| Aphis craccivora Koch (Homoptera: Aphididae) is an important pest of alfalfa in the world. The chemicals have mainly been used to control the black bean aphids for a long time, which not only pollutes the environment but also causes the pests to produce the resistance to pesticides, and kills natural enemies. Occurring and harming of the black bean aphids have not decreased, and there is a more serious trend instead. Insect-resistant varieties are one of the most economic and effective measures to control the black bean aphids. However, there are rare researches on the resistance to aphids and mechanism of the alfalfa until now. Therefore, the resistance and mechanism of six alfalfa varieties to Aphis craccivora Koch is studied in induced resistance aspect. The result shows as follows:1 Six different alfalfa varieties were evaluated for resistance to aphid (Aphis craccivora Koch) by aphid number ratio in the flied. Deguo-daye and Meiguo-zajiao that with high resistance were found, especially; Ameristand 201+Z and Alfagraze have middle resistance, especially; while Zhongmu No.1 and Xinmu No.1 exhibited high susceptibility.2 Throughout the period of short-team herbivore stress by aphid, the MDA contents of the high susceptible varieties were higher than that of the middle and high resistant ones, and the MDA contents displayed a rising trend in all varieties. The SOD, POD and PAL activities in the high susceptible varieties appeared lower than that of the middle and high resistant ones. The activities of SOD and POD of all varieties rose firstly and then declined. However, the PAL activity of all varieties rose to a peak and remained high during the testing period. The CAT activity formed cycles of rising and declining, with the activity peaks and bases showing opposite positions in the all varieties, respectively. The PPO activity of the high susceptible varieties became higher compared to the high resistant ones in the earlier period but lower in the later part, and the PPO activity of the middle resistant ones located between the high susceptible ones and the high resistant ones. Therefore, under the stress of the aphid, the contents of MDA and the activity of SOD, POD, PAL and PPO in susceptible and resistant varieties are closely related to the aphid-resistance of alfalfa. These biochemical markers may be used as physiological index for aphid-resistance appraisal. However, the relationship between CAT and aphid-resistance of alfalfa is worth of further research.3 Throughout the period of short-team herbivore stress by aphid, the Chla+Chlb, Pn, and Gs in all varieties displayed the dropping tendency, but the dropping tendency in the middle and high resistant ones was much less. The persistent decrease of Ci and persistent increase of Ls in the middle and high resistant varieties, which indicated stomatal factor was the main reason of the decrease of Pn; the tendency of Ci was first decreased and then increased but the tendency of Ls was opposite in the high susceptible varieties, this clearly showed that the predominate influence factor in prophase was stomatal factor but in anaphase was not stomatal factor.4 Throughout the period of short-team herbivore stress by aphid, the Fv/Fm, Fv/Fo,ΦPSⅡ, ETR and qP in the high susceptible varieties all showed incessant downtrend, but the downtrend in the middle and high resistant varieties became much less, especially. TheΦPSⅡand ETR of Alfagraze and Deguo-daye showed little uptrend tendency. The NPQ in all varieties has the same tendency in prophase and metaphase, which was first rose and then dropped down, but in anaphase the high susceptible varieties had the ascending tendency while the middle and high resistant varieties had the descending tendency. In conclusion, under the herbivore stress by aphid, the variational tendency and range of chlorophyll content, photosynthetic characteristics and chlorophyll fluorescence in different varieties were closely allied to aphis-resistance ability of alfalfa.
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