Abstract:【Objective】The kiwifruit is highly appreciated by consumers because of its delicious taste and high nutritional value. Although the global kiwifruit industry has grown rapidly in recent years, it is still facing the great challenge of kiwifruit bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa). The disease can cause large scale death of kiwifruit because of its fast transmission and strong pathogenicity. It has caused serious yield and economic losses in many countries and has become the major limiting factor in the development of the kiwifruit industry. Utilization of resistant kiwifruit cultivars has always been recognized as the most cost-effective and environment-friendly strategy for disease control, but there is still lack of knowledge about the disease resistance of different cultivars in kiwifruit production.The analysis of the resistance of different kiwifruit germplasm to bacterial canker and the correlation between different evaluation indexes are of great significance to breeding new kiwifruit varieties resistant to the disease.【Methods】The kiwifruit germplasm resources used in this study are the hybrid populations of Actinidia rufa × A. chinensis var. chinensis, all of which are preserved in the National Kiwifruit Resource Nursery, with consistent ploidy and tree age. Psa M228 was provided by the laboratory of Pathogen Biology and Integrated Control of Fruit Tree Diseases Research Team, Northwest A & F University, China. Psa was diluted to 1.0×109 CFU·mL-1 before inoculation. The one-year old detached branch, approximately 0.8 cm in diameter, were sterilized with 75% alcohol and then cut into 12-14 cm, the ends of the shoots were dipped in candle wax to reduce dehydration. A wound of about 3 mm was made and Psa was added to the wound. Subsequently, all of the branches were put on a draining board on which two layers of sterile absorbent paper had previously been placed. The lower tray was filled with sterile water close to the bottom of the draining board, and another two layers of sterile absorbent paper placed over the cane pieces. Germplasm with differences in disease resistance were selected, and leaf tissue structure and stomatal characteristics were observed by paraffin section method and scanning electron microscope technique, and total phenol, soluble sugar and lignin content of leaves were determined to screen out the indicators significantly related to canker disease resistance. After 42 d of incubation, the outer cortex of the branches was peeled off with a sterile knife to observe and measure the lesion; the germplasm resistance was classified according to the length of the lesion: Resistant (R): lesion length ≤7.0 mm; Tolerant (T): 7.0 mm＜lesion length ≤9.0 mm; Susceptible (S): 9.0 mm＜lesion length ≤11.0 mm; High susceptible (HS): lesion length >11.0 mm. The data were analyzed by one-way ANOVA (one-way ANOVA) Duncan's New Compound Extreme Variance method and Pearson correlation analysis using SPSS 21.0 software. 【Results】Significant difference in the level of resistance of different kiwifruit germplasm were found, and there was a significant correlation between the results of two years' branch lesion. 84 germplasm contained 67 disease-resistant germplasm, which accounted for 79.76%, 13 disease-tolerant germplasm, which accounted for 15.48%, 3 susceptible germplasm, which accounted for 3.57%, and 1 highly susceptible germplasm, which accounted for 1.19%. There were significant differences in the thickness of leaves spongy tissue of different kiwifruit germplasm, and the susceptible varieties were generally higher than the resistant varieties, with a maximum stomatal density of 855.2 stomata/mm2 in the same field of view. The distribution of stomatal apparatus length ranged from 16.78 to 7.68 μm.The total phenol content of the most resistant germplasm was highest at 52.53 mg·g-1. The soluble sugar content varied significantly among germplasm, with a minimum of 33.05 mg·g-1 and a maximum of 51.05 mg·g-1. The higher the lignin content, the more resistant to the disease. The thickness of the upper epidermis was greater than that of the lower epidermis in all the germplasm, but it was not related to disease resistance, and the width of the fenestrated tissues and stomata were also not related to branch disease resistance. Leaf spongy tissue thickness, stomatal density, and stomatal length were significantly and negatively correlated with resistance, while total phenolic content, soluble sugar content, and lignin content were significantly and positively correlated with resistance.【Conclusion】In this study, we evaluated the disease resistance of 84 germplasm, which were the hybrid populations of A. rufa and A. chinensis var. chinensis, 67 resistant germplasm were screened out, and found that the six indexes including spongy tissue thickness, stomatal density, stomatal length, lignin content, soluble sugar content, and total phenol content can be used as disease resistance indicators. The result laying a foundation for the study of kiwifruit resistance breeding and resistance mechanism.
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