Water Transport Ability Research Articles

Xylem hydraulic and photosynthetic function of Gnetum (Gnetales) species from Papua New Guinea

Gnetum (Gnetales) species are suggested to be unique extant gymnosperms that have acquired high photosynthetic and transpiration capacities as well as greater xylem hydraulic capacity and efficiency compared with all other extant gymnosperms. This is because Gnetum is the only extant gymnosperm lineage that combines vessels, broad pinnate-veined leaves and an ecological distribution in wet, productive lowland tropical rainforest habitats. Yet, field-based observations on the group's ecophysiological performance are lacking. To test a hypothesis that Gnetum species are ecophysiologically analogous to light-demanding woody tropical angiosperms, stem xylem hydraulic performance, photosynthesis and stomatal conductance were investigated in Gnetum as compared with a diverse group of co-occurring woody plants in a lowland tropical rainforest. It was found that Gnetum species combined low photosynthetic capacity and low stomatal conductances with a low stem water transport ability. The physiological observations are consistent with the general occurrence of Gnetum species in shady, primary forest habitats. These results on Gnetum ecophysiology indicate that the coupling of vessels, broad pinnate-veined leaves and the liana habit do not signal the evolution of a highly opportunistic, light-demanding life history in gymnosperms.

Root Hydraulic Conductivity and Its Components, with Emphasis on Desert Succulents

AbstractPlant growth and development are often limited by the ability of roots to extract water and nutrients from the soil and to transport them to the shoots. The water absorption and transport ability of roots can be quantified by the hydraulic conductivity, LP. This quantity can be considered in terms of two components, the radial conductivity (LR) for water movement into the root and the axial conductance per unit pressure gradient (Kh) for water movement along the root xylem. The pathways for water movement through roots are discussed, and the hydraulic conductivities are expressed mathematically. The techniques currently used for determining LP, LR, and Kh are presented and conductivities for various plant species are compared. Recent experimental results on structural factors controlling LP and Kh together with the influences of two major environmental factors, temperature and soil water availability, are analyzed for roots of desert succulents. Interpreting the anatomical and physiological controls for LP, LR, and Kh and the variations caused by environmental factors is essential to understand the functioning of root systems for water uptake.

On resistance to water transport in crop plants for estimating water uptake ability under intense transpiration.

Root water uptake ability is supposed to greatly affect the degree of midday depression in the photosynthetic rate of crop plants. However, there is no useful indicator for estimating it quantitatively despite necessity. This study investigated whether resistance to water transport calculated according to Ohm's law can be adopted in estimating water uptake ability by using rice, corn and soybean plants. The resistance to water transport from soil through root to leaf decreased markedly as the transpiration rate increased, especially at low transpiration rates, but became constant at high transpiration rates where there were close linear correlations between transpiration rate and leaf xylem water potential and the linear extrapolation of the regression lines down to zero transpiration rate gave the water potentials quite near to thc origin. By measuring transpiration rate and leaf xylem water potential for the leaves on the intact stem and the excised stem with its cut end kept under water, it was supposed that root have a relatively high resistance in the water pathway of plants and this resistance could be attributed to the variable resistance of whole plants. Water uptake decreased and the resistance to water transport increased significantly when the root system was partly excised and when treated with NaN3. On the other hand, the resistance was not affected by lowering the water potential of the culture solution, although water uptake of the plants decreased markedly. Our conclusions are that resistance to water transport in crop plants under intense transpiration could be adopted in estimating water uptake and transport ability of plants and that the differences in root water uptake abilities among plants could be compared by calculating the resistance, since the root water uptake ability very much affects whole plant resistance.