The Resistance Comparison Of Six Watermelon Cultivators Against Aphis Gossypii Glover

Melon aphids（Aphis gossypii Glove）are one of the most threatening pests to watermelon plants.Because of the extensive and frequent use of pesticides,melon aphids have been reported to develop high resistances to common pesticides and environmental pollution.In recent years,breeding aphid-resistant watermelon varieties has been selected as one of the most active and effective methods for the less use of pesticides.This study analyzed the resistance or susceptibility of six watermelon varieties based on aphid performance,including the population abundance,offspring per female,and mortality rate of melon aphids,and determine plant defenses at different cell layers,including plant defenses at the plant surface（wax content,trichome density,and stomata density）,mesophyll tissue（ROS,JA,SA contents）,and phloem（callose contents）,in the aphid resistant/susceptible watermelon plants.The results were shown as follows:1.Differences in the melon aphid resistance of six watermelon varietiesFor population abundance,the total number of aphids was the highest on Xinong8（XN）plants（934.25）and was the lowest on Jinmeidu（JMD）plants（225）.The number of offspring per aphid was also significantly higher on XN plants（109.53）than that on JMD plants（56.40）.The mortality rate of aphids was no significant difference among six watermelon varieties.According to the aphid ratio,JMD was identified as a medium resistant variety with the aphid ratio of 0.39,XN was identified as high susceptible variety with the aphid ratio of 1.63,and Badaojipinjingxin（JX）,Zhonghuatianwang（TW）,Zaojia8424（ZJ）,Meifulai（MFL）were all identified as susceptible watermelon varieties with the aphid ratio of 0.92,0.91,1.15,and 0.99,respectively.2.Differences in plant defenses at plant surface among six watermelon varietiesThe wax content and leaf trichome density had no significant difference among six watermelon varieties.The stomatal density was 71.10/mm²in XN plants,which was significantly higher than52.46/mm²in JMD plants.Further correlation analyses showed that there were no significant correlation relationship between the aphid ratio and leaf surface physical parameters（wax content,trichome density,and stomata density）,respectively.3.Differences in plant defenses at mesophyll tissue among six watermelon varietiesCompared to non-infestation plants,aphid infestation significantly increased the ROS content at 6hour post-infestation（hpi）and 12 hpi,but had no significant effects on ROS content at 24 hpi and 48hpi in all six watermelon varieties.The foliar SA content was significantly increased by aphid infestation at 48 hpi,but had no differences between aphid infestation and non-infestation plants at 6,12,24 hpi in all six watermelon varieties.For foliar JA content,there were no significant differences between aphid infestation and non-infestation plants of all six watermelon varieties.4.Differences in plant defense at phloem among six watermelon varietiesThe aphid infestation had different effects on the callose content in six watermelon varieties.Aphids infestation significantly increased the callose content in JMD plants with the highest callose content at 6 hpi（16532.03 g/g FW）,but significantly decreased the callose content in JX,TW,ZJ,MFL,XN plants.Furthermore,compared to aphid non-infestation plants,the callose synthase activity was also increased by aphid infestation in JMD plants,and was decreased by aphid infestation in JX,TW,ZJ,MFL,XN plants at four different time points.Differ to callose synthase,theβ-1,3-glucanase activity was not affected by aphids infestation in six watermelon varieties at 6,12,24 hpi,but it was significantly increased by aphid infestation at 48 hpi.To confirm the effects of callose deposition on the aphid performance in different watermelon varieties,we performed the aphid performance assays in a moderate resistant watermelon variety（JMD）and a high susceptible watermelon variety（XN）with H₂O and callose synthesis inhibitor2-deoxy-d-glucose（2-DDG）treatments.For both JMD and XN plants,2-DDG treatment significantly decreased the callose content and callose synthase activity at 6 hpi,but had no obvious effects on theβ-1,3-glucanase activity.Furthermore,compared to JMD plants with H₂O treatment,the total number of aphids and the number of offspring per female were significantly increased by 55%and by 34%in JMD plants with 2-DDG treatment.Similarly,the number of aphids and the number of offspring per female were 1.34-fold higher and 1.40-fold higher in XN plants with 2-DDG treatment than those in XN plants with H₂O treatment.As a result,2-DDG neutralized JMD resistance and exacerbated XN susceptibility to aphids.