INSTANTANEOUS AND DEVELOPMENTAL EFFECTS OF LOW-TEMPERATURE ON THE CATALYTIC PROPERTIES OF ANTIOXIDANT ENZYMES IN 2 ZEA SPECIES

When Zea mays cv. LG11 plants were grown 14 degrees C (close to the lo wer thermal limit for leaf expansion), three of the five enzymes of th e active oxygen scavenging cycle (Halliwell-Asada Pathway) showed chan ges in total leaf activity compared with growth at 25 degrees C. Two o f these enzymes, ascorbate peroxidase (APX) and glutathione reductase (GTR), were selected for further investigation. The effects of assay t emperature on three kinetic parameters (V-max, K-m, V-max/K-m) were de termined in extracts from Z. mays and compared with extracts from its low temperature tolerant relative, Z. diploperennis Iltis, Doebley & G uzman. The kinetic power (V-max/K-m) was determined because the Km alo ne 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 degrees C was much smaller in Z. diploperennis than in Z. mays. T his suggests that the Z. diploperennis APX is better able to remove H2 O2 at severely reduced temperatures than is APX from Z. mays. Z. diplo perennis had a twofold greater ascorbate pool than the chilling-suscep tible Z. mays. Only minor differences were seen in the kinetic propert ies of GTR and the size of the glutathione pool between the genotypes.