Differential Effects of Glycine Betaine and Spermidine on Osmotic Adjustment and Antioxidant Defense Contributing to Improved Drought Tolerance in Creeping Bentgrass

Glycine betaine (GB) and spermidine (Spd) are known to play roles in plant adaptation to stresses that induce dehydration, including drought stress. The objectives of this study were to examine whether improved drought tolerance by exogenous application of GB or Spd is associated with the increases in the endogenous accumulation of those solutes under drought stress in cool-season turfgrass species, and to determine the relative effects of those solutes on osmotic adjustment (OA) and antioxidant metabolism for improving drought tolerance. Creeping bentgrass (Agrostis stolonifera cv. Penncross) plants were treated with 200 mM GB or 0.1 mM Spd for 3 weeks by weekly foliar application before the exposure to drought stress; those plants were subsequently subjected to drought stress by withholding irrigation. The endogenous content of GB and Spd increased significantly through the exogenous application of either compound, to a greater magnitude for GB. The comparison of GB-or Spd-treated plants to untreated plants exposed to drought stress for growth [turf quality (TQ)] and physiological responses (water relations, membrane stability, and antioxidant metabolism) demonstrated that both compounds led to significant improvement in drought tolerance in creeping bentgrass. The improved drought tolerance by exogenous GB could be due to its contribution to OA by significant accumulation of endogenous GB, and activation of antioxidant enzymes with the greatest effects on ascorbate peroxidase (APX). Exogenous Spd did not cause increases in leaf OA despite of the increased endogenous accumulation, but significantly enhanced antioxidant enzyme activities, with the most pronounced effects on catalase (CAT). This study demonstrated that GB and Spd had different effects on OA and activated different antioxidant defense pathways, protecting plants from drought damages in creeping bentgrass.