3.2.5  Roots responding to soil constraints

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Roots respond to selection pressures imposed by temporal and spatial patterns in water and nutrient supply such as those shown in Figures 3.6 and 3.7. For example, seasonally fluctuating water tables are reflected in shifts in root growth of swamp paperbark in order to maximise extraction of non-saline water (Chapter 18). Similarly, heterogeneous spatial distributions of water and individual nutrients call for root structures able to extract all resources required for growth. In many tree species, superficial roots deplete the enriched surface layers of soil-immobile nutrients while sinker roots tap water and soil-mobile nutrients (e.g. nitrate) which leach deeper into the soil profile. For example, dimorphic root systems of Banksia prionotes (Figures 3.2) are characterised by sinker roots which have almost constant resistance (high conductance) along their axes and are therefore well adapted to remove water from deep in the soil. Finer lateral roots with very low conductances appear to play a more minor role in water uptake (Figure 3.2b). B. prionotes also has proteoid roots which absorb much of the plant’s nutrient requirements. Thus, roots have become genetically modified to accommodate the variability in resource availability that characterises each soil type and climate. Photoassimilate costs of producing such complex root systems can be high.