| Pear is one of the most important fruit species in China, and it has pivotal position in fruit production in our country. However, pear production often suffers serious losses from black spot disease caused by Alternaria kikuchiana Tanaka. To reduce the incidence of black spot disease, lots of chemical pesticides are used. Although the common chemical pesticides have positive effects on the control of the disease, it would cause environmental pollution and pesticide residues for a long time. Seeking for new methods to control pear black spot disease is an important topic for researchers and pear growers. In this paper, the young leaves of Pyrus bretschneider Kehd cv. Yali and Pyrus pyrifolia cv. Hwangkumbae were collected as the trial materials to research the effect of induced resistance by salicylic acid treated. At the same time, the physiological and biochemical characteristics of detached pear leaves were measured after application of SA, and the changes in microstructure were observed by paraffin section. The induction of SAR-associated PR genes expression in pear leaves after SA treatment was tested with Northern blotting technique.The main results were as follow:1. Salicylic Acid treatment could significantly improve Yali pear and Hwangkumbae pear resistance to black spot disease. The result shown that SA at 0.02 mmol·L-1 sprayed onto young leaves of Yali pear could significantly degrade disease index and the induced resistance reached up to 63.9%. While the treatment concentration of SA was 0.2 mmol·L-1, the disease index of Hwangkumbae pear was the lowest and it was reduced to 59.0% compared to control. The induced resistance of SA could sustainable at least 10d both on Yali pear and Hwangkumbae pear. PDA culture results showed that SA at lower concentration could inhibit the growth of Alternaria kikuchiana Tanaka. The effect of resistance was not by SA killing the fungi directly, but come from SA activated the systemic acquired resistance of the plants. SA treatment on the different position leaves showed that SA could protect not only the local treated leaves but also the untreated right leaves. It was suggested that SA induced the expression of not only local resistance but the systemic acquire resistance.2. After exogenous Salicylic Acid sprayed on the leaves of pears, the physiological and biochemical characteristics (POD,SOD,PPO,PAL,MDA and·02-) were measured. The results showed that SA (0.02mmol·L-1) dealing with Yali pear 2d later, the activities of antioxidant enzymes were induced in varying degrees. Simultaneously, the content of MDA and·02- dropped down. While Hwangkumbae pear was treated by SA (0.2 mmol·L-1) the similar result was obtained. The treatment of SA not only induced resistance index of local position, but also the effect could transmit to nearby leaves. Paraffin sections' result shown that SA treatments had the positive effect on keeping cell integrity,improving the amount of chlorophyll. An intact cell was helpful to stop the invasion of pathogens and enhance the plant resistance.3. The result of Northern blotting suggested that SA could induce PR-1 and PR-5 gene expression in the leaves of Yali pear locally and systemically. The effect of expression of PR gene was related to the concentration and application period of Salicylic Acid. With the treatment SA at different concentration on the leaves of Yali pear, the expressions of PR gene strengthened at first and gradually weakened after reaching to the peak. When the concentration of SA was 0.02 mmol·L-1, expression of PR gene was reached the peak. After SA (0.02 mmol·L-1) treated the Yali pear five days, the expression of PR-1 gene achieve the strongest and then weakened gradually. After SA treatment, PR-5 gene appeared two peaks at 5d and 15d, respectively. To the leaves that were not treated by SA directly, SA also could induce PR gene expression by way of activating SAR.In conclusion, SA could induce the local and systemic resistance of Yali pear and Hwangkumbae pear to defense the black spot disease. And the effect of induced resistance was related to the resistance ability of physiological characteristics and the establishment of systemic resistance.
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