Cactus physiological ecology, emphasizing gas exchange of Platyopuntia fruits

Net CO2 uptake and transpiration help characterize how plants and their organs respond to the environment. To interpret environmental responses of fruits of platyopuntias, comparisons with the gas exchange and water potentials of the underlying cladodes are also important. Fruits of Opuntia ficus-indica have substantial amounts of chlorophyll, photosynthetic enzymes, nocturnal stomatal opening, and net CO2 uptake. The higher water potential of the fruits for six species of platyopuntias implies that water moves from the underlying cladodes to the fruits via the phloem, not the more expected pathway of the xylem. The phloem delivers a dilute solution supplying most of the photosynthate required for fruit growth (90% midway in the period between floral bud appearance and fruit maturation) and all of the water required. The net CO2 uptake ability of fruits of O. ficus-indica decreases 10-fold from anthesis to fruit maturation 75 days later. To understand why the total daily net CO2 uptake by fruits of this species varies and what happens to soluble sugars and polymers during the final stage of fruit ripening requires further research.