Asparagus (Asparagus officinalis) is an herbaceous perennial plant in which the above-ground fern senesces and dies in the fall, leaving only the below-ground crown to overwinter. In southern Ontario, where air and soil temperatures of –20 and –5 °C, respectively, are common for extended periods, freezing tolerance is essential for survival, and growers require cultivars adapted to the region for optimum productivity. Consequently, winterhardiness is an important objective in an asparagus breeding program.
Compounds with cryoprotective properties such as proline, glucose, sucrose, and fructans may contribute to freezing tolerance of the overwintering asparagus crown. Proline is known to stabilize protein synthesis, increase water-binding capacity of the cell, stabilize proteins during water stress, and promote expression of cold-responsive genes (Kandpal and Rao, 1985; Nikolopoulos and Manetas, 1991; Steponkus, 1984; Venekamp et al., 1989). Glucose and sucrose protect plant cells from freeze-induced dehydration and reduce ice formation by increasing the intracellular solute concentration as well as preventing structural changes to proteins (Caffrey et al., 1988; Vagujfalvi et al., 1999; Wilson et al., 2008). Fructans can have direct cryoprotective effects (Herman et al., 2006) through interaction with membranes to prevent dehydrative stress (Hincha et al., 2007) and their hydrolysis results in simple sugars (Gasecka et al., 2008) that may also act as cryoprotectant agents to increase freezing tolerance (Allison et al., 1999; Hincha et al., 2007).
Asparagus cultivars Guelph Millennium (GM) and Jersey Giant (JG) show different patterns of fall senescence and longevity in southern Ontario (Landry and Wolyn, 2011). GM senesces with fern turning yellow by mid-October and has sustained high yields over several years. JG has delayed senescence, is often green at the first killing frost, and suffers yield decline after several harvest seasons. Timely fall senescence was hypothesized to contribute to freezing tolerance and consequently improved longevity and yield. GM showed earlier accumulation of nitrogen and proline in the rhizome and earlier decrease in crown water percentage and fern chlorophyll concentration, indicators of dormancy and potentially increased freezing tolerance, compared with JG. The late initiation of senescence or active growth in the late fall was also negatively correlated with yield in asparagus (Bai and Kelly, 1999). In other plants such as alfalfa [Medicago sativa (Dhont et al., 2006)] and mayapple [Podophyllum peltatum (Watson and Lu, 1999)], early senescing lines showed greater freezing tolerance and long-term yield than those demonstrating late senescence. Asparagus cultivars with late senescence may be unadapted in southern Ontario as a result of the lack of metabolites flowing from the fern to the crown that could be important for both winterhardiness and spring vigor.
In seedling assays simulating fall acclimation under controlled conditions, GM had greater freezing tolerance than JG based on LT50 values (temperature at which 50% of plants die) (Landry and Wolyn, 2012; J. Kim and D.J. Wolyn unpublished data). In experiments with hybrids showing varying adaptation to southern Ontario, LT50 was highly correlated with seedling crown reducing sugar (Landry and Wolyn, 2012), proline, sucrose, and fructan concentrations and percentage water, and fern chlorophyll concentration (J. Kim and D.J. Wolyn, unpublished data).
The association of specific parameters with LT50 suggests that indirect measures of freezing tolerance could be used in a breeding program, especially to predict the trait in large populations acclimated in the fall under natural conditions. Effective breeding requires both genetic variation and heritability for attributes of interest. Therefore, the objectives of this study were to assess variation for biochemical and physiological parameters associated with freezing tolerance in asparagus germplasm and determine the heritability of these measures such that they may be useful for selection under field conditions.
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