Study Of Physiological And Biochemical Response Of Alfalfa To Odontothrips Loti Damage

Odontothrips loti is a dominant pest that influences the production of alfalfa （Medicago sativaL.） in northern China, the damage of which impeded alfalfa growth, decreased its yield and quality.The breeding of thrip-resistant alfalfa variety is the most fundamental approach for solving thripdamage economically and effectively, while making clear the mechanism of thrip resistance isprerequisite. This study aims at exploring the change of growth indices, photosynthetic indices andresistant indices of thrip resistant clone R-1and thrip susceptible clone I-1after the alfalfa plantsbeing damaged by Odontothrips loti of different densities to disclose the resistant mechanism ofalfalfa to Odontothrips loti damage. The results are listed as follows:（1） Damage indices for the inoculated treatments of R-1clone and I-1clone increased as theincrease of Odontothrips loti damage pressure; internode numbers for both R-1clone and I-1cloneplants increased, while plant height, internode length, stem diameter, leaf area, leaf-stem ratio andindividual biomass decreased; both the damage index and dry weight loss of I-1clone were higherthan R-1clone on condition of the same Odontothrips loti density, indicating that the resistance andcompensational growth capabilities in response to Odontothrips loti of R-1clone were stronger thanI-1clone.（2） As the time increase of thrip damage, chlorophyll content for R-1clone increasedcontinuously on condition of low pest density, chlorophyll contents of treatments for5and7pestsper branch increased significantly at the primary stage of Odontothrips loti damage, followed by adecrease process; for1pest per branch treatment, the chlorophyll content change of I-1clone wasnot conspicuous, while as the increase of pest density, the chlorophyll content of which decreasedcontinuously. During the process of Odontothrips loti damage, chlorophyll contents for R-1treatments were found higher than I-1treatments. Simultaneously, the corresponding indices as netphotosynthetic rate （P_n）, water use efficiency （WUE） and transpiration rate （T_r） presented the sametendency, indicating that R-1clone is capable of compensating the yield loss according to increasethe assimilation of photosynthesis and WUE after being damaged by Odontothrips loti.（3） F₀for R-1treatments decreased firstly and then increased, while F_v/F₀, F_v/F_m, NPQ andqP presented a reversed tendency. F₀for I-1treatments increased, F_v/F₀, F_v/F_m and qP decreased,and the NPQ increased firstly and then decreased. On the5^th day of damage, the amplification of F₀ and the decreasing amplitudes of F_v/F₀, F_v/F_m, NPQ and qP for all the R-1treatments were foundlower than I-1clone, indicating that the damage of PSII reaction center for I-1leaves was moreserious than that of R-1, and R-1could generate the adaptability to Odontothrips loti according tothe internal adjustment, and therefore to protect PSII from damage.（4） Soluble protein contents decreased while the free proline contents increased for all the R-1and I-1clones during the damage process, with the changes of the two mentioned indices of all theinoculated treatments for R-1within the rangeability for that of I-1. On the5^th day of damage, thedecrement of soluble protein and the increment of free proline for all the inoculated treatment ofR-1clone were found significantly lower than I-1clone. NR activity of all the R-1treatmentsincreased, followed by a stable tendency, nitrate nitrogen content increased firstly, and thendecreased, and the free proline content keep rising continuously; for I-1treatments, NR activityincreased firstly, and then decreased, nitrate nitrogen contents decreased firstly and then increased,which was opposite to that of NR, and the free proline contents were found changed in variousdegrees, indicating that R-1clone was with relatively stronger adaptability and lesser damage, whilethe nitrogen metabolism of I-1clone became imbalanced after being damaged.（5） H₂O₂contents of all the R-1treatments had no obvious change or increased. The OH^-1concentration and the superoxide radical generation rate presented a firstly rapid increase, thenfollowed by a decrease or rapid increase-decrease-rapid increase tendency. H₂O₂contents andsuperoxide radical generation rate of all the I-1treatments presented an increase tendency; MDAcontents for both R-1and I-1increased. During the damage process, the increment of MDA for R-1clone was found lower than I-1clone, and on the5^th day of damage, the increment of MDA contentfor all the R-1treatments was found obviously lower than that of I-1, indicating that the activeoxygen eliminating capability of R-1was stronger than that of I-1, and the damage of thrips lead tothe imbalance of active oxygen metabolism for I-1clone, which made the destructiveness of cellmembrane system for I-1more serious than that of R-1clone.（6） As the time increase of thrip damage, the activities of SOD, APX, GPX and GR for all theR-1treatments increased continuously, then stopped or decreased slowly, the CAT activitydecreased, and the PPO activity increased slowly or increased rapidly followed by a slow decrease,PAL activity increased slowly or then followed by a rapid decrease. SOD, APX and GPX activitiesfor all the I-1treatments presented an increase-rapid decrease tendency, the CAT activity decreased, the GR activity had no obvious change or increased, the PPO activity increased slowlyfollowed by a slow decrease or increased rapidly followed by a rapid decrease, the PAL activity hadno obvious change, or increased slowly and then keep stable or increased rapidly and then followedby a rapid decrease, indicating that R-1clone could start the antioxidant enzyme system anddefensive enzyme system rapidly and maintain them at a relatively high activity level to resist thedamage caused by thrips, while the antioxidant enzyme system and defensive enzyme system of I-1clone got damaged, the activity of which was lower than that of R-1clone. Simultaneously, as thetime increase of thrip damage, GSH and AsA contents for all the R-1treatments increasedcontinuously, and then kept stable or decreased slowly, while the GSH contents for all the I-1treatments had no obvious change-increased rapidly or then followed by a slow decrease, the AsAcontents increased rapidly firstly and then decreased rapidly, which furtherly indicated that theoxidative resistant capability of R-1clone was stronger than that of I-1, and the thrip resistantcapability of R-1was also stronger than I-1.