Vernalization and photoperiod requirements regulate the timing of the vegetative/reproductive transition in plants. Cereals adapted to cold winter climates regulate this developmental transition mainly through vernalization requirements, which delay transition from the vegetative to the reproductive growth stage. Recent research indicates that vernalization requirements also influence the expression of low-temperature (LT) tolerance genes in cereals exposed to acclimating temperatures. The objective of the present study was to determine if LT tolerance expression was also developmentally regulated by photoperiod response. The nonhardy, short day (SD) sensitive, wheat (Triticum aestivum L. em Thell) cultivar AC Minto, the LT tolerant, highly SD sensitive barley (Hordeum vulgare L.) cultivar Dicktoo, and a barley selection with very low sensitivity to SD were subjected to 8-h (SD) and 20-h (LD) days at cold acclimating temperatures over a period of 98 d. Final leaf number (FLN) was used to measure photoperiod sensitivity and determine the vegetative/reproductive transition point. The LT tolerance of the less SD sensitive barley genotype was similar for LD and SD treatments. In contrast, a delay in the transition from the vegetative to the reproductive stage in AC Minto and Dicktoo grown under SD resulted in an increased level and/or longer retention of LT tolerance. These results support the hypothesis that not only the level, but also the duration of gene expression determines the degree of LT tolerance in cereals. Consequently, any factor that lengthens the vegetative stage, such as vernalization or photoperiod sensitivity, also increases the duration of expression of LT tolerance genes.
