Long-term compound interest effect of CO₂ enrichment on the carbon balance and growth of a leafy vegetable canopy

The response of crops to elevated atmospheric CO₂ concentrations (C_a) is of great concern for horticultural vegetable production in greenhouse facilities, where C_a levels are often artificially elevated. To elucidate the long-term effect of the elevated C_a 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 C_a (approximately 800μmolmol^-1) and ambient C_a (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 C_a on A, which was evaluated with enhancement ratios (i.e., A in the elevated C_a treatment divided by A in the ambient C_a treatment; A_elev/A_amb), 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 A_elev/A_amb was attributed to the enhancement of A_elev brought on by not only the short-term enhancing effect of elevated C_a but also the more rapidly increasing and thus larger L in the elevated C_a treatment compared with that in the ambient C_a treatment. Both the long-term increase in A_elev/A_amb and the more rapidly increasing L in the elevated C_a treatment were caused by the “compound interest effect” of elevated C_a, where the enhancing effects of elevated C_a 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 C_a 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 C_a 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 C_a on canopy photosynthesis, carbon balance, and growth.