Ion Transport in Wetland Roots

Of wetland plants, rice has been studied the most extensively, and nitrogen has been the most extensively studied element. In this section the rates at which rice roots can absorb nitrogen are discussed and whether this is affected by the morphological and physiological adaptations to anoxic soil conditions. 

The aim is to measure the influx of the nutrient into a root for a given concentration of the nutrient in the soil solution at the root surface. This is a seemingly simple matter. But there are well-known difficulties in obtaining unequivocal information (Marschner, 1995 Tinker and Nye, 2000). The main problem is that the influx of the nutrient is closely regulated by the plant and depends sensitively on the current nutrient content of the plant as well as the external concentration the root is exposed to. Over time the plant will adjust its intake to the new external concentration, so the measured influx will be a function of how long the plant has been exposed to the new concentration. Measurements should therefore be made as rapidly as possible following exposure to the new concentration. 

It is apparent that the roots have considerable flexibility in their response to the external N concentration. Influx of NH4+ is up-regulated as the plant s internal N status decreases, but suppressed as the N status increases. Hence there 

Source Wang et al. (1993b). Reproduced by permission of the American Society of Plant Biologists. 

These results indicate it may be possible to improve the efficiency of absorption and assimilation by altering the process of regulation. However the mechanisms governing regulation are poorly understood. It is not known whether the regulation is linked to the concentration of NH4+ or NOs itself or to the concentrations of products of N assimilation downstream from NH4+ or NOs , such as particular amino acids. Nor is it known what the targets of the resulting feedback mechanisms are. 

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