Lack of downregulation of photosynthesis in a tropical root crop, cassava, grown under an elevated CO2 concentration

Abstract

We evaluated the effects of an elevated [CO₂] on photosynthesis and growth of cassava plants grown in open-top chambers with an adequate supply of water and N and a sufficient rooting volume. Cassava plants (Manihot esculenta Crantz. cv. Motilona) showed higher photosynthetic rates (P_n) when grown and measured at elevated [CO₂] (680 µmol mol^-1) than when grown and measured at ambient [CO₂] (480 µmol mol^-1). No downregulation of photosynthesis due to elevated [CO₂] was found, since carboxylation efficiency increased after 220 d in spite of a decrease in leaf soluble protein, Rubisco, and leaf N content. Soluble sugar and starch contents decreased with time under elevated [CO₂], the decrease in starch content coinciding with the beginning of the increase in root mass. Canopy P_n by leaf area decreased with time under elevated [CO₂] but, when canopy P_n was expressed by ground area, higher and constant rates were observed, suggesting a higher productivity in plants grown at elevated [CO₂]. The absence of differences between growth [CO₂] in root : shoot ratio observed suggests that elevated [CO₂], while causing increases in the shoot as well as the root, did not affect the pattern of biomass allocation. Acclimation responses of gas exchange parameters changed during the experiment. The absence of downregulation of photosynthesis was associated with a decrease in leaf sugar and starch contents of plants grown at elevated [CO₂], which suggests a favourable source/sink relationship.