Gauging submersed plant response to CO2 enrichment: Pot size matters

• Growth response to CO2 enrichment was greater in larger pots than in smaller pots.
• CO2 enrichment alters biomass allocation to belowground parts more in larger pots.
• Plant growth increased with pot size at high [CO2], but not at low [CO2].
• Growth measured as final dry weight, change in dry weight, and RGR all showed the same patterns.
• Experimenters should use larger pots to test effects of CO2 enrichment.

The availability of a photosynthetic carbon source may limit submersed plant growth, and experimental assessment of this limitation may depend on the volume available to belowground plant parts. To test the effect of pot size on submersed plant growth response to CO2 enrichment, we grew the freshwater macrophyte Vallisneria americana in pots containing 1.01, 2.25, and 4.04 L of lake sediment in pH- and [CO2]-controlled greenhouse tanks for nine weeks. A highly significant (P < 0.001) interaction between [CO2] and pot size for final biomass, root mass fraction, dry weight gain, and relative growth rate (RGR) indicated that growth response to CO2 enrichment depends on pot size. The ratios of growth at high [CO2] to that at low [CO2] increased from 3.2 in small pots to 8.1 in large pots for dry weight gain, and from 1.8 in small pots to 3.3 in large pots for RGR. Response to CO2 enrichment was thus substantially greater for plants grown in larger pots. Further, increasing pot size from small to large did not increase dry weight gain at low [CO2], but yielded 69% greater dry weight gain at high [CO2]. Macrophyte experimentalists should carefully consider pot size in relation to available sediment resources, which can influence growth rate and growth response to CO2 enrichment.