Relationship among phenological development, physiological indices and freezing tolerance in winter wheat and rye under field conditions in moderate and cold regions

Low temperature (LT) acclimation, phenological development and vernalization requirement are known to influence the expression of frost tolerance (FT) in winter cereals. A phenotypic study was undertaken to determine the interrelationship between developmental stages and FT in hardy winter wheat and rye cultivars acclimated under field condition in the cold and temperate regions of Iran. Winter wheat Norstar (Triticum aestivum L.) and winter Puma rye (Secale cereale L.) with long vernalization requirements were planted at the second decade of October in 2008 at Karaj and Zanjan as temperate and cold regions, respectively. Plant development (morphological stage of shoot apex), vernalization requirement (days to heading and final leaf number) and FT (survival of plants exposed to freezing conditions) were evaluated during the autumn and winter seasons in 2008-09. Furthermore, the ratio, the quantum efficiency of photosystem II and chlorophyll content of upper fully expanded leaves were measured at the early stage of reproductive growth in Karaj. Result showed that in both species, FT increased during the autumn in 2008 and the maximum FT in the both regions was achieved during the second decade of January 2009, which was about the same time as vernalization fulfillment occurred. Puma rye plants acclimated in Zanjan showed the highly significant level of FT (~-24ºC) in March in comparison to Norstar wheat (LT50= -17 ºC). A highly significant decrease (P