Physiological Changes Associated with Drought-Induced Freezing Tolerance of Perennial Ryegrass.

Studies have shown that drought stress may increase plant freezing tolerance in the absence of cold acclimation.  Previous research using six perennial ryegrass (Lolium perenne L.) cultivars demonstrated that wilt-based irrigation, which induced mild drought stress, resulted in an enhancement in freezing tolerance; however, this response was dependent on the genotype.  Therefore, the objectives of this research were to determine the physiological basis for differences in drought-induced freezing tolerance for two cultivars of perennial ryegrass and to quantify the degree of freezing tolerance of plants with drought-induced acclimation compared to plants with conventional cold acclimation at 2 °C. Perennial ryegrass cultivars Sunkissed and Bucaneer were moved into a controlled environment chamber (20/15 ^oC day/night temperature, 10 h photoperiod, and photosynthetic photon flux density of 500 μmol m^-2 s^-1) and subjected to the following treatments: (i) well-watered control, irrigated three times per week (WW), and (ii) wilt-based irrigation, plants irrigated at wilt prior to any severe symptoms of drought stress (WB). After five wilt events, a group of plants were maintained under the same treatments (WW and WB) at 2 °C for an additional three weeks.  Leaf and crown tissues were harvested at the end of five wilt events and following cold acclimation for analysis of osmotic adjustment, water soluble carbohydrates (WSC), proteins, and freezing tolerance (LT₅₀ killing temperature).  Mild drought stress associated with WB irrigation caused an improvement in freezing tolerance (lower LT₅₀) for ‘Sunkissed’, but had no significant effect on freezing tolerance of ‘Bucaneer’.  The differences in drought-induced freezing tolerance among perennial ryegrass cultivars were primarily associated with enhanced osmotic adjustment and accumulation of WSC for ‘Sunkissed’ compared to ‘Bucaneer’.