The effect of growth at low temperature on photosynthetic characteristics and mechanisms of photoprotection of maize leaves

Abstract

We examined the photosynthetic properties, the activity of antioxidant enzymes and the amount of caroten-oids of two maize genotypes characterized by different sensitivity to low temperature. Plants of the low-temperature-sensitive genotype A-619 and of the low-temperature-resistant genotype VA-36 were grown at 25/20°C (control plants) and at 16/14°C (plants grown at low temperature). Twenty-five days after seeding, the exposure to low temperature caused, in all plants, a reduction of leaf chlorophyll and carotenoid content, but an increase in the activity of the oxygen-detoxifying enzymes, superoxide dismutase and ascorbate per-oxidase. However, the response of photosynthesis, stomatal conductance and the fluorescence properties to light and temperature were not affected by growth at low temperature. Fifty days after seeding, photosynthesis, stomatal conductance, and fluorescence properties of A-619 leaves grown at low temperature were significantly reduced with respect to control plants at all temperatures and light intensities. In the leaves of A-619 plants grown at low temperature, the electron transport rate was not tightly down-regulated by carbon metabolism and an excess of electrons was shown by the increased ratio between the quantum yield of electron transport of photosystem II and the quantum yield of CO2 assimilation. On the contrary, VA-36 leaves grown at 16°C maintained the same photosynthetic characteristics and photochemical properties as control plants. The chlorophyll content of both genotypes and carotenoid content of A-619 plants were lower in leaves of plants maintained at 16°C than in those grown at 25°C. In contrast, the carotenoid content of VA-36 leaves of plants grown at low temperature were higher than in plants grown at 25°C. The activity of superoxide dismutase and ascorbate peroxidase of VA-36 plants grown at low temperature were higher than in controls. In A-619 plants grown at fow temperature the activity of superoxide dismutase was higher than in controls, but the activity of ascorbate peroxidase was lower than in controls. Our findings suggest that when maize plants are grown at low temperature the electron transport rate may be in excess of carbon metabolism and electrons may be used to reduce oxygen. A co-ordinate increase of pigment amounts and of the activity of oxygen-detoxifying enzymes is necessary to protect maize leaves from the accumulation of oxygen radicals at low temperature. In A-619 plants, the carotenoid content did not increase and the activity of ascorbate peroxidase was low when plants were exposed to low temperature for 50 d. As a result, the photochemical apparatus of A-619 leaves was damaged and photo-oxidation occurred. These experiments also indicated that when photosynthesis was reduced by a transitory reduction of temperature, the electron transport was still tightly down-regulated by carbon metabolism and the photosynthetic apparatus of both genotypes was not damaged.