Background
Weiss et al. introduce the report of their newest study by writing that the crassulacean acid metabolism or CAM cacti Hylocereus undatus (red pitaya) and Selenicereus megalanthus (yellow pitaya) "are cultivated in a dozen countries around the world and were introduced into and developed in Israel as export fruit crops (Mizrahi et al., 1997)." They say that "both species consist of elongated, three-ribbed stems that cling to trees and rocks in their natural habitat, which, for S. megalanthus, is characterized by humid tropical forests (Colombia, Ecuador, Peru and the Amazon Basin," but that H. undatus grows naturally in the dry forests of Mexico, indicating that it is better adapted to semi-arid conditions."

What was done
Working in the northern Negev Desert of Israel, the authors grew rooted shoot cuttings of the two vine-cactus fruit-crop species in pots filled with volcanic gravel for a period of one full year (August 2006 to August 2007). This was done within vented chambers maintained at either ambient or elevated atmospheric CO2 concentrations (380 or 1000 ppm, respectively) in a cooled greenhouse, where the plants were "fertigated" twice weekly with a 0.5-strength Hoagland's solution, and where the researchers measured net photosynthesis on four days in mid-April and made final biomass determinations at the conclusion of the study. In addition, they conducted a second one-year study of more mature eight-year-old plants in order to investigate their fruit development responses to atmospheric CO2 enrichment; and this work was done in open-top chambers maintained within the same greenhouse.

What was learned
Weiss et al. report that "H. undatus plants enriched with CO2 demonstrated 52%, 22%, 18% and 175% increases, relative to plants measured in ambient CO2, in total daily net CO2 uptake, shoot elongation, shoot dry mass, and number of reproductive buds, respectively," while corresponding responses for S. megalanthus were 129%, 73%, 68% and 233%. In addition, they found there was a slight (7%) increase in the fruit fresh mass of H. undatus and a much greater 63% increase in the fruit fresh mass of S. megalanthus, due to the extra 620 ppm of CO2 enrichment of the air in which the plants had been grown.

What it means
The three Israeli researchers write that "to the best of our knowledge, this is the first study of the reproductive responses of CAM plants to CO2 enrichment," and they state that their experiments demonstrate "the vast potential of possible increases in the yields of CAM crops under CO2 enrichment."

Reference
Mizrahi, Y., Nerd, A. and Nobel, P.S. 1997. Cacti as crops. Horticultural Reviews 18: 321-346.