Figure 4.

Root system architecture responses to different environmental conditions. (a) Ideal condition: crown roots make up the bulk of the mature grass root system. (b) Drought: Proliferation of the primary root system branches, with steeper root angles. Crown root growth is arrested to conserve water. (c) Flooding: Shallow root system to avoid growing in deeper areas where oxygen is less available. Ethylene accumulation stimulates additional aerenchyma formation, which allows oxygen transport in roots. In addition, the formation of radial oxygen loss barriers helps to reduce the loss of oxygen transported via the aerenchyma. (d) Salinity: Reduction of root elongation. Formation of suberized barriers for salt exclusion from transport into the shoot. (e) Nutrient availability: Foraging for mobile (e.g., nitrogen) and immobile (e.g., phosphorus) nutrients either through searching far or through exploring local areas by increased branching. (f) Soil compaction: Thickening of primary root and older crown roots facilitates soil penetration but is not sufficient to allow for growth in more compacted areas. Crown roots from newer nodes are thicker from the time of emergence due to increased cortex cell area and are less sensitive to soil compaction. Increased aerenchyma counteracts the effects of hypoxia in roots growing in compacted soil