Effects of Drought Preconditioning On Freezing Tolerance of Perennial Ryegrass.

Previous research has shown that prior exposure to mild drought stress, or drought preconditioning, could enhance plant tolerance to different abiotic stresses. Therefore, the objectives of this research were to (i) examine the influence of drought preconditioning on freezing tolerance of two perennial ryegrass (Lolium perenne L.) cultivars under non-cold acclimating (20°C) and cold acclimating (2°C) conditions; and (ii) determine the physiological basis for changes in freezing tolerance in response to drought preconditioning, with a focus on compatible solute accumulation and antioxidant scavenging capacity.  Plants of ‘Buccaneer’ and ‘Sunkissed’ perennial ryegrass were moved to a controlled environment chamber (20°C) and subjected to the following treatments: (i) well-watered control (WW), plants irrigated three times per week, and (ii) drought preconditioned (DP), plants irrigated at first signs of wilt prior to any severe symptoms of drought stress. After five wilt events, a group of plants were maintained under the same treatments (WW and DP) at 2°C for an additional three weeks.  Leaves and crowns were harvested at the end of five wilt events and following cold acclimation for analysis of freezing tolerance (lethal temperature for 50% kill, LT₅₀), carbohydrates, and antioxidant enzyme activities.  Drought preconditioning resulted in an improvement in freezing tolerance (lower LT₅₀) for ‘Buccaneer’ at 20°C and for both cultivars at 2°C.  The differences in freezing tolerance among perennial ryegrass cultivars were primarily associated with changes at the crown level including increases in total nonstructural carbohydrates and specific antioxidant enzymes such as ascorbate peroxidase, guaiacol peroxidase and catalase.  In addition, increases in crown proline and soluble protein concentrations were detected for both cultivars in response to drought preconditioning.