| Poplar is an important afforestation and greening tree species in Northern China. Frequent cold injury and the triggered disease are mainly limited factors for poplar industrial development. Therefore, it is necessary to study physiological and biochemical response of different cultivars to low temperature in autumn and specific expression of function proteins. It has the significance to elaborate the mechanism of cold resistance, distinguish the adaptability of different cultivars, and keep the balance of mass growth and adaptability for the selective breeding of cross parent, as well as select the sites for contrast test and compartmentation test of hybridal clones.Four poplar cultivars [P. ussuriensis Kom,populus simonii×P. nigra, (P. psudosimonii×P. nigra)×P. nigra CL.'A5'and populus alba×P. davidian CL.'1333'] grown in Harbin city were selected to investigate their cold resistance to natural low temperature in autumn; cold-resistance mechanisms were studied by investigating on the changes of growth, phenology, cell membrane permeability, chlorophyll fluorescence parameters, osmotic adjustment, phenolic compounds and changes of specific proteins in natural cooling process. The results were shown as follows:(1) Different poplar cultivars grown in the same site had various resistances to the changing temperature; genetic basis was a determining factor for the difference in growth, phenology, and adaptability. P. ussuriensis, belonging to the local native tree species, forms some adaptive characteristics to local climate in a long-term evolution process, such as earlier occurrence of capping and higher survival rate of terminal buds, thus this cultivar had almost no cold injure to occur. (P. psudosimonii×P. nigra)×P. nigra CL.'A5'cap slightly later than that of P. ussuriensis, its survival rate of the terminal buds in winter was 97%; the capping of Populus simonii×P. nigra and populus alba×P. davidian CL.'1333'occurred later; this led to the difficulty of the formation of dormant buds and a poor adaptability of its photoperiod to local climate. Thus the two cultivars could not avoid chilling injury by timely access dormant.(2) Under the average minimum temperature of 5℃, significant changes in relative conductivity and chlorophyll fluorescence parameters were observed in the four cultivars. Under the stress of low temperature above zero, the injured degree of P. ussuriensis was the highest, followed by (P. psudosimonii×P. nigra)×P. nigra CL.'A5', Populus simonii×P. nigra and populus alba XP. davidian CL.'1333'. However, when air temperature dropped to-3℃, relative conductivity of the four poplar cultivars exceeded 50%, of which the injured degree of P. ussuriensis was the lowest, populus alba×P. davidian CL.'1333'the highest. It indicates that timely access dormant may be a better way to resist low temperature for the poplar cultivars.(3) Under chilling stress, osmotic regulation substances and phenolic compounds of the poplar leaves were increased, indicating that there is a close correlation between protection substances and cold resistance. In general, osmotic regulation substances and phenolic compounds of in leaves were increased with a decreasing temperature. The strongest response was found in populus alba×P. davidian CL.'1333'leaves, followed by populus simonii×P. nigra and(P. psudosimonii×P. nigra)×P. nigra CL.'A5'. A relatively weak response occurred only at low temperature above zero for P. ussuriensis. A large decrease in osmotic regulation substances and phenolic compounds occurred for all the four cultivars before defoliation. It implies that P. ussuriensis adapts the low temperature stress mainly through the way of stress avoidance, whereas populus alba×P. davidian CL.'1333'resists the chilling stress by improving protective substances.(4) P. ussuriensis had an earlier capping period. Before defoliation, osmotic regulation substances were transferred largely between organs, moreover, relative conductivity, Fv/Fm, and Fv/F0 were decreased slowly before freezing temperature arrival. These characteristics are adaptive performance, formed in a long-term evolution. The other two cultivars (populus simonii×P. nigra and(P. psudosimonii×P. nigra)×P. nigra CL.'A5') also showed a similar changing trend.(5) Low temperature stress had significant effects on the protein expression. Under the condition of pH 4-7,94 protein spots in gel electrophoresis map of P. ussuriensis were found to have the change in abundance of more than two times, of which 45 spots were identified. GO classification showed that 23 spots (42% of total protein spots) are stress response proteins,12 (23%) spots might be related to energy metabolism, nine spots (17%) related to photosynthesis, five spots (10%) catabolism associated, the rest four spots (8%) unidentified for the function. Among the identified 45 proteins,41 spots were up-regulated expression; only four spots were down-regulated. These changes in protein abundance may be closely related to chilling adaptation mechanisms in P. ussuriensis.(6) Compared with the 45 protein spots identified in gel electrophoresis map of P. ussuriensis,28 protein spots were matched in gel electrophoresis map of populus simonii×P. nigra and populus alba X P. davidian CL.'1333'. Among them, the expressions of 15 proteins were up-regulated in three cultivars in the three stages of temperature drop, the other 13 protein spots showed an inregulatory express, of which nine proteins in P. ussuriensis were up-regulated expression and four proteins were down-regulated expression; the change in populus simonii×P. nigra and populus alba×P. davidian CL. '1333'are not significantly. Variation in protein expression in the three cultivars may be closely related with different low-temperature adaptation mechanisms, this assumption needs to be studied further.(7)The expression differences of heat shock protein (spot 4,10,67), bisphosphoglycerate-independent phosphoglycerate mutase (spot 21), filamentation temperature-sensitive h 2b (spot 44), ATP synthase delta chain (spot 53) in the three poplar species may be closely related to low temperature stress response mechanisms. |
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