THE INTERRELATIONSHIP OF GROWTH AND FROST TOLERANCE IN WINTER RYE

The reduction in growth of winter cereals that occurs in the fall is thought to be required for the development of frost resistance. In the present study, the interrelationship of freezing tolerance and growth was examined by raising winter rye (Secale cereale cv. Puma) plants at 20/16-degrees-C (day/night) and at 5/3-degrees-C under 8-, 16- and 24-h daylengths to vary growth rates and frost tolerance. Temperature and irradiance were quantified as thermal time, photothermal time and photosynthetic photon flux and examined by multiple linear regression in order to determine their effects on growth and frost tolerance of rye shoots. At low temperature, both growth and frost tolerance were markedly influenced by daylength and irradiance. Plants grown at 5/3-degrees-C with a short daylength accumulated shoot dry weight and increased frost tolerance at a greater rate per unit photothermal time or photon flux than plants grown at longer daylengths. Moreover, 5/3-degrees-C plants grown with a 16-h day grew more slowly and were less frost tolerant than plants grown with a 24-h day. We conclude that the interrelationship between growth and frost tolerance is a quantitative one. Frost tolerance is induced only by low temperature, but the development of frost tolerance is dependent upon both irradiance, which affects the amount of photoassimilate available, and daylength, which may affect the partitioning of photoassimilates between growth and frost tolerance.