Tropical maize is not suitable for growth in the cold climates at high latitudes or altitudes, where daily temperatures around zero and bright sunlight are common. Such conditions predispose the plants to a series of events that can ultimately lead to destruction of the photosynthetic apparatus of exposed leaves. Photogeneration of active oxygen species, following the formation of chlorophyll triplets and singlet oxygen in the light‐harvesting antennae when light in excess of that usable for photosynthesis is absorbed, is the key event for this oxidative destruction. Under optimal or mild stress conditions, xanthophylls can contribute to efficient dissipation, as heat, of this excess energy, with a high proportion of de‐epoxitated xanthophylls indicating enhanced energy dissipation. A well‐organized antioxidant system can also efficiently scavenge the AOS eventually formed. However, these defence mechanisms need to be efficiently regulated and photosynthesis is likely to be involved in the regulatory mechanism.
Under severe stress conditions, however, the capacity of the protective mechanisms can be overwhelmed. An effective strategy for protection under such conditions could be to reduce the amount of light arriving at the chloroplasts rather than to have the defence mechanisms working at full activity for an unpredictable length of time. An interesting example of such a strategy comes from some highland races of maize from Mexico that grow successfully where lowland tropical maize cannot be grown. Among the phenotypic characteristics shown by highland races is that they tend to accumulate anthocyanin pigments in stems and leaves. These pigments are water‐soluble flavonoids that are concentrated in cell vacuoles and absorb visible light in the range 400-550nm. Their synthesis is determined by inherited factors and enhanced by conditions such as a com‐bination of low temperature and high light.
According to Gould the role of anthocyanins is not clear and may depend on whether their location is in the vacuoles of the abaxial or adaxial leaf epidermis, in the cytosol of mesophyll cells, in roots, or in stems. Gould suggested that in, Begonia pavonina and Triolena hirsuta, anthocyanin accumulation in leaves prevented photo‐inhibition by shading chlorophyll b and, as a result, photosynthesis of red leaves was higher than photosynthesis of green leaves. In contrast, anthocyanin accumulation seems to reduce photosynthesis in many other species. Recently Havaux & Kloppstech have shown that, in flavonoid Arabidopsis mutants, anthocyanin exerts very little protection against photo‐inhibition and or photo‐oxidation, giving flavonoids absorbing UV and blue light a more important role for photoprotection. These observations suggest that the role of anthocyanins should be further investigated, particularly with respect to its effects on photosynthesis and other protective mechanisms.
In this article we address the question of how anthocyanin accumulation in maize leaves may reduce the risk of photo‐inhibition at low temperature by enhancing the photoprotection capacity, without further limitation to photosynthesis.
