Plant nutrition between chemical and physiological limitations: is a sustainable approach possible?

The estimate of world population growth and the extent of malnutrition problems due to lack of food or to deficit of specific micronutrients bring to light the importance of plant nutrition in the context of a sustainable development. Beside these aspects, which force to use fertilizers, the topic of nutrient use efficiency of by plants is far from being solved: recent estimates of world cereals productions indicate that use efficiency of nitrogen fertilizers is not higher than 35%. These values are even smaller for phosphorus fertilizers (estimate of use efficiency between 10 and 30%), worsen by the fact that, with the present technology and on the basis of present knowledge, it is expected that the phosphorus reserves used for fertilizer production will be sufficient for less than 100 years. Efficiency problems have also been recently raised concerning the use of synthetic chelates to alleviate deficiency of micronutrients: these compounds have been shown to be extremely mobile along soil profile and to be only partially utilizable by plants. The low uptake efficiency of nutrients from soil is, in one hand, caused by several intrinsic characteristics of the biogeochemical cycle of nutrients, by the other, seems to be limited by biochemical and physiological aspects of nutrient absorption. Only recently, the complexity of these aspects has been apprehended and it has been realized that the programs of breeding had neglected these problematic. In this review aspects related to the acquisition of a macro- (N) and a micro- (Fe) nutrient, will be discussed. The aim is to show that improvements of mineral nutrient use efficiency can be achieved only through a scientific approach, considering the whole soil-plant system. Particularly emphasis will be put on aspect of molecular physiology relevant to the improvement of nutrient capture efficiency; furthermore, the role of naturally occurring organic molecules in optimizing the nutritional capacity of soil, will be considered.