Techniques that can handle large numbers of genotypes include: germination or plant survival in high salinity, leaf injury as measured by membrane damage (leakage of ions from leaf discs), premature loss of chlorophyll (using a hand-held meter), or damage to the photosynthetic apparatus (using chlorophyll fluorescence). These methods can identify genotypes able to germinate, or survive, in very high salinities (over 200 mM NaCl), but do not discriminate between genotypes in their ability to tolerate the low or moderate salinities typical of many saline fields (50-100 mM NaCl). A major limitation to the use of injury or survival to identify salt-tolerant germplasm arises when the cause of injury is not known.

1.1.1.1 Screening methods based on physiological mechanisms

Because of the complex nature of salinity tolerance, as well as the difficulties in maintaining long-term growth experiments, trait-based selection criteria are recommended for screening techniques (Noble and Rogers, 1992). Traits used for screening germplasm for salinity tolerance have included Na+ exclusion, K+/Na+ discrimination (Asch et al., 2000) and Cl — exclusion (Rogers and Noble, 1992). The relationship between salinity tolerance and K+/Na+ discrimination was also considered, because K+/Na+ rather than Na+ alone has been used as an index of salinity tolerance for cultivar comparisons in wheat (Chhipa and Lal, 1995) and rice (Zhu et al., 2001). One of the mechanism of salinity tolerance that could be considered was tissue tolerance of high internal Na+ concentrations. Tissue tolerance cannot be measured directly, and is difficult to quantify. Yet it is clearly important; overexpression of vacuolar Na+/H+ antiporter that sequesters Na+ in vacuoles improved the salinity tolerance of Arabidopsis, tomato and brassica (Aharon et al., 2003).

1.2 Breeding

Breeding programs for new varieties of sweet sorghum suited to semi arid tropics, temperate areas with rainy summer, Mediterranean areas with dry summer and soil salinity, are under development (Cosentino, 1996).