Seed Germination Characteristics of Rhus tripartitum (Ucria) Grande and Ziziphus 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. 1. Introduction Drought stress is considered to be the main environmental factor limiting plant growth and yield of many agronomic and horticultural crops, especially in semi-arid areas. In Mediterranean-type ecosystems, seasonal water shortage is the main factor constraining survival and growth of plants. Soil depth and texture are considered the most important edaphic properties that influence the moisture regime in arid environments with episodic rainfall. To date, a great deal of effort has been focused on physiological process underlying plant responses to drought stress. The current situation of arid and desert areas of Tunisia (three quarters of the area) is in rapid decline of natural vegetation cover associated with an erosion of biodiversity [1, 2]. This decline is attributed to particular stressful environmental conditions, land clearing, and overgrazing, resulting in effects of increasingly adverse ecological (desertification) and economical. In this area, the steppic vegetation is dominated by tall perennial grasses [3]. But, sparse trees and shrubs are among the most threatened species because of their excessive use for domestic purposes and their poor regeneration performances [4, 5]. On the other hand, under arid bioclimate, the flora has been subjected to a high, permanent increase of human pressure since at least the last century. Such a situation contributed and might have induced the phenomenon of desertification [5, 6]. In Tunisia, several attempts have been made to restore degraded