Fall defoliation affects acquisition of freezing tolerance and spring regrowth in asparagus

Asparagus (Asparagus officinalis L.) acquires freezing tolerance during a period of fall acclimation when both photoperiod and temperature decrease. The above-ground vegetative growth may be important for sensing changing environmental conditions and translocating compounds to the below-ground crown. Defoliation experiments, repeated over 2 yr, were conducted by removing fern in mid-August, -September, and -October and evaluating crown metabolites and LT50, the temperature at which 50% of plants die, at monthly intervals to mid-November. Spring emergence and vigor were also assessed in separate experiments. In the first year, only mid-August defoliation affected LT50 values, decreasing freezing tolerance, which was associated with diminished rhizome proline concentration and storage root low- and high-molecular-weight fructan concentrations. All defoliation treatments in the second year decreased LT50 values, or increased freezing tolerance, possibly resulting from an interaction between defoliation and drought which increased rhizome sucrose concentrations. Defoliation decreased spring vigor in both experiments; the response was proportional to the earliness of the treatment and associated with rhizome and storage root fructan levels. Crowns of plants defoliated in mid-August had increasing proline concentrations during the fall, similar to control plants, suggesting the below-ground organs may have sensed soil temperature to cold acclimate. Autumn defoliation to control disease, harvest seed, or implement other cultural practices can reduce vigor and likely attenuate long-term performance of a plantation.