The Meaning of Matric Potential

The commonly used equation, = P - + , which describes thepartitioning of plant water potential, , into components ofhydrostatic pressure, P, osmotic pressure, , and matric potential,, is misleading. The term , which is supposed to show the influenceof a solid phase on , is zero if a consistent definition ofpressure is used in the standard thermodynamic derivation. However,it can be usefully defined by = + _D, where _D is the osmoticpressure of the equilibrium dialysate of the system. The practicaland theoretical significance of this definition is discussed.     

The Transport of Water from Soil to Shoot in Wheat Seedlings

A technique is described for precisely measuring the drop inwater potential, , between the soil and the leaf xylem of wheatseedlings. The technique was used to explore the relation betweentranspiration rate and at various times during the monotonicdrying of the soil in which the plants were growing. When thesoil was wet, the relation was linear, but, as the soil dried,nonlinearities appeared which were, in the main, explicablein terms of simple soil physical models describing the flowof water through the soil to the roots. There was no sign ofthe major hydraulic resistance at the root: soil interface thatother people have recently found.     

Stored xylem sap from wheat and barley in drying soil contains a transpiration inhibitor with a large molecular size

Xylem sap was collected from wheat and barley growing in a drying soil, and the effect of the sap on transpiration was detected by a bioassay with detached wheat leaves. The inhibitory activity of fresh sap was small, and could be largely accounted for by the abscisic acid content (about 2×10^-5mol m^-3). When fresh sap was stored at -20°C for several days, the activity increased. Maximum activity developed after a week. This increase in activity was due to a compound that increased in size with storage at -20°C. When fresh sap was fractionated with filters of different molecular size exclusion characteristics, and the separated fractions stored at -20°C for a week, activity developed only in the fraction containing compounds smaller than 0·3 kDa. However, when sap already stored at -20°C was fractionated, activity was only in fractions containing compounds larger than 0·3 kDa. The increase in activity and in size did not occur with storage in liquid nitrogen (-196°C) or at -80°C. These results suggest that storage at -20°C causes the aggregation or polymerization of a small compound with low activity to form a large compound with high activity. This change is not catalysed by an enzyme because it can occur in a fraction from which molecules larger than 0·3 kDa are removed. It is probably promoted by high solute concentrations when ice crystals form. Sap collected from plants in soils of high water potential had little or no activity after storage at -20°C.     

The water relations of the root–soil interface

The difference in hydrostatic pressure between the xylem of the leaf and the soil depends, for a given transpiration rate, on the series of hydraulic resistances encountered along this pathway. Many studies have shown that the sum of the resistances in the plant and the soil is too small to account for the fall in water pressure between the leaf xylem and the soil, especially when plants are growing in sandy soils, which are prone to dry rapidly. A resistance at the root–soil interface, caused possibly by poor contact between the roots and the soil, has been proposed to account for the discrepancy. We explored the resistance in the pathway from soil to leaf using a technique that allows precise and continuous non-destructive measurement of the hydrostatic pressure in the leaf xylem. When the soil was leached with water, the fall in leaf water status as the soil dried was reasonably well described by a simple physical model without the need to invoke an interfacial resistance. However, when the soil was flushed with a nutrient solution with an osmotic pressure of 70kPa, the hydrostatic pressure in the leaf xylem fell several times faster than that in the soil. We suggest that solutes accumulated either in the root or just outside it, creating large osmotic pressures, which gave the appearance of an interfacial resistance.     

Comparative Leaf Epidermal Anatomy of Mutants of Barley (Hordeum vulgare L. 'Himalaya') which Differ in Leaf Length

We wished to determine the nature of differences in epidermalcell numbers and dimensions between leaves of different lengthin mutants of barley (Hordeum vulgare L. ‘Himalaya’).Three comparisons were made: leaf one (L1)vs. leaf four (L4);wild typevs. nine dwarf mutants and wild typevs. a slender mutant.L1 was shorter than L4, and for most lines this was associatedwith a change in epidermal cell number for the blade, and inboth cell number and length for the sheath. Compared to wildtype, the smaller leaves of dwarf plants generally had shorterand fewer cells in both blade and sheath. The blade of slenderplants was the same length (L1) or longer (L4) than wild type,while the sheath was longer than that of wild type for bothL1 and L4. Slender plants had longer but fewer cells than thewild type along the blade of L1, and shorter but more cellsfor the blade of L4. In the sheath, slender plants had longerand more (L1) or fewer (L4) cells than did the wild type. ForL1, variation in blade width amongst the barley lines was associatedwith a change in file width and file number. For L4, blade widthvaried only with file number, except for slender plants wherenarrow blades were associated with reduced file width. Hencethere was no consistent correlation between changes in cellsize or cell (or file) number with changes in leaf length orwidth. Differences depended on the leaf (L1vs. L4), leaf part(bladevs. sheath), and the nature of the mutation (dwarfvs.slender). Barley (Hordeum vulgare L. ‘Himalaya’); leaf epidermis; dwarf mutant; slender mutant     

Water relations under root chilling in a sensitive and tolerant tomato species

The shoots of cultivated tomato (Lycopersicon esculentum cv. T5) wilt if their roots are exposed to chilling temperatures of around 5 °C. Under the same treatment, a chilling‐tolerant congener (Lycopersicon hirsutum LA 1778) maintains shoot turgor. To determine the physiological basis of this differential response, the effect of chilling on both excised roots and roots of intact plants in pressure chambers were investigated. In excised roots and intact plants, root hydraulic conductance declined with temperature to nearly twice the extent expected from the temperature dependence of the viscosity of water, but the response was similar in both species. The species differed markedly, however, in stomatal behaviour: in L. hirsutum, stomatal conductance declined as root temperatures were lowered, whereas the stomata of L. esculentum remained open until the roots reached 5 °C, and the plants became flaccid and suffered damage. Grafted plants with the shoots of one genotype and roots of another indicated that the differential stomatal behaviour during root chilling has distinct shoot and root components.