Screening Malus seedlings for cold resistance
Mathers, Hannah Mary
Cold stress is the most important factor limiting the northern expansion of Malus cultivation, and breeding for improved cold resistance is time-consuming and laborious. Members in the genus Malus are characteristically heterozygous and reveal a high variability in the hardiness of their seedlings. This study was conducted to develop an appropriate acclimation routine to identify cold hardy transgressive segregants from Malus seedling populations. The effects of frosts (16 hours at -3Â°C) and lag-times (10 and 11 days at +3 to +5Â°C), short (8 hour) day and cool (+3 to +5Â°C) temperature exposures, and different screening temperatures (-20, -30, and -40Â°C) were investigated on seedlings grown in a greenhouse from open-pollinated Golden Delicious (Malus pumila), Antonovka (Malus baccata) X (Malus pumila) and Rescue Crab (Malus baccata) X (Malus pumila). Differentiation in hardiness response of the seedling populations was not achieved until after exposure to short days at cool temperatures for six weeks. Further population differentiation was achieved by exposure to one or more frosts compared to no frost exposures. Rescue gave a 74% increase in survival, Antonovka a 62% increase, and Golden Delicious a 51% increase, when compared to survival with no frosts. After the acclimation response had been initiated by exposure to short days at cool temperatures, up to 11 days in the same conditions caused no significant decrease in hardiness. Additionally, no significant decrease in survival was observed in seedlings held at cool temperatures and short days for up to 10 days after a frost exposure. Hardiness levels of acclimated and non-acclimated seedlings agreed with known inherent hardiness responses for all three cultivars evaluated. Cultivar seedling response to different freeze temperatures was pronounced. For crosses representing a full range of cold hardiness capabilities, a screening temperature close to -30Â°C was found to be most effective. The response of the different cultivar seedlings to the three different freeze temperatures indicated that the screen could be tailored to fit the minimum survival requirements of a particular region. A binomial form of regrowth data collection, percent seedling survival, was determined to be the most efficient and precise measure of evaluation. The controlled three-step freeze procedure and thawing rate that was developed, will facilitate rapid, repeatable screening of large numbers of progeny. An examination of replication means revealed that the probability of survival was influenced by when the seedlings grew in the greenhouse. It is recommended that, for subsequent screens, all the material should be grown at the same time.