3.6.5  Transport of water and solutes

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Water and nutrients are acquired by roots in a mutually dependent fashion because bulk flow of soil solution carries with it a cargo of nutrients to root surfaces. Furthermore, most essential macronutrients become more concentrated during absorption, creating water potential gradients and inducing water flow into roots (‘root pressure’). Indeed, if roots fail to take up nutrients as fast as water, water uptake is gradually restricted (Section 3.6.2).


Figure 3.25 Transpiration (---) and phosphate absorption (circles) by sugar cane plants over a five-day period showing diurnal fluctuations in transpiration and much smaller oscillations in phosphate absorption. Tight regulation of phosphate inflow removes the extremes of phosphate supply to shoots which would follow if uptake were passive (Based on van den Honert et al. 1955)


Figure 3.26 Osmotic pressure (reflecting total solute concentration) of xylem sap of young barley plants at a range of transpiration rates. Transpiration rates were imposed by varying vapour pressure deficit around the shoots and xylem sap was sampled by applying sufficient pressure to roots to cause a cut leaf tip to bleed (Based on Munns and Passioura 1984)

Inflow of water is driven by two processes, transpirational pull and osmotically derived ‘root pressure’. Gradients in water potential generated by transpiration from shoots (suction) are sufficient to draw soil solution to the roots (Section 3.2). For example, phosphate and water uptake coincide in time when followed over several days in sugar cane plants (Figure 3.25). The degree to which this ionic mixture is modified before entering the xylem will be determined partly by ionic interactions in cell walls (Donnan Free Space) and to a much greater extent by membrane transport properties as ions enter the symplasm. Imbalance between water and ion uptake can generate apoplasmic solute levels high enough to drought a plant growing in fertile, damp soils under sunny conditions (Section 3.6.3). Reserves of ions in root cell vacuoles can help to buffer deficits of ions in the uptake stream: for example, ions of potassium, nitrogen and phosphorus stored in vacuoles can represent up to 90% of the cell’s reserves. None the less, plants which are transpiring rapidly generally have a nutrient-depleted (low osmotic pressure) xylem stream compared to slowly transpiring plants (Figure 3.26).

At the other extreme, plants which have had shoots removed so xylem exudates can be sampled from cut stumps have very concentrated xylem fluid which is released under hydrostatic pressure from the roots (‘root pressure’). This flow is osmotically driven (water follows ions into the roots) and apart from the droplets seen on the margins of guttating leaves in early morning, it is an inconspicuous contributor to sap flow. The contribution of ‘root pressure’ to sap flow in rainforest species may be significant but in the brighter environments of more open canopies, transpirational pull is the dominant force in sap flow