Screening of Tomatoes Germplasm for Heat Stress Tolerance Under Controlled Conditions

Abstract

Forty four tomato (Solanum lycopersicum) lines originating from USA including three wild accessions were evaluated under controlled conditions for heat tolerance using the three physiological parameters electrolyte leakage, chlorophyll fluorescence and stomatal conductance. Tomato plants in their vegetative stage two months after sowing were placed in a growth chamber at 27 degrees for one week. Control measurements were taken prior to the heat shock treatment of 44°C for four hours. As selection of heat tolerant and sensitive genotypes proved difficult with stomatal conductance, a preliminary selection of twelve genotypes was made based on electrolyte leakage and fluorescence,, and measurements were repeated in triplicate.Chlorophyll fluorescence, i.e. the ratio of variable to maximal fluorescence in both dark-adapted (Fv/Fm) and light-adapted leaves (Fv’/Fm’), was measured using an infra-red gas analyser (IRGA, Licor 6400) fitted with a fluorescence chamber. Both Fv/Fm and Fv’/Fm’ decreased in the heat-sensitive genotypes, however Fv’/Fm’ was a better predictor for heat-shock sensitivity.Electrolyte leakage was measured by collecting the leaf discs from control and heat-shocked plants. Electrical conductivity (EC) was measured as an indication of cell injury due to electrolytes leakage. Electrolyte leakage was significantly increased in heat-sensitive genotypes and it was 32.92μmhos/cm in sensitive genotype compare with 22.1μmhos/cm in tolerance genotype.Significant variation in heat tolerance was observed between genotypes. Electrolyte leakage proved to be a more reliable parameter compared to chlorophyll fluorescence in differentiating heat tolerant and heat sensitive genotypes of tomato under controlled conditions. This method can potentially be used to short list larger sets of germplasm under controlled conditions that will reduce cost of testing the bigger sets under field settings.