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Tammy Schlichenmaier, Department of Biology, University of Nebraska Kearney, Kearney, Nebraska 68849, USA
Mentor: Joseph T. Springer, Professor, Department of Biology, University of Nebraska at Kearney, Nebraska 68849, USA. Email: firstname.lastname@example.org)
To Cite This Paper:
Schlichenmaier, T. J. 1997. Photosynthetic rates of C3 and C4 plants under two light types. Senior Research Thesis, Department of Biology, University of Nebraska at Kearney, Kearney, Nebraska.
Another factor found by Kalberer et al. (1967) to contribute to low rates of CO2 assimilation was the old procedure for grinding leaves, washing and centrifuging chloroplasts to separate them from other cellular particles. They found that this method was no longer essential.
This work is being done for the purpose of finding what light intensity, if any, causes the highest rates of photosynthesis in selected species of C3 and C4 plants and to find out whether a plant light, sold for increased plant growth, is really any better than ordinary incandescent light at stimulating photosynthesis. These findings could be applied to crop production and green house operations, especially when agriculturalists are concerned with productivity of plant parts, not necessarily the entire plant (Salisbury and Ross 1985).
Then they were ground on high speed for an additional 30 seconds. An ice bath was then prepared. At this point, 3 50 ml centrifuge tubes, a plastic funnel, and a triple layer of cheesecloth was gathered. After the sample was ground to completion, the homogenates were filtered through the cheesecloth into the funnel and into a centrifuge tube that was held in the ice bath. The green filtrate was then collected from the sample until the tube was about 3/4 full. The filtrate was then held in the ice bath until it was to be used.
The chloroplasts were separated using differential centrifugation. The filtered homogenate was centrifuged for one minute at 1000x g. The supernatent from this was then collected in another centrifuge tube and the pellet from the previous tube was discarded. The supernatent was then centrifuged for 10 minutes at 1000x g. The supernatent from this spin was then discarded and the pellet was resuspended in 20 ml 2% NaCl, 0.02 M Tris-HCl. This final solution was held in the ice bath throughout the rest of the procedure.
The next step to this procedure was the dye reduction by illuminated chloroplasts. The initial trial run was done in a spec 20 cuvette containing 2.0 ml 0.04 M Tris-HCl, 1.7 ml distilled water, 0.2 ml of the chloroplast suspension, and 0.1 ml 2.5 mM DCIP, giving a total volume of 4.0 ml. The blank for the first run was assembled by replacing the DCIP with distilled water. This solution was used to zero the Spec 20 at 630 nm. In another clean cuvette, the reaction tube for the first trial run was mixed using the ingredients previously described.
Before the addition of the DCIP, the reaction tube was mixed gently by inversion. The tube was then wrapped in aluminum foil and the DCIP was then added and the tube was mixed again by gently inverting. The absorbency of the mixture was then read and recorded as the absorbency at time zero. After recording the reading, the tube was quickly placed back into the foil sleeve.
The first part of the experiment was performed at 100% illumination under the plant light which was measured to be 1940 Fc. The foil was removed and the tube was allowed to be exposed to the light for 30 second increments until a total time of 2 minutes had been reached. After each increment of 30 seconds, the tube was placed back into the foil sleeve and the absorbency was read and recorded.
The same procedure was performed for all four plants used. It was repeated at 75% illumination (1455 Fc), 50% illumination (970 Fc), and 25% illumination (485 Fc). The same light intensities were used for the incandescent light, but since the light was brighter, the distance the tubes were held away from the light were farther away.
In the process of photosynthesis, CO2 and water are substrates and carbohydrates and oxygen are the products (Jakob and Heber 1996). There are two different methods by which plants will photosynthesize, C3 and C4 photosynthesis. The C3 path involves the Calvin cycle, whereas the C4 path uses a cycle where 3-phosphoglyceric acid is not the first product (Salisbury and Ross 1985). Due to the differential compartmentalization of photosynthetic functions between mesophyll and bundle sheath cells of C4 plants, C4 photosynthesis provides a mechanism for high rates of carbon assimilation (Edwards, et al. 1979). C4 photosynthesis, unlike C3 photosynthesis, is more resistant to the process of photo respiration.
