Investigations of the physiological and morphological changes that occur in plants in response to herbivore damage have identified several potential mechanisms of tolerance. However, it is unlikely that all physiological changes that occur following damage are tolerance mechanisms. Tiffin reviewed some of the empirical data related to several potential tolerance mechanisms, including compensatory photosynthesis, compensatory growth, activation of dormant meristems, utilization of stored reserves, and plant architecture. Tiffin notes that at this time it is difficult to determine the relative importance of these mechanisms give that few empirical studies have investigated the relationship between the expression of any one of these mechanisms and the expression of tolerance, let alone examined the relationship between the expression of multiple mechanisms and the expression of tolerance. Tiffin concludes that recognizing that identification of tolerance mechanisms requires investigating the relationship between the expression of a trait and the expression of tolerance will help to increase understanding of tolerance mechanisms. In addition, several empirical approaches, including artificial selection, phenotypic manipulations, and QTL analysis have been used either not at all or only to a limited extent in the study of tolerance.
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