Analysis of correlation between leaf structure and resistance to leaf spot in Eriobotrya

Abstract:【Objective】Loquat (Eriobotrya japonica Lindl.), originated in China, is an evergreen tree species belonging to Maloideae (Rosaceae). It is one of the earliest fruits available in the market. Leaf spot disease is a serious infectious disease in loquat The disease would lead to premature decline of tree vigor and seriously reduce the yield of loquat. The resistance to it will be extremely valuable in loquat production. The resistance is believed as a quantitative trait with complex inheritance, and the resistance of the hybrid offspring tends to be maternal inheritance. The study aimed to explore the relationship between the leaf structure and the resistance to leaf spot and to clarify the mechanism of disease resistance in loquat.【Methods】A total of 52 loquat germplasm accessions were phenotyped in the field and inoculated (indoor) for two consecutive years, including 15 wild Eriobotrya species and 37 culti-vars. Field evaluations followed the grading standards of “Description Specifications and Data Stan-dards for Loquat Germplasm Resources”. The incidences of leaf spot disease in 52 loquat germplasm accessions were investigated by visual inspection and Photoshop software for two consecutive years. The symptoms of loquat leaf spot disease were identified by comparing with the“Diagnosis and Treatment Atlas of Loquat Diseases and Insects”. Each plant was investigated in five directions, including east, south, west, north, and middle, and 10 leaves were investigated from top to bottom. Disease resistance was evaluated by disease index. The indoor inoculation was carried out by acupuncture, referring to the method described by Wan Jiang. Finally, a total of 11 loquat materials were selected, including three highly resistant accessions (E. serrate, E. fragrans, and Baili), four resistant accessions (E. fulvicoma Chun & Liao, E. deflexa f. koshunensis, Mogi, and Shiromogi), two susceptible accessions (E. kwangsiensis and Dahongpao), and two highly susceptible accessions (E. cavaleriei and Guangrongben).The mature leaves of 11 loquat germplasm accessions with different resistance performance, such as E. fragrans, E. serrate, E. deflexa f.koshunensis, E. fulvicoma Chun & Liao, E. kwangsiensis, E. cavaleriei, Baili, Mogi, Shiromogi, Dahongpao, and Guangrongben were used as experimental materials. The stomata and microanatomical structure of the leaves were observed by the method of epidermal tearing and conventional paraffin sectioning, and the measurements were accomplished by Image J software. The stomatal density, stomatal length, stomatal width, guard cell length, guard cell width, leaf thickness, palisade tissue thickness, spongy tissue thickness, upper epidermal thickness, lower epidermal thickness, upper stratum corneum thickness, lower stratum corneum thickness, spongy ratio (SR) and cell tense ratio (CTR) were recorded. The variance analysis and correlation analysis were conducted on the leaf structure and resistance to leaf spot disease for these accessions.【Results】There were sig-nificant differences in stomatal density among 11 accessions with different resistance. The correlation coefficient was 0.870. The stomatal density of E. cavaleriei was the highest (887.47 per mm2 ), and that of E. fragrans was the lowest (249.33 per mm2 ). The order of stomatal density from high to low was E. cavaleriei > Dahongpao > Guangrongben > E. kwangsiensis > Shiromogi > Mogi > E. fulvicoma Chun & Liao > E. deflexa f. koshunensis > Baili > E. serrate > E. fragrans. The stomatal length, stomatal width, guard cell length, and guard cell width were not significantly correlated with disease resistance, and there were no significant differences in these indexes among the different accessions. The thickness of leaf, palisade tissue, spongy tissue, upper epidermis and lower epidermis, upper stratum corneum and lower stratum corneum were different among 11 accessions. The leaf thickness and spongy tissue thickness were closely related to the disease resistance. The order of the leaf thickness is arranged as Guangrongben > E. cavaleriei > Dahongpao > E. kwangsiensis > Shiromogi > E. deflexa f. koshunensis > E. fulvicoma Chun & Liao > Mogi > Baili > E. serrate > E. fragrans. There was a significant and positive correlation between the leaf thickness and the disease index (r=0.964). There was also a significant and positive correlation between the spongy tissue thickness and the disease resistance (r= 0.683). Among 9 anatomical structure indexes, the upper stratum corneum thickness, lower stratum corneous thickness, and cell tense ratio (CTR) had negative correlations with the disease resistance (p<0.05).【Conclusion】 The leaf thickness and stomatal density and sponge tissue thickness were significantly and positively correlated with the disease resistance (r=0.964 0, r=0.870 0, r=0.683 0, respectively). The leaf thickness, stomatal density and spongy tissue thickness could be used as the preliminary identification index-es for the resistance to loquat leaf spot disease. Key words: Eriobotrya; Leaf spot; Resistance; Microstructure