Relationships between the nutrient contents and activities of defense enzymes in pear leaves and its resistance to powdery mildew

【Objective】Pear is one of the main fruits in the world and is the third largest fruit after apple and citrus in China, the top player in both output and area in the world. However, the pear producing areas in China are often affected by many diseases. Among them, powdery mildew caused by Phyllactinia pyri is one of the most common and serious diseases in pear producing areas in northern China. This disease mainly damages the leaves and causes early leaf falling, resulting in the reduction of the fruit quality, and also decreases production and thus serious economic losses. This study explored the relationship between nutrient contents and activities of defense enzymes and resistance to powdery mildew.【Methods】A total of eight varieties, including New Hongxing, Yuluxiang, Yuanhuang, Ganli No.3, Jin Twentieth Century, Longyuanyang red pear, Zaosu and Hanhong were collected from Yuzhong experimental station in Lanzhou city, Gansu province. The orchard management was extensive, and powdery mildew was common over the years. The incidence of powdery mildew caused by P. pyri was investigated in different varieties each with three trees. Two new shoots in each in five different directions of the cano-py were investigated, and 10-15 leaves from top to bottom in each new shoot were observed and the number of diseased leaves in each shoot was recorded. The damage level in pear leaves was assessed using a scale of 0, 1, 3, 5, 7 and 9, corresponding to a percentage of damaged area in pear leaves of 0, 0%- 5%, 5.1%-25%, 25.1%-50%, 50.1%-75%, and 75.1%-100%, respectively. The resistance of pear varieties to powdery mildew was classified based on the disease index (DI) as follows: Immunity (I, DI = 0); High Resistance (HR, DI ≤ 5); Resistance (R, 5＜DI ≤ 25); Susceptibility (S, 25＜DI ≤ 50); High Susceptibility (HS, DI＞50). The resistance level of eight pear varieties was determined by the field observation for consecutive 3 years, in addition to determination of nutrient content and defense enzyme activity in pear leaves. The correlations between nutrient content and defense enzyme activity and resistance to powdery mildew in pear leaves were analyzed.【Results】Different pear varieties showed different resistance levels to powdery mildew. Yuluxiang, Yuanhuang, Ganli No.3 and new Hongxing had the highest resistance to powdery mildew, while Zaosu had the lowest resistance, followed by Hanhong. The difference in sensory resistance between the varieties was obvious. The contents of soluble proteins, soluble sugars and chlorophylls varied in leaves of different pear varieties were different, and but seemed to have no relation to disease resistance. The highest chlorophyll content was found in New Hongxing at 2.34 mg·g-1 and the lowest was in Yuanhuang at 1.66 mg·g-1 . The highest contents of soluble sugar and soluble protein were in Hanhong (43.63 mg · g- 1 ) and Yuluxiang (6.42 mg · g- 1 ), and the lowest in Jin Twentieth Century, at 33.75 mg·g-1 and 4.84 mg·g-1 , respectively. The contents of proline and MDA varied greatly among different varieties, but were not related to resistance to powdery mildew. The highest contents of proline and MDA were found in Longyuanyang red pear and Yuluxiang, 6.49 mg · 100 g- 1 and 288.97 nmol · g- 1 , respectively, and the lowest in Zaosu and new Hongxing, 3.59 mg · 100 g- 1 and 133.63 nmol·g-1 , respectively. The activities of various defense enzymes varied significantly among varieties with significant differences in powdery mildew resistance. The highest activities of CAT and POD were found in Zaosu (74.88 U · g- 1 and 27 017.34 U · g- 1 , respectively) and the lowest were Ganli No.3 and Jin twentieth century, 39.13 U·g-1 and 5 025.26 U·g-1 . New Hongxing had the highest SOD activity (1 093.98 U · g- 1 ), and Hanhong had the lowest (561.08 U · g- 1 ). The highest PPO and PAL were found in Hanhong (195.77 U· g-1 ) and Jin twentieth centuries (204.69 U· g-1 ). SOD activity was significantly negatively associated with the disease index of powdery mildew, with a correlation coefficient of -0.755. Pear varieties with high SOD activity demonstrated high resistance to powdery mildew. The activities of CAT, POD, PPO and PAL varied significantly, but there were no significant correlations with powdery mildew resistances.【Conclusion】Different pear varieties showed significant differences in powdery mildew resistance. Yuluxiang, Yuanhuang, Ganli No. 3 and New Hongxing had higher resistance to powdery mildew, while Zaosu and Hanhong were more sensitive. SOD activity is highly correlated to resistance, suggesting it can potentially be used as a main physiological index for powdery mildew resistance identification.