ABSTRACT
Plants that grow in cold climates are well adapted to the ambient chilling temperatures, but the adaptation to low temperatures in these plants is directed more towards survival rather than to performance (harvestable yield). For crop plants such as maize, which are grown outside their natural environment, performance is the only valuable criterion. Greaves (1996) defined a suboptimal stress for maize as any reduction in growth or induced metabolic, cellular, or tissue injury that results in limitations to the genetically determined yield potential, caused as a result of exposure to temperatures below the thermal thresholds for optimal biochemical and biophysical activity or morphological development. Optimal growth of maize crop occurs in climates with midsummer temperatures between 21°C and 27°C and the optimal temperature for maximum grain yields lies around 25°C. Maize plants that are subjected to temperatures below 20°C are believed to undergo already physiological and biochemical changes. Of course, the damage will increase according to the duration and severity of the chilling conditions. For example, maize plants grown continuously at 17/15°C (day/night temperatures) are seriously retarded in growth. The stressed plants reach the same developmental stage 10 days after nonstressed plants. Besides the disruption of plasma membranes and decreased activities of metabolic enzymes, photooxidative stress is considered the main cause of damage during chilling stress in chilling-sensitive species such as maize.
