Comparative ecophysiology of CAM and C3 bromeliads. II. Field measurements of gas exchange of CAM bromeliads in the humid tropics

Abstract

Abstract The results described represent the first detailed measurements of gas exchange of epiphytic plants with crassulacean acid metabolism (CAM) in the humid tropics. A portable steady‐state CO2 and H2O porometer was used to measure net exchange rates of CO2 and H2O vapour (JCO2, JH2O), leaf temperature (T1), air temperature (TA), air relative humidity (RH) and photosynthetically active radiation (PAR) for bromeliads in the field during the dry season in February and March 1983 on the tropical island of Trinidad. Different lengths of tubing (up to 25 m) were used so that the gas exchange could be measured of bromeliads in situ in their epiphytic habitats. Derived parameters such as leaf‐air water‐vapour‐concentration difference (Δw), water‐vapour conductance of leaves (g) and internal CO2 partial pressure (piCO2) could be calculated.The particular problems of making such measurements in the humid tropics due to high relative humidities and high dew‐point temperatures are discussed. The long and often broad, strap‐like leaves of bromeliads are well suited for measurements with the steady‐state porometer. It is shown that CAM activity varies along the length of individual leaves, and variability between different leaves is also demonstrated.The major phases of CAM, i.e. nocturnal stomalal opening, CO2 uptake and dark fixation as malic acid (Phase I), daytime stomatal closure and light‐dependent assimilation of CO2 derived from decarboxylation of the malic acid (Phase III), and late‐afternoon stomatal opening with direct light‐dependent assimilation of atmospheric CO2 (Phase IV) were all clearly shown by CAM bromeliads in situ. Their expression and magnitude depended on the environmental conditions. An early‐morning peak of CO2 uptake as is characteristic of Phase II of CAM was not detected during the night‐day transition. A bromeliad intermediate between C3 and CAM, Guzmania monostachia, showed substantial net CO2 uptake in the early morning but no net uptake integrated over the whole of the night.