Photosynthetic characterization of Jerusalem artichoke during leaf expansion

Gas exchange, chlorophyll a fluorescence and modulated 820 nm reflection were investigated to explore the development of photosynthesis in Jerusalem artichoke (Helianthus tuberosus L.) leaves from initiation to full expansion. During leaf expansion, photosynthetic rate (Pn) increased and reached the maximal level when leaves were fully expanded. The same change pattern was also found in the stomatal conductance and chlorophyll content. Lower Pn could not be ascribed to the higher stomatal resistance in developing leaves, as intercellular CO₂ concentration was not significantly lower in these leaves. Lower Pn partly resulted from the lower actual photochemical efficiency of PSII in developing leaves, as more excited energy was dissipated through non-photochemical quenching. The development of primary photochemical reaction and electron transport in the donor side of PSII was completed in the initiating leaves. However, the development of electron transport in the acceptor side of PSII was not accomplished until leaves were fully expanded, indicated by the change in probability that an electron moves further than primary quinone (ψo). PSI activity changed in parallel with ψo suggesting that PSI cooperated well with PSII during leaf expansion. It should be stressed that the development of carbon fixation process was later than primary photochemical reaction but earlier than photosynthetic electron transport during leaf expansion. The later development of photosynthetic electron transport may reduce the production of reactive oxygen species from Mehler reaction, particularly under low carbon fixation.