Robinson, R. W., and E. Kowalewski  

Transgressive segregation for frost tolerance in interspecific crosses with the tomato.

An early frost in the fall of 1972 brought a premature end to the tomato season in New

York, sharply reducing yield since all but the very earliest varieties had a large proportion of immature fruit when they were killed by frost. All of the numerous varieties being tested were killed when the temperature dropped to -3°C, but some F₂ segregants of interspecific crosses survived this and several subsequent frosts.

            For more than a decade, a search had been made at Geneve, N. Y. for a source of frost tolerance, but previously without success. In some years male sterile and other unfruitful plants escaped injury from very light frosts, and occasionally plants with heavy vine cover would survive when only their upper layer of leaves were killed, but none appeared practical for breeding a frost tolerant tomato. They were unsuitable in yield or plant habit and were not able to survive temperatures as low as occurred in the fall of 1972.

            The frost tolerant plants found in 1972 were in F₂ populations of crosses of L. esculentum with L. hirsutum, L. glandulosum and Solanum penellii. The most promising selections were from crosses with L. hirsutum. All parental species were killed by the frost, as were each of the interspecific F₁ hybrids. The small proportion of F₂ survivors suggested that frost tolerance is recessive, of complex inheritance, and resulting from transgressive segregation.

            The first frost of the following season was not cold enough to harm any tomato varieties in the field. The temperature recorded in the weather station was 0°F and likely was colder near the ground level by, the tomato plants, but none of the Lycopersicon species was injured. Some F₂ segregants from L. esculentum x L. glandulosum, however, were killed by frost. Evidently transgressive segregation frost tolerance had occurred once again, but this time plants more susceptible to frost than either parent occurred whereas plants more tolerant to frost than either parent had been found the previous year.

            It was confirmed in 1974 that derivatives of interspecific crosses have a promising degree of frost tolerance. A large assortment of breeding material was tested, including species collected by C. M. Rick at elevations of 2,000 meters. The best degree of tolerance to a frost of -6°F was by the progeny of the L. esculentum x L. hirsutum selections that were frost tolerant in 1972.

            Frost tolerance has also been evaluated in growth chambers. Super cooling of the plants occurred before injury resulted at constant temperatures of -3°C and lower. Breeding lines tolerant to frost in the field appeared to have a greater extent of super cooling than normal tomatoes before the formation of ice in their plant tissues.