Normal Cyclic Variation in CO₂ Concentration in Indoor Chambers Decreases Leaf Gas Exchange and Plant Growth

Area of Science:

• Plant physiology
• Agricultural science
• Environmental science

Background:

• Research is increasingly focused on identifying crop genetic material for enhanced growth under elevated atmospheric CO2.
• Short-term CO2 concentration variations in free-air CO2 enrichment plots may reduce photosynthesis and yield.
• The impact of typical indoor CO2 cyclic variation on plant growth requires investigation.

Purpose of the Study:

• To determine if normal CO2 cyclic variation in indoor chambers affects photosynthesis and growth.
• To investigate the mechanisms behind observed responses to CO2 variation.
• To assess the impact of different CO2 variation amplitudes on crop development.

Main Methods:

• Wheat, cotton, soybeans, and rice were grown in indoor chambers with a mean CO2 of 560 μmol mol⁻¹.
• Triangular cyclic CO2 variation with standard deviations of 4.5 or 18.0 μmol mol⁻¹ was applied.
• Leaf gas exchange systems were used to test stomatal responses to light and CO2 step changes and square-wave cycling.

Main Results:

• Larger CO2 variation (18.0 μmol mol⁻¹) significantly reduced photosynthesis, stomatal conductance, and above-ground biomass in all four species.
• Reduced stomatal conductance was observed even in soybeans and rice, despite their equal stomatal opening/closing rates.
• Gas exchange data suggest that factors beyond reduced mean stomatal conductance contribute to decreased photosynthesis under variable CO2.

Conclusions:

• Cyclic CO2 variation, even at levels typical for indoor experiments, can persistently impact crop photosynthesis and growth.
• The amplitude of CO2 fluctuations is a critical factor influencing plant responses.
• Further research is needed to fully understand the physiological mechanisms limiting photosynthesis in variable CO2 environments.