Cold Acclimation Attributes among Three Asparagus Cultivars under Controlled Conditions

Southern Ontario is one of the coldest asparagus production areas in the world. Cultivars must demonstrate adequate cold acclimation before the first killing frost and continue to demonstrate freezing-tolerance throughout the freeze-thaw cycles in the early spring. In the field, cultivar Guelph Millennium (GM) senesces in mid-October while fern of Jersey Giant (JG) and UC157 often remain green until a killing frost. Superior longevity of GM compared to the other cultivars could be related to proper cold acclimation in the fall and improved freezing tolerance. A seedling experiment was conducted in growth chambers to assess the effects of temperature and photoperiod on the induction of senescence, freezing tolerance, and biochemical parameters related to cold acclimation. Ten-week old seedlings of the three cultivars were acclimated in a factorial combination of two temperatures (23 °C, 8 °C) and two photoperiods (16 hr, 8 hr) or acclimated followed by sub-freezing (3 °C/–3 °C, day/night). Low temperature acclimation induced the greatest loss of chlorophyll in GM compared to the other two cultivars. Median lethal temperature (LT50), the temperature at which 50% of the population dies, decreased for all cultivars when acclimated at low temperature; GM showed the lowest LT50 (highest freezing tolerance) followed by JG and UC157. After subsequent subfreezing acclimation GM maintained its LT50, whereas values for the other two cultivars increased (decreased freezing tolerance). Biochemical analyses indicated low crown percentage water, and high concentrations of sucrose, raffinose and proline are associated with freezing tolerance. Results also suggest a seedling screen may be useful to select for freezing tolerant germplasm in a breeding program.