Introduction

1. Transpiration is “the uptake of water by plant roots, transport through the plant and evaporation from the leaf through pores called stomata” (Roberts et al., 2006). Transpiration is important because it enables the plant to absorb nutrients while taking up water and the effect of evaporation prevents the temperature within the plant from reaching “supra-high levels” (Roberts et al., 2006).

2. Plants regulate transpiration through “partial or complete stomatal closure” (Roberts et al., 2006).

3. Transpiration is affect by many abiotic factors such as “water stress, flooding, light intensity, high temperatures, cold and freezing” (Nilsen and Orcutt, 1996). Water Stress refers to drought conditions which can lead to dehydration of vegetation. Flooding can cause the nutrients to be washed away as well as smother the plant roots. High light intensity could cause irreparable damage to the leaves due to radiation where as low light intensity could inhibit photosynthesis. Under freezing conditions, water will turn to ice which prevents the plant from absorbing it. Also, as water becomes ice, the volume will expand and can cause physical damage to the plant itself. High temperatures increases evaporation of water and can lead to the plant dehydrating. In our experiment we will be exploring the abiotic factors heat as well as humidity, closed stomata, and surface area of the leaves. Humidity can affect transpiration because under high humidity, the water vapor in the air can greatly reduce the evaporative rate that drives transpiration. Because transpiration occurs through the stomata, closing the stomata can greatly reduce the rate of transpiration. Lastly, the surface area of the leaves affects the number of stomata as well as the amount of veins present on the plant leaves which can also affect transpiration rates. Because heat is one of the major abiotic stresses on plants, we will be looking into the specific effect that heat has on transpiration rate through statistical testing.

4. I predict that under humid conditions, the effect of transpiration will be significantly reduced because the high amount of water vapor can slow the evaporation rate which drives transpiration. While under high heat conditions I predict that the transpiration rate will increase significantly because the heat will expedite the evaporative effect. If the stomata are closed, I predict that the rate of transpiration will be significantly slowed because evaporation of water which drives transpiration occurs through the stomata. Lastly, I predict that if the surface area of the leaves is increased, the amount of water loss will also significantly increase because larger leafs contains more stomata.

Materials & Methods

1. A potometer measures the amount of water that is displaced during the experiment and in this case that represents the “uptake of water” (Sadava et al., 2009) by a plant. By simulating environmental conditions and using the potometer, scientists can understand how a plant reacts to different environmental stresses.

2. The potometer setup consisted of a single rubber tubing which was used to hold the water while a pipette was used as a measuring scale. After attaching the rubber tubing with the pipette, the joint was liberally coated with petroleum jelly. The apparatus was then submerged under water. A branch of the plant Iresine was cut and the surface area of the leaves are measured and summed. Afterwards, the stem of the branch was submerged underwater and an oblique cut was made to the end of the branch. While the stem and apparatus was submerged, the stem was placed in the opened end of the rubber tubing and coated liberally with petroleum jelly. The apparatus was then gently taken out of the water and moved to a ring stand fixed with clamps to hold the potometer in place. The leaves are cleaned, dried, and the apparatus was checked for leaks. After two minutes, the starting volume of the water in the pipette was recorded. After 10 minutes, the final reading of the water in the pipette was read and recorded as the control. For the humidity treatment, a bag was sprayed with water and draped over the plant. Then we waited for 2 mins. before taking the initial reading on the pipette. After another 10 mins. a final reading was taken and we moved on to the heat trial of our experiment. For the heat trial, all of the leaves were patted dry. Then a lamp was added as a heat source shining down from 10cm above the plant. After 2 mins., we recorded the initial water volume in the pipette and 10 mins. after that we recorded the final volume. Moving into the final treatment, petroleum jelly was liberally used to cover the stomata of all the leaves. We then waited 5 mins. before taking the initial volume of water and 10 mins. after that to take the final volume. By subtracting the initial reading from the final for each trial, the displaced water amount was found and recorded for each trial. By combining our data with the data of our class, we were able to obtain 9 data samples for each trial and can then use the t-test or t’-test to compare each data to the control to determine if the treatments created a significant difference from the control. We also used a regression analysis to analyze the correlation between the heat treatment data sample and the leave size data sample.

