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 cu ltivated for its fruit in Jalisco, Mexico, was compared to that for ot her Crassulacean acid metabolism (CAM) species of different morphology and cultivation history whose monthly growth is proportional to month ly 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 mon thly net CO2 uptake, based on daily net CO2 uptake measured under vari ous conditions in a glasshouse and the environmental conditions at the field site, was relatively constant from July to October. The increas e in monthly stem elongation was accompanied by a decrease in the cont ent of total sugars and an increase in reducing sugars in the outer ap proximately 10 mm of the stems. The monthly changes in sugars were muc h less than the monthly net CO2 uptake, suggesting that most photoassi milates were converted to polysaccharide reserves or translocated to o ther parts of the plant, presumably resulting in maximal growth occurr ing 3 months after environmental conditions became favorable for net C O2 uptake. The maximal daily net CO2 uptake (317 mmol m(-2) day(-1)) a nd the maximal relative growth rate (0.0018 day(-1)) for S. queretaroe nsis were both lower than for some highly productive CAM species, cons istent with the low levels of nitrogen (7.8 mg g(-1)) and of certain m icronutrients (Fe, Mn) in its stem tissues, but were higher than for v arious undomesticated CAM species.