Instantaneous and developmental effects of low temperature on the catalytic properties of antioxidant enzymes in two Zea species

When Zea mays cv. LG11 plants were grown 14°C (close to the lower thermal limit for leaf expansion), three of the five enzymes of the active oxygen scavenging cycle (Halliwell-Asada Pathway) showed changes in total leaf activity compared with growth at 25°C. Two of these enzymes, ascorbate peroxidase (APX) and glutathione reductase (GTR), were selected for further investigation. The effects of assay temperature on three kinetic parameters (V(max), K(m), V(max)/K(m)) were determined in extracts from Z. mays and compared with extracts from its low temperature tolerant relative, Z. diploperennis Iltis, Doebley and Guzman. The kinetic power (V(max)/K(m)) was determined because the K(m) alone may not be a useful predictor of an enzyme's effectiveness in situ. The decrease in the kinetic power of APX on lowering the temperature to 5°C was much smaller in Z diploperennis than in Z mays. This suggests that the Z. diploperennis APX is better able to remove H₂O₂ at severely reduced temperatures than is APX from Z. mays. Z. diploperennis had a twofold greater ascorbate pool than the chilling-susceptible Z. mays. Only minor differences were seen in the kinetic properties of GTR and the size of the glutathione pool between the genotypes.