3.6.1  Radial uptake: a dynamic component of resource acquisition

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Water plants are bathed in a solution buffered against sudden variation but land plants must extract water and ions from soil, a much more stochastic environment. Sharp variations in soil composition occur through time and space. Soil water content can, for example, swing from dryness to saturation over minutes while sharp gradients in nutrient concentration occur over less than a centimetre. Integrating water and nutrient extraction from such a heterogeneous source is achieved through a high degree of plasticity in root structure and function. Structural modifications might include local root branching to deplete nutrient-rich zones (Figure 3.4) or pockets of moisture. Function can alter even more quickly in response to concentration variations, seen, for example, in the rapid up-regulation of K+ and orthophosphate absorption when roots are deprived of these ions (Drew et al. 1984).

Water and nutrient acquisition are also influenced by shoot factors. Water status in leaves helps determine hydraulic gradients through the soil–plant–atmosphere continuum and thereby influences water uptake. Shoots are also essential for maintenance of ion uptake activity in roots because they supply photoassimilates for energy production in roots.

So, radial flow of water and ions across roots should be viewed as one important part of the entire pathway for resources entering a plant. Experiments on radial flow of water and ions reveal some of the critical sites of control for uptake of these soil-borne resources.