Possibilities and limitations of utilizing 28Mg for studying Mg‐specific problems in higher plants

The possibility of utilizing artifical radionuclides in agricultural biological research enabled studies to be implemented in which the detection sensitivity of the nutrients to be investigated was one to two powers of ten higher than with physicochemical methods. Although usable radionuclides are available for numerous plant nutrients, their radiochemical properties partly restrict their utilization. For example, the relatively short half‐life of ²⁸Mg of 21.3 hours only permits short‐term studies. This is all the more disadvantageous the lower the specific activity of ²⁸Mg is. The ²⁸Mg produced from a lithium target in the experimental reactor Merlin at the KFA Jülich achieves a maximum specific activity of 1 mCi/g Mg. In contrast, specific activities of over 1 Ci/g Mg are achieved in the isochronous cyclotron where ²⁸Mg is generated from an aluminium target. This more or less carrier‐free ²⁸Mg is especially suitable for short‐term studies (minutes or hours) or for problem areas in which the ²⁸Mg uptake rates are very low. It was able to be demonstrated on the basis of autoradiographs that Mg is a relatively mobile nutrient in higher plants and is transported both in the xylem and also in the phloem. Its uptake and transport rate does not only depend on the presence and concentration of competing cations but is also considerably influenced by the type of plant and its nutritional status, especially by its K content. In studies on strawberry plants and grapevines it was demonstrated that the transport rate of Mg in the fruits was in part considerably greater than that of Ca. However, this relative preference for Mg in comparison to Ca largely depends in the case of grapevines on the uptake conditions and the type of application (dipping or trough application). In comparative studies of the leaf uptake of ²⁸Mg it was established that considerably more ²⁸Mg was taken up from ²⁸MgCl₂ than from a complete nutrient solution or leaf dressing solution.