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WATER INTAKE IN THE DARK
Do Flowers of Varying Stem Lengths Absorb Significantly
Different Amounts of Water When Light Deprived?
Megan Sullivan
University of Florida
Do Flowers of Varying Stem Lengths Absorb Significantly
Different Amounts of Water When Light Deprived?
Abstract This study uses quantitative data to determine whether the length of a cut flower’s stem has any effect on how much water it absorbs over a five-day period. Flowers were cut to stem lengths of 5, 6, 7, and 8 inches and placed in test tubes marked at every mL. Every day each flower started with exactly 10mL of water and the levels were measured every day at the same time before the displaced water was refilled. In the dark experiment, this method was followed exactly and a paper bag was placed over the setup to block out the light. This design was set up to test the hypothesis that when there is light present, stem length affects water intake and when light is not present it does not. Both of these hypotheses were confirmed as the intake total and rate of intake varied linearly in the light experiment and randomly in the dark. This is important because it helps us understand the relationships between water intake, light availability, and photosynthesis.
Introduction
Previously I conducted a study on whether the length of a flower’s stem affects its intake of water. I placed four flowers varying in size by one inch, in the same amount of water each day for five days and graphed the amount of water they took in as well as the rates of water absorption over five days. I determined that as flowers increase in stem length, the amount of water they absorb also increases. This continued for the first few days until eventually all but the largest flower started taking in close to the same amount of water in a day. This occurred after the rate of water intake for each flower began to decrease steadily until the third day of my study. The rate of decrease was largest in the tallest flower while the rates of decrease between the three remaining flowers were very similar to one another.
After analyzing these results, I began to speculate why this could be the case. Does the length of the stem affect the amount of water absorbed because there is more volume for the water to be stored in? My most logical point of speculation was that because the larger flower has more cells both to carry out processes and that require nutrients, the larger flower must photosynthesize faster than the smaller ones. This made me wonder if limiting the light available to these flowers and thus giving them a limiting resource in photosynthesis would lessen the gap in the amount of water take in by each flower. I hypothesize that limiting the light reaching the flowers will cause the intake of each of the flowers to be more similar to one another than when the flowers had excess light.
The effects of light exposure on plant water intake has been widely studied. Inamoto, Nagasuga, Yano, and Yamazaki (2015) studied the growth of plants, which depends on the accumulation of carbon dioxide via photosynthesis, under different light intensities. They tested the rates of photosynthesis on 30 lilies placed in 0% shade, 40% shade, or 60% shade. They determined that decreasing the light intensity decreased the rate of photosynthesis. The researchers noted that this result did not concur with those of Sorrentino et al. (1997) but offered that this may be because they conducted their experiment in an area that was often cloudy while the other research was conducted in an area of high natural light intensity.
Murchie and Niyogi (2011) conducted a study that could confirm the reason for this discrepancy. Murchie and Niyogi found that photosynthetic rate and light could best be represented by a light response curve. Photosynthetic rate rises as light intensity increases until it reaches its maximum when it is saturated with light.
Thus, because Sorrentino et al. conducted their study in an area of naturally high light intensity, the shading may not have lowered the light intensity enough to put the amount of light below saturation levels for the lilies. This theory explains why the shading could have not affected the photosynthetic rate of the lilies in this naturally bright environment as it did in the typically cloudy environment.
Xu, Ibrahim, and Harvey (2016) also studied the effects of light on photosynthetic rate. However, instead of shading, they implemented different length photoperiods. They set up algae cultures under varying light and dark periods that summed to 24 hours. Based on the observation that cell volume increased during the cycles of light and decreased during the periods of dark, the researchersdetermined that the rate of photosynthesis was enhanced at higher light intensities. Lu et al. (2011) conducted a similar study using continuous measurements of water intake during light and dark periods on cut flowers. Lu et al. determined that water intake was much less during the dark periods and that it also decreased with vase time as I saw in my previous experiment.
Scheer (2012) found that the primary stems in photosynthesis are photovoltaic meaning they are driven by light. This study, along with the aforementioned ones, indicates that as light intensity increases, so must the photosynthetic rate and water intake.
Method
Observational Logbook
In my first study, I used four flowers of the same type, each having only a different stem length. The stem lengths were five inches, six inches, seven inches, and eight inches. All of the stems had the same diameter and were cut at the same angle to prevent the surface area of the stem cut from affecting water intake. All of the flowers were from the same bouquet ensuring identical pre-experiment conditions. Each flower was placed in a marked test tube that was filled to 10 mL at observation every day, which was always at the same time. The flowers were placed in the same location so that each was exposed to identical external conditions such as temperature and amount of light. Every day, I observed the amount of water displaced from each test tube and graphed the total amount of water taken in by each flower and the rate of water intake over the duration of the study.
To test my new hypothesis, all of the procedures from my previous experiment will remain the same, except this time I will cover the flowers and test tubes with a paper bag to block light from reaching them. I will only remove the paper bag at the time of observation every day to check the results and refill the test tubes to their original values.
