Genetic analysis of cold tolerance during vegetative growth in tomato, Lycopersicon esculentum Mill

The genetic basis of cold tolerance (CT) during vegetative growth in tomato was investigated by evaluating plants of a cold-tolerant primitive cultivar (PI120256) and a cold-sensitive breeding line (UCT5) and their reciprocal F₁, F₂, F₃, BC₁P₁ and BC₁P₂ progeny under two temperature regimes of 15/10°C (cold stress) and 25/15°C (control). Plants were evaluated for shoot dry weight (DW) under cold stress and by a tolerance index (TI) measured as the ratio of DW under cold stress (DW_s) to DW under control (DW_c) conditions. Shoot DW was reduced in all genotypes in response to cold stress. However, PI120256 exhibited the highest CT (TI = 90.5%) and UCT5 the lowest (TI = 38.9%). The TIs of the filial and backcross progeny were intermediate to the parents. Across generations, there was a positive correlation (r = 0.76, p < 0.01) between DW_s and DW_c indicating that growth under cold stress was influenced by plant vigor. However, the absence of a significant correlation (r =0.47, p >0.05) between DW_c and the TI and, in contrast, the presence of a significant correlation (r =0.92, p <0.01) between DW_s and the TI suggest that plant vigor was not a determining factor in the expression of CT in PI120256 and its progeny. Generation means analyses of DW_s and TI indicated that the variation among generations was genetically controlled, with additive effects accounting for most of the variation. There were no significant dominance effects, and epistatic effects were minor and involved only additive x additive interactions. The results suggest that the inherent CT of PI120256 should be useful for improving CT of commercial cultivars of tomato.