Episodic association of strict halophytes and glycophytes in a saline, hydromorphic ecosystem in semi-arid zones.

The biomass production and the uptake of mineral nutrients (N, P, K) in an ungrazed area edging the sebkha of Enfidha (100 km south-east of Tunis; semi-arid bioclimatic zone) were studied for 2 successive years; the first year was particularly dry, the second relatively rainy. Half the aerial biomass was produced by perennial halophytes. Annual plants were responsible for a large part of the ecosystem productivity in the second year. Among them, Medicago ciliaris (L) Krock, M polymorpha L, M truncatula Gaertn and M minima Grufb were the dominant species (40% of ecosystem primary production). We studied their response to salinity in controlled culture conditions. Vegetative growth (dry matter production) was reduced by NaCl (110 or 160 mM) in all Medicago species. This effect was related to a reduction in leaf area rather than in assimilation efficiency. The number of seeds produced per plant and per unit biomass was severely reduced in the presence of salt. In contrast, individual seed weight, as well as their germinating power, were only weakly affected. In spite of their origin (edge of sebkha crowded by halophytes), the Medicago species studied appeared to be glycophytic, on the basis of their growth and nutritional responses to NaCl. Their reproduction depended on the maintenance of the salt concentration at low levels. These results suggest that these plants exploit the upper horizon layer, which is less salty than the deeper ones, which support the halophytes. The presence of halophytes might contribute to maintaining a fertile and less salty superficial layer, occupied periodically by annual Medicago species.