The response of crops to elevated atmospheric CO2 concentrations (Ca) is of great concern for horticultural vegetable production in greenhouse facilities, where Ca levels are often artificially elevated. To elucidate the long-term effect of the elevated Ca on canopy photosynthesis and growth, we conducted long-term continuous measurements of the canopy photosynthesis (A) and leaf area index (L) of hydroponically grown spinach canopies under elevated Ca (approximately 800μmolmol-1) and ambient Ca (approximately 400μmolmol-1) treatments by combining the open chamber method and image analysis of the top of view of canopies. There existed a positive feedback loop between A and L, where an increase in L caused an increase in A, which subsequently accelerated the increase in L. The enhancing effect of elevated Ca on A, which was evaluated with enhancement ratios (i.e., A in the elevated Ca treatment divided by A in the ambient Ca treatment; Aelev/Aamb), showed a short-term increase under high-light conditions during the daytime and a long-term increase with canopy growth toward the harvest. The long-term increase in Aelev/Aamb was attributed to the enhancement of Aelev brought on by not only the short-term enhancing effect of elevated Ca but also the more rapidly increasing and thus larger L in the elevated Ca treatment compared with that in the ambient Ca treatment. Both the long-term increase in Aelev/Aamb and the more rapidly increasing L in the elevated Ca treatment were caused by the “compound interest effect” of elevated Ca, where the enhancing effects of elevated Ca on A and L were gradually amplified over a long-term period through the positive feedback loop between A and L. This compound interest effect of elevated Ca also caused a long-term increase in canopy nighttime respiration in proportion to daytime photosynthesis. Through these changes in the canopy-scale carbon balance, the compound interest effect detected in the elevated Ca treatment likely contributed to a substantial increase in the final aboveground dry weight at the harvest time. This study highlights the importance of the long-term compound interest effect of elevated Ca on canopy photosynthesis, carbon balance, and growth.
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