The relation between low K+/Na+ ratio and salt-tolerance in the wild tomato species Lycopersicon pennellii

The cultivated tomato Lycopersicon esculentum cultivar M82 and its wild salt-tolerant relative L. pennellii accession Atico were analyzed for: (a) dry weight, content of Na+ and K+ ions (in young and old leaves), and dry weight/potassium ratio (in young leaves) in plants grown in standard Hoagland and K-free solutions, without or with 100 mmol L-1 NaCl; and (b) dry weight in plants grown in K-free medium, in which K+ was substituted by the same concentration (5 mmol L-1) of Na+. Plants of L. pennellii, as expected: (a) responded better than the cultivated species to the high salinity in the standard solution with respect to biomass production; and (b) accumulated less K+ and more Na+ under this condition. Furthermore, L. pennellii plants were characterized by: (a) a more efficient substitution of K+ function by Na+, as expressed by the bet that Na+ added to the K-free medium caused a greater increase of dry weight in the wild species; (b) a higher retranslocation of K+ from old to young leaves, and consequently (c) a higher K-efficiency (dry weight/K+) ratio. It is suggested that: (a) the use of less energy in L. pennellii, as compared with the cultivated species, to exclude Na+ and to accumulate K+ under salt stress contributes to the mechanism of salt tolerance in that species, and (b) the higher Na+ in the wild species is used as a cheap osmoticum in the vacuole and, possibly, as a partial substitute for K+ in some of its functions, and the lower K+ is compensated, at least in part, by the higher K-efficiency ratio.