Calcium Stimulates the Adaptation of Cultured Liquorice Cells to PEG-Induced Water Stress

Calcium may be involved in plant tolerance to water deficit by regulating antioxidant metabolism or/and water relations. This study was designed to examine whether external Ca²⁺ would stimulate drought tolerance in cultured liquorice cells. Water stress induced by 15% PEG significantly reduced fresh weight and relative water content in liquorice cells, but external Ca²⁺ markedly increased them after stress for 7 days. The activities of catalase (CAT), superoxide dismutase (SOD) declined and activity of peroxidase (POD) slowly increased during water stress imposition. External calcium significantly enhanced SOD and CAT activities, but the effect on POD activity was weak. The effect of external Ca²⁺ on water deficit tolerance in liquorice cells was not due to the osmotic adjustment in culture medium. Under nonstress conditions, external calcium slightly increased the activities of SOD, CAT, and POD. Ca²⁺ signal in liquorice cells may be different under stress and nonstress conditions. Under water stress, Ca²⁺ signal involves in reactive oxygen species transduction pathway and affects the processes participating in regulation of antioxidative enzymes; under nonstress conditions, Ca²⁺ signal coming from external calcium might not participate in ROS signal transduction pathway resulting in antioxidative defense response in liquorice cells. Less malondialdehyde was accumulated after water stress for 7 days in Ca²⁺-treated cells than in untreated cells. It was proposed that external calcium could reduce the damage of water deficit and stimulate tolerance to it in liquorice cells by mitigating oxidative stress.