International Journal of Modern Botany 2013, 3(2A): 1-4 DOI: 10.5923/s.ijmb.201310.01 Defense and Tolerance: Is the Distinction between these Two Plant Strategies Useful? Kirk A. Stowe Interdisciplinary Studies, Beacon College, Leesburg, FL, 34748 USA Abstract Resistance to herbivores has typically been divided into two categories: defense and tolerance. However, defense and tolerance may not be distinct traits. In this opinion paper, I will argue that the distinction between these two categories of resistance may inhibit our understanding of how these traits might evolve. That is, they may evolve in conjunction and those traits that confer defense may also confer tolerance to herbivores. A number of examples are given to support this argument Keywords Resistance, Defense, Tolerance Plants possess a multitude of traits that allow them to contend with consumers. These traits include leaf physical and chemical characteristics, phenology, and/or resource storage[1, 2, 3, 4, 5, 6, 7, 8, 9]. Further, these traits appear to be adaptations to those consumers[3]. Originally, these traits were grouped under the umbrella term, resistance[10, 11, 12]. This term included both defense and tolerance as subsets[10, 11, 12, 13, 14]. While defense has more recently been used as the inclusive term, with resistance and tolerance as subcategories[15, 16, 17, 18], in this paper I will follow the former view, i.e., I will interpret defense and tolerance as types of resistance[14]. Defense traits are those that decrease the probability of damage to the plant[5, 10, 11]. In contrast, tolerance traits do not decrease damage, instead they decrease the fitness impact of the incurred damage and in some instances may increase fitness[10, 11, 12, 13, 14, 18, 19]. While defense and tolerance should both evolve in response to selection imposed by consumer damage, the evolution of either of these groups of traits may depend on the phenotypic expression of the other[14, 20, 21, 22]. That is, there will be correlated selection[23] between defensive and tolerance traits. Since defended individuals receive low levels of damage, they should undergo weaker selection for tolerance. In contrast, tolerant individuals would not undergo selection for increased defense[21]. Such patterns of selection should result in a negative genetic correlation, or a trade-off between defense and tolerance traits[13, 14, 20, 22, 24]. However, a trade-off between tolerance and defense traits may also be directly due to the pattern of resource allocation within a plant[13, 25]; but see reference[26]. * Corresponding author: kstowe@beaconcollege.edu (Kirk A. Stowe) Published online at http://journal.sapub.org/ijmb Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved Thus, defense and tolerance have often been considered different anti-herbivore strategies[9, 14, 17, 19, 20, 27, 28, 29], but I will argue that this may not always be the case. While in theory, these two groups of traits can be considered alternative, distinguishable strategies for coping with consumer damage[9, 13, 16, 17, 19, 20, 29, 30, 31], in practice, defense and tolerance traits may not always be distinguishable. As discussed above, their phenotypic expression and evolution may not be independent[9, 14, 21, 29]. Thus, it can be argued that the distinction between ‘defense’ and ‘tolerance’ may be an artificial construct and biologically unimportant. In this commentary, I will illustrate several cases where defense may actually contribute to tolerance, which may limit our ability to distinguish between these two strategies. Thus, examining tolerance without considering defense may potentially lead to inaccurate interpretations of how plant resistance against consumers evolves[9, 13, 14, 25, 29]. Tolerance models have often assumed that stored resources are packaged in a manner that is inaccessible to consumers, i.e., stored resources that escape damage and as such are defended which can then be reallocated to growth and/or reproduction following damage (for rev. see[14]). Reallocation of 'defended' resources then allows individuals to tolerate consumer damage. Thus, characteristics that appear to confer tolerance may actually be contingent on defensive traits which allow plants to store resources in a manner such that they escape damage (i.e., defended). Some examples of this are demonstrated in scarlet gilia, Ipomopsis aggregata[6, 32, 33, 34, 35, 36, 37], the common milkweed, Asclepias syriaca[38], goldenrod Solidago altissima[39], and Arabidopsis thaliana[40]. In Ipomopsis, individuals withhold differentiation of meristems, i.e., in a defended form, into reproductive branches until the apical meristem has been removed by consumers. As a result