Comment: Interpretation of Recycling Indices
Response: An Alternative Interpretation of the Appropriateness and Correct Means for the Evaluation of CO2 Recycling Indices.

Abstract

Pretreatment of maize (Zea mays L.) seeds withCaCl₂ solution significantly raised calcium andcalmodulin levels and enhanced intrinsic heat tolerance in maize seedlings.This enhancement is specific to Ca²⁺ and cannot besubstituted by Mg²⁺. In contrast, treatments with theCa²⁺ chelator EGTA or plasma membraneCa²⁺ channel blockers La³⁺ orverapamil lowered the intrinsic heat tolerance. Concurrent treatment withCa²⁺ and Ca²⁺ -channel blockers(La³⁺, verapamil) also decreased theCa²⁺ -enhanced heat tolerance. In addition, calmodulinantagonists chlorpromazine or W7 treatment eliminated theCa²⁺ -enhanced heat tolerance, whilst W5, a less activeanalogue of W7, had little effect on the Ca²⁺ -enhancedheat tolerance. Measurement of antioxidant enzyme activities and lipidperoxidation showed that heat stress induced an oxidative stress in maizeseedlings. External Ca²⁺ treatment enabled the seedlingsto keep relatively higher activities of superoxide dismutase (SOD), catalase(CAT) and ascorbate peroxidase (APX) and lower levels of lipid peroxidationthan Ca²⁺ -deficient treatments; on the contrary, EGTAtreatment led to more rapid loss of SOD, CAT and APX activities and higherlevels of lipid peroxidation in the seedlings under heat stress. In addition,concurrent Ca²⁺ and W7 treatment weakened the effects ofCa²⁺ treatment on SOD, CAT and APX activities and causedmore severe lipid peroxidation. These results suggested that externalCa²⁺ can enhance the intrinsic heat tolerance of maizeseedlings. This enhancement requires the entry of externalCa²⁺ into cells across plasma membranes and themediation of intracellular calmodulin, and is associated with the increase ofantioxidant system activity.