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Spring 2010 Biology 3B Paper Dark Stomatal Response and Carbon Dioxide Levels in Dudleya lanceolata at Various Humidities Jennifer A. Oberholtzer and Amanda C. Swanson Department of Biological Sciences Saddleback College Mission Viejo, California 92692 Crassulacean Acid Metabolism (CAM) is a photosynthetic adaptation carried out by various plants that thrive in arid conditions, such as cacti and succulents. In order to reduce water loss, CAM plants undergo a diurnal cycle of opening their stomata at night and then closing them during the day. This experiment was constructed to determine whether humidity levels, in the absence of light, would affect the carbon dioxide consumption and stomatal response of Dudleya lanceolata. The leaves of eight D. lanceolata plants were acclimated to four different humidities. Carbon dioxide levels and stomatal imprints were taken. The metabolic rates for each humidity level were averaged and the percentage of open stomata was determined. No significant difference was found for the metabolic rates (p=0.655, ANOVA) nor the percentage of open stomata (p=0.292, ANOVA). Introduction There are several methods of carbon fixation a plant can use to convert carbon dioxide into glucose. In C 3 plants carbon fixation is initially performed via the enzyme rubisco. Rubisco takes carbon dioxide and adds it to ribulose bisphosphate, initiating the first step of the Calvin Cycle. During the presence of light, these plants open their stomata during the day to allow for gas exchange. Stomata then close at night when light is no longer available (Hartsock & Nobel, 1975). Crassulacean Acid Metabolism (CAM) is a photosynthetic adaptation carried out by various plants that thrive in arid conditions, such as cacti and succulents. In order to reduce water loss, CAM plants undergo a diurnal cycle of opening their stomata at night and then closing them during the day. Carbon dioxide is stored primarily as malic acid in vacuoles until light is available. Once light is present, carbon dioxide is released and the Calvin cycle begins (Ting, 1985). In C 3 plants, light stimulates potassium ions to enter guard cells causing them to become turgid, a hyposmotic effect that causes water to move into the cell. This results in the opening of stomata and the entrance of carbon dioxide into the cell. CAM plants have an endogenous “photoperiodic circadian rhythm” meaning they perform a 24-hour daily cycle in which they open and close their stomata according to external cues, the presence/ absence of light and water (Lee 2010). There are numerous plant species that exhibit the ability to “switch” from CAM to C 3 or take on characteristics from both methods of carbon fixation. Plants that possess the ability to switch from CAM to C 3 are termed facultative CAM plants and can open and close their stomata either during the day or night. This switch in carbon fixation is dependent on various environmental factors such as availability and salinity of water, temperature, or photoperiod (Sayed, 2001; Cushman, 2001 and Lange & Medina, 1979; Nobel & Zutta, 2007). Previous studies have also observed the influence of humidity on stomatal response and carbon dioxide levels. The difference in leaf-to-air vapor pressure directly affects stomata and the actual conductance of carbon dioxide. As the difference between vapor pressures increases, carbon dioxide consumption decreases (Hubbart et al, 2007; Lange & Medina, 1979). This difference between leaf and air vapor pressures largely depends on the relative humidity level surrounding a plant. This experiment was constructed to determine whether humidity levels, in the absence of light, would affect carbon dioxide consumption and stomatal response of the facultative CAM plant Dudleya lanceolata. Materials and Methods Eight Dudleya lanceolata plants were purchased from Tree of Life Nursery (San Juan Capistrano, CA). The plants, all of similar size, were placed outside in direct sunlight during the day and then moved inside at dusk to prevent freezing. They were watered every other evening with 25 milliliters (mL) of deionized water. Trials began on March 24, 2010 and continued through April 4, 2010. The testing was conducted at the house of Amanda Swanson (Laguna 1 Saddleback Journal of Biology Spring 2010
