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

Cold tolerance (CT) of 31 tomato accessions (cultivars, breeding lines, and plant introductions) representing six Lycopersicon L. sp. was evaluated during seed germination and vegetative growth. Seed germination was evaluated under temperature regimes of 11 ± 0.5 °C (cold stress) and 20 ± 0.5 °C (control) in petri plates containing 0.8% agar medium and maintained in darkness. Cold tolerance during seed germination was defined as the inverse of the ratio of germination time under cold stress to germination time under control conditions and referred to as germination tolerance index (TI(G)). Across accessions, TI(G) ranged from 0.15 to 0.48 indicating the presence of genotypic variation for CT during germination. Vegetative growth was evaluated in growth chambers with 12 h days/12 h nights of 12/5 °C (cold stress) and 25/18 °C (control) with a 12 h photoperiod of 350 mmol.m^-2.s^-1 (photosynthetic photon flux). Cold tolerance during vegetative growth was defined as the ratio of shoot dry weight (DW) under cold stress (DW(S)) to shoot DW under control (DW(C)) conditions and referred to as vegetative growth tolerance index (TI(V)(G)). Across accessions, TI(VG) ranged from 0.12 to 0.39 indicating the presence of genotypic variation for CT during vegetative growth. Cold tolerance during vegetative growth was independent of plant vigor, as judged by the absence of a significant correlation (r=0.14,P>0.05) between TI(V)(G) and DW(C). 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 TI(V)(G) and TI(G). A few accessions, however, were identified with CT during both seed germination and vegetative growth. Results indicate that for CT breeding in tomato, each stage of plant development may have to be evaluated and selected for separately.