The process of photosynthesis can be damaged by too much light or inactivated by it, known also as photo inactivation. Chloroplasts in leaves are reasonably well protected against photo inactivation, but isolated chloroplasts are highly sensitive to bright illumination (Jakob and Heber 1996).
According to the research by Jakob and Heber (1996), the electron transport chain may be largely inactivated during less than one hour of exposure to sunlight conditions, owing damage to both PSI and II if in the presence of oxygen. In the absence of oxygen, they found that PSI is left unaffected, but PSII is rapidly damaged. They also stated that photo inactivation of the chloroplast electron transport chain is fast under high intensity illumination and insignificant at low light.
An important factor in the rate of photosynthesis, according to Kalberer et al. (1967), is the age of the leaf. They stated that chloroplasts isolated from young leaves had a higher rate of activity than those from mature leaves. This finding was confirmed by Salisbury and Ross (1985). They state that as leaves grow, their ability to photosynthesize increases for a time and then before maturity begins to decrease.
For every type of plant used in this experiment, the plant light did give higher rates of photosynthesis. This was especially true for the C3 plants of lettuce and spinach. These data are shown in Fig. 1.
Fig. 1. Rates of photosynthesis. Diffeent plant types show a higher rate of photosynthesis under a plant light than under floodlight. C3 plants show the most dramatic change in photosynthetic rate.
Lettuce showed a SHRR of only 0.0130 µmoles DCIP reduced min-1 mg chlorophyll-1 under the floodlight, but this was increased to 0.0202 µmoles DCIP reduced min-1 mg chlorophyll-1 with the use of a plant light. The spinach results showed that the floodlight gave a rate of 0.0021 µmoles DCIP reduced min-1 mg chlorophyll-1 and a rate of 0.0195 µmoles DCIP reduced min-1 mg chlorophyll-1 for the plant light.
There was a great deal of discrepancy in the readings that were obtained for the spinach under the floodlight. The absorbency of each tube exposed to the light actually decreased as the tube was exposed longer to the light source. These readings gave a poor line fit when graphed and resulted in a poor reading in the final graph.
The C4 plants, corn and sorghum, also increased their rates of photosynthesis when exposed to the plant light. These increases were not near as dramatic as those experienced by the C3 plants. This could be due to the fact that C3 plants are generally cool season plants and tend to operate best at cooler temperatures that the C4 plants, which are generally warm season plants. In the procedure the isolated chloroplasts were kept in ice baths to prevent them from rupturing. By doing this, the photosynthetic mechanism of the C4 plants could have been inhibited since these plants use a different photosynthetic pathway.
Sorghum had a greater increase in photosynthesis than corn had. The sorghum showed a rate of 0.0018 µmoles DCIP reduced min-1 mg chlorophyll-1 under the floodlight and 0.0030 µmoles DCIP reduced min-1 mg chlorophyll-1 for the plant light for an increase of 0.0012 µmoles DCIP reduced min-1 mg chlorophyll-1. The corn only showed an increase of 0.0002 µmoles DCIP reduced min-1 mg chlorophyll-1 between the floodlight (0.0037 µmoles DCIP reduced min-1 mg chlorophyll-1) and the plant light (0.0039 µmoles DCIP reduced min-1 mg chlorophyll-1). This difference may not be statistically significant, but no tests were done to show significance.
As far as the intensity of the light, the 100% illumination did not always increase the rate of photosynthesis in the chloroplasts and in some cases it actually decreased the rate. This would be due to the damage imposed on the chloroplasts under very high light intensities that was found by Jakob and Herber (1996).
According to the data presented, the plant light does appear to increase the rate of photosynthesis, especially in C3 plants. Itdoes, to a lesser degree, stimulate photosynthesis in C4 plants. The use of the plant light may be a useful investment for large scale greenhouse operations or labs, depending on the types of plants that are being grown. The plant lights are more expensive than a normal incandescent bulbs and would not be a good investment for household use or small private greenhouses.
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