Dynamical Regulation of Photosynthetic Components Related to Photosynthesis and Photoprotection in Maturing Blueberry Fruit

Fruit photosystem not only performs photosynthesis for carbon fixation but is also related to photoprotection. This study is to investigate the changes within the fruit photosystem during maturation and their possible effects in photosynthesis and photoprotection. We investigated the changes of fruit photosynthesis during the three phases (green, pink, and blue) of blueberry fruit maturation via examining CO2 gas exchange and chlorophyll fluorescence. Also, we quantified chlorophylls, carotenoids, anthocyanins, phenolics using spectrometry, and determined photosynthetic components using LC-MS/MS. As a result, fruit photosynthesis contributed in supplying a part of the carbon requirements during the maturation process. The gross photosynthetic rate in green fruit accounted for 26.36% of that in the leaf, and 15.47% and 7.59%, respectively, in pink and blue fruit. Maximum photochemical quantum yield of PSII (Fv/Fm = 0.77) in green fruit was comparable to that in leaf (0.78) and dropped to 0.65 in pink fruit (data unavailable in blue fruit). Notably, in early stage of fruit maturation, the components of photosystem reaction center (PS core), electron transport chain (ETC), and CO2 fixation were significantly downregulated, while the components in light-harvesting complex (LHC) were more highly stable than others. This led to a decrease in the ratio of PS core/LHC. Thus, more light energy absorbed by LHCs had to be dissipated as heat, since it could not be converted to chemical energy through enough PS core, ETC, and CO2 fixation. This led to an increase in non-photochemical quenching (NPQ), indicated by increased qN of 35.53%. The increased NPQ became necessary for photoprotection at the early stage of fruit maturation, while increased anthocyanin provided another kind of photoprotection at the late stage of maturation. The results suggest that the changes of photosynthetic components could be related to the transition from photosynthesis to photoprotection during fruit maturation.