Mobilization of rock phosphate by rape (Brassica napus L.)

<p>Rape <em>(Brassica napus)</em> is known as an effective user of sparingly soluble rock phosphates. The research reported in this thesis aimed to establish the cause of this phenomenon.<p>With the help of an agar plate technique it was established that phosphate-deficient rape plants grown with nitrate as nitrogen source acidify their rhizosphere. The acidification is restricted to a root zone of about 1.5 cm behind the root tip, and is not related to nutrient uptake. Enzymatic analyses revealed that more malic and citric acid is exuded in the acidified part of the rhizosphere than in the alkalinized part and that the concentrations of these acids in the exuding root segments are higher. It is concluded that acidification of the rhizosphere by exudation of organic acids might enable rape to mobilize rock phosphate.<p>The concentration of citric acid in the shoots of phosphate-deficient rape plants is also higher. The results of experiments in which the shoots of rape plants were exposed to labelled carbon dioxide indicated that the exuded acids originate from the shoot.<p>To calculate the effect of organic acid exudation on phosphate uptake from rock phosphate a simulation model was used. The first version model describes the uptake of a growth-limiting, dissolved nutrient from soil by a growing root system. The uptake of the nutrient depends on the rate of nutrient supply towards the roots by mass flow and diffusion. Allowance is made for both time-dependent root density and inter-root competition. Each root is assigned a finite cylindrical soil volume delivering nutrients. This soil volume per unit root length declines with increasing root density.<p>Simulated and experimental results agreed well when uptake of nitrate or (dissolved) phosphate from a quartz sand/nutrient solution mixture was described at growth-limiting supply (a).<p>An experimentally determined relation was used to describe the effect of decreasing pH on the solubility of Mali rock phosphate (b). Parameters on exudation were measured in rape plants grown without phosphate. The values of these parameters were assumed to describe the potential of rape plants to mobilize rock phosphate. It appeared that simulated phosphate uptake was greater than observed uptake (c). From (a), (b) and (c) it is concluded that measured rates of organic acid exudation are more than sufficient to explain the relatively large uptake of phosphate from rock phosphate by rape