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Abstract
Within photosynthetically active radiation (PAR), green photons had a lower quantum yield of photosynthesis than blue and red photons at low PPFD. Once absorbed, green photons drove photosynthesis as efficiently as red photons and more efficiently than blue photons. At high PPFD, low absorptance of green photons by leaves allowed more even distribution of green photons throughout the depth of leaves, resulting in higher quantum yield of photosynthesis than blue or red photons. Far-red photons supplemented to background white light induced leaf expansion and canopy enlargement of ‘Cherokee’, ‘Green Saladbowl’ and ‘Little Gem’ lettuces. Supplemental far-red photons only increased light interception of ‘Cherokee’ throughout the whole life cycle and increased that of ‘Green Saladbowl’ and ‘Little Gem’ transiently, which resulted in increased dry weight of ‘Cherokee’ and ‘Little Gem’ but not ‘Green Saladbowl’. Our study with ‘Little Gem’ lettuce grown under lights with the same ePAR but different fractions of far-red photons (%FR), showed that far-red photons affected canopy-level photosynthesis depending on plant density. At high plant density, far-red photons resulted in limited increase in light interception. The effect of higher light interception on canopy-level photosynthesis was likely compensated by lower absorptance of far-red photons by lettuce plants and %FR did not affect canopy-level photosynthesis. At low plant density, far-red photons increased light interception more dramatically, which resulted in increased canopy-level photosynthesis despite lower absorptance. Far-red photons also had lower canopy-level quantum yield of photosynthesis than red photons (0.57 times of that of red photons), due to lower absorptance and the fact that they can only excite PSI. The change in canopy-level photosynthetic rates was not reflected in the dry weight of the plants. Carbon use efficiency changed dramatically through life cycle of ‘Little Gem’ lettuce plants, ranging from 0.05 to 0.76. CUE initially increased with plant size, then decreased steadily during active vegetative growth. This steady decrease in CUE was due to the increasing percentage of maintenance respiration in total respiration, which competed with growth and growth respiration for carbon. CUE is important for modelling crop growth, as canopy-level photosynthetic rates alone cannot accurately predict crop growth and yield.