| We found that both sides of the lower reaches of Tarim river had formed floodplain in the process of the implementation of ecological water conveyance, and the floodplain had experienced a complex moisture change from drought to flood, so in order to explore the tolerance of critical soil moisture content of Tamarix ramosissima seedling in the process of water change, we determined the growth, photosynthesis and chlorophyll fluorescence parameters of Tamarix ramosissima seedling under the Z(soil water content was between 19.4% and 21.8%), L(soil water content was between 12.1% and 14.5%), M(soil water content was between 7.3% and 9.7%) and S(soil water content was below 4.8% weight, no longer moisturizing) and different soil moisture content(A: 16.9%, B: 9.6% and C: 8.5%) in the field. the following conclusions:(1) When the soil moisture content was between 12.1%-16.9%, the net photosynthetic rate(Pn), stomatal conductance(Gs), the actual optoelectronic(FPSII), photosynthetic electron transport efficiency(ETR) and chemical quenching(qp), and the maximum light quantum yield(Fv/Fm) of Tamarix ramosissima seedlings reached peak, and when the water content was between 19.4%-21.8% or 7.3%-21.8%, its corresponding value were lower, but they were higher than 1.4-1.5%; And the Fv/Fm value of Tamarix ramosissima under 1.4%-1.5% was significantly lower than 0.75, but other treatments were higher than 0.75, and the non photochemical quenching(NPQ) and water use efficiency(WUE) of Tamarix ramosissima were highest. This suggests that the photosynthetic growth state of Tamarix ramosissima seedling under 12.1%-14.5% was best, and it would be inhibited under high moisture(19.4%-21.8%) or lower(7.3%-9.7%) by a certain extent, however, with drought stress increasing(1.4%-1.5%), the Pn and FPSII of Tamarix seedling was significantly declined, and Fv/Fm value of Tamarix ramosissima seedling was significantly lower than 0.75, so the photosynthetic institution of Tamarix ramosissima seedling had been irreversible damage, if maintaining the water condition, the photosynthetic growth of Tamarix ramosissima seedling will be limited, at the same time,it may lead Tamarix ramosissima seedling to death.(2) When the soil moisture content was between 19.4%-21.8% and 1.4%-1.5%, the root-shoot ratio and root length of Tamarix ramosissima seedling was bigger than other treatments. This suggests that the roots under higher moisture content could produced a large number of adventitious roots which absorbed nutrients in order to adapt to the high moisture condition, and when they were in drought stress, they could also increase the number of fine roots to get more water resources and adapt to drought stress environment。(3) In the wild natural experiment, larger stength ecological water conveyance could not only lead a large number of seedlings to death, but also make them be updated to a great extent, so in this state of continuous ecological water conveyance, Tamarix ramosissima of floodplain was always in seedling stage, and can not stably grow.(4) Combining with the indoor experiment and field natural water stress, this study shows too much water(>19.4%)can not be good for growth, and the tolerance of critical soil moisture content(1.4%-1.5%) can provide certain theoretical basis for the implementation of ecological water conveyance in the lower reaches of Tarim river, which can ensure the normal growth of vegetation, and make the full use of the water in arid areas, avoid the waste water, save energy and make better service for the production. |
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