AbstractDue to photorespiration, carbon fixation can be extremely costly to C3 plants, particularly

in warm and dry conditions. C4 plants have developed a way to fix carbon that is more energetically costly, but fixes many of the shortcomings of C3 plants. These adaptations should make C4 plants better suited for growing in warm and dry places. In this experiment, we tested this idea by growing two C3 plants, sunflower and tomato, and two C4 plants, corn and amaranth, in different soils and water conditions. The C3 plants, contrary to what we believed would happen, grew better in low water conditions that had some amount of drainage. For C4 plants, drainage was an important factor in growth. The reason that we may have received these results is that the high water conditions that we used for the experiment was too much for any of the plants that we were testing. The lack of adequate drainage could also adversely affect the plants, as it inhibits gas exchange and can encourage root rot and other fungal diseases in the plants. This is important because regardless of the photosynthetic pathway, if a plant does not have the adaptations to deal with soil saturation, it cannot grow as well in that soil.

IntroductionPlants can use one of three photosynthetic pathways to fix carbon; C3, C4, and CAM. In

the C3 pathway, a three-carbon molecule is created as a product of photosynthesis. This pathway uses RuBP to capture CO2, but it is inefficient due to its low affinity for carbon dioxide (Gurevitch, 25). Photorespiration is the process where oxygen is taken up instead of carbon dioxide, which is more common at higher temperatures. To counteract this, a plant using the C3 pathway must have a high concentration of RuBP in its leaves to maintain rates of photosynthesis to grow, which can be nutrient prohibitive to some plants.

The C4 pathway evolved as a way to address many of the drawbacks of the C3 pathway, but not without its own drawbacks. C4 pathways use a four-carbon molecule that has a much higher affinity to RuBP than carbon dioxide does, but the creation of this molecule requires an extra step that uses more ATP than is required in a C3 pathway (29). This means that while C4 plants are able to photosynthesize more and do so with a better water use efficiency, it requires more energy to fix carbon. This is not a problem when there is plenty of light, but when there is less light, such as when the plant is in the shade, there is no need for a costly method of carbon fixation because light is the limiting factor.

Because of differences in C3 and C4 pathways, one pathway can be more dominant in an environment over another (35). For example, a grassland that receives warm weather, little rain, and full sun is going to have predominantly C4 plants. This is the best condition for them because the C4 pathway decreases the effect that temperature has on the affinity of carbon dioxide, conserves water better than the C3 pathway, and there is enough sunlight for them to photosynthesize all day long. C3 plants would have some amount of difficulty with these environments because the RuBP would lose affinity for carbon dioxide as temperatures increased, the plants would lose too much water, and most C3 plants become light saturated before the day is complete, which means they lose the opportunity to photosynthesize more where C4 plants take advantage of it (29).

Despite its shortcomings, the C3 pathway is still the most dominant pathway. In places where the temperatures do not become too extreme, there is no need to produce the four-

carbon molecule at extra cost. When water is not the limiting factor in photosynthesis, it can be expended in exchange for light. Also if a plant is not going to be in full sun, it cannot afford the cost for more efficient carbon dioxide capture that it cannot use.

For our experiment we tested what soil and water conditions allow C3 and C4 plants to grow the best. From what I knew of C3 and C4 pathways, my hypothesis was that C3 plants would grow better in soils that were more moist and C4 plants would grow better in conditions that were more dry. With our soil and water combinations, the combination that should be the most moist is the low sand, high water groups, and the combination that should be the driest is the high sand, low water groups. Sand consists of large particles that do not capture water, instead allowing it to drain at a much faster rate than smaller particles, so soils that are high in sand should have less water.

The purpose of this experiment is to see if these assumptions are correct. Because all other factors will be controlled for, the differences in growth should be due to the amount of water in the soil. We will use three different measures of growth for the plant, the height, number of leaves, and photosynthetic rate, so that we can get a better idea of how soil moisture affects the growth of C3 and C4 plants.

MethodsThree different soil types were made. Soil A was intermediate sand, which consisted of

3 parts topsoil, 2 parts vermiculite, and 2 parts sand. Soil B was high sand, which consisted of 3 parts topsoil, 1 part vermiculite, and 3 parts sand. Soil C was low sand, which consisted of 3 parts topsoil, 3 parts vermiculite, and 1 part sand. The amount of topsoil was kept constant to ensure an equal amount of nutrients which can be found in the topsoil would be in all soil types. Some fertilizer was added to the soils and they were mixed until completely uniform. Equal amounts of the soils were put into the pots with the appropriately labeled soil type. Seeds from two C3 plants, tomato and sunflower, and two C4 plants, amaranth and corn, were seeded in a seed starting mix in each of the pots. The pots were randomized in blocks using a random selection technique. There were six soil and water combinations, as well as four species, which meant that there were 24 plants per block. Block 1-3 were on the first bench and block 4-5 were on the second bench. All plants that were in the low water treatments were marked with an orange tape, but all other plant identification was left under the soil line.High water treatments were watered every two days with 320 mL of water and low water treatments were watered every four days with 160 mL water. Plants were also monitored for changes in health during these times. Plants were measured three weeks after planting, and again seven weeks after planting. The height was measured using a measuring tape from the base of the plant to the tip. The number of leaves were counted as well using a standardized method. Soil moisture was measured using a Luster Leaf 1820, where moisture was measured on a scale from 1 to 10, where 10 was the most saturated and 1 was the least. Photosynthetic rate was measured with a Decagon SC-1 Leaf Porometer. This device gave the stomatal conductance of the leaf in mmol per m2 per sec, which is a good indicator of relative photosynthetic rate.Results

