The Physiology of Flowering Plants - KHAM PHA MOI

WATER RELATIONS OF WHOLE PLANTS AND ORGANS 77could be pumped up under pressure, or it could be pulled up undertension, negative pressure. Capillary rise (a surface tension phenomenon)could not account for a rise of more than 1 m in the finestconducting elements; many plants are much taller than this, treesreaching 90–100 m.Root pressureIn certain circumstances, the xylem sap is under positive hydrostaticpressure; when the plant is decapitated just above root level, thestump exudes sap, a phenomenon often called ‘bleeding’. A manometerfitted over such a bleeding stump registers a pressure knownas the root pressure, usually in the range of 0.1 to 0.2 MPa, exceptionallyreaching 0.5 to 0.6 MPa. The development of this pressure isdependent on the metabolic activity of the roots. No positive rootpressure is found when the roots are subjected to treatments inhibitingmetabolic activity, such as lack of oxygen, application of respiratoryinhibitors, low temperature, or starvation. The mechanism isthought to be osmotic. The exuded sap has a C value below that of thesoil because of a higher concentration of solutes, mainly inorganicions, but sometimes including organic solutes, too. It is postulatedthat the living parenchyma and transfer cells of the xylem secrete thesolutes into the conducting cells using respiratory energy. The loweringof C causes water to follow the ions into the xylem, building upa pressure – the classical osmotic pressure – which pushes up the sap.The xylem C does not equilibrate with that of the soil, since the ionsare continuously swept away with the water movement, and the rootcells continuously secrete more. In favour of this hypothesis is thefact that when the C of the medium around the roots is suddenlylowered, reversing the gradient, exudation rate falls and may evenbecome negative, so that externally applied liquid is sucked in at thecut stump. Root pressure can result in guttation, drops of liquidappearing at leaf tips and edges where the xylem sap is forced outthrough pores overlying vein endings. This liquid is much moredilute than bleeding sap at a stump, solutes having been absorbedby leaf cells. The manifestation of root pressures in temperate climatesis most frequent during warm humid weather and the pressureshows a diurnal rhythm with maxima at nights, often dropping tonear zero by day. The drops of guttation fluid are easily mistaken fordew in the morning, but an attentive examination shows that thedrops are arranged regularly, corresponding to the positions of thepores. In a strawberry (Fragaria ananassa) leaf, for instance, there is aneat droplet at the tip of every tooth of the leaf edge. In tropicalrainforests, where it is warm and humid all the time, guttation fluiddrips from shrubs and small trees, mimicking rain.Although the development of root pressures is well authenticated,in the majority of cases root pressure cannot account forwater movement. Some species apparently never develop root pressure.The observed pressures are too low to raise water to therequired level in tall plants. A pressure of 0.2 MPa can raise water