Studies On Response Mechanisms Of Two Eculypts To Low Temperature Stress

The low-temperature injury limited the distribution of plants and the low-temperature stress is one of the natural disasters prevailed in China.Therefore,studies on response mechanisms of plants to low-temperature stress are very important,which will not only be a guide to introduction and cultivation of plants but also help to breed cold-tolerance plants.The intact seedlings of Eucalyptus urophylla and E.dunnii were exposed to low temperature under humid conditions,and effects of low-temperature stress on physiology and biochemistry of the two species were tested.The results showed as follows:(1)The activities of SOD and contents of MDA in both species increased with low-temperature stress,it indicated that the anti-oxidative systems played an important role in potentiating cold-resistance of plant and limiting the production of free radicals to protect membrane integrity.E.dunnii showed a better adaptability for low-temperature stress in the higher SOD activity and lower MDA content than E.urophylla in order to develop the tolerance to low temperature.(2)Photosynthetic characteristic and chlorophyll fluorescence of two species were very important parameters of measuring and estimating the low-temperature stress influence degree. Following the low-temperature stress,Pn,Gs,Tr,A_max,Φ,LSP,CE,F_v/F_m and F_v/F_o of two species decreased obviously,while CO₂ compensation points and F_o increased.Compared to under contral,chlorophyll content of both species was no obvious difference.Changes of photosynthetic parameters showed that the decline of photosynthetic enzyme activities as the main non-stomata limitation was the main factor which resulted in the decline of photosynthetic rate.When low-temperature prolong,effect of stomata limitations became more and more important.E.dunnii showed a prevailing strategy in the changes of Pn,Gs,Tr,A_max,Φ,LSP, CE,F_v/F_m,F_v/F_o and F_o than E.urophylla in order to adapt low-temperature stress.(3)2-D electrophoresis patterns of total proteins in the leaves of E.urophylla and E.dunnii seedlings were affected by low-temperature stress,there were some diffenences between two eucalypts.Most of changed proteins located in low weight area,and the proteins of high weight area kept stable.Down-regulated proteins of E.dunnii were less than E.urophylla,however, up-regulated proteins and special expressing proteins of E.dunnii were more than E.urophylla, all of this differences might suggested that changes of leaf proteins were an very important physiology response to low-temperature stress. (4)It was showed that ultrastructure of mesophyll cells of two eucalypts changed obviously after low-temperature stress.Cell membrane systems were damaged,the shape of chloroplasts changed gradually from fusiform to round,starch grains decomposed slightly,cell inclusion increased.following the temperature decreased,cell ultrastructure of E.urophylla were more serious than E.dunnii.After 24 hours stress of subzero five,cell organelles of E.urophylla were damaged badly,but the cell structure of E.dunnii still good.More stable chloroplasts and later decomposed starch grains in E.dunnii under low-temperature stress might account for its greater capacity for cold tolerance than E.urophylla.(5)A comparative study was carried out on the EM-cytochemical localization of calcium in mesophyll cells between E.urophylla and E.dunnii.Electron microscopic observations revealed that the electron dense calcium antimonite deposits,an indication of the calcium localization, were localized mainly in the vacuoles and intercellular space,and few was found in the cytosol and nuclei.When both species were cultured at 0℃for 24h,an elevation of Ca²⁺level in the cytosol was observed,moreover,the calcium antimonite deposits of Ca²⁺in the vacuoles was closed to vacuole membrane.When both species were cultured at 0℃for 48h,Ca²⁺in the vacuoles was closed to vacuole membrane obviously,Ca²⁺level in the cytosol of E.dunnii reduced slightly,a few was found in the plasmalemma,while Ca²⁺level in the cytosol of E. urophylla still increased,and many were found in the plasmalemma.These results suggested that the changes of calcium might be one of the important factors of cold tolerance capacity.(6)With electromicroscopic-cytochemical assay,comparative observation on the ultracytochemical localization of ATPase in the mesophyll cells of E.urophylla and E.dunnii was carried out.The electron-dense lead phosphate deposits,an indication of ATPase activity, were localized mainly in the plasmalemma,vacuole membrane and plasmodesmata,few was found in nuclei and chloroplasts and the other organelles.The localization and activity of ATPase in the mesophyll cells of both species had no obvious differences expect the ATPase in the plasmodesmata of E.dunnii was slight more than E.urophylla,when both species were treated at 0℃for 24h,the ATPase activity of E.urophylla in plasmalemma decreased,and few was observed in the vacuole membrane and plasmodesmata,however,the ATPase activity of E. dunnii in plasmalemma decreased slightly,and no obvious changes in the vacuole membrane and plasmodesmata,at the same time,a few was observed in the nuclei.More stable ATPase under low-temperature stress was important adaptive mechanisms of E.dunnii in order to develop cold tolerance capacity.