Effects of decreasing photoperiod on cold acclimation of asparagus seedlings

Cold acclimation is a vital process for surviving subzero temperatures in asparagus (Asparagus officinalis L.). A winter hardy cultivar, "Guelph Millennium" (GM), cold acclimates, senesces, and develops freezing tolerance earlier in the fall compared to "UC157" (UC), a cultivar adapted to warm climates. Decreasing photoperiod can induce timely senescence and freezing tolerance in both woody and herbaceous perennials, although its effects on asparagus are unclear. The objectives of this research were to study the effects of daylength and cold, and their interaction, on the induction of freezing tolerance in asparagus. Seedlings of GM and UC were subjected to four treatments: constant (15.5 h) or decreasing (15.5-14 h) photoperiods with either high (23 degrees C) or low (11 degrees C) temperatures. Decreasing daylengths did not impact the acquisition of freezing tolerance at either temperature. For UC, low temperature accompanied by decreasing photoperiod diminished root:shoot ratio and increased crown water percentage, which are both indicators of reduced dormancy. A decreasing photoperiod and high temperature increased crown fructan (total and high-molecular weight) and decreased glucose and proline concentrations in GM compared to UC. These results suggest that decreasing daylengths, similar to those occurring in late summer in Southern Ontario, may be a signal for crown metabolite partitioning. This effect may be a contributing factor to the timely senescence and cold acclimation seen in GM compared to UC.