The short-term effects of sodium azide (NaN(3)) on water flow in red-osier dogwood (Cornus stolonifera Michx.) seedlings were examined in excised roots at a constant pressure of 0.3 MPa. NaN(3) significantly decreased root water flow rates (Q(v)). It also induced a significant reduction in root respiration and reduced stomatal conductance to a greater extent in intact seedlings than in excised shoots. Apoplastic flow of water increased with the NaN(3)-induced decreases in Q(v). Mercuric chloride (HgCl(2)) was also used to characterize the water flow responses and respiration of dogwood roots. Similarly to NaN(3), 0.1 and 0.3 mM HgCl(2) decreased root respiration rates and Q(v). The lower, 0.05 mM HgCl(2) treatment, reduced Q(v), but had no significant effect on root oxygen uptake. The reduction of Q(v) in HgCl(2)-treated plants was only partly reversed by 50 mM mercaptoethanol. The mercurial inhibition of Q(v) suggested the presence of Hg-sensitive water channels in dogwood roots. The results indicate that root-absorbed NaN(3) metabolically inhibited water channel activities in roots and in shoots and resulted in stomatal closure. It is suggested that the inhibition of respiration that occurs in plants stressed with environmental factors such as flooding, cold soils, and drought may be responsible for the closure of water channels in root cells and inhibition of root water flow.

Water transport was examined in solution culture grown seedlings of aspen (Populus tremuloides) after short-term exposures of roots to exogenous ethylene. Ethylene significantly increased stomatal conductance, root hydraulic conductivity (L p ), and root oxygen uptake in hypoxic seedlings. Aerated roots that were exposed to ethylene also showed enhanced L p . An ethylene action inhibitor, silver thiosulphate, significantly reversed the enhancement of L p by ethylene. A short-term exposure of excised roots to ethylene significantly enhanced the root water flow (Q v ), measured by pressurizing the roots at 0.3 MPa. The Q v values in ethylene-treated roots declined significantly when 50 m HgCl 2 was added to the root medium and this decline was reversed by the addition of 20 mm 2-mercaptoethanol. The results suggest that the response of Q v to ethylene involves mercury-sensitive water channels and that root-absorbed ethylene enhanced water permeation through roots, resulting in an increase in root water transport and stomatal opening in hypoxic seedlings.Hypoxia, a condition of oxygen deficiency in plant roots, is the main consequence of flooding or waterlogging. Plants respond to hypoxia with reduced root permeability, closure of stomata, hypertrophy of lenticels, epinasty, formation of aerenchyma, and adventitious roots (Vartapetian and Jackson, 1997). Ethylene accumulation is often assumed to be the factor responsible for many of the responses observed in plants exposed to hypoxia (Mattoo and Suttle, 1991; Abeles et al., 1992). Hypoxia induces the formation in roots of the immediate precursor of ethylene, 1-aminocyclopropane-1-carboxylic acid, which is transported in the xylem to the shoots and there rapidly oxidized to ethylene (Mattoo and Suttle, 1991). The synthesis of ethylene and the response of plants to ethylene differ among tissues and different plant species (Abeles et al., 1992), and can be affected by different internal and environmental factors (Sharp et al., 2000; Grichko and Glick, 2001).The effects of ethylene on stomatal closure are not clear. Several studies on the effects of exogenous ethylene on stomatal movements demonstrated differential responses between the examined species (Taylor and Gunderson, 1986; Woodrow et al., 1988; Gunderson and Taylor, 1991; Abeles et al., 1992). Exogenous ethylene is known to increase membrane permeability in petal cells (Mayak et al., 1977; Borochov and Woodson, 1989); however, its impact on cell-to-cell water transport has not been thoroughly examined.Water transport across intact higher plant cell membranes occurs predominantly through water channels (aquaporins; Chrispeels et al., 1997). Aquaporins are located in root cell membranes (Chrispeels and Maurel, 1994) at a high density (Johansson et al., 1998). In our previous work (Wan and Zwiazek, 1999, 2001;Kamaluddin and Zwiazek, 2001), we showed that the root water channels in aspen (Populus tremuloides) and Cornus stolonifera rapidly responded to changes in root metabolism. Phosphorylation of the aquapor...

We investigated the effects of root medium pH on water transport in whole-plant and detached roots of paper birch (Betula papyrifera Marsh.). Exposure of seedling roots to pH 4 and 8 significantly decreased root hydraulic conductivity (Lp) and stomatal conductance (gs), compared with pH 6. When roots of solution-culture-grown (pH 6) seedlings were transferred to pH 4 or 8, their steady-state water flow (Qv) declined within minutes, followed by a decline in gs. The root oxygen uptake rates were not significantly affected by the pH treatments. Treatment of roots with mercuric chloride resulted in a large decrease in Qv at pH 6; the extent of this decrease was similar to that brought about by pH 4 and 8. Lowering root temperature from 21 to 4 degrees C decreased Qv irrespective of medium pH. Low root temperatures did not offset the effects of medium pH 4 on Qv and the roots in this treatment had a high activation energy for water flow. Conversely, roots exposed to pH 8 had a low activation energy, similar to that at pH 6. When 2 micro M abscisic acid, (+/-)-cis-trans-ABA, was added to the root medium, Qv increased in roots that were incubated at pH 6. It also increased slightly in roots incubated at pH 4, but not at pH 8. The increase at pH 4 and 6 was temperature-dependent, occurring at 21 degrees C, but not 4 degrees C. We suggest that the pH treatments are responsible for altering root water flow properties through their effects on the activity of water channels. These results support the concept that ABA effects on water channels are modulated by other, possibly metabolic- and pH-dependent factors.

The effects of sodium fluoride (0.3, 5 and 10 mM NaF) on root hydraulic conductivity, and gas exchange processes were examined in aspen (Populus tremuloides Michx.) seedlings grown in solution culture. A long-term exposure of roots to NaF significantly decreased root hydraulic conductivity (Lp) and stomatal conductance ( gs). Root absorbed NaF significantly affected electrolyte leakage in leaf tissues and substantially restricted leaf expansion. NaF did not significantly affect leaf chlorophyll contents but decreased net photosynthesis (Pn). A short-term exposure of excised roots to 5 mM NaF and KF significantly decreased root water flow (Qv) with a concomitant decline in root respiration and reduced gs when applied through intact roots or excised stems. The same molar concentration of NaCl also decreased Qv and gs in intact seedlings, but to a lesser extent than NaF or KF, and did not significantly affect root respiration. The results suggest that fluoride metabolically inhibited Qv or Lp, probably by affecting water channel activity. We suggest that the metabolic inhibition of Lp by root-absorbed fluoride affected gas exchange and leaf expansion in aspen seedlings.

Effects of sodium naphthenates (NAs) on root hydraulic conductivity (Lp) and gas exchange processes were examined in aspen (Populus tremuloides Michx.) seedlings grown in solution culture. Exposure of roots to NAs for 3-5 weeks significantly decreased Lp and stomatal conductance. Root-absorbed NAs also decreased leaf chlorophyll concentration, net photosynthesis and leaf growth. Short-term (< or = 2 h) exposure of excised roots to NAs significantly decreased root water flow (Qv) with a concomitant decline in root respiration. We conclude that NAs metabolically inhibited Lp, likely by affecting water channel activity, and that this inhibition could be responsible for the observed reductions in gas exchange and leaf growth.