Contrast Response of Apple and Pear Genotypes to Salicylic Acid under Invasion of Causal Agent of Fire Blight (Erwinia amylovora)

Previous researches demonstrated contrast response of apple and pear to systemic acquired resistance trigger by salicylic acid (SA) against fire blight disease. This research was conducted to understand how SA causes these responses، considering bi-lateral role of SA on catalase activity and (SAR= System acquired resistance activation. First، effect of SA were studied on catalase activity of extracted enzyme as well as in the host tissues of one and seven days pre-treated in vitro shootlets of MM111 (tolerant) and MM106 (moderately-susceptible) apples; and Harrow Sweet (tolerant) and Spadona (moderately -susceptible) pear cultivars. Salicylic acid had similar inhibitory effect on extracted catalase of both species، but increased this activity on pre-treated hosts at different rates in the tissues. Considering similar catalase behavior of two species to SA induction، subsequently، this response was evaluated in low (0. 1 to 0. 001 mgl-1) and high (50 to 50000 mgl-1) SA concentrations in pears and apples، respectively. By doing this the disease susceptibilities were inversed in comparison to the normal SA concentrations (1 to 50 mgl-1)، in both host species. Also، catalase activity comparison in resembled concentrations of total protein extracts showed significantly higher activity in apples. The contrast responses of two hosts seemed to be derived from the balance between catalase inhibition and SAR activation by SA.