Resource Allocation Among Cladodes of Opuntia Robusta From East-central Mexico

Abstract The Optimal Defense Hypothesis (ODH) predicts that younger, more valuable plant organs should be better defended. We tested this hypothesis in Opuntia robusta Wendl. since its sequential, hierarchical and segmented architecture permits the consideration of a possible reallocation of secondary metabolites among cladodes with different age. We performed a field study taking samples of vegetative tissues from cladodes of different orders during eight months, and a field experiment, by covering either apical or basal cladodes with a fabric. We determined the content of 4-hydroxybenzoic acid (4-HBA), chlorogenic acid (CGA), quercetin (QUE) and salicylic acid (SA). 4-HBA and CGA followed the predictions of ODH. QUE followed an inverse tendency. SA did not show any tendency related either with the position of the cladode or the treatment (light vs. shade) however, its concentration was positively correlated with the concentration of 4-HBA. As we detected SA only in a low proportion of cladodes and 4-HBA in all cladodes, we hypothesize the conversion of the latter metabolite to the former one. Contrary to ODH, CGA presented lower concentrations in apical than in basal cladodes when co-occurred with SAL, and QUE was transferred from apical (younger) to basal (older) cladodes. In this study, we found contradictions in the premises and outcomes of both the Resource Availability Hypothesis (RAH) and the ODH, suggesting that a new hypothesis concerning the plant defense against stress factors should be proposed.