Phenological development and expression of freezing resistance in spring and winter wheat under field conditions in north-west Iran

Low temperature (LT) stress is one of the primary environmental factors limiting the expansion of winter wheat (Triticum aestivum L.) production in northern temperate climates. The objective of this study was to clarify the interrelationship between developmental stage and freezing resistance (FR) in winter and spring habit cultivars acclimated under field conditions in regions with long mild winters. Four cultivars with different vernalization and photoperiod requirements were planted in the field at the recommended autumn seeding dates in 2002-2003 and 2003-2004 at Maragheh in north-west Iran. Freezing resistance, as measured by LT50, and stage of phenological development, as estimated from final leaf number (FLN) and shoot apex developmental morphology, were determined during the autumn and winter seasons. 'Kohdasht' spring wheat, which does not have a vernalization or day length requirement, quickly entered the reproductive stage and had a limited ability to LT acclimate. In contrast, a significant decrease in FLN associated with LT treatment and delayed double ridge formation under field conditions indicated that 'Norstar', 'Sardari' and 'Azar2' had vernalization responses. Sardari increased its FLN from 9 to 12 when grown at 16 h compared to 12 h day indicating that it was also day length sensitive. Azar2 had little day length sensitivity, but it had a higher unvernalized FLN with the result that both Sardari and Azar2 achieved their vegetative/reproductive transition and maximum FR(similar to-15 degrees C) by early December. Norstar had a longer vernalization requirement and reached vernalization saturation and maximum FIR (similar to-28 degrees C) near the end December. The delay in the phenological development associated with a vernalization requirement, photoperiod sensitivity and/or increased leaf numbers was accompanied by increased expression of LT tolerance confirming that the length of the vegetative phase determines a plants ability to maintain a high level of LT tolerance gene expression. These observations suggest that more detailed studies should be initiated to establish the importance of interactions among factors that influence the time to vegetative/reproductive transition with the objective of identifying genetic combinations and management systems that extend the vegetative stage and provide longer term protection from LT stress in regions with long mild winters like those normally experienced in the north-west Iran. (c) 2005 Elsevier B.V. All rights reserved.