关键词: 根系, 水流导度, 水孔蛋白, 水曲柳

Abstract:

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 (L_p) 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 L_p, 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 L_p is an effective index in determining the relationship between aquaporins and water transport. However, most studies on L_p have used the same uptake solution, (e.g., deionized water), and variation of L_p in solutions of different substances has been ignored. The objectives of this study were to 1) compare differences in L_p in deionized water and various concentrations of NH₄NO₃ solutions, and 2) examine changes of L_p 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 min and L_p 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 L_p measured.
The results showed that the L_p 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, L_p increased with increasing concentrations of NH₄NO₃, and the maximum L_p occurred at 8 mmol·L^-1 NH₄NO₃. The average L_p in NH₄NO₃ solutions was 77% higher than in deionized water After HgCl₂ treatment, L_p 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 studies, the percent L_p reduction in nutrient solutions was higher than in deionized water after HgCl₂ 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 HgCl₂, nutrient ions can stimulate aquaporins and increase their activity. After aquaporins were inhibited by the HgCl₂ treatment, the L_p 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.

Key words: Root system, Root hydraulic conductance, Aquaporins, Fraxinus mandshurica