Determination of screening techniques to salinity tolerance in tomatoes and investigation of genotype responses

Abstract

In order to determine the predictive screening parameters that can be applied at early development stages of tomato plants, 55 tomato genotypes were grown in nutrient solution with 200 mM NaCI. The relationships among the salinity scale classes based on the visual appearance and shoot Na⁺ accumulation, K⁺/Na⁺ and Ca²⁺/Na⁺ ratios and shoot-root dry weights were investigated. Tomato genotypes differed greatly for shoot Na⁺ concentration and salinity scale classes were significantly correlated with Na⁺ concentrations. Higher shoot Na⁺ concentrations indicated higher shoot damage. Shoot K⁺/Na⁺ and Ca²⁺/Na⁺ ratios were significantly correlated with the salinity scale classes. The higher shoot K⁺/Na⁺ and Ca²⁺/Na⁺ ratios indicated lower shoot damage. Tomato genotypes grown under 200 mM NaCI stress showed significant variations in shoot and root dry weights. However, no significant correlations were found between the shoot-root dry weight and the scale classes and Na⁺ concentration and the ion ratios investigated. These may indicate that plant shoot and root dry weights were independent of salt tolerance at the growth stage reached in this study.

Introduction

Salinity stress, which usually occurs in arid and semiarid regions, is a major environmental constraint to crop productivity. Low rainfall, high evaporation, native rocks, saline irrigation water and poor water managements can cause salinity problems in agricultural areas. The majority of crop plants are susceptible and cannot survive under conditions of high salinity or can survive only with decreased yields. Plants are stressed in three ways in saline soils; (1) low water potential of the root medium leads water deficit, (2) the toxic effects of the ions mainly Na⁺ and Cl⁻, (3) nutrient imbalance by depression in uptake and/or shoot transport [1], [2], [3]. To alleviate the deleterious effects of salinity some rehabilitations such as reclamation of salinized lands, improving of irrigation saline water and special cultural techniques are applied. Corrections of the salinity problems are usually expensive and considered only a temporary solution. Selection and breeding of the cultivars that can grow and produce economic yield under the saline conditions are more permanent and complementary solutions to minimize the detrimental effects of the salinity [4], [5]. Plant species and cultivars within a crop species differ greatly in their response to salinity [3]. Genetic variability within a species is a valuable tool for screening and breeding for higher salt tolerance. Tomato (Lycopersicon esculentum Mill.), one of the important and widespread crops in the world, is sensitive to moderate levels of salt in the soil. So many authors have reported large variation among tomato genotypes in their response to salinity [5], [6], [7], [8].

Ion regulation is an essential factor of the mechanism of salt tolerance in tomato; salinity raises Na⁺ concentration in roots and leaves of tomato plants, Ca²⁺ and K⁺ concentrations in roots of salinized tomato plants change little under salinity whilst they arc greatly reduced in leaves [8]. While K⁺ and Ca²⁺ play key roles in several physiological processes, Na⁺ does not function as a macronutrient, and thus the substitution of K⁺ by Na⁺ and decrease in Ca²⁺ concentration may lead to nutritional imbalances. The control of Na⁺ accumulation, by exclusion strategy, and high shoot K⁺/Na⁺ and Ca²⁺/Na⁺ ratios may enhance salt tolerance or resistance in tomato crops [9].

In many crop species, salt tolerance is reported to be a developmental regulated; stage specific phenomenon, i.e. the tolerance at one stage of plant development may not be correlated with tolerance at other developmental stages [10], [11]. However, typically in many plant species, including tomato, screening methods and the physiological works for salinity are based on the young plant stages [9], [12], [13], [14], [15], Qureshi et al. [16] have studied the procedures for a quick screening of wheat cultivars for salt tolerance at germination, seedling and maturity stages. The authors showed significant correlation between the seedling stage in nutrient solution and maturity stage in saline soil and suggested that the screening at seedling stage is not only less laborious, less time consuming and less expensive, but also has high reliability. Screening under natural field conditions is not feasible due to the high degree of soil heterogeneity.

The main objective of this study was to determine predictive screening parameters by judging the relationships among the visual appearance of the genotypes and shoot Na accumulation, maintenance of K/Na and Ca/Na ratios, shoot-root dry weights, that can be applied at early development stages of tomato plants. Sub-objective of the study was determination of differential responses of 55 tomato genotyes to salinity stress.