Hydraulic properties (half-time of water exchange, T1/2, and hydraulic conductivity, Lp; T1/2 approximately 1/Lp) of individual cells in the cortex of young corn roots were measured using a cell pressure probe for up to 6 h to avoid variations between cells. When pulses of turgor pressure of different size were imposed, T1/2 (Lp) responded differently depending on the size. Pulses of smaller than 0.1 MPa, which induced a small proportional water flow, caused no changes in T1/2 (Lp). Medium-sized pulses of between 0.1 and 0.2 MPa caused an increase in T1/2 (decrease in Lp) by a factor of 4 to 23. The effects caused by medium-sized pulses were reversible within 5-20 min. When larger pulses of more than 0.2 MPa were employed, changes were not reversible within 1-3 h, but could be reversed within 30 min in the presence of 500 nM of the stress hormone ABA. Cells with a short T1/2 responded to the aquaporin blocker mercuric chloride (HgCl2). The treatment had no effect on cells which exhibited long T1/2 following a mechanical inhibition by the large-pulse treatment. Step decreases in pressure resulted in the same inhibition as step increases. Hence, the treatment did not cause a stretch-inhibition of water channels and was independent of the directions of both pressure changes and water flows induced by them. It is concluded that inhibition is caused by the absolute value of intensities of water flow within the channels, which increased in proportion to the size of step changes in pressure. Probable mechanisms by which the mechanical stimuli are perceived are (i) the input of kinetic energy to the channel constriction (NPA motif of aquaporin) which may cause a conformational change of the channel protein (energy-input model) or (ii) the creation of tensions at the constriction analogous to Bernoulli's principle for macroscopic pores (cohesion-tension model). Estimated rates of water flow within the pores were a few hundred micro m s-1, which is too small to create sufficient tension. They were much smaller than those proposed for AQP1. Based on literature data of single-channel permeability of AQP1, a per channel energy input of 200 kBxT (kB=Boltzmann constant) was estimated for the energy-input model. This should be sufficient to initiate changes of protein conformation and an inactivation of channels. The data indicate different closed states which differ in the amount of distortion and the rates at which they relax back to the open state.

Effects of root zone temperature on growth, shoot water relations, and root water flow were studied in 1-year-old aspen (Populus tremuloides Michx.) seedlings. Seedlings were grown in solution culture and exposed to day/night air temperatures of 22/16 degrees C and solution culture temperatures of 5, 10, or 20 degrees C for 28 days after bud flush. Compared with root growth at 20 degrees C, root growth was completely inhibited at 5 degrees C and inhibited by 97% at 10 degrees C. The 5 and 10 degrees C treatments severely reduced shoot growth, leaf size, and total leaf area. Root water flow was inhibited by the 5 and 10 degrees C treatments. However, when seedlings were grown for 28 days at 5 degrees C and root water flow was measured at 20 degrees C, there was an increase in flow rate. This increase in root water flow was similar in magnitude to the decrease in root water flow observed when seedlings were grown for 28 days at 20 degrees C and root water flow was measured at 5 degrees C. Reduced root water flow of seedlings grown at 5 and 10 degrees C resulted in decreased stomatal conductance, net assimilation, and shoot water potentials. Root water flow was positively correlated with leaf size, total leaf area, shoot length, and new root growth. Transferring seedlings from 5 to 20 degrees C for 24 h significantly increased root water flow, shoot water potential, and net photosynthesis, whereas transferring seedlings from 10 to 20 degrees C resulted in only a slightly increased shoot water potential. Transferring seedlings from 20 to 5 degrees C greatly reduced root water flow, stomatal conductance, and net photosynthesis, whereas shoot water potential decreased only slightly.

Low root temperatures significantly reduced root hydraulic conductivity and increased resistance to water flow through the roots of aspen (Populus tremuloides Michx.) seedlings. Increased resistance to water flow could not be fully explained by the corresponding increase in water viscosity at low temperatures. The shapes of Arrhenius plots of root water flow and the activation energies were dependent on the direction, sequence and extent of temperature change. The Arrhenius plots suggested that the effect of low root temperature on root water flow was mediated by an effect on root metabolism. The low root temperatures tested did not induce root electrolyte leakage normally associated with cell membrane injury. Although a decrease in root temperatures to 7 or 4 degrees C induced a reduction in stomatal conductance, this reduction lagged the decline in root water flow by several hours. In contrast, when soil temperatures were raised from 4 or 7 degrees C to 25 degrees C, root water flow presumably increased, and stomatal conductance responded rapidly and was temporarily higher than before the cold treatment was imposed.

Temporal and spatial variations in the water status of walnut trees (Juglans regia L.) and the soil in which they were growing were traced by analyzing the differences in hydrogen isotopes during spring and summer in a 7-year-old walnut stand. Walnut root dynamics were measured in both dry and wet seasons. Walnut roots were mainly distributed in the upper soil (0-30 cm depth), with around 60% of the total root mass in upper soil layers and 40% in deep soil layers (30-80 cm depth). The upper soil layers contributed 68% of the total tree water requirement in the wet season, but only 47% in the dry season. In the wet season, total roots, living roots and new roots were all significantly more abundant than in the dry season. There were significant differences in pre-dawn branch percentage loss of hydraulic conductance (PLC), pre-dawn leaf water potential and transpiration between the dry and wet seasons. Water content in the upper soil layers remarkably influenced xylem water stable-hydrogen isotope (δD) values. Furthermore, there were linear relationships between the xylem water δD value and pre-dawn branch PLC, pre-dawn leaf water potential, transpiration rate and photosynthetic rate. In summary, J. regia was compelled to take a larger amount of water from the deep soil layers in the dry season, but this shift could not prevent water stress in the plant. The xylem water δD values could be used as an indicator to investigate the water stress of plants, besides probing profiles of soil water use.

It is well known that xylem embolism can be repaired by bark water uptake and that the sugar required for embolism refilling can be provided by corticular photosynthesis. However, the relationship between corticular photosynthesis and embolism repair by bark water uptake is still poorly understood. In this study, the role of corticular photosynthesis in embolism repair was assessed using Salix matsudana branch segments dehydrated to −1.9 MPa (P 50 , water potential at 50% loss of conductivity).The results indicated that corticular photosynthesis significantly promoted water uptake and nonstructural carbohydrate (NSC) accumulation in the bark and xylem during soaking, thereby effectively enhancing the refilling of the embolized vessels and the recovery of hydraulic conductivity. Furthermore, the influence of the extent of dehydration on the embolism refilling enhanced by corticular photosynthesis was investigated. The enhanced refilling effects were much higher in the mildly dehydrated (−1.5 MPa) and moderately dehydrated (−1.9 MPa) branch segments than in the severely dehydrated (−2.2 MPa) branch segments. This study provides evidence that corticular photosynthesis plays a crucial role in xylem embolism repair by bark water uptake for mildly and moderately dehydrated branches.