3. In order to determine if heat significantly increases the rate of transpiration in Iresine herbstii leaves, a two sample t-test was used (α = 0.05).
In analyzing if the heat treatment was affected by the surface area, a regression test was used (α = 0.05).

Results 1.

Transpiration, under high heat, is decreased as compared to the control.

The regression analysis shows a positive correlation between the size of the leaves and the rate of transpiration under high heat treatment. However the R2 values are quite low due to the outliers above 450 cm3.

Our results of the t-test were that the amount of water taken up by Iresine Herbstii was significantly reduced under the heat treatment compared to the control treatment (T = 0.035 p = 0.634). The mean and standard deviations for the control and heat treatment data samples were respectively µControl = 0.051 + 0.026 and µHeat = 0.047 + 0.025. Also, as the surface area of the leaves are increased, the water uptake in the heat trials also increased (P-Value = 0.219; R-Squared = 20.64%; Regression Equation: heat (ml) = 0.00005x – 0.0286). The mean of the Surface Area = 363.0 + 233.0.

Discussion

1. Transpiration is a very important biological process in plants. Understanding the conditions that would help plants use the resources in its environment efficiently could give us a better understanding of that plant and how to help it thrive. In our experiment, the result of heat on the rate of transpiration in Iresine Herbstii is a significant decrease in water uptake from the plant. In a study that measured sap flows in savannah shrubs and herbs using a sap flow gauge (Allen and Grime, 1994), the study shows during high net radiation hours, the rate of transpiration increased while during low net radiation hours, the rate of transpiration decreased. The study of Dr. Allen and Dr. Grimes received the opposite results as ours and I believe that our results might have been contaminated by the previous trial. As the heat trial came after the humidity trial, the leaves might not have been completely dabbed dry which caused the plants to think that it was still in a high humidity condition. I believe that the results of our experiment are skewed due to the limitations of the study, human error, as well as the equipment that we have access to.

2. The limitations of our study are that the potometer measures water uptake and not the rate of transpiration. The water that is taken up by the plant are used for other processes such as photosynthesis as well as to maintain its turgor pressure within the plant and only a portion of the water uptake goes to the leaves as transpiration. Also, the twig we used might have lost is activity soon after it was cut from the main plant which could also affecting the result of our study. Additionally, the potometer set up is very sensitive therefore easily subjected to leaks and mistakes in measurements and set up. Lastly, since we only had one plant each to do 4 trials on, the order of the trials become an important factor. By doing the humidity trial first could have caused residual water on the leaves which contaminated the heat trial. All these limitations could greatly affect our results.

3. In this study I learned the importance of transpiration in plants as well as how to use the potometer. This experiment could be improved by using a better way to measure the amount of water displaced. Also if a larger piece of the sample plant might yield a more accurate result. If possible, instead of a rubber tube, a glass U shaped tube would minimize the sensitivity of the apparatus to mistakes. Also, rearranging the order of the trials could prevent unwanted contamination from previous trials. Under optimal conditions, having multiple plants samples for each separate trial might also provide the best possible result.

Literature Cited

Allen, S.J., Grime V.L.. 1994. Measurements of transpiration from Savannah shrubs using sap flow gauge. Agricultural and Forest Meteorology 75 (1995) 23-41

David E. Sadava, David M. Hillis, H. Craig Heller, May Berenbaum. 2009. LIFE. 9th ed.

Nilsen E.T., Orcutt D.M.. 1996. The Physiology of Plants Under Stress. John Wiley & Sons, New York. Roberts, John M., Rosier, Paul T.W., Gowing, David J.. 2006. Seasonal and interannual variability of canopy transpiration of a hedgerow in southern England. Tree Physiology.