Carbon exchange rate of pitahaya (Hylocereus undatus L. Harwort) in contrasting seasons and irrigation regimes

The aim of this investigation was to evaluate the effect of two contrasting seasons (fall and spring) and irrigation regimes (well-watered and drought-stressed plants) on the carbon exchange rate (CER) of pitahaya grown in a cold greenhouse in the Valley of México. Carbon exchange rate (CER) of one-year-old stems of pitahaya was studied over two periods of 24 h (November and April), in well-watered plants (every 15 days) and drought-stressed plants (irrigation suspended for four and five months before CER measurements in November and April, respectively). In November, when the relative humidity (RH) during the night was between 65 and 85%, watered plants with 88% of relative water content (RWC) showed two maximum CER peaks. The first CER peak declined slowly when the air temperature dropped from 11 to 7.4 °C and sharply at 6 °C. The second CER peak was reached when the air temperature was 12.2 °C. Drought-stressed plants (RWC = 60%) also showed two maximum peaks but at different times. The first one was when the air temperature was 8.8 °C and the second peak appeared when the air temperature was 6 °C. In April, when the night RH was between 35 and 59%, the watered plants (RWC = 88%) followed a similar pattern to the RH variation. Two peaks were recorded; the first declined at a slow rate when the air temperature shifted from 18 to 13.2 °C, and the second declined sharply when the air temperature shifted from 10 to 6 °C. Plants under drought stress (RWC = 34%) showed a trend towards CAM-idling. Results showed that both season and irrigation regime affected the carbon exchange rate of pitahaya. The destructive method, using the portable photosynthesis system (LI-6200), was shown to be as reliable as the analysis of in-vitro activity of PEP-C in the spring (April).