ABA-Mediated Stomatal Response in Regulating Water Use during the Development of Terminal Drought in Wheat

End-of-season drought or "terminal drought," which occurs after flowering, is considered the most significant abiotic stress affecting crop yields. Wheat crop production in Mediterranean-type environments is often exposed to terminal drought due to decreasing rainfall and rapid increases in temperature and evapotranspiration during spring when wheat crops enter the reproductive stage. Under such conditions, every millimeter of extra soil water extracted by the roots benefits grain filling and yield and improves water use efficiency (WUE). When terminal drought develops, soil dries from the top, exposing the top part of the root system to dry soil while the bottom part is in contact with available soil water. Plant roots sense the drying soil and produce signals, which on transmission to shoots trigger stomatal closure to regulate crop water use through transpiration. However, transpiration is linked to crop growth and productivity and limiting transpiration may reduce potential yield. While an early and high degree of stomatal closure affects photosynthesis and hence biomass production, a late and low degree of stomatal closure exhausts available soil water rapidly which results in yield losses through a reduction in post-anthesis water use. The plant hormone abscisic acid (ABA) is considered the major chemical signal involved in stomatal regulation. Wheat genotypes differ in their ability to produce ABA under drought and also in their stomatal sensitivity to ABA. In this viewpoint article we discuss the possibilities of exploiting genotypic differences in ABA response to soil drying in regulating the use of water under terminal drought. Root density distribution in the upper drying layers of the soil profile is identified as a candidate trait that can affect ABA accumulation and subsequent stomatal closure. We also examine whether leaf ABA can be designated as a surrogate characteristic for improved WUE in wheat to sustain grain yield under terminal drought. Ease of collecting leaf samples to quantify ABA compared to extracting xylem sap will facilitate rapid screening of a large number of germplasm for drought tolerance.

Mannitol permeation and radial flow of water in maize roots

• The postulated nonselective hydraulic route through the root apoplast has not yet been supported by experimental findings on solvent drag. • Therefore, mannitol transport from the medium to the shoot of young maize plants was studied at different rates of transpiration in hydroculture. The concentration of mannitol was determined by gas chromatography. • Mannitol utilization in shoot metabolism was not detectable. Experiments with exuding roots showed that the radial transport of mannitol was mainly apoplastic. The ratio α between the mannitol concentration in xylem vessels and that of the external medium was calculated from mannitol translocation to the shoot or measurement of the mannitol concentration in the root exudate, where it reached c. 0.07 in the steady state. In transpiring plants, α decreased with increasing water flux from 0.04 to values below 0.01. These findings demonstrate that the root reflection coefficient for mannitol is above 0.99. • It is concluded that the radial movement of water to the vessels is under complete protoplastic control, whereas solutes can diffuse on an apoplastic path. The absence of a significant volume flux through the root apoplast is of physiological importance as it prevents the coupling of the apoplastic permeation of ballast solutes, such as NaCl, to transpiration.

Xylem development in relation to water uptake by roots of grapevine (Vitis vinifera L.)

Axial resistance (R_a ) was estimated from xylem diameter measurements obtained from periodic acid and toluidinc blue O (PAS-TBO) stained sections of Vitis vinifera L. ev. Shiraz. Multiple linear regression showed a strong negative relationship between axial resistance and either root diameter or distance from the root tip. Water stress treatments did not affect the relationships, but plant age significantly influenced the intercept of the regression. The use of both lignin and cytoplasm stain showed that some vessels retained degenerating protoplasm which would impede water flow. Inclusion of these vessels in calculations of axial resistance could account for the underestimation reported by some authors in comparison with experimental determinations. Calculations based on the assumption that all xylem vessels, large and small, are involved in axial water conduction showed that use of mean xylem diameter might result in overestimating values by a magnitude of between 1.7 and 4.4. The use of individual xylem vessel diameters gave more accurate estimation of axial resistance. Some adjacent secondary xylem and metaxylem vessels were observed to merge into single vessels as a result of breakdown of the wall between them. Implications of such a developmental phenomenon are discussed.

