Monthly stem elongation for Stenocereus queretaroensis: Relationships to environmental conditions, net CO2 uptake and seasonal variations in sugar content

Growth of 7-year-old plants of Stenocereus queretaroensis, a cactus cultivated for its fruit in Jalisco, Mexico, was compared to that for other Crassulacean acid metabolism (CAM) species of different morphology and cultivation history whose monthly growth is proportional to monthly CO2 uptake. Stem elongation commenced at the beginning of the rainy season in the late spring, increased 6-fold from July to October 1993, and then declined during an ensuing drought. However, calculated monthly net CO2 uptake, based on daily net CO2 uptake measured under various conditions in a glasshouse and the environmental conditions at the field site, was relatively constant from July to October. The increase in monthly stem elongation was accompanied by a decrease in the content of total sugars and an increase in reducing sugars in the outer approximately 10 mm of the stems. The monthly changes in sugars were much less than the monthly net CO2 uptake, suggesting that most photoassimilates were converted to polysaccharide reserves or translocated to other parts of the plant, presumably resulting in maximal growth occurring 3 months after environmental conditions became favorable for net CO2 uptake. The maximal daily net CO2 uptake (317 mmol m-2 day-1) and the maximal relative growth rate (0.0018 day-1) for S. queretaroensis were both lower than for some highly productive CAM species, consistent with the low levels of nitrogen (7.8 mg g-1) and of certain micronutrients (Fe, Mn) in its stem tissues, but were higher than for various undomesticated CAM species. Zapotitlán 1995 Elsevier Science Ltd