Physiological responses of the Crassulacean acid metabolism (CAM) plant Opwutia ficus-indica (Cactaceae) were studied on a commercial plantation in central Chile. Young cladodes (flattened stems) and flower buds exhibited daytime stomatal opening, whereas mature cladodes and fruit exhibited the nocturnal stomatal opening characteristic of CAM plants. Severe water stress suppressed the nocturnal stomatal opening by mature cladodes, but their high water vapor conductance occurring near dawn was not affected. Nocturnal acidity increases were not as sensitive to water stress as was the nocturnal stomatal opening. The magnitude of the nocturnal acidity increases depended on the total daily photosynthetically active radiation (PAR), being 90% PAR-saturated at 27 moles per square meter per day for a mean nighttime air temperature of 5°C and at 20 moles per square meter per day for 18°C. Inasmuch as the PAR received on unshaded vertical surfaces averaged about 21 moles per square meter per day, nocturnal acidity increases by the cladodes were on the verge of being PAR-limited in the field. The net assimilation rate, which was positive throughout the year, annually averaged 3.4 grams per square meter per day for 1.0-and 2.0-year-old plants. Plants that were 5.4 years old had 7.2 square meters of cladode surface area (both sides) and an annual dry weight productivity of 13 megagrams (metric tons) per hectare per year when their ground cover was 32%. This substantial productivity for a CAM plant was accompanied by the highest nocturnal acidity increase so far observed in the field, 0.78 mole H' per square meter.CAM is generally associated with succulent species such as cacti that conserve water and are relatively slow growing (10,23,29 Mg (dry weight) ha-', of which about 30%o is harvested as grain (2,3,14). Relatively little is known about the physiology of the platyopuntia 0. ficus-indica, although it has been shown that its mature cladodes (flattened stems) can have a fairly high nocturnal acidity accumulation of 0.63 to 0.67 mol H+ m-2 in the field (19,26). In the current study, attention was focused on the diurnal pattern of stomatal opening for various plant ages and parts under well watered and drought conditions. The influence of PAR and nocturnal temperature on diurnal acidity changes was also examined during field studies in central Chile from 1980 to 1982. Productivity measured as dry weight gain per unit area was found to vary markedly with time of year and with plant age.
MATERIALS AND METHODSPlant Material, Soil, and Climate. Opuntia ficus-indica (L.) Miller (Cactaceae) is a perennial succulent native to Mexico, but also widely cultivated in a variety of soils ranging from an almost pure sand-gravel mixture to clay loam. Plants of various ages occur at the commercial plantation used as the study site in Til Til, Chile (33°2' south latitude, 70°54' west longitude, 570-m elevation, which is 50 km north-northwest of Santiago, Chile). The initial planting at the study site consisted of placing four c...