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In most crop species, growth of the shoot is more sensitive to salt stress than root growth. Avocado (Persea americana Mill.) is very sensitive to NaCl stress. Even low concentrations of salt (15 mM) inhibit tree growth and decrease productivity. Observations in experimental orchards have suggested that root growth in avocado might be more restricted by salinity than shoot growth. In the present study, we evaluated quantitatively the inhibitory effects of salt stress on growth of the avocado root in comparison to the shoot. Seedling plants of the West-Indian rootstock 'Degania 117ʼ were grown in complete nutrient solution containing 1, 5, 15, or 25 mM NaCl. The threshold NaCl concentration causing root and shoot growth reduction occurred between 5 and 15 mM. At all concentrations, root growth was much more sensitive to salinity than shoot growth. A concentration of 15 mM NaCl, which did not affect the rate of leaf emergence on the plant and decreased leaf biomass production only 10%, induced a 43% reduction in the rate of root elongation and decreased root volumetric growth rate by 33%. Under 25 mM NaCl, leaf biomass production, leaf initiation rate and leaf elongation rate were reduced 19.5%, 12%, and 5%, respectively, while root volumetric growth and root elongation rate were reduced 65% and 75%, respectively. This strong root growth inhibition is expected to infl uence the whole plant and therefore root growth under salinity should be considered as an important criterion for rootstocksʼ tolerance to NaCl. Salt stress is one of several environmental stresses that induces changes in growth and morphology of plants. Inhibition of veg- etative development of shoots and roots is the primary response to salt stress (Lazof and Bernstein, 1998; Munns, 1993). Root growth is usually less sensitive to the stress than is growth of the shoot (Bernstein and Kafkafi , 2002; Cheeseman, 1988; Rawson et al., 1988) and root to shoot ratio therefore typically increases under salinity. Avocado is known to be the most salt-sensitive cultivated fruit tree. Even low levels of salt inhibit the vegetative growth of the avocado shoot (Bar et al., 1997; Bernstein et al., 2001; Bingham and Fenn, 1966; Haas, 1950) and its fruit production (Ayers et al., 1951; Bingham et al., 1968; Downton, 1978; Steinhardt et al., 1995.) Chloride and sodium concentrations in irrigation water, which are considered tolerable for many crops, induce severe leaf damage in avocado. Salt-induced leaf damage is considered to reduce the photosynthetic leaf area and hence yield potential in avocado (Ayers et al., 1951; Bingham et al., 1968). The extent of sensitivity of avocado rootstocks to salt stress is therefore conven- tionally quantifi ed by stress impact on the appearance of chloride and sodium-induced leaf-burn damage. However, the extent of stress induced leaf damage did not correlate with stress-induced inhibition of shoot vegetative development (Bernstein et al., 2001). Additionally, in a long-term selection project, minimizing leaf damage under salt stress did not prevent the negative effects on avocado productivity (Ben-Yaʼacov et al., 1979; Ben-Yaʼacov and Michelson, 1995). This suggests that the utilization of leaf damage as a major marker for salt tolerance should be reevaluated, and other plant factors might contribute signifi cantly to the tree response to salinity, and to decreased productivity.

Root Growth of Avocado is More Sensitive to Salinity than Shoot Growth