In temperate climates, asparagus phenology includes the induction of fern senescence and crown dormancy in the fall, quiescence in the winter, and regrowth in the spring. For optimal survival, plants must acquire adequate freezing tolerance, which is maintained during the winter, and dormancy must be released after spring freeze-thaw cycles.
In Ontario, Canada, sub-zero air and soil temperatures can occur for extended periods, consequently, cultivars must be winter hardy. Local trials of asparagus hybrids bred in different climatic zones indicated varying levels of adaptation (D.J. Wolyn, unpublished data). ‘Guelph Millennium’, bred in southern Ontario, is highly adapted with sustained high yields over many years. ‘UC 157’, developed in a mild California climate, usually dies after 3 to 4 years. ‘Jersey Giant’, bred in New Jersey, has moderate adaptation to southern Ontario and declines 3 to 4 years after establishment. The varied adaptation of these three cultivars could be related to different levels of winterhardiness.
The establishment of freezing tolerance in asparagus may be associated with physiological and biochemical changes including the production of cyroprotective compounds. Simple sugars such as sucrose and glucose can increase ψS to reduce ice formation and protect proteins from structural changes (Vagujfalvi et al., 1999); however, high and low sucrose concentrations in asparagus rhizomes and buds can indicate growth and dormancy, respectively (Pressman et al., 1993). Fructans have a role in producing high ψS (Livingston, 1996) and can also maintain cellular integrity by binding membranes to keep them flexible and unimpaired during freezing (Cairns, 1992; Valluru and Van den Ende, 2008). Proline can protect cell membranes from desiccation (Kishor et al., 1995) and stabilize proteins (Verbruggen and Hermans, 2008). Expression of cold-regulated and antifreeze proteins and antioxidant enzymes can also increase protein concentrations with the acquisition of freezing tolerance (Herman et al., 2006; Patton et al., 2007; Zhang and Ervin, 2008). Increasing solute concentrations in the cell can depress freezing point and limit water availability for ice formation (Fujikawa et al., 1999; Guy, 2003).
Analysis of fall acclimation suggested GM asparagus had greater freezing tolerance than JG due to its early senescence and decreased percentage water, increased low-molecular-weight nonstructural carbohydrate concentration, and decreased sucrose concentration in the rhizome (Landry and Wolyn, 2011). For a seedling experiment in controlled environments, where cold acclimation induced early senescence of GM as observed in the field, this cultivar had lower LT50 values (increased freezing tolerance) compared with JG after subfreezing acclimation, which was associated with high proline levels in crowns (Landry and Wolyn, 2012). In a subsequent analysis of seedling freezing tolerance, both cold acclimation and cold acclimation followed by subfreezing acclimation differentiated the three cultivars as predicted based on field adaptation (Kim and Wolyn, 2015). GM, JG, and UC had the lowest, moderate, and highest LT50 values, respectively. Freezing tolerance was associated with low percentage water and high proline, sucrose, and low- and high-molecular-weight fructan (HF) concentrations in the crown. Analysis of LT50 during fall acclimation for the three cultivars in the field indicated that on specific sampling dates in early fall, GM crowns had greater freezing tolerance than those of UC, and values of JG were intermediate (Panjtandoust and Wolyn, 2016). Interestingly, all three cultivars had similar LT50 values at the start of winter.
Direct measurements of LT50 and associated physiological parameters among the asparagus cultivars during natural fall or artificial cold acclimation suggested that varying levels of freezing tolerance can explain differences in persistence and adaptation (Kim and Wolyn, 2015; Landry and Wolyn, 2012; Panjtandoust and Wolyn, 2016). However, the timing of deacclimation or dehardening in the spring may also be important for survival. The early release of dormancy and loss of freezing tolerance from increasing temperatures could make asparagus plants susceptible to damage from late freeze-thaw cycles and result in low winterhardiness and poor adaptation, as observed in alfalfa (Medicago sativa), another herbaceous perennial (Suzuki, 1981), and grasses (Hoffman et al., 2014; Jørgensen et al., 2010).
Genetic variation for the minimum temperature at which dormant asparagus buds begin to grow has been demonstrated in controlled environments (Ku et al., 2007). Since values for JG and UC were 12.5 and 10 °C, respectively, one can hypothesize that UC releases dormancy and loses freezing tolerance earlier than JG and is more susceptible to frost damage from spring freeze-thaw cycles.
The identification of three asparagus cultivars, GM, JG, and UC, with varying levels of adaptation to the cold climate of southern Ontario allows detailed investigations into how release of dormancy and loss of freezing tolerance in the spring may affect winterhardiness. The objectives of this research were to assess freezing tolerance and associated biochemical and physiological parameters of these three cultivars under field conditions during the spring. The differing spring climates over the 2 years of this experiment, where soil temperatures increased unusually early then decreased in year 1, and increased later than normal in year 2, offered insight into genetic variation for deacclimation and its role in adaptation of asparagus cultivars to cold temperate regions.
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