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We determined changes in cell-wall peroxidase activities and isoform patterns in response to wounding in seedlings of Prosopis tamarugo Phil. (an endemic species of the Atacama Desert) and Prosopis chilensis (Mol.) Stuntz (a native species of central Chile), to assess tolerance to predation. In seedlings of both species, the maximal increase in peroxidase activity occurred 48 h after wounding, reaching three times the control value in P. tamarugo and twice the control value in P. chilensis. The activity of ionically bound cell-wall peroxidases increased only locally in wounded embryonic axes, whereas the activity of soluble peroxidases increased systemically in unwounded cotyledons. Analysis of ionic peroxidases by isoelectrofocusing revealed two groups of peroxidases in the cell walls of both species: four distinct acidic isoforms and a group of basic isoforms. In response to wounding, there was a large increase in activity of the acidic isoforms in P tamarugo, whereas there was an increase in the activity of the basic isoforms in R chilensis. In P chilensis, the wound-induced increase in activity of the basic isoforms corresponded with one of the two isoforms detected in P tamarugo prior to wounding. Experiments with protein and RNA synthesis inhibitors indicated that a preexisting basic peroxidase is activated in P chilensis after wounding. Assays of ionically bound peroxidase activity with four different substrates corroborated the differences found in isoform patterns between species. In P tamarugo, the largest increases in activity were found with ortho-phenylenediamine and ferulic acid as substrates, whereas in R chilensis the largest increase in activity was found with guaiacol as substrate. Because the same basic cell-wall peroxidase that accumulated after wounding in R chilensis was present in P tamarugo prior to wounding, and the activity of acidic cell-wall peroxidases increased after wounding in P tamarugo but not in P chilensis, we conclude that R tamarugo is more tolerant to wound stress than R chilensis.