Cost of Defense in the Context of Plant Competition: Brassica rapa May Grow and Defend

Abstract

Theory on costs of plant defense against herbivory in stressful environments predicts that costs should increase when competition is intense. This amplifies a fundamental dilemma that plants are thought to face: allocate limited resources to grow fast enough to compete, or invest these resources in secondary metabolites to maintain defense. We studied costs associated with genetic and environmental variation in secondary metabolite production of Brassica rapa in the presence and absence of the generalist competitor Lolium perenne. We used experimental quantitative genetics (artificial selection) to manipulate genetic variation, and herbivore-induction treatments to produce environmental variation in myrosinase and glucosinolate concentrations and resistance. Glucosinolates, and their byproducts after breakdown by myrosinase, are known to affect herbivory on plants in the Brassicaceae family. Defense costs were significant in the absence of competitors, but in contrast to theoretical predictions, costs of constitutive defense (measured as growth rates) were not detectable and the cost of induced defense remained the same in the competitive environment. To understand what factors made constitutive defense costs not detectable under competition we conducted several experiments to assess the effects of limited resources and allelopathy on costs and benefits of the defense chemicals. None of the experiments involving nutrient supply and weak competition supported the hypothesis that the lack of defense costs in competitive environments was due to limited resources. Instead, the breakdown products of the glucosinolate-myrosinase reaction appeared to function as allelopathic agents, which may benefit B. rapa plants in competition, thereby reducing net costs of chemical defense. We found that: (1) the effects of exogenous glucosinolates on Lolium root length depended on the presence of myrosinase. (2) In the absence of nutrients, Lolium root lengths were shorter when seeds germinated with B. rapa. (3) Genetic increases in glucosinolate concentration negatively affected Lolium seedling growth only when there were simultaneous genetic increases in myrosinase concentration. Activated carbon treatments designed to neutralize allelopathic effects and restore costs in the competitive environments were, however, not statistically significant. When plant defenses also function to benefit plants in competitive interactions, plants may evolve to compete and defend.

Invasive exotic plants suffer less herbivory than non-invasive exotic plants

“…Examples of phytochemicals with multiple activities include cnicin in spotted knapweed (Kelsey & Locken 1987;Landau et al 1994), glucosinolates in the Brassicaceae (Siemens et al 2002) and monoterpenes in Thymus vulgaris (Linhart & Thompson 1999). If potent novel phytochemistry commonly manifests itself through multiple biological activities, leaf herbivory could be a useful proxy for allelopathy or antimicrobial activity in predicting invasiveness because it is simple to measure.…”

Changes in growth, oxidative metabolism and essential oil composition of lemon balm (Melissa officinalis L.) subjected to salt stress

“…It suggests that the production of isoprenoids (monoterpenes and sesquiterpenes) occurs even under stress conditions, when photosynthesis is almost or completely inhibited or when carbon is unavailable (Monson and Fall, 1989;Brili et al, 2007). This suggests that under stress conditions isoprenoids have benefited plants in adverse environments (Siemens et al, 2002). These compounds can activate responses to oxidative stress since they are lipophilic and can physically stabilize hydrophobic interactions in cell membranes, minimizing lipid peroxidation, reducing oxidative stress and ROS accumulation (Vickers et al, 2009).…”

Survey of glucosinolate variation in leaves of Brassica rapa crops

“…includes a great array of crops commonly used in human diet, particularly in Asian cuisine, such as Chinese cabbage, pak-choi, among many other leafy vegetables, as well as turnip and rapeseed. Limited evaluations of gsl have been performed in different tissues of a few B. rapa crops with the objective of determining their potential biological activity (Siemens et al 2002;Cipollini et al 2003;Kang et al 2006;Bellosta et al 2007). Padilla et al (2007) determined the gsl diversity of B. rapa varieties restricted to turnip greens from northwestern Spain.…”