Mechanisms of Photosynthetic Apparatus Acclimation of C4 Plants to Different Irradiances

Abstract

The regulation by light of the photosynthetic apparatus and specifically the composition of light-harvesting complexes in mesophyll and bundle sheath chloroplasts was investigated in the C4 plants: Zea mays (NADP-ME type), Panicum miliaceum (NAD-ME type) and Panicum maximum (PEP-CK type). Chloroplasts ultrastructure, chlorophyll contents, fluorescence analysis, PQ level, transcripts accumulation, amount of thylakoid proteins and Lhc polypeptides were determined for plants grown under high, moderate and low irradiance. Results demonstrate that mesophyll and bundle sheath chloroplasts of plants grown under the same light conditions differ significantly with respect to the amount of light-harvesting complexes. However, Z. mays leaves acclimation to different light caused changes in LHCII complexes in both types of chloroplasts, while in both Panicum species the abundance of LHC proteins was similarly affected although some changes occurred at the chloroplast organizational level. For all examined species light intensity changed significantly the PQ level and its oxidation states in both types of chloroplasts. In our opinion redox balance derived from electron transport is a main factor playing an important regulatory role in acclimation of C4 subtypes to light and they accomplished this by balancing the amounts of thylakoid components and their activity or/and by the chloroplast ultrastructure.