Relationship between cold tolerance during seed germination and vegetativegrowth in tomato: Germplasm evaluation

Cold tolerance (CT) of 31 tomato accessions (cultivars, breeding lines, and plant introductions) representing six Lycopersicon L. sp. was evaluated du ring seed germination and vegetative growth. Seed germination was evaluated under temperature regimes of 11 +/- 0.5 degreesC (cold stress) and 20 +/- 0.5 degreesC (control) in petri plates containing 0.8% agar medium and main tained in darkness. Cold tolerance during seed germination was defined as t he inverse of the ratio of germination time under cold stress to germinatio n time under control conditions and referred to as germination tolerance in dex (TIG). Across accessions, TIG ranged from 0.15 to 0.48 indicating the p resence of genotypic variation for CT during germination. Vegetative growth was evaluated in growth chambers with 12 h days/l2 h nights of 12/5 degree sC (cold stress) and 25/18 degreesC (control) with a 12 h photoperiod of 35 0 mmol.m(-2).s(-1) (photosynthetic photon flux). Cold tolerance during vege tative growth was defined as the ratio of shoot dry weight (DW) under cold stress (DWS) to shoot DW under control (DWC) conditions and referred to as vegetative growth tolerance index (TIVG). Across accessions, TIVG ranged fr om 0.12 to 0.39 indicating the presence of genotypic variation for CT durin g vegetative growth. Cold tolerance during vegetative growth was independen t of plant vigor, as judged by the absence of a significant correlation (r= 0.14,P>0.05) between TIVG and DWC. Furthermore, CT during vegetative growth was independent of CT during seed germination, as judged by the absence of a significant rank correlation (r(R) = 0.14, P > 0.05) between TIVG and TI G. A few accessions, however, were identified with CT during both seed germ ination and vegetative growth. Results indicate that for CT breeding in tom ato, each stage of plant development may have to be evaluated and selected for separately.