Changes of Photochemical and CO 2 Assimilation Activities in Response to Growth at Low Temperature and High Light in a Chilling Sensitive Maize Genotype as Compared with a Tolerant Maize Genotype

The photosynthetic mechanism of C4 plants is generally less tolerant to chilling stress than the mechanism of C3 plants. Visible oxidative injuries on the leaf tissue of C4 plants exposed for long periods to the combination of chilling and high light suggest that these plant types are not adequately photoprotected. The energy absorbed under stress at the chloroplasts might be in excess of that required for the carbon reduction metabolism. A process of non-radiative emission disposes of most of this excess but a pseudocyclic flux of electrons to oxygen might also occur acting as a dissipation process. The strong increase of chloroplast antioxidant activities (1) indicates that the oxygenated species formed at the PSI or at the other sides must be quickly and completely scavenged to avoid an irreversible damage to the photochemical apparatus and thus a limitation to photosynthesis. In alternative, a limitation might be initiated by an enzymatic impairment in the carbon reduction metabolism with a cascade of events leading to photochemical over-reduction, inactivation and oxidative stress damage. In this paper we report the investigation on the response to low temperature and high light of photosynthesis in two maize genotypes with contrasting chilling tolerance. Differences in the regulation of photosynthetic metabolism and of the mechanisms that protect and cope with the excess of reducing power at chloroplasts were found.