Seed Germination Characteristics ofRhus tripartitum(Ucria) Grande andZiziphus lotus(L.): Effects of Water Stress

Ziziphus lotus (L.) Lam. (Rhamnaceae) and Rhus tripartitum or Sumac (Anacardiaceae) are two indigenous species from arid and semiarid regions of Tunisia, characterized by a severe climate where dry seasons are very long. The combined action of anthropogenic factors and climate in arid regions caused a gradual threat of plant assets. In this context, an experimental study of the effects of water stress (0 to −1 MPa) on seeds has identified the water requirements germinal stage of both species. The results showed that both species were able to germinate at relatively low water potentials. However, beyond −0.6 MPa, germination was completely inhibited for R. tripartitum, when it reached for another 50% for Z. lotus. Increasing the concentration of PEG6000 progressively inhibited germination in both species. Only Z. lotus could be considered tolerant of water stress, because, to −1 MPa, seeds germinated with a rate of 17%. It resulted in that the species Z. lotus presented an adaptive capacity to aridity much greater than that observed for R. tripartitum