Effects of abscisic acid (ABA) and ABA analogs on freezing tolerance,low-temperature growth,and flowering in rapeseed

Brassica napus and B. campestris are grown in Western Canada in areas subject to unseasonable frosts. At the seedling stage, cultivars of Brassica are very sensitive to frosts of -2° to-5°C, which are either lethal or delay the development of the plant. Seedlings of B. napus and B. campestris, germinated and grown at 10°C (16-h photoperiod), were treated with a foliar spray of either 100 M racemic abscisic acid (ABA), 100 M of various ABA analogs, 0.1% acetone, or were untreated. Freeze tests indicated 2°C of frost tolerance could be gained in B. napus following an application of three ABA analogs. In B. campestris, three analogs also increased freezing tolerance approximately 1.5°C. The analogs 2,3 dihydro ABA and acetylenic divinyl methyl-ABA were effective in both species. Plant fresh weight and dry weight increased in treated plants relative to control or acetone-treated plants after 3 weeks at 10°C. The effect of frost and/or analog treatment on flowering was determined in both species. In B. campestris and B. napus, a mild frost advanced flowering by approximately 2 days compared with nonfrozen control plants. The promotive effect of frost on flowering decreased with increasing severity of the frost. Several of the analog treatments, particularly 2,3 dihydro ABA and acetylenic divinyl ABA, advanced flowering by 2–3 days in both species. The benefit of these ABA analog treatments on flowering was enhanced additionally by a mild frost. Plants treated with either ABA, 2,3 dihydro ABA, 2,3 acetylenic dihydro ABA, or acetylenic divinyl ABA flowered up to 5 days earlier than control plants.