The influences of photosynthetically active radiation (PAR) and water status on nocturnal Crassulacean acid metabolism (CAM) were quantitatively examined for a widely cultivated cactus, Opuntia ficus-indica (L.) Miller (4, 6-8, 18, 24, 25, 27). For instance, Sale and Neales state that the acid-to-CO2 ratio was 1.8 for pineapple, Ananas comosus (21). They refer to a previous publication for their method (7), where acidity levels are expressed as equivalents of H+ (one H+ per acid), but they indicate that Nobel and Hartsock (14) had-an acid-to-CO2 ratio of 1.0 for Agave deserti, which means two H+ per acid, since the latter authors argued that 2 mol of H+ would be produced per mol of CO2 taken up. Szarek and Ting (25) presumed "a direct stoichiometric relationship between organic acid synthesis and CO2 uptake" for certain ecological considerations. However, the ratio might be variable in the field. In the present study, the influences of both PAR and water status on the H+-to-CO2 ratio were assessed in a quantitative fashion. The ratio was found to vary with PAR level, time during the night, and plant water status. MATERIALS AND METHODSOpuntiaficus-indica (L.) Miller (Cactaceae) was obtained from a commercial plantation at Fillmore, CA (34°24'N, 1 18°53'W, 90 m above sea level). Mature cladodes (flattened stems) were placed in loamy sand and maintained in environmental chambers with day/night air temperatures of 25°C/15°C. PAR between 400 and 700 nm was provided for 12 h each day by 300-w cool-beam tungsten lamps and was measured with a Lambda Instruments LI-190S quantum sensor placed in the planes of the cladodes. The PAR on each surface of the cladodes routinely averaged 560 ,umol m-2 s-1 (24 mol m-2 day-'), which is typical of the PAR on unshaded vertical surfaces during most of the year in the field (I 1, 12). Other PAR levels were provided by adjusting the distance from the cladodes to the lamps. The RH averaged 40%7o during the daytime and 60%o at night, and the CO2 content of the air averaged 340 ,ul I-'. The plants were routinely watered each week with 1/10 Hoagland solution no. 1 (2) so that the soil water potential in the root zone was -0.2 ± 0.1 MPa (-2 ± 1 bars), as determined with Wescor PT 51-05 soil thermocouple psychrometers. (14). Air temperatures, RH, PAR, and CO2 levels were the same as in the environmental chamber. The water vapor conductance was equated to the transpiration rate divided by the water vapor concentration drop from leaf to air. To measure tissue acidity, two 1.14-cm2 samples extending from the cladode surface to 1 mm interior to the chlorenchyma (about 4 mm overall) were removed at dawn or dusk with a cork borer, ground with sand in 30 ml distilled H20, and then titrated to an endpoint of pH 6.8 using 0.010 N NaOH (1; all acidity data refer to H+ amounts). Similar samples were removed for Chl extraction in 80o acetone/20% water (v/v). Actual cladode surface area was used in the expressions for PAR, CO2 exchange, acidity, and Chl. The transmittance of the epidermis plu...

Under optimal photosynthetic conditions the leaf liquid phase resistance for CO2 uptake is generally considerably larger than the gas phase resistance in series with it (5, 13), and so I1 2 is usually the main determinant of the upper limit ofphotosynthetic capacity of a leaf. In general, when the mesophyll region is thicker, the area available for CO2 to diffuse into the cells is greater, and consequently r?c"2 is then lower. A useful parameter quantifying such an effect of leaf anatomy on photosynthesis is Ame8/A, the surface area of the Chl-containing mesophyll cells exposed to the (1,24). Here, the photosynthetic consequences of the thick chlorenchyma or A. deserti were examined and the properties of the cellular resistance for CO2 uptake, about which little is known for any CAM plant, were investigated. MATERIALS AND METHODSPlant Material. Mature plants of A. deserti Engelm. (Agavaceae) having 17 to 23 leaves were transplanted from the western Colorado desert near Palm Desert, California, and then maintained in desert soil in growth chambers. Unless indicated otherwise, the chambers provided 14-hr days with leaf temperatures of 26 ± 1 C, a water vapor concentration of 8 ± 1 ,ug cm-3, and a daily average of 1.7 mE cm-2 of PAR in the planes of the leaf surfaces. PAR (400-700 nm) was provided by warm-white fluorescent lights supplemented with tungsten-filament lamps and was measured with a Lambda Instruments LI-190S quantum sensor. For the 10-hr nights the leaf temperatures were 15 ± 1 C and the water vapor concentration was 8 ± 1 ,ug cm-3. Soil water potential was measured with a Wescor HR-33T dewpoint microvoltmeter using PT5 1-05 soil thermocouple psychrometers placed 10 cm below the soil surface; it averaged -3 ± 1 bar just before the weekly watering with one-tenth Hoagland solution and -0.5 + 0.2 bar a day later.

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The influences of various day/night air temperatures on net CO2 uptake and nocturnal acid accumulation were determined for Opuntia ficus‐indica, complementing previous studies on the water relations and responses to photosynthetically active radiation (PAR) for this widely cultivated cactus. As for other Crassulacean acid metabolism (CAM) plants, net nocturnal CO2 uptake had a relatively low optimal temperature, ranging from 11°C for plants grown at day/night air temperatures of 10°C/0°C to 23°C at 45°C/35°C. Stomatal opening, which occurred essentially only at night and was measured by changes in water vapor conductance, progressively decreased as the measurement temperature was raised. The CO2 residual conductance, which describes chlorenchyma properties, had a temperature optimum a few degrees higher than the optimum for net CO2 uptake at all growth temperatures. Nocturnal CO2 uptake and acid accumulation summed over the whole night were maximal for growth temperatures near 25°C/15°C, CO2 uptake decreasing more rapidly than acid accumulation as the growth temperature was raised. At day/night air temperatures that led to substantial nocturnal acid accumulation (25°C/15°C.). 90% saturation of acid accumulation required a higher total daily PAR than at non‐optimal growth temperatures (10°C/0°C and 35°C/25°C). Also, the optimal temperature of net CO2 uptake shifted downward when the plants were under drought conditions at all three growth temperatures tested, possibly reflecting an increased fractional importance of respiration at the higher temperatures during drought. Thus, water status, ambient PAR, and growth temperatures must all be considered when predicting the temperature response of gas exchange for O. ficus‐indica and presumably for other CAM plants.

Leaves of twelve C3 species and six C4 species were examined to understand better the relationship between mesophyll cell properties and the generally high photosynthetic rates of these plants. The CO2 diffusion conductance expressed per unit mesophyll cell surface area (gCO2cell) cell was determined using measurements of the net rate of CO2 uptake, water vapor conductance, and the ratio of mesophyll cell surface area to leaf surface area (Ames/A). Ames/A averaged 31 for the C3 species and 16 for the C4 species. For the C3 species gCO2cell ranged from 0.12 to 0.32 mm s‐1, and for the C4 species it ranged from 0.55 to 1.5 mm s‐1, exceeding a previously predicted maximum of 0.5 mm s‐1. Although the C3 species Cammissonia claviformis did not have the highest gCO2cell, the combination of the highest Ames and highest stomatal conductance resulted in this species having the greatest maximum rate of CO2 uptake in low oxygen, 93 μmol m‐2 s‐1 (147 mg dm‐2 h‐1). The high gCO2cell of the C4 species Amaranthus retroflexus (1.5 mm s‐1) was in part attributable to its thin cell wall (72 nm thick).