Roots are indeterminate organ systems that grow apically, potentially indefinitely, and that form lateral roots at a distance from the growing apex. The patterns and rates of root system growth are influenced by the distribution and concentration of mineral nutrients in the soil, the availability of water and the degree of soil compaction. The distribution of some mineral nutrients such as phosphate and iron is very heterogeneous, due to their strong ionic interactions with the soil matrix and the strong pH-dependency of their solubility. The abundance of such immobile minerals can vary by an order of magnitude at scales of a 100 ^m (Strawn et al. 2002). In contrast, other nutrients such as nitrate and potassium are at least tenfold more mobile in the soil (Marschner 1995), and therefore tend to accumulate as solutes above water-impermeable clay layers. The distinct physicochemical properties of plant mineral macronutrients implies that there should be at least two distinct growth or foraging strategies in response to limitation of soil minerals: for immobile minerals, the most efficient response to enhance uptake is to increase the surface area of the root to directly contact soil particles in previously unexploited domains of the soil. Increased branching, radial thickening, and growth of root hairs, while suppressing primary root growth, best accomplish this objective. In contrast, for mobile elements, the optimal strategy is to enhance root apical growth to reach deeper layers where such solutes accumulate.
Both syndromes are observed: under conditions of phosphate starvation, reduced primary root growth, enhanced lateral root formation, and stimulation of root hair growth is observed (Lopez-Bucio et al. 2002), while during iron starvation, mostly root hair growth is stimulated (Muller and Schmidt 2004). Both iron and phosphate have low mobility in the soil column. In contrast, root apical growth is stimulated in low nitrate (0.1-2.5 mM), when compared to higher concentrations, and this is due to a larger population of dividing cells and a delayed phase I/II transition (Dubrovsky and Doerner, unpublished). At lower concentrations (< 50 ^M), primary root growth per sists for a while without stimulation of lateral root growth (Lopez-Bucio et al. 2003). Nitrate is relatively mobile in the soil column and accumulates above water-impermeable layers in the ground.
The analysis of root growth parameters is in many ways more straightforward than in shoots: organ growth is essentially anisotropic, and because the different processes contributing to organ growth occur in spatially distinct domains, they can be more readily analyzed. Kinematic analysis is very powerful in this respect, but has surprisingly only been used in a few cases for root growth analysis in Arabidopsis (Beemster and Baskin 1998,2000; Beems-ter et al. 2002). In Sect. 4.1, I will focus on growth responses to phosphate starvation as these have been analyzed in greater detail than for other mineral nutrients.
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