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Soil salinity is a serious concern for productivity and landscape quality in arid and semi-arid regions, as a consequence of high concentrations of ions (Benata et al, 2008) .Salt tolerance has been partially linked to the regulation of shoot Cl- and Na+ concentration (Teleisnik and Grunberg, 1994), and it has been suggested that the growth under salinity stress is result of processes such as ion transport and compartmentalization and synthesis and accumulation of osmotic solutes (Viegas et al, 2001).The processes lead to osmotic adjustment and particularly exclusion and/or compartmentation of specific ions such as Na+ at a level compatible to plant growth (Munns and Termaat,1986; Delauney and Verma,1990; Silveira et al,2003).In saline environments, adaptations of plants to salinity during germination and early seedling stages, is crucial for the establishment of species ( Ungar,1995).Seedlings are the most vulnerable stage in the life cycle of plants (Gutterman,1993; Kigel,1995).It is established that salt stress decreases growth in most plants, including halophytes (Munns,2002).
Soluble salts affect seedling establishment and growth by osmotic effect or by toxicity of certain ions present in the soil solution (Greenway and Munns, 1980).Salts can produce decreased water uptake, disturbances in metabolism and nutrient intake and reduced growth (Parman and Moore, 1996; Mayer and Poljakoff-Mayber, 1982; Volkmar et al, 1998). Felker et al (1981) have studied the growth of different spp. At a maximum concentration of 0.5M NaCl with almost no growth of P.chilensis at this concentration.The relative importance of these effects depends not only on the level of salinity but also on the time plants are exposed to salinity (Munns, 2002). For overcoming salt stress, plants have evolved complex mechanisms that contribute to the adaptation to osmotic and ionic stress caused by high salinity (Meloni, 2004). Some mechanisms like osmotic potential regulation, control of stomatal opening, regulation of ion uptake and transport and induction of anatomical changes in roots and stems are considered to be very important in plant adaptations to salinity stress (Levit, 1980; Zhao and Harris, 1992; Catalan et al, 1994; Volkmar et al, 1998; Munns, 2002).Prosopis species can extract water from saline soils. The ability to assimilate water elevated salt content is aided by the capacity of Prosopis species to exclude salt from its aboveground biomass, accumulating it in the roots instead (Jarell an Virginia, 1984; Zhao and Harris, 1992). P.chilensis, P.fleoxusa and P.alba seedlings exposed to NaCl developed more root than shoot tissue in order to evade and elevated saline content (Latinoamericana, 2010).
STATEMENT OF THE PROBLEM
Prosopis is an invasive species in Botswana. These species are indigenous to South America and were purposely introduced into Botswana by Ministry of Agriculture with the objectives of controlling desertification and the spread of sand dunes (BCAPR, 2004). They can grow in all soil types (Klingen and Campbell, 2001) that allow root penetration. Alkalinity, salinity and water deficit will not limit its growth (Helsnkis, 2007). The Prosopis weed problem has had a dramatic impact on the environment and livelihoods of pastoral communities, and borders on being considered a national disaster (HDRA, 2005).The Prosopis is only invasive where there is relatively deep soil with groundwater near the surface, and rapid spread occur after years of above-average rainfall (Harding and Bate,1991). These invasive species form dense impenetrable thickets and render the land useless for normal farming practices. The impenetrable thickets that characterized most Prosopis infestations have mostly out-competed grass and related rangeland forage, making its threat far outweigh any current benefits (HDRA, 2005). The negative effects are dramatic for dwellers for some include the following; blockage of boreholes due to the dense root system, depletion of water tables and nutrient deep soil profiles, bush thickets along road side contribute to an increase in road accidents that involve livestock and vehicles and pricks by Prosopis thorns cause wounds that are painful and difficult to heal (BCAPR, 2004).
To assess the effects of different levels of salinity (NaCl) on growth of Prosopis species
MATERIALS AND MTHODS
0, 50,100,200,300,400,500 and 600mM NaCl solutions
Incubator at 25oC
Seeds of Prosopis species were collected from Tsabong Area, South Western Botswana .Seeds were manually separated from their pods and preserved using following methodology proposed by Cazebonne et al (1999) for other species of the genus. The seeds were germinated on petri dish covered with cotton wool (with distilled water) and kept under continuous light during all germination period. Germination was conducted in incubators set at 25oC.
Once seedlings are two weeks old they will be exposed to the following NaCl concentrations in culture solutions 0 mM NaCl (control), 50mM NaCl; 100mM with NaCl; 200mM NaCl; 300mM NaCl; 400mM NaCl; 500mM NaCl and 600mM NaCl. The plants will be raised in plant pots containing potting soils (one plant per pot) and the experiment will be replicated three times.
The growth parameters such as plant height, leaf area and chlorophyll concentration will be measured after every 2 days. At the end of the experimental period, the plants will be harvested and separated into roots and shoot. The dry weights of the shoot and roots will be determined by oven drying at 60 oC.(Khan et al,1987). Proline content will also be determined.