Antioxidant and Hormone Responses to Drought Stress in Two Perennial Ryegrass Cultivars Contrasting in Drought Tolerance.

Water deficit is a major limiting factor for grass culture in many regions with physiological mechanisms of tolerance not yet well understood.  This study was designed to investigate antioxidant enzymes, isozymes, abscisic acid (ABA), and indole-3-acetic acid (IAA) responses to deficit irrigation in two perennial ryegrass (Lolium perenne L.) cultivars contrasting in drought tolerance. The plants were subjected to well-watered (100% container capacity, 34.4%±0.21 volumetric moisture content (VWC)) or deficit irrigation (30% evapotranspiration (ET) replacement; 28.6% ±0.15 to 7.5% ±0.12 VWC) conditions for up to 8 d and re-watering for 4 d for recovery in growth chambers. Deficit irrigation increased leaf malondialdehyde (MDA) content in both cultivars, but drought tolerant ‘Manhattan-5’ exhibited lower levels relative to drought sensitive ‘Silver Dollar’. Superoxide dismutase (SOD) activity declined and then increased during water deficit treatment. ‘Manhattan-5’ had higher SOD activity and greater abundance of SOD1 isozyme than ‘Silver Dollar’ under water deficit. Deficit irrigation increased catalase (CAT) and ascorbate peroxidase (APX) activity in ‘Manhattan-5’, but not in ‘Silver Dollar’. ‘Manhattan-5’ had higher CAT, APX, and peroxidase (POD) activity than ‘Silver Dollar’ during water limitation. Deficit irrigation increased mRNA accumulation of cytolic cupper/zinc SOD (Cyt Cu/Zn SOD) whereas gene expression of manganese SOD (Mn SOD) and peroxisome APX (pAPX) were not significantly altered in response to water limitation. No differences in Cyt Cu/Zn SOD, Mn SOD and pAPX gene expression were found between the two cultivars under deficit irrigation. Water limitation increased leaf abscisic acid (ABA) and indole-3-acetic acid (IAA) contents in both cultivars, with ‘Silver Dollar’ having a higher ABA content than ‘Manhattan-5’. Change in ABA level may regulate stomatal opening and oxidative stress which may trigger antioxidant defense responses. These results indicate that accumulation of antioxidant enzymes and ABA are associated with perennial ryegrass drought tolerance. Activity and isozyme assays of key antioxidant enzymes under soil moisture limitation can be a practical screening approach to improve perennial ryegrass drought tolerance and quality.