Hydraulic resistance to radial water flow in growing hypocotyl of soybean measured by a new pressure-perfusion technique

Hydraulic resistances to water flow have been determined in the cortex of hypocotyls of growing seedlings of soybean (Glycine max L. Merr. cv. Wayne). Data at the cell level (hydraulic conductivity, Lp; half-time of water exchange, T 1/2; elastic modulus, ɛ; diffusivity for the cell-to-cell pathway, D c) were obtained by the pressure probe, diffusivities for the tissue (D t) by sorption experiments and the hydraulic conductivity of the entire cortex (Lpr) by a new pressure-perfusion technique. For cortical cells in the elongating and mature regions of the hypocotyls T 1/2=0.4-15.1 s, Lp=0.2·10(-5)-10.0·10(-5) cm s(-1) bar(-1) and D c=0.1·10(-6)-5.5·10(-6) cm(2) s(-1). Sorption kinetics yielded a tissue diffusivity D t=0.2·10(-6)-0.8·10(-6) cm(2) s(-1). The sorption kinetics include both cell-wall and cell-to-cell pathways for water transport. By comparing D c and D t, it was concluded that during swelling or shrinking of the tissue and during growth a substantial amount of water moves from cell to cell. The pressure-perfusion technique imposed hydrostatic gradients across the cortex either by manipulating the hydrostatic pressure in the xylem of hypocotyl segments or by forcing water from outside into the xylem. In segments with intact cuticle, the hydraulic conductance of the radial path (Lpr) was a function of the rate of water flow and also of flow direction. In segments without cuticle, Lpr was large (Lpr=2·10(-5)-20·10(-5) cm s(-1) bar(-1)) and exceeded the corticla cell Lp. The results of the pressure-perfusion experiments are not compatible with a cell-to-cell transport and can only the explained by a preferred apoplasmic water movement. A tentative explanation for the differences found in the different types of experiments is that during hydrostatic perfusion the apoplasmic path dominates because of the high hydraulic conductivity of the cell wall or a preferred water movement by film flow in the intercellular space system. For shrinking and swelling experiments and during growth, the films are small and the cell-to-cell path dominates. This could lead to larger gradients in water potential in the tissue than expected from Lpr. It is suggested that the reason for the preference of the cell-to-cell path during swelling and growth is that the solute contribution to the driving force in the apoplast is small, and tensions normally present in the wall prevent sufficiently thick water films from forming. The solute contribution is not very effective because the reflection coefficient of the cell-wall material should be very small for small solutes. The results demonstrate that in plant tissues the relative magnitude of cell-wall versus cell-to-cell transport could dependent on the physical nature of the driving forces (hydrostatic, osmotic) involved.

Water-relation parameters of epidermal and cortical cells in the primary root ofTriticum aestivum L

Water-relation parameters of root hair cells, hairless epidermal cells, and cortical cells in the primary root of wheat have been measured using the pressure-probe technique. Under well-watered conditions the mean cell turgor of cortical cells was 6.8±1.9 (30) bar (mean±SD; the number of observations in brackets). In hairless epidermal and root hair cells the mean cell turgor was 5.5±1.9 (22) and 4.4±1.5 (15) bar, respectively. Despite the large variability, turgor pressure was significantly lower (confidence interval=0.95) in epidermal cells relative to cortical cells. This may be a consequence of the ultrafiltration of ions by the external cell wall and-or plasmalemma of epidermal cells. The volumetric elastic modulus of the cells ranged from 10 to 150 bar. This parameter was dependent on cell volume, but within experimental accuracy, was independent of cell type. No pressure dependence of the volumetric elastic modulus was observed in these cells. The half-times for water exchange ranged from 1.8 to 48.8 s. The mean value increased in the order root hair < hairless epidermal < cortical cells and was directly related to volume to surface area ratio. Thus the hydraulic conductivities of the three cell types were similar and averaged 1.2±0.9·10(-6) (170) cm s(-1) bar(-1). No polarity was observed between inwardly and outwardly directed water flow. The similarity of the hydraulic conductivities of root hairs to those of other cells indicates that the membranes of root hairs are not particularly specialized for water transport. The overall hydraulic conductivity for radial water flow across the root was estimated from the pressure-probe data using a simple model and was compared with that measured directly on whole roots using an osmotic backflow technique. It was tentatively concluded that upon sudden osmotic perturbation, the major pathway for water transfer across the root may be through the symplasm and involve net flow from vacuole to vacuole.