The Regulation of Citric Acid Accumulation and Carbon Recycling During CAM inAnanas comosus

Publisher Website
Google Scholar
Abstract
The carbon balance of shade-grown Ananas comosus was investigated with regard to nitrogen supply and responses to high PAR. Net dark CO2 uptake was reduced from 61.2 to 38.5 mmol CO2 m−2 in N limited (−N) plants grown under low PAR (60 μmol m−2 s−1) and apparent photon yield declined from 0.066 to 0.034 (mol 02.mol−1 photon), although photosynthetic capacities (measured under 5% CO2) were similar. Following transfer for 7 d to high PAR (600. μmol m−2 s−1), net CO2 uptake at night increased by 14% in +N plants, and daytime photosynthetic capacity was higher, with a maximum value of 7.8 μmol m−2 s−1. The magnitude of dark CO2 fixation during CAM was measured as dawn—dusk variations in leaf-sap titratable acidity (ΔH+) and as the proportion of malic and citric acids. The contribution from re-fixation of respiratory CO2 recycling (measured as the difference between net CO2 uptake and malic acid accumulation) varied with growth conditions, although it was generally lower (30%) than reported for other bromeliads. Assuming a stoichiometry of 2H+: malate and 3H+: citrate, there was a good agreement between titratable protons and enzymatically determined organic acids. The accumulation of citric acid was related to nitrogen supply and PAR regime, increasing from 7.0 mol m−3 (+N plants) to 18 mol m−3 (−N plants) when plants were transferred to high PAR; malate: citrate ratios decreased from 13.1 to 2.5 under these conditions. Under the low PAR regime, leaf-sap osmotic pressure increased at night in proportion to malic acid accumulation. However, following the transfer to high PAR for 7 d, there was a much greater depletion of soluble sugars at night which corresponded to a decrease in leaf-sap osmotic pressure. Although a role for citric acid in CAM has not been properly defined, it appears that the accepted stoichiometry for CAM in terms of gas exchange, titratable acidity, malic acid and osmotic pressure may not hold for plants which accumulate citric acid.

This publication has 17 references indexed in Scilit: