Examination of some physiological mechanisms and phenological aspects of cold tolerance in bread wheat genotypes (Triticum aestivum L.)

The flowering of many plants is either dependent on or promoted by prior exposure to the prolonged cold of a winter season. The process by which exposure to cold promotes flowering is known as vernalization-requirement that is an important adaptive feature which delays heading by postponing the transition from the vegetative to the reproductive phase. Similarly, photoperiod requirement is an adaptation that allows the plant to flower at the optimum time. Wheat also have the ability to low-temperature acclimate. Low-temperature acclimation is a cumulative process that is initiated once temperatures drop below 10 to 15 úC. Exposure of wheat to low-temperature produces a myriad of measurable changes in morphological, biochemical and physiological characteristics that are often highly correlated with plant cold tolerance. The objective of present study was to investigate the physiological mechanisms and the regulatory role of vernalization on expression of cold tolerance and seasonal changes in cold acclimation potential of wheat. The responses of four bread wheat cultivars (Triticum aestivum L.) to low-temperatures were examined in controlled environment and field conditions. Prolonged cold treatment accelerated the transition to reproductive development in two winter wheats (cv. Norstar and cv. Shahryar) and facultative wheat (cv. Alvand), but not in a spring wheat (cv. Kavir). Exposure to low-temperatures also enhanced cold tolerance of the winter and facultative wheats. Maximum frost tolerance was achieved around the point where further cold treatment caused no additional acceleration of flowering time; the vernalization fulfillment. This greatest cold acclimation potential was observed in the winter wheat Norstar, which required the longest cold treatments to saturate the vernalization response. The increased cold tolerance observed after exposure to low-temperatures (cold acclimation) was associated with reduced water content and with increased sugar content, chlorophyll content, MDA and proline, there was a strong association between cold tolerance and increased fructan content in the crowns; fructan levels increased proportionally with the length of cold treatment until the vernalization fulfillment was reached. These data support the hypothesis that vernalization and cold tolerance pathways are interconnected in cereals and that the delay of floral development until spring is critical to allow acclimation to low-temperatures during winter.