LONG-TERM RESPONSES OF MELILOTUS-SEGETALIS TO SALINITY .2. NUTRIENT ABSORPTION AND UTILIZATION

Specific absorption rates (SAR) and specific utilization rates (SUR) o f sodium, chloride, potassium, calcium, magnesium and phosphate ions w ere determined for Melilotus segetalis (Brot) Ser. (annual sweetclover ) grown under both control and salinized conditions (NaCl treatment of CE = 15 dS m(-1)) for a complete life cycle with sequential harvests. The behaviour over time of the SARs and SURs of the mineral elements was in general correlated with relative growth rate (RGR) kinetics, wi th a parabolic trend during the vegetative phase and a progressive lin ear decrease during the reproductive stage. Salinity significantly red uced the SARs of K and Mg but did not affect the SARs of Ca and P duri ng the vegetative phase. During the reproductive stage, however, the S ARs of K, Ca and P of salt-stressed plants were higher than in control plants. The similar SARs of total cations (TC) found in control and s alt-stressed plants may indicate compensatory mechanisms to maintain a constant total cation content. Salt-stressed plants showed lower SURs of K, Ca and P during the vegetative phase, and lower SURs of K and P but a higher SUR of Mg during the reproductive stage. A nutrient imba lance, caused by a lower root efficiency in absorbing K and Mg and a l ower leaf efficiency in producing biomass per unit of K, Ca and P, app arently contributed to the salt-induced reduction in growth during the vegetative phase of M. segetalis. The switch to nonreduced, compensat ed growth during the reproductive phase may have been caused by a high er nutrient demand which increased the root efficiency in absorbing K, Ca and P and the leaf efficiency in utilizing Mg.