Differential Shoot Nutrient Stoichiometry in Sulla carnosa Ecotypes Exposed to Salt Stress under Greenhouse Conditions

Soil salinity is a major environmental constraint in arid and semi-arid regions, impairing plant growth by inducing osmotic and ionic stress and disrupting nutrient balance. Understanding how halophytic species regulate nutrient stoichiometry under salinity is crucial for improving both their ecological and agronomic use. This study investigated the effects of salt stress on growth and shoot nutrient stoichiometry in four Tunisian ecotypes of Sulla carnosa, a halophytic forage species of major ecological and economic significance in saline ecosystems. Salinity significantly reduced growth in all ecotypes, although the decrease magnitude was ecotype dependent. Based on the sensitivity index, Kalbia was identified as the most tolerant ecotype, whereas Sidi Khlif was the most sensitive. As hypothesized, salt stress triggered marked changes in nutrient stoichiometry; however, the extent and type of these changes varied among ecotypes: Kalbia maintained relatively stable nutrient ratios and ionic balance under salinity, indicating strong homeostatic regulation. In contrast, Sidi Khlif and Thelja exhibited strong alteration in nutrient relationships, reflecting the occurrence of ionic imbalance concomitant to low salt tolerance. Thus, our findings highlight ecotype-specific strategies of nutrient regulation under salt stress and demonstrate that stoichiometric stability is closely associated with improved growth and salt tolerance. In addition, we provide new insights into the mechanisms of ionic homeostasis in S. carnosa and emphasize the ecological significance of selecting tolerant ecotypes for sustainable management and rehabilitation of saline environments.