Physiological Comparison Between Genotypes With Contrasting Tolerance To Citrus Canker Disease And SA-induced Disease Resistance

Citrus canker disease caused by Xanthomonas axonopodis pv. Citri (Xac) is one of the most devastating biotic stresses confronting citrus industry. However, satisfactory results have not been achieved yet to date. This study focused on dissecting the relationship among the ability of citrus canker resistance and disease defense-related enzymes as well as plant morphology especially the stomata, using canker-resistant and susceptible genotypes Meiwa (Fortunella crassifolia) and Newhall (Citrus sinensis Osbeck) respectively. Moreover, SA-induced citrus canker resistance was investigated. The main results are as follows:1 Relationship between the resistance of citrus canker and leaf structure. Several indexes were compared with the two cultivars and the results revealed that, wax content of Meiwa is significantly higher than Newhall, while stomatal density and area are dramatically smaller than Newhall. As for stomotal morphology, guard cell of Meiwa is invaginated and covered by heavy wax, while Newhall the opposite. Meiwa had disordered arrangement of epidermal keratinocytes, which is identified as a typical characteristic of disease resistance, but Newhall ordered. Our results also suggested that cell structure as well as the ratio of Palisade and spongy tissue have no correlation with resistance of citrus canker.2 Response of stomata to bacteria invasion. Within the first one hour invasion, we observed a marked reduction in the number of open stomata on both genotypes, two hours later, stomata of Meiwa remained closed while Newhall showed a reverting trend.3 Induction of H2O2 generation in stomata by bacteria invasion. Level of H2O2 showed steadily increasing tendency throughout two-hour invasion experiment in stomata of Meiwa, as to Newhall, H2O2 only showed an apparent increase in stomata during the first one hour invasion, then decreased gradually.4 Relationship between the resistance of citrus canker and disease defense-related enzymes. Before inoculation, PPO, PAL, POD activities are significantly higher in Meiwa than in Newhall, while CAT activity showed no difference between two genotypes. After inoculation, PAL, POD, CAT activities increased more profoundly in Meiwa than in Newhall. All these illuminated a significant correlation with the resistance of citrus canker and these enzymes.5 Effect of exogenous salicylic acid on citrus canker disease. Exogenous SA can enhance the resistance of Newhall to citrus canker, which may be as a result of stomatal closure by inducing the generation of H2O2 temporarily. Investigation of the pathogen-related enzymes and genes, we found that SA is capable of inducing the activity of PAL andβ-1,3 glucanase, enhancing the expression of CHI, AOS, GPX, PR4, increasing the level of endogenous SA especially GSA, which implied partial explanations for SA-induced disease resistance.