The second phase of cold hardening: Freezing tolerance and fructan isomer changes in winter cereal crowns

Cold-hardening plants at above freezing temperatures significantly contributes to their overall winter hardiness. However, little research has been conducted on hardening at temperatures below freezing, before freezing injury results. To determine the effect of hardening at below freezing temperatures, barley (Hardeum vulgare L.) and oat (Avena sativa L.) were grown and hardened under controlled conditions and freeze tested after being held at -3 degrees C from 1 to 7 d. A significant hardening effect was observed after exposure to below freezing temperatures. The biggest change, a reduction of 7 degrees C in the temperature at which 50% of the population survives, occurred after 7 d at -3 degrees C in the winter hardy oat cultivar, Wintok. The additional hardening appeared related to changes in carbohydrate concentration. While wheat (Triticum aestivum L.) and rye (Secale cereale L.) wore not freeze tested, their changes in carbohydrates were even greater than oat and barley. In oat, the concentrations of all 15 fructan isomers of degree of polymerization 3 to 5 were lower after the below freezing treatment while the concentration of fructose and sucrose were higher. Some carbohydrate concentrations were highly correlated with freezing survival under these conditions, but the exact mechanisms behind this relationship are not understood. Controlled freeze tests which consistently quantitate the effect of individual mechanisms will allow plant breeders and geneticists to more effectively screen germplasm for winter hardiness genes.