Effect of growing season on the photosynthetic apparatus and leaf antioxidative defenses in two maize genotypes of different chilling tolerance

The effect of the growing season on the photosynthetic apparatus and on leaf antioxidants was investigated in a chilling-tolerant genotype (Z7) and a chilling-sensitive genotype (Penjalinan) of maize (Zea mays). Maize was sown in the field in Switzerland in 3 consecutive weeks, so plants sown early were exposed to chilling conditions, whereas those sown later developed under more favorable growth conditions. Measurements of the quantum efficiency of CO2 fixation (Phi(CO2)) and the photosystem II (Phi(PSII)) were made simultaneously on the third fully expanded leaves. The activity of scavenging enzymes and the content of pigments and antioxidants were also determined. Leaves that developed under chilling conditions showed typical chill-induced alterations, namely low photosynthetic capacity and efficiency, reduction in the pigment contents, and a decrease in catalase activity. These alterations were more pronounced in the chilling-sensitive than in the chilling-tolerant genotype. Determining the Phi(PSII)/Phi(CO2) ratio indicated that it was very unlikely that alternative electron sinks, such as the Mehler reaction, were activated to a significant extent in either of the genotypes. This was supported by the measurements of the activity of enzymes involved in the Mehler ascorbate peroxidase reaction. However, a comparison of the genotypes showed that chilling tolerance might be correlated with an increase in the cr-tocopherol and glutathione contents as well as in the activity of glutathione reductase. (C) 1999 Elsevier Science B.V. All rights reserved.