Cotton Plant, Gossypium hirsutum L., Defense in Response to Nitrogen Fertilization

Abstract

Plants respond to insect herbivory by producing dynamic changes in an array of defense-related volatile and nonvolatile 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 H(1), H(2), H(3), and H(4)), and volatile production in response to beet armyworm (Spodoptera exigua) herbivory. Additional bioassays assessed parasitoid (Cotesia marginiventris) host-searching success 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. 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 among N treatments. This suggests that, despite significant N fertilization effects on herbivore-induced plant defenses, at short range, the parasitoids were unable to differentiate between S. exigua larvae feeding on physiologically different cotton plants that share large constitutive volatile pools releasable when damaged by herbivores.

Nitrogen Deficiency Affects Bottom-Up Cascade Without Disrupting Indirect Plant Defense

“…Looking at induced VOCs, higher levels were found also in N deficient, hydroponically cultivated maize, and in soil-grown cotton. In both plants, the accumulation of the phytohormone jasmonic acid (JA), an important factor in the signaling cascade leading to volatile biosynthesis, correlated negatively with N (Schmelz et al 2003;Chen et al 2008a). It has been argued that N starved plants produce greater induced defense responses because N deficiency commonly leads to higher levels of leaf sugars and starch.…”

“…Both factors are intertwined as N availability may affect the synthesis of constitutive and induced defensive secondary compounds. Depending on the metabolites and plant species in question, N availability may lead to changing levels of secondary compounds (e.g., Dudt and Shure 1994;Stout et al 1998;Hemming and Lindroth 1999;Cipollini et al 2002;Lou and Baldwin 2004;Chen et al 2008a). …”

“…Maize seedlings, for example, show decreased emissions of induced plant VOCs when N-P-K fertilization was reduced (Gouinguené and Turlings 2002). Manipulating N availability alone, however, resulted in enhanced levels of induced VOCs in maize (Schmelz et al 2003) and cotton (Chen et al 2008a) but not in Nicotiana attenuata at low levels of N (Lou and Baldwin 2004). Concentrations of the phytohormone jasmonic acid (JA) correlated negatively with N availability and positively with VOC induction, thus, it was suggested that changes in JA provide a mechanism to regulate the magnitude of plant defense responses (Chen et al 2008a).…”

Interactive Influence of Leaf Age, Light Intensity, and Girdling on Green Ash Foliar Chemistry and Emerald Ash Borer Development

“…Regarding defenses, it is widely accepted that plant parts with higher fitness values should be better defended (Optimal Defense theory;McKey 1979). The greater fitness value of young expanding leaves over older leaves has been demonstrated experimentally in some plants (McKey 1979;Strauss et al 2004), as has the elevated accumulation of chemical defenses in these leaves (Ohnmeiss and Baldwin 2000;Chen et al 2008b).…”

“…No significant changes in protease inhibitors and total phenolics were observed following girdling. This is surprising since a variety of plant defensive chemicals are known to be induced after mechanical wounding and insect herbivory inflicted on leaves (Stout et al 1998;Chen et al 2008b). One possible explanation is that plants may respond differently to foliar damage and phloem injury.…”