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Four local Cenchrus ciliaris L. accessions (P1: Bou Hedma, P2: Tozeur, P3: Raas Jedir, and P4: Sidi Toui) were planted in a common garden experiment in Tunisia in autumn 2002. Net photosynthesis (Pn), stomatal conductance (gs) and leaf water potential (Psi) were measured between September 2003 and September 2004. All accessions showed two distinct periods of growth, interspersed by summer and winter dormancy periods. Initiation of the cycle occurred in autumn and appeared to be associated with soil water availability. There were significant differences (p < 0.001) between seasonal measures of net photosynthesis (Pn) and stomatal conductance (gs) of each accession. The highest value was observed for accession P3 (42 mu mol m(-2) s(-1)). P1 and P4 showed the same value of Pn (35 mu mol m(-2) s-(1)) in spring. Photosynthetic rate (Pn) of all accessions decreased with decreased stomatal conductance (gs). The annual course of predawn water potential (Psi(pd)) and minimum water potential (Psi(md)) observed during the year, showed that lower water potentials were observed at the end of spring (-3.0 MPa). As would be expected, higher water potentials (-1.0 MPa) were observed during winter, due to the high level of rainfall during that season. It has been observed that seasonal water availability and air temperature influence the seasonal dynamics of photosynthesis and leaf water potential. Cenchrus ciliaris accessions originated from different regions of Tunisia, and growing under the same conditions did not differ in their net photosynthesis and water status.