Water relations and photosynthesis of a barrel cactus, Ferocactus acanthodes, in the Colorado desert

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Abstract
The structural characteristics, water relations, and photosynthesis of Ferocactus acanthodes (Lemaire) Britton and Rose, a barrel cactus exhibiting Crassulacean acid metabolism (CAM), were examined in its native habitat in the western Colorado desert. Water storage in its succulent stem permitted nighttime stomatal opening ot continue for about 40 days after the soil water potential became less than that of the stem, a period whe the plant would be unable to extract water from the soil. After 7 months of drought and consequent unreplenished water loss from a plant, diurnal stomatal activity was not observed and the stem osmotic pressure was 6.4 bars, more than double the value measured during wet periods with nighttime stomatal opening. F. acanthodes had a shallow root system (mean depth of 8 cm) which responded within 24 h to rainfall. When the nocturnal stem surface temperature was raised from 8.0° C to 35.0° C, the stomatal resistance increased 4-fold, indicating that cool nighttime temperatures are advantageous for gas exchange by F. acanthodes. Moreover, the optimal temperature for CO2 uptake in the dark was only 12.6° C. CO2 uptake at night became maximal for 3.0 mEinsteins cm-2 of photosynthetically active radiation incident during the preceding day, and the minimum number of incident quanta absorbed per CO2 fixed was 68. The transpiration ratio (mass of water transpired/mass of CO2 fixed) had the relatively low value of 70 for an entire year, consistent with values obtained for other CAM plants. The total amount of water annually diverted to the floral structures was about 6% of the stem wet weight. The annual growth increment estimated from the net CO2 assimilation corresponded to about 10% of the stem mass for barrel cacti 34 cm tall, in agreement with measured dimension changes, and indicated that such plants were about 26 years old.