Nutrient deficiency limits crop growth and yields worldwide. Fertilizer application alone cannot always ameliorate this limitation. Certain nutrients, such as phosphorus, become unavailable to plants by forming insoluble complexes at both high and low pH. Further, as with iron, diffusion rates are generally low because the ions readily bind to soil particles.
One mechanism by which the plant can augment uptake of limiting nutrients is to increase the area over which ion uptake is possible. Production of root hairs, which have been shown to increase the total root surface area between two- and fivefold over the root surface area alone, is a function of both hair length and hair density (Gahoonia et al., 1997).
Hair length can vary considerably between ecotypes of the same species. In wheat and barley, such variation correlates with zones of phosphorus depletion in the soil around the root (Gahoonia et al., 1997). There is direct evidence that this zone of phosphorus depletion is due to the activity of root hairs. In experiments with the cereal grass rye (Secale cereale), roots were grown against a mesh through which root hairs alone could protrude. As they grew across an air gap they contacted soil labelled with phosphorus (32P). After 2 days, 32P was detected in the shoot, showing that phosphorus is absorbed by root hairs and translocated to the shoot (Gahoonia and Nielsen, 1998).
Variation in root hair density directly affects the amount of phosphorus a plant can acquire and assimilate (Bates and Lynch, 1996; Gahoonia and Nielsen, 2004). Wild-type Arabidopsis, a mutant with reduced root hair number (rhd6), and a mutant that develops very short hairs (rhd2) were grown at a range of phosphate concentrations. Shoot biomass of all genotypes was higher at increased phosphorus levels, suggesting that hairs were not important under conditions where phosphate was not limiting. When grown in low phosphorus, wild-type plants had significantly more shoot biomass than rhd6, which in turn, had more shoot biomass than rhd2. When grown in increasing phosphate, rhd6 shoot biomass increased to wild-type levels. The biomass of rhd2 mutants also increased to wild-type levels, but required more phosphate than rhd6 plants. At the highest phosphorus concentrations, there was no significant difference in biomass between the three plant types. As root length did not differ significantly between the plants in these experiments, these results suggest that, under limited phosphorus availability, increased root hair length and density lead to enhanced phosphate uptake per unit root length.
Such observations in the model Arabidopsis have direct implications for field crops. Cultivars of barley with longer root hairs sustain high grain yields under low phosphate conditions - there is a positive correlation between the volume of soil explored by the root and phosphate uptake (Gahoonia and Nielsen, 2004). In field trials, cultivars with longer root hairs sustained grain yield at low phosphorus levels, and yield levels did not significantly increase with phosphate addition. By contrast, cultivars with shorter root hairs had lower grain yield under the P limited conditions, but yield was responsive to phosphate addition.
The relative increase in surface area by root hair proliferation thus increases the zone of soil around the root that the plant can mine for limiting nutrients. Root hairs can take up these nutrients directly and, under limiting conditions, longer haired ecotypes can take up and assimilate more nutrients than shorter haired ecotypes.
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