Control of intracellular Na⁺ and K⁺ concentrations in plant tissues under saline conditions is crucial for salt tolerance. In this study, two Na⁺ ‐selective class I‐HKT transporters, HKT1;1 and HKT1;2, from tomato were found to be closely linked in proximity to the position with the maximum LOD score for major QTLs in chromosome 7 (at a distance of 35 kb), thus contributing to stem and leaf Na⁺ and K⁺ in two populations of tomato RILs derived from two salt‐tolerant lines from wild tomato species S. pimpinellifolium and S. cheesmaniae. This result, together with their patterns of expression and phenotypic effects on a pair of near isogenic lines (NILs) differing in terms of HKT1‐like alleles, support the hypothesis that HKT1;1 and/or HKT1;2 genes underlies a major tomato QTL for shoot Na⁺ / K⁺ .

In this study we have identified the first plant K⁺/H⁺ exchanger, LeNHX2 from tomato (Lycopersicon esculentum Mill. cv. Moneymaker), which is a member of the intracellular NHX exchanger protein family. The LeNHX2 protein, belonging to a subfamily of plant NHX proteins closely related to the yeast NHX1 protein, is abundant in roots and stems and is induced in leaves by short term salt or abscisic acid treatment. LeNHX2 complements the salt- and hygromycin-sensitive phenotype caused by NHX1 gene disruption in yeast, but affects accumulation of K⁺ and not Na⁺ in intracellular compartments. The LeNHX2 protein co-localizes with Prevacuolar and Golgi markers in a linear sucrose gradient in both yeast and plants. A histidine-tagged version of this protein could be purified and was shown to catalyze K⁺/H⁺ exchange but only minor Na⁺/H⁺ exchange in vitro. These data indicate that proper functioning of the endomembrane system relies on the regulation of K⁺ and H⁺ homeostasis by K⁺/H⁺ exchangers.

Endogenous abscisic acid levels and induced heat shock proteins were measured in tissue exposed for 6 hours to temperatures that reduced their subsequent chilling sensitivity. One-centimeter discs excised from fully expanded cotyledons of 11-day-old seedlings of cucumber (Cucumis sativus L., cv Poinsett 76) were exposed to 12.5 or 37 degrees C for 6 hours followed by 4 days at 2.5 or 12.5 degrees C. Ion leakage, a qualitative indicator of chilling injury, increased after 2 to 3 day exposure to 2.5 degrees C, but not to 12.5 degrees C, a nonchilling temperature. Exposure to 37 degrees C before chilling significantly reduced the rate of ion leakage by about 60% compared to tissue exposed to 12.5 degrees C before chilling, but slightly increased leakage compared to tissue exposed to 12.5 or 37 degrees C and held at the nonchilling temperature of 12.5 degrees C. There was no relationship between abscisic acid content following exposure to 12.5 or 37 degrees C and chilling tolerance. Five heat shock proteins, with apparent molecular mass of 25, 38, 50, 70, and 80 kilodaltons, were induced by exposure to 37 or 42 degrees C for 6 hours, and their appearance coincided with increased chilling resistance. Heat shock treatments reduced the synthesis of three proteins with apparent molecular mass of 14, 17, and 43 kilodaltons. Induction of heat shock proteins could be a possible cause of reduced chilling injury in tissue exposed to 37 or 42 degrees C.