Abscisic acid (ABA) is a potent molecule that certainly modifies stomatal behaviour and plant water loss and probably acts to modify the growth of leaves. The hormone is synthesized both in the leaves and the roots of the plant and in the soil and may move freely from plant to soil and soil to plant. It can also move rapidly through the plant in both the xylem and the phloem and will partition between different compartments in different tissues largely as a function of pH. It is described here how perturbations in soil conditions around the roots and the water status of the air can modify the fluxes of ABA around the plant and its accumulation in different compartments and different tissues. These fluxes can be interpreted as signals of different stresses imposed on the plant and consideration is given to how different perturbations can exert subtle changes which are manifest as modified shoot growth rates and functioning. Most emphasis in the discussion is placed upon the plant's responses to the imposition of soil and atmospheric drought.

Abscisic acid conjugate concentrations increased in barley xylem sap under salinity, whereas it remained at a low level in the intercellular washing fluid (IWF) of barley primary leaves (Hordeum vulgare cv. Gerbel). Here it is shown that IWF contains beta-glucosidase activity which releases abscisic acid (ABA) from the physiologically inactive ABA-glucose conjugate pool in the leaf apoplast. The following data support this conclusion and give the first biochemical and physiological characterization of the extracellular glucosidase activity in barley. Free ABA was released by the incubation of ABA glucose ester with IWF. The product exhibited the retention time of authentic ABA upon separation by thin layer chromatography and was identified by ABA-ELISA. p-Nitrophenol-beta-D-glucopyranoside (pNPG) was used as the substrate for beta-glucosidases. The K(M)(pNPG) was 1.8 mmol l(-1). The activity was affected by ABA glucopyranoside in a competitive type of inhibition with a K(I) of 400 micromol l(-1). Various hormone conjugates were compared with respect to their inhibitory effect on beta-glucosidase activity. Inhibition was highest for the ABA glucopyranoside and the zeatin riboside, but insignificant for ABA methyl ester and zeatin-9-beta-D-glucoside. The specific activity of the beta-glucosidase was 16-fold greater in IWF as compared to crude leaf extracts confirming its extracellular compartmentation. The activity of beta-glucosidase was strongly increased after growth in hydroponic medium supplemented with NaCl. The data support the hypothesis that the glucose conjugate is a long-distance transport form of ABA.

Abscisic acid is a hormonal stress signal that moves in the xylem from the root to the different parts of the shoot where it regulates transpirational water loss and leaf growth. The factors that modify the intensity of the ABA signal in the xylem are of particular interest because target cells recognize concentrations. ABA(xyl), will be decreased as radial water flow through the roots is increased, assuming that radial ABA transport occurs in the symplast only. Such dilutions of the plant hormone concentration can be compensated in different ways, which help to keep the ABA-concentrations in the xylem constant: (i) apoplastic bypass flows of ABA, (ii) ABA flows between the stem parenchyma and the xylem during transport and (iii) the action of beta-D-glucosidases that release free ABA from its conjugates to the root cortex and the leaf apoplast. The significance of reflection coefficients (sigma(ABA)), permeability coefficients of membranes (P(S)(ABA)) and apoplastic barriers for ABA is discussed.

Abscisic acid (ABA) conjugates, predominantly their glucose esters, have recently been shown to occur in the xylem sap of different plants. Under stress conditions, their concentration can rise substantially to levels that are higher than the concentration of free ABA. External ABA conjugates cannot penetrate apoplastic barriers in the root. They have to be hydrolysed by apoplastic enzymes in the root cortex. Liberated free ABA can then be redistributed to the root symplast and dragged directly across the endodermis to the stele. Endogenous ABA conjugates are formed in the cytosol of root cells, transported symplastically to the xylem parenchyma cells and released to the xylem vessels. The mechanism of release is unknown; it may include the action of ABC-transporters. Because of its extremely hydrophilic properties, ABA-GE is translocated in the xylem of the stem without any loss to the surrounding parenchyma. After arrival in the leaf apoplast, transporters for ABA-GE in the plasmalemma have to be postulated to redistribute the conjugates to the mesophyll cells. Additionally, apoplastic esterases can cleave the conjugate and release free ABA to the target cells and tissues. The activity of these esterases is increased when barley plants are subjected to salt stress.

Abscisic acid is a hormonal stress signal that moves in the xylem from the root to the different parts of the shoot where it regulates transpirational water loss and leaf growth. The factors that modify the intensity of the ABA signal in the xylem are of particular interest because target cells recognize concentrations. ABA(xyl), will be decreased as radial water flow through the roots is increased, assuming that radial ABA transport occurs in the symplast only. Such dilutions of the plant hormone concentration can be compensated in different ways, which help to keep the ABA-concentrations in the xylem constant: (i) apoplastic bypass flows of ABA, (ii) ABA flows between the stem parenchyma and the xylem during transport and (iii) the action of beta-D-glucosidases that release free ABA from its conjugates to the root cortex and the leaf apoplast. The significance of reflection coefficients (sigma(ABA)), permeability coefficients of membranes (P(S)(ABA)) and apoplastic barriers for ABA is discussed.