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: springerj@unk.edu)

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.