Plant physiology nutrients uptake

Soil anaerobiosis also affects plant nutrient uptake in wetland environments. Anaerobiosis in the rhizosphere, a dominant factor in wetland areas, causes physiological stresses that can limit active nptake of essential elements snch as nitrogen. The nitrogen status, in turn, can affect photosynthetic activity in plants. In addition to effects on plant growth, both intensity and capacity of reduction... 

Rhizosphere modeling remains difficult and complex, as it combines technical know-how from several fields such as plant physiology, soil physics, soil chemistry and mathematics. Mechanistic rhizosphere models do not always operate with adequate precision (Rengel, 1993 Darrah and Roose, 2001). Two main fields of application of mechanistic rhizosphere models are carbon flow in the rhizosphere and nutrient uptake by plants. While carbon flow models study the exudation of carbon compounds into the soil and its consequences on the microbial population, uptake models focus on the transport and uptake of ions by roots. In the following sections, we will concentrate on uptake models on the single root scale. 

Of more relevance to the uptake of radionuclides by plants is the question of discrimination between radionuclides and their nutrient analogues. Indeed, the question of whether the physiological mechanisms of ion uptake within the root can discriminate between the radioion and its analogue has been central to the elucidation of the environmental behaviour of 137Cs and Sr since the late 1950s. Comar et al. (1957) devised a measure of the degree of discrimination by plants between strontium and calcium which they termed the observed ratio (OR), defined as ... 

In solution culture experiments both strontium and caesium show hyperbolic absorption isotherms with respect to the external concentration of the element. Figure 7-15 (a) shows an example of a typical uptake isotherm for Sr while Shaw and Bell (1989) have demonstrated a similar isotherm for Cs. Baker (1981) has referred to such plant uptake responses as accumulator functions and has identified these as being typical of the absorption of elements over which plants can exert some degree of physiological control. Typically, the nutrient elements, including K and Ca, exhibit such isotherms and it can be postulated from the similarity in the uptake patterns of K and Cs on the one hand and Ca and Sr on the other that the radioions share, to some extent, the same uptake mechanisms as the nutrient ions. This has several important implications. Firstly, the direct competition for uptake sites between radioions and nutrient ions means that the external (soil) concentration of one is increased at the expense of the uptake of the other as the nutrient ions in question are vastly more abundant in soils than radioions it is K and Ca which will be effective in competitively excluding Cs and Sr, respectively. Secondly, the kinetics of this competition are concentration-dependent, so the assumption of first order kinetic movements of... 

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