Heat tolerance of cold acclimated Puma winter rye seedlings and the effect of a heat shock on freezing tolerance

An increase in tolerance to one form of abiotic stress often results in an increase in tolerance to another stress, The heat tolerance of Puma rye (Secale cereale) was determined for seedlings either not cold hardened or hardened under either controlled environmental or natural conditions. The heat tolerance was determined either as a function of time at 42 degrees C or the ability to tolerate a maximum temperature. The seedlings were either not heat preconditioned or heat preconditioned before the heat stress. In all cases cold hardened seedlings were more heat tolerant than non or partially cold hardened seedlings, Heat preconditioning had no effect on the heat tolerance of naturally cold hardened seedlings. In contrast, seedlings cold hardened in a controlled environment chamber, then heat preconditioned, were more heat tolerant than non preconditioned seedlings. A heat shock of 36 degrees C for 2 h increased the freezing tolerance of non hardened seedlings from -2.5 degrees C to -4.5 degrees C. Analysis of heat shock protein 70 (HSP70) gene expression indicated that the HSP70 gene was not induced by cold acclimation and therefore not directly involved in the increased thermo tolerance observed. A number of heat stable proteins, simple sugars and long chain carbohydrate polymers accumulated during the cold acclimation process and may have a role in increasing heat tolerance as well as freezing tolerance. These data suggest cold hardening increases heat tolerance, however, a heat shock to non acclimated seedlings only marginally increased the freezing tolerance of Puma rye seedlings.