Spring 2010 Biology 3B Paper Hills, CA). Located on site were four Pasco GLX data loggers with carbon dioxide probes and photosynthesis tanks provided by Saddleback College Department of Biology. A photosynthesis tank is a two chambered tank in which the inner portion can be sealed off, via a rubber stopper, to create a sealed, isolated environment. The four probes were set up and calibrated, prior to each testing, to verify that all equipment was functioning properly. At the beginning of the experiment, each plant was given a number for identification and a control was established to ensure that the plants were not performing carbon fixation. The control was set up during the daylight at ambient temperature and humidity (35%). A leaf from each plant was removed and placed into the inner chambers of the photosynthesis tanks. Carbon dioxide levels were measured. Appropriate solutions for the various humidities were predetermined by referencing previous studies (Greenspan, 1976; Sweetman, 1933; Winston & Bates, 1960); saturated solutions were prepared. The 0% humidity environment was produced by placing 15 mL of drierite in the bottom center portion of the inner photosynthesis chamber to absorb all the moisture. A rubber stopper was placed in the middle of the drierite to prevent direct contact with the leaf. To produce a 33% humidity environment, 15mL of saturated MgCl was placed at the bottom of the inner chamber. The 75% humidity environment had 15mL of saturated NaCl. The 100% humidity level was obtained by placing 15mL of deionized water at the bottom of the center chamber. Each humidity condition had a rubber stopper in the chamber so that the leaves could rest without contamination or damage from the solutions. Leaves were removed from the plant to eliminate the effect of soil water potential on stomatal response during acclimation. Leaves, seven centimeters (cm) long, were removed by cutting with scissors. The cut portion of the plant was covered with parafilm to prevent any potential water loss. In the absence of light, the leaves with the parafilm were weighed (grams) and placed in the inner chamber of the appropriate photosynthesis tank. Two leaves, from separate plants, were placed in a photosynthesis chamber to ensure that sufficient carbon dioxide levels could be detected. The leaves were paired consistently throughout all trials and allowed to acclimate for three hours at the respective environment. Pasco GLX data loggers with carbon dioxide probes were turned on to record data for three hours. Once carbon dioxide data were collected, leaves were removed from the photosynthesis tanks and immediately reweighed. Stomatal imprints were obtained initially by adding a drop of Superglue to a blank glass slide. The top of the leaf was then firmly pressed into the wet glue and pressure was applied for ten seconds. The leaf was carefully removed from the slide, leaving an imprint behind. The glue was allowed to dry and the slides were analyzed under a compound light microscope magnified at 100x. Trials were repeated with freshly cut leaves for a total of four trials, to ensure that each plant was rotated through every humidity level. Stomatal data was analyzed by photographing a 1 mm 2 area and determining the percentage of open stomata. Statistical analyses were conducted using Microsoft Excel 2003; all data were analyzed by converting parts per million (ppm) of carbon dioxide to grams of carbon dioxide produced per gram of plant. Results The metabolic rate of the leaves for each humidity level was averaged and graphed (Figure 1). There was no significant statistical difference between plant metabolic rate and humidity level (p=0.655, ANOVA). At 0% humidity the average metabolic rate was 7.25x10 -7 ± 4.18x10 -7 ; at 33% humidity average metabolic rate was 1.20x10 -6 ± 8.62x10 -7 ; at 75% humidity average metabolic rate was 9.50x10 -7 ± 1.43x10 -6 ; at 100% humidity average metabolic rate was 1.58x10 -6 ± 2.59x10 -6 . There was no significant statistical difference between stomatal response and humidity level (p=0.292, ANOVA). At 0% humidity the average percentage of open stomata was 16.96% ± 30.1%, at 33% humidity the average percentage of open stomata was 34.69% ± 8.92%, at 75% humidity the average percentage of open stomata was 34.20% ± 19.62%, and at 100% humidity the average percentage of open stomata was 36.65% ± 32.19% (Figure 2). Average Metabolic Rate (g CO2 • g plant mass -1 • s -1 ) 5.00E-06 4.00E-06 3.00E-06 2.00E-06 1.00E-06 0.00E+00 -1.00E-06 -2.00E-06 0% 33% 75% 100% Humidity Level Figure 1. The mean metabolic rates for each humidity. ANOVA shows no significant difference between humidities (p=0.655). Error bars indicate mean ± SEM 2 Saddleback Journal of Biology Spring 2010
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