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 fact 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.