Response of the photosynthetic apparatus in maize leaves grown at low temperature on transfer to normal growth temperature
Low temperatures are known to restrict chloroplast development and prevent the attainment of photosynthetic competence in maize leaves. The responses of the photosynthetic apparatus of mature maize leaves grown at 14 °C on transfer of the plants to 25 °C are examined. The synthesis of thylakoid proteins increased immediately on transfer of leaves from 14 to 25 °C, with a dramatic accumulation of thylakoid proteins and chlorophylls occurring after 3 d at 25 °C. Thylakoid structure and organization also became similar to those observed in leaves grown at 25 °C over this period. However, no comparable development of photosynthetic competence in photosystems I and II or in the rate of CO 2 assimilation was observed on transfer of leaves from 14 to 25 °C. Immunocytological analyses demonstrated heterogeneity in the distribution of a range of thylakoid proteins (cytochrome f, the α and β subunits of the coupling factor, DI of the photosytem II reaction centre, the 33 kDa protein of the extrinsic oxygen-evolving complex of photosystem II, and subunit II of photosystem I) between mesophyll cells in leaves grown at 14 °C, and in the responses of individual proteins to transfer of the leaves to 25 °C. Such heterogeneity between mesophyll cells would account for the inability of the leaves to develop the expected degree of photosynthetic competence on transfer to 25 °C. The effects of low growth temperatures on chloroplast biogenesis are complex, as are the changes induced by the transfer of leaves grown at low temperatures to optimal growth temperature, and both these factors may limit the canopy development and photosynthetic productivity of crops in temperate regions