The Physiology of Flowering Plants - KHAM PHA MOI

78 WATER RELATIONSmaximally to 10 m, so root pressures in the usual range could raisewater no higher than 10 m. The maximal values of 0.6 MPa could raisewater to 30 m, still far short of the height of many trees. The quantityof water that can be moved by root pressure is small: e.g. wheatseedlings (Triticum aestivum), which transpired about 3 mL water perhour, exuded only 0.5 mL per hour by root pressure. In manyinstances maximum bleeding rates are only 1–2% of the water lossoccurring by transpiration. Root pressure persists only as long as thewater-yielding capacity of the soil is high, but plants can still extractand transport water after root pressure becomes inactivated througha lowering of the soil C. As stated, root pressures in temperateclimates are most frequently developed during warm nights; mostof water transport occurs during daytime. It is moreover mostlyherbaceous species that develop root pressures. In deciduous treesroot pressures are demonstrable in the spring before the buds open,but once the leaves have expanded and rapid water movementthrough the plant begins, root pressures can no longer be detected.During the periods of rapid water movement associated withrapid transpiration, the vast majority of evidence in fact indicatesthat the water is not under positive pressure but under tension. Insuch circumstances, a cut in the xylem does not result in sap exudation,but if the cut is made under water, the water is sucked in (as seenif a dye is added to the water). Transpiring twigs can pull wateragainst an artificial resistance more effectively than a vacuumpump; leafy twigs can raise a column of mercury to heights greaterthan can be supported by atmospheric pressure. On the basis of suchevidence it is generally accepted that water movement in plants,particularly in woody species, is the result of water being pulled upto replace that lost by transpiration. We therefore need to explainhow water can be pulled up under tension to the topmost leaves ofthe tallest trees.The cohesion–tension theory (transpiration–cohesion theory)for the ascent of xylem sapA theory of the ascent of sap based on the cohesive properties ofwater was advanced independently in 1894 by Dixon and Joly, and in1895 by Askenasy. This theory, as described below, is still supportedin its essentials by most plant physiologists, although alternativeshave also been postulated (Section 3.4.4).The motive force for root pressure is generated at the root end ofthe plant. The generation of tension takes place at the leaf end. Theliving cells of the leaf contain solutes and commonly have a C wellbelow 0, say down to –2 MPa under ‘average’ conditions in a temperateclimate. But they are still relatively water-saturated comparedwith the atmosphere for most of the time in most climates. The C ofthe atmosphere is usually very much lower than that of the leaf cells,say –10 to –50 MPa. To put it another way, the atmospheric vapourpressure is usually very much lower than would be in equilibriumwith the leaf cells. There are of course occasions when the