水曲柳幼苗根系在不同浓度NH_4NO_3溶液中水流导度的变化

VARIATIONS OF ROOT HYDRAULIC CONDUCTANCE OF MANCHURIAN ASH SEEDLINGS IN DIFFERENT CONCENTRATIONS OF NH_4NO_3 SOLUTIONS

水分吸收过程是根系重要的生理过程。水孔蛋白在根系水分径向运输中起着重要的作用,根系水流导度(Lp)的测定是研究水孔蛋白的重要途径。该研究采用压力流的方法,对相同生长条件下的水曲柳(Fraxinusmandshurica)幼苗根系进行研究,测定了根系在去离子水和不同浓度NH4NO3溶液中的Lp。结果表明:未经处理的水曲柳幼苗根系,Lp随NH4NO3浓度的增加而上升,而且NH4NO3溶液中的Lp比去离子水中的Lp平均高77%;经HgCl2处理后,水曲柳幼苗根系的Lp仍然随NH4NO3浓度的增加而增大,但是根系Lp在去离子水下降了22%,而在NH4NO3溶液中下降了68%,与以前的研究相比发现,经HgCl2处理后,以营养液为吸水基质的根系Lp的降低值普遍高于以去离子水为基质的试验。因此,基质中养分离子的存在对根系中水孔蛋白活性产生了重要的影响,进而影响根系水分的吸收过程。

Roots absorb water from the soil and play an essential role in their water balance. In wet soil, the root system is the primary limitation for plant water uptake and can contribute up to approximately 50% of the overall hydraulic resistance of the plant. The root hydraulic conductance （Lp） based on the root surface area has a major influence on the shoot water status, plant growth and development. Recent studies show that radical water transport is an important determinant of root Lp, because aquaporins in the protoplasm and vacuole membranes play a key role in the radial water transport of plant roots, i.e., higher activity of aquaporins favors higher water transport, and Lp is an effective index in determining the relationship between aquaporins and water transport. However, most studies on Lp have used the same uptake solution, （e. g., deionized water）, and variation of Lp in solutions of different substances has been ignored. The objectives of this study were to 1 ） compare differences in Lp in deionized water and various concentrations of NH4NO3 solutions, and 2） examine changes of Lp when treated with deionized water and NH4NO3 solutions after HgCl2 treatment to inhibit aquaporins. The experiment was conducted in May 2002. Manchurian Ash （ Fraxinus mandshurica ） seedlings were grown for 40 - 45 d on a bench in a greenhouse at the Northeast Forestry University. Greenhouse temperatures were approximately 25℃ day maximum and 18℃ night minimum, and average photosynthetic photon flux was 500 - 800 μmol photons· m^- 2· s^-1 during this period. Roots were placed in treatments of deionized water and NH4NO3 solutions with concentrations of 16, 8, 4, and 1 mmol·L^-1 for 15 rain and Lp measured by the pressure-flux approach. Roots were treated with HgCl2（50 μmol·L^-1） and then again placed in the different treatment solutions and Lp measured. The results showed that the Lp of roots in deionized water was 22% higher than when treated with HgCl2 and 16% higher than both HgCl2-treated and mercaptoethanol-treated plants. In NH4NO3 solutions of various concentrations, Lp increased with increasing concentrations of NH4N03, and the maximum Lp occurred at 8 mmo· L^-1l NH4NO3. The average Lp in NH4NO3 solutions was 77% higher than in deionized water After HgCl2 treatment, Lp still increased from lower to higher NH4NO3 concentrations but was reduced by 68%, which was three folds higher than the 22% reduction in HgCl2-treated roots in deionized water. In contrast to other stud ies, the percent Lp reduction in nutrient solutions was higher than in deionized water after HgC12 treatment. These results suggest that aquaporins are very important in regulating water movement in the roots of Manchurian ash, and nutrient ions can have a significant effect on the activity of aquaporins. In plants not treated by HgC12, nutrient ions can stimulate aquaporins and increase their activity. After aquaporins were inhibited by the HgCl2 treatment, the Lp of roots in different solutions decreased significantly and inhibition of HgCl2 increased with ion solution concentrations. Our results indicate that there are HgCl2-sensitive aquaporins and ion channels in the protoplasm and vacuole membranes of the roots. However, these need to be experimentally tested in the future.