizer regimes. At low N input (42 ppm N), herbivore damage resulted in significant increases in local leaf tissue concentrations of JA and volatiles, and in systemic accumulation of terpenoid aldehydes. However, increased N fertilization of cotton plants suppressed S. exigua-induced plant hormones and led to reduced production of various terpenoid aldehydes in damaged mature leaves and undamaged young leaves, consistent with predictions of the carbon/nutrient balance (CNB) hypothesis. While increased N fertilization significantly diminished herbivore-induced leaf volatile concentrations, the parasitism of S. exigua larvae by the parasitoid C. marginiventris in field cages did not differ between N treatments. This suggests that the parasitoids were unable to differentiate at short range among the varying VOC titers induced by different N treatments, possibly due to either qualitative or quantitative differences, or a combination of the two.