The mean effective water self-diffusion coefficient in maize root segments under the effect of aquaporin blocker (mercuric chloride, 0.1 mM) was measured using the spin-echo NMR method with pulsed magnetic field gradient within the temperature range from 10 to 35 °С. HgCl 2 caused the reduction in water diffusion by 30 % as compared to the control samples. Temperature dependences of water self-diffusion coefficients showed two linear regions with different values of Q 10 and activation energy, E a . As the temperature reduced from 20 to 10 °С, E a values calculated from the Arrhenius plots were close to those of bulk water (20 ± 3 kJ mol -1 ) and slightly changed for the sample pretreated HgCl 2 . Within the temperature range from 25 to 35 °С the slope of temperature dependences became steeper and E a values were 31 ± 3 kJ mol -1 for the control and 40 ± 4 kJ mol -1 for the treated sample. In the vicinity of 20 o С, the temperature dependence of water diffusion via the mercury-sensitive water channels showed extreme value. In the region, the specific area of the mercury-sensitive aquaporins was 0.004 % of the total cell surface area. The data indicate that water transfer via aquaporins is sensitive to temperature, and the contributions of the transmembrane pathways (aquaporins, lipid bilayer) differ in different temperature ranges.

The influence of inhibitor of water channels, HgCl 2 , on water diffusion in maize (Zea mays L.) seedling roots was investigated with the pulsed nuclear magnetic resonance (NMR) method. Blocking of water channels decreased the water permeability of cell membranes by 1.5 -2 times. This effect of HgCl 2 was exhibited only in the roots of seedlings grown in a nutrient solution containing Ca 2+ and was reversed with Hg-scavenging agent β-mercaptoethanol. Subsequent incubation of Ca 2+ -deficient roots in the nutrient solution with Ca 2+ recovered the sensitivity to HgCl 2 . The water stress decreased water diffusion rates similarly to HgCl 2 and the effects of water stress and HgCl 2 were not additive. The obtained data demonstrate the possibilities of the pulsed NMR method for study of the transmembrane water exchange in vivo in connection with water channel functioning.

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Changes in the total water permeability of two cell membranes (plasmalemma and tonoplast), estimated by the effective diffusion coefficient of water (D ef), were controlled using the NMR method. The time dynamics of D ef in maize (Zea mays L.) root cells was studied in response to (i) root excision from seedling and the following 6-h incubation in the growth medium (wound stress) and (ii) the superposition of wound stress plus paraquat, which induces the excess of reactive oxygen species (ROS). The dynamics of lipid peroxidation, oxygen consumption, and heat production was studied to estimate general levels of oxidative stress in two variants of experiments. Under wound stress (the weak oxidative stress), the reversible by dithiothreitol increase in cell membrane water permeability was observed. The applicability of mercury test to aquaporin activity in our experiments was verified. The results of wound stress effect, obtained using this test, are discussed in terms of oxidative upregulation of aquaporin activity by ROS. The increase of oxidative stress in cells (wound-paraquat stress), contrary to wound stress, was accompanied by downregulation of membrane water permeability. In this case, ROS is supposed to affect the aquaporins not directly but via such processes as peroxidation of lipids, inactivation of some intracellular proteins, and relocalization of aquaporins in cells.

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