Comparative Study On The Salt-Tolerance And Mechanisms Of Resisting Salt For Three Poplar Varieties

We compared effects of soil salinity (NaCl) on seedlings of three poplar cultivars (Bailin2; Bailin 3; Xijiajialin). The contents of Na⁺, K⁺ proline and soluble sugar, net photosyntheticrate (P_N), stomatal conductance (g_s), Intercellular CO₂ concentration (C_i), transpiration rates(E), water utilizing efficiency, dry weight, fresh weight, water content, leaf number, leaf areaand branch length were determined, to elucidate the mechanism of salt-stress damage toplants and the physiological adaptive mechanism of plants to salt-stressAfter comprehensive analysis of the various growth indexes, it is detected that Xijiajialinshow more salt-resistance than Bailin2 and Bailin3. None of Bailin2 and Bailin3 couldsurvive from the concentration of 20mM NaCl; however Xijiajialin showed no significantgrowth inhibition. This result not only indicated their different salt endurance and variedtolerance limits, but also implicated the existence of distinct salt-tolerant mechanisms.Additionally, the regression analysis of the growth index and salt stress also demonstrated thatthe salt stress caused the similar harmful effects on the growth of Bailin2 and Bailin3, whichwas more serious than that on Xijiajialin. Salt-stress reduce sharp No. and extension of leavesin the three cultivars. But moderate salt stress did not affect the average area of the leaves inBailin2 and Bailin3. Oppositely, the high intensity did significantly prohibit their leavesextension, but this not was observed in Xijiajialin.For all the three poplar cultivars, proper salt stress will enhanced the net photosynthesisrate and photosynthesis productivity, which is important for the adaptive process of the plants.However, the high intensity of salt can lead to dramatically decrease of these indexes. It wasobserved that with the increasing salinity, the Intercellular CO₂ concentration (C_i) of Bailin2and Bailin3 showed the increasing tendency, albeit this was not detected in Xijiajialin group.For Bailin2 and Baitin3 group, the high intensity possibly damaged the structure and functionof their photosynthesis which were not affected in Xijiajialin group, because the rate ofphotosynthesis decrease with the lightly dropping Intercellular CO₂ concentration (C_i) in thehigh salt concentration. The latter decreasing photosynthesis rate was mainly restricted by thestoma.The sodium contents within the leaves of the three poplar cultivar significantly increasedwith the increasing salt concentration, but meanwhile the potassium did not drop. All these areobviously distinct with the other plants and are valuable for the cultivation of the salt-resistantligneous plants.Within the leaf cells of the three poplar cultivars, the contents of sodium and proline bothincreased with the increasing salt concentration. Based upon the analysis, great accumulationof sodium and proline may be the main way by which the three poplar cultivars resists to saltstress. In this experiment, it was detected that within the leaves of the cultivars there was greatamount of soluble carbohydrates, which occupied topmost 18.4ï¼…of the dry weight. However,the carbohydrates played different roles in the adaptive process to the salt stress: The solublecarbohydrates in Bailin2 and Bailin3 both showed decreasing tendency with the increasingsalt concentration, but in Xijiajialin group, they increased and then slightly dropped whichwas still higher than the control plants. The soluble carbohydrates in Xijiajialin possiblyparticipated in the osmotic regulation of the cells. The decreasing contents of solublecarbohydrates in Bailin2 and Bailin3 may be the result of the metabolism interruption or transition. Plants which can be accommodated to the severe environment probably adjust theirmetabolism pathway, strategy for the life history and the length of the life cycle. Thedistribution of substance and energy among growth, development, reproduction and toleranceto stresses are significantly correlated with the plants' tolerance to the stresses.