Resistance of Phaseolus species to ice crystallization at subzero temperature

Dry bean ( Phaseolus vulgaris L.) cultivars possess little or no freezing tolerance and are killed at the temperature of ice formation in their tissues. An increase in frost tolerance by 2–3°C would expand dry bean production in the short growing seasons of the Canadian prairies and possibly to higher altitudes in the tropics where episodic frosts occur during the growing season. The objective of this study was to determine the differences in frost resistance of Phaseolus species in both controlled and field environments. Leaflets of dry bean cv. CDC Nighthawk, and wild relatives from the primary gene pool ( P. vulgaris var. mexicanus Freytag and P. vulgaris var. aborigineus (Burkart) Baudet) and the tertiary gene pool ( P. acutifolius var. tenuifolius A. Gray, P. filiformis Bentham, P. angustissimus A. Gray and P. ritensis M.E. Jones) were subjected to subzero temperatures with and without ice nucleation to determine the levels of tolerance and avoidance, respectively. The lethal temperature at which 50% of the leaflets were killed (LT₅₀) was 0.5–1°C lower for species of the tertiary gene pool compared to those from the primary gene pool. Leaflets of species from the tertiary gene pool were also characterized by extensive supercooling compared to leaflets of species from the primary gene pool. Resistance of Phaseolus species to spring and autumn frosts were determined on seedlings transplanted to the field. Phaseolus angustissimus , a species of the tertiary gene pool had the highest seedling survival in response to both autumn and spring frosts, when the minimum air temperatures were ?5 and ?7°C, respectively. Frost resistance of Phaseolus angustissimus , if successfully introgressed into bean germplasm, may enable the development of frost resistant dry bean cultivars.