Selectivity of the root absorption of nutrient ions in grain sorghum

Root absorption selectivity, which refers to the fact that the mineral elements are not found in the same proportions in the plant as in the growth medium, was characterized using plots where the proportions between the elements were considered. Grain sorghum was grown hydroponically in nutrient solutions where the cationic macroelements were increasingly replaced with sodium and the anionic macronutrients with chloride. The data obtained from the analyses of the nutrient solutions and of the whole plants show that root selectivity promotes the absorption of K+ and NO3- -N and impedes that of Ca++ whereas there is nearly no selectivity towards the other nutrients (Mg, P, S).The high negative selectivity towards unessential elements (Na and Cl) is reflected by their very low proportions within the plant.The comparison between the various plant organs shows that the proportions of the essential elements are nearly identical in the roots, the sheaths, the younger leaves and in the adult leaves (only for the anions). The differences observed for the older leaves are to be assigned to their senescence status which conditions the possibilities of translocation of the elements within the plant.