Cotton aphids (Aphis gossypii Glover) are important pests of cucurbit crops. Plant defenses at different cell layers, including the leaf surface, mesophyll cells, and phloem, are employed to defend aphids. Here, we assessed differences in aphid resistance among six watermelon varieties and elucidated the defense mechanisms underlying aphid-resistant/susceptible watermelon varieties. The population abundance, offspring number per female, and meantime of phloem-feeding (E2 phase) of aphids were the highest on XiNong (XN), followed by JingXin (JX), TianWang (TW), ZaoJia (ZJ), and MeiFuLai (MFL), and these parameters were the lowest on JinMeiDu (JMD). Further analyses showed that there was no correlation between the aphid resistance of six watermelon varieties and defenses at the leaf surface. For defenses at mesophyll cells, aphid infestation increased salicylic acid (SA) content at 48 h post-infestation (hpi) and ROS accumulation at 6 and 12 hpi in six watermelon varieties. For phloem defenses, aphid infestation increased callose content in JMD plants but decreased callose content in JX, TW, ZJ, MFL, and XN plants at 6, 12, 24, and 48 hpi. Moreover, callose deposition suppressed by 2-deoxy-d-glucose (2-DDG) neutralized the resistance of JMD plants and exacerbated the susceptibility of XN plants to aphids, which exhibited a higher population abundance and E2 phase time. Collectively, phloem defenses regulated by aphid-induced callose deposition were responsible for differences in aphid resistance among the watermelon varieties.

棉蚜（Aphis gossypii Glover）是葫芦科作物的重要害虫。植物不同细胞层的防御，包括叶表面、叶肉细胞和韧皮部，用于防御蚜虫。在这里，我们评估了六个西瓜品种之间的抗蚜性差异，并阐明了抗蚜/易感西瓜品种的防御机制。蚜虫的种群丰度、雌性后代数和韧皮部取食时间（E2期）在西农（XN）上最高，其次是京新（JX）、天王（TW）、枣甲（ZJ）和美芙来（ MFL），这些参数在金美度（JMD）上最低。进一步分析表明，6个西瓜品种的蚜虫抗性与叶面防御能力之间不存在相关性。对于叶肉细胞的防御，蚜虫侵染增加了六个西瓜品种在侵染后 48 小时 (hpi) 时的水杨酸 (SA) 含量以及在 6 和 12 hpi 时的 ROS 积累。对于韧皮部防御，蚜虫侵染增加了 JMD 植物中的胼胝质含量，但降低了 JX、TW、ZJ、MFL 和 XN 植物中 6、12、24 和 48 hpi 的胼胝质含量。此外，2-脱氧-d-葡萄糖（2-DDG）抑制的胼胝质沉积中和了JMD植物的抗性，并加剧了XN植物对蚜虫的敏感性，表现出较高的群体丰度和E2期时间。总的来说，由蚜虫诱导的胼胝质沉积调节的韧皮部防御是西瓜品种间抗蚜性差异的原因。