Effect of physical characteristics of leaf on resistance of cucumber to aphis

With laboratory experiments and field trials, the survival rate, reproduction rate and population index of Aphis gossypii in 13 cucumber varieties were investigated. The aim of the study was to determine the role of cucumber leaf in resistance to aphid, the correlation between the physical characteristics of cucumber (e.g., hair density, vascular bundle depth and surface wax content) and the population index of aphids. The results showed that the survival rate and fecundity of aphid nymph in different cucumber varieties were significantly different (P < 0.05). The aphid nymph survival rate was lowest (40.16%) in resistant variety 'Axin' and highest (55.86%) in susceptible variety 'pepino'. Significant differences were observed in fecundity rate among different cucumber varieties. Farrowing number of single aphid in susceptible varieties exceeded 60 while it was less than 48.25 in resistant variety 'EP6392'. Physical leaf characteristics (e.g., hair density, vascular bundle depth and surface wax content) were significantly different among different cucumber varieties. Leaf hair density of high-susceptible variety 'Sichuancunjin' (372.63 number·cm^{-2) was 1.96 times that of high-resistant variety 'Axin' (190.50 number·cm-2). Leaf vascular bundle depth of high-resistant variety 'Axin' (0.50 mm) was significantly deeper than those of high-susceptible variety 'Xiuyan' (0.38 mm) and 'pepino' (0.40 mm). Also leaf surface wax content in high-resistant variety 'Axin' (4.56 mg·g-1) was 3.80 times that of susceptible variety 'Yihuang' (1.19 mg·g-1). The correlation among hair density, vascular bundle depth and surface wax content of cucumber leaf and aphid population index was also analyzed in the study. A significant positive correlation existed between leaf hair density and population index of A. gossypii. This correlation was strongly expressed in the regression equation of Y = 8.312 1x + 10.700 0, with r = 0.819 8 and P = 0.000 6. Also significant negative correlations were noted between aphid population index and cucumber leaf vascular bundle depth and surface wax content. The correlations were clearly expressed by the regression equations of Y = 0.004 2x + 0.557 1 with r = 0.682 9 and P = 0.010 1 for leaf vascular bundle depth and aphid population index, and then Y = 0.181 0x + 8.286 8 with r = 0.752 7, P = 0.003 0 for surface wax content and aphid population index. The multi-step regression equation of population index (Y) and hair density (X₁), vascular bundle depth (X₂) and surface wax content (X₃) was Y = 38.624 6 + 0.043 1X₁ - 39.165 3X₂ - 1.044 3X₃ with r2 = 0.830 7. The respective path coefficients of the equation were →Y = 0.414 2, →Y = 0.166 0, →Y = 0.206 7. All the resultssuggested that vascular bundle depth was the most critical factor influencing the resistance of cucumber to aphid.}