Involvement of calcium and calmodulin in the acquisition of thermotolerance in Amaranthus Lividus L.

Heat caused reduction in membrane protein thiol level and increased accumulation of thiobarbituric acid reactive substances in 72 hour old germinating tissues thereby reducing germination and early growth performances. Calcium chelator (EGTA), calcium channel blocker (LaCl₃) and calmodulin inhibitor (trifluroperazine) aggravated the effects of heating and added calcium reversed them, implying that protection against heat induced oxidative damage and improvement of germination in Amaranthus requires calcium and calmodulin. Imposition of heat stress during early germination also causes accumulation of reactive oxygen species (ROS) like O₂^.-and H₂O₂. Calcium treatment significantly reduced the accumulation of both the toxic ROS, while EGTA, LaCl₃ and TFP treatment enhanced the accumulation. Activities of antioxidative enzymes catalase (CAT), ascorbate peroxidase (AP) and glutathione reductase (GR) and total thiol content decreased under heat stress in germinating Amaranthus seedlings. Seedlings raised with Ca²⁺ treatment under heat stress exhibit higher activities of CAT, GR and AP and total thiol level than the untreated plants. EGTA, LaCl₃ & TFP treatment, on the other hand significantly reduce the activities of all anti-oxidative enzymes and total thiol level. The work supports the view that Ca²⁺-signalling pathway plays significant role in limiting heat induced oxidative injury during recovery phase of post germination event in Amaranthus lividus.