In general, water was highest in pots with the smallest amount of sand and the high water treatment. Both times that the soil water content was tested, the soil moisture was about

Fig. 1: Soil moisture is higher in soil treatments that have high water treatments (labeled as H on the axis). Soil C, which had the lowest percentage of sand, had the highest average soil moisture in both the high and low treatments. Soil B, which had the highest average amount of sand, had the lowest average soil moisture in all high and low water level groups. Data bars represent one standard deviation from the mean.

the same in the intermediate sand, high water group and the low sand, low water group. These two soil types were compared together as soils that had similar soil moisture types, where when compared between groups, intermediate sand high water soil only had on average 0.1 measures of moisture levels higher than the other soil type. The high sand soils and intermediate sand soils also had similar moisture levels.

The C3 plants, tomato and sunflower, tended to grow better in soils that were drier. The soil and water treatment that resulted in the tallest plants was Soil A/low water. In both groups, height was lower in high water groups than in low water groups. In all cases but sunflower in high water, tomato and sunflower did the best in the intermediate sand soils and the worst in low sand soils. For tomato, the height of plants was significant in growing conditions comparing soil A/low water to soil C/low water (p=<0.0001), Soil A/low water to Soil B/low water (p=0.0247), Soil A/low water to Soil C/low water (p=<0.0001), and Soil A/high water to Soil C/high water (p=0.0042). For sunflower, the height of plants was significant in growing conditions comparing Soil A/high water to Soil A/low water (p=<0.0001). The p values for both tomato and sunflower in the two soil conditions that offered the most similar soil moistures, AH and CL, were 0.1622 and 0.0070 respectively.

Fig. 2: Sunflower is represented by blue bars and tomato is represented by red bars. Sunflower and tomato both grew better in low water conditions. Plants typically grew better in Soil A and worst in Soil C. Error bars represent one standard deviation.

The C4 plants, amaranth and corn, had about the same height across water conditions. For corn, plants grew better in soil with higher amounts of sand. Amaranth grew taller in soil with

higher amounts of sand and low water, but also in the most moist soil, Soil C/high water. For both corn and amaranth, the lowest average heights were in the Soil C/low water group. For amaranth, the height of plants was significant in growing conditions comparing Soil A/high water to Soil A/low water (p=0.0024) and Soil A/low water to Soil C/low water. For corn, the height of plants was significant in growing conditions comparing Soil A/high water to Soil B/high water (p=<0.0238). Neither of the p values for amaranth and corn in the two soil conditions that offered the most similar soil moistures, AH and CL, were significant (p=0.1546, p=0.8088).

Fig. 3: Amaranth is represented by blue bars and corn is represented by red bars. They both tended to grow better in dry soils. Both plants grew the worst in Soil C/low water. Error bars represent one standard deviation.

DiscussionMy hypothesis that C3 plants would grow better in more moist soil conditions and C4

plants would grow in drier soil conditions was not supported with this data. C 3 plants did best in soils that received low water with better drainage. Corn plants did best in the soils with the most drainage, but height between the two water treatments was insignificant, while amaranth did not exhibit these same patterns. One reason for this could be that the low water treatments provided the optimal amount of water to the plants, while the high water treatments provided too much. Too much water can be harmful to the plant because it affects root gas exchange and can cause root rot. This could have inhibited the growth for the plants grown in those conditions. Another problem that arises when there is too much water draining from the soil is the loss of nutrients from the soil. This could have also affected the growth in the high water treatments for all four species of plants.

Fig.4: On the left, a pot is shown directly after watering in the high water treatment. There is standing water in the pot and most of that water will drain out of the bottom of the pot. The picture on the right shows how saturated Soil C could get in the high water treatments.

The corn that was grown did not have a difference between high and low water. This could possibly be due to that fact that soil type is more important for the growth of corn than water availability. Soil that has more sand has more air spaces, providing more space for gas exchange, and resists compaction that can happen with too much water. It is possible that the drainage of Soil B was what allowed it to grow taller in those soils.

Another potential problem with our experiment is that we used cultivars of C3 and C4

plants. Usually these kinds of plants are bred with specific conditions in mind, particularly drought resistance. The C3 plants that were used could have behaved differently than C3 plants one would find in the wild. The C4 plants could have been bred to grow in conditions that offered more water than is typically available in wild C4 plants because they would be growing in conditions that would have plenty of water to increase the yields of the plants.

The large variation in data and the lack of more replicates meant that the standard deviations in each group were unusually high. More replicates would be needed to decrease the standard deviations to achieve more significant results.

ConclusionDespite the typical trends that are seen in the distribution of C3 and C4 plants, the plants

themselves can vary in favorable growth conditions. In this experiment, drainage and lack of water saturation provided some of the best growing conditions in all species. Because there was not enough disparity between the low water treatment we used and what can be considered drought conditions, we did not receive some of the results that were expected in this experiment. Regardless of whether a plant is C3 and C4, if the plant does not have the adaptations to facilitate growth in conditions when soil is completely saturated, it will not do as well in that environment.

AcknowledgementsI would like to thank my peers, TAs, and professor for helping design, conduct, and

analyze data for this experiment.

CitationGurevitch, Jessica, Samuel M. Scheiner, and Gordon A. Fox. The Ecology of Plants.

Sunderland, Mass: Sinauer Associates, 2006.