Limitations
A limitation of this experiment is that I am unable to make sure that absolutely no light reaches the flowers, as I will need to remove the covering to observe the test tubes. Also, because this sample of flowers came from a different bouquet than those in my previous study, I cannot compare total intakes of water between stems of the same length in each study. I can only compare the intakes of flowers in the same study and then must compare these relationships to those of the flowers in my previous study.For example, I cannot compare the water intake of both of the 5-inch flowers but I can compare the difference in water intake between the 5-inch and 8-inch flowers of one study to the difference between these two flowers in the other study.
Results
The results from my original study provided me quantitative data that was best analyzed graphically.
Figure 1 Total Water Intake
I found that the total water intake by each flower increased with the length of the stem. The intakes of the shortest flowers varied linearly with one another. The difference between water intake between the shortest two flowers and that of the middle two lengths were almost equal. The largest flower took in almost double the amount that would have been necessary to fit this linear relationship.
Figure 2 Rate of Water Intake
The rate of water intake decreased in all of the flowers for the first three days.The rate of decrease of intake for the largest flower was about two times that of the other three whose intakes decreased at about the same rate. By around days three and four, the rate of intake for every flower began to steady. The constant intake that they began to steady to was close to equal for the three shortest flowers and about three times this value for the largest flower.
Figure 3 Total Intake in the Dark
In my follow up experiment it seems that these trends were not followed. Contrary to the results in the first experiment, the shortest flower actually took in the most water over the five-day dark period. The largest flower took in the second largest amount of water followed by the 6-inch stem and then the 7-inch stem. From this data, there seems to be no correlation between stem length and water intake when flowers are deprived of light.
Figure 4 Rate of Intake Over 5 Dark Days
Much like the results of my first experiment and that of Lu et al., it seems, that except for the results of day two for the 7-inch flower and day 4 for the 6-inch flower, the water intake decreased with vase time. The shortest and largest flowers have decreasing levels of intake that are very similar as do the 6 and 7-inch flowers. Thus, it appears that when the flowers are placed in the dark, rate of water intake does not depend greatly on the length of the stem.
Discussion
Based on the results of this experiment, I have determined that although stem length and water intake have a positive correlation when the plants have access to light, when this light is removed, the stem length seems to have less of an effect on the water intake of the flowers. Thus, I have proved my hypothesis accurate, as the variation between the total intakes of the flowers is less in the dark experiment.
This result also leads me to believe that the speculation I made in my observational logbook was on the right track. I speculated that in the presence of light, the longer stems absorb more water as there are both more cells to carry out the photosynthesis process and that need the products of photosynthesis to thrive. When light was removed from the picture in the second experiment, the flowers could only photosynthesize as long as there was enough light as this became the limiting resource. Because there was no light, a crucial reactant in the photosynthesis process, none of the flowers were able to photosynthesize as much as they would have if light were in excess. This resulted in there being no correlation between stem size and water intake because there was no longer a correlation between stem size and amount of photosynthesis taking place in the plant.
The observation that the rate of water intake decreased with vase time in each case can be attributed to the fact that as the time passed, the flowers became more saturated. Because the photosynthetic rate was so small, the flowers had less water to replace each day before they were again fully saturated. I speculate that the shortest and tallest flowers took in the most water because they were on either edge of the experiment closer to the edge of the bag blocking the light. Because of their placement, they probably received the most light.
If I had more time to conduct this experiment, I would have wanted to extend the duration and number of flower species I studied. I would have also liked to run both the light and dark experiments side by side with flowers from the same pre-experiment conditions so that I could directly compare the results of the light and dark experiments, as this was one of the limitations I experienced.
Overall, these two experiments show that light availability is related to water intake. This is because both light and water are necessary inputs to perform photosynthesis and as light is now limited the same amount of water is no longer needed. If in a different experiment, the amount of water was limited and light was not, we would see that there was an excess of light for the amount of photosynthesis occurring but I am unable to accurately measure that without access to the proper equipment.
References
Lu, P., Huang, X., Li, H., Liu, J., He, S., Joyce, D., & Zhang, Z. (2011). Continuous automatic measurement of water uptake and water loss of cut flower stems.Hortscience,46(3), 509-512.
Murchie, E. H., & Niyogi, K. K. (2011). Manipulation of photoprotection to improve plant photosynthesis. Plant Physiology, 155(1), 86-92.
Inamoto, K., Nagasuga, K., Yano, T.,Yamazaki, H. (2015). Influence of light
intensity on the rate of photosynthesis and dry matter accumulation in Oriental hybridlily ‘Siberia’ at different developmental stages.The Journal of Horticultural Science andBiotechnology, 90:3, 259-266. DOI: 10.1080/14620316.2015.11513180
Scheer, H. (2012). Light-harvesting in photosynthesis.Acta Physica Polonica a,122(2), 247-251.
Sorrentino, G., Cerio, L., & Alvino, A. (1997). Effect of shading and air temperature on leaf photosynthesis, fluorescence and growth in lily plants.Scientia Horticulturae,69(3), 259-273. doi:10.1016/S0304-4238(97)00016-2
Xu, Y., Ibrahim, I. M., & Harvey, P. J. (2016, September). The influence of photoperiod and light intensity on the growth and photosynthesis of Dunaliella salina (chlorophyta) CCAP 19/30. Plant Physiology and Biochemistry,106, 305-315. doi:10.1016/j.plaphy.2016.05.021