Measuring the effects of wind and light on the transpiration rate in a leaf

by weager (student)

International Baccalaureate Biology

Alexander Templet

Transpiration Rate in

Solanum lycopersicum

Biology 156

Summer 2008

Mr. Leith Adams, Instructor

Abstract

Plants draw water up through their roots and out through their leaves. This process is known as transpiration. The transpiration rate is a major determining factor in how quickly plants absorb water, and is thus critically important to understand for agriculture. In order to study how varying weather conditions affect the rate of transpiration, we conducted experiments using stems of the tomato Solenum lycopersicum. Our results showed increased transpiration when the plants were subjected to wind and also when subjected to light. Interestingly, wind and light combined did not increase transpiration as greatly as light acting alone.

Introduction

Plants draw water in through their roots, and then transport it through the xylem up to the branches and leaves. Water exits the leaves through the stomata in the form of water vapor. Polarity causes the water exiting through the stomata to draw after it the water in the xylem, which then pulls in more water through the roots. This process is known as transpiration (Raven et al., 2002).

Transpiration is a vitally important process in plants, and to study it further we designed and conducted an experiment to measure the rate of transpiration in the tomato plant, Solenum lycopersicum. In order to study the effects of varying weather conditions on the process, we studied how the transpiration rate would be affected by light, wind, and both light and wind (Vodopich et al., 2002).

We hypothesized that these variables would increase the rate of transpiration. Wind can blow water vapor away faster than it would escape normally, and light can increase the plant’s temperature and thus increase evaporation from the leaves. Tartachnyk and Blanke (2007) showed that increased sunlight increases transpiration, thus seeming to confirm our hypothesis. We therefore predicted that both light and wind would increase the transpiration rate, with the greatest increase being seen in the plant that was exposed to both light and wind.

Materials and Methods

We began by flushing the tanspirometer with water to remover all air from ...

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Materials and Methods

We began by flushing the tanspirometer with water to remover all air from the pipette and tubing. We opened the screw valve below the reservoir to flush the transpirometer, and continued flushing until no air bubbles remained in the tubing or pipette. We sealed off the pipette with a spring clamp and inserted the stem of the S. lycopersicum, which was wrapped in a paper towel and clamped into place. We opened the spring clamp sealing off the pipette, and opened the screw valve just enough to allow water to drip from the pipette. We sealed the screw valve again and blotted the lingering drop of water from the open tip of the pipette. If there were no leaks in the tubing, no water should continue dripping through the pipette. Satisfied that there were no leaks, we measured the distance of the water from the tip of the pipette and used this as the zero measurement for our readings.

As a control, we first measured the transpiration rate without any simulated weather variables. We measured the distance of the water from the tip of the pipette at five minute intervals for fifteen minutes, giving us three measurements after our zero reading. Each measurement was recorded in millimeters, and once the fifteen minutes were complete, we calculated the average transpiration rate by subtracting the zero measurement from the final measurement. This difference was then divided by fifteen to give the rate of millimeters per minute, and we multiplied this figure by 1.136 to convert our result into microliters per minute.

After completing our control set of readings, we flushed the transpirometer again as described above to set it up again for our weather variable. My group was assigned to conduct our second reading using a small electric fan to simulate wind. We placed the fan 30 centimeters from the plant, and blew it onto the leaves while we repeated the process used to measure the transpiration rate in the control.

Upon completion of our second set of measurements, we removed the leaves from the stem and measured their surface area. We then divided this figure into our transpiration rates to give the rate of microliters per minute per square millimeter. We then subtracted the rate of the control from the rate of the variable, to determine exactly how much of a difference the simulated wind had on the transpiration rate.

Two other groups in our laboratory conducted the same experiment. All three groups conducted their control measurements in exactly the same manner descriped above. One group used a lamp to simulate sunlight as their weather variable, while the other group used both a lamp and a fan to simulate both sunlight and wind. All three groups calculated their final transpiration rates and we compared the results to test our hypotheses.

Results

My group calculated a final transpiration rate of 2.36x10-4μL/min/mm2 for our control, and the addition of wind altered the transpiration rate to 2.73x10-4μL/min/mm2. This showed that the presence of wind had increased the transpiration rate by 3.70x10-5μL/min/mm2. The group that used light as their variable measured a transpiration rate of 2.30x10-4 μL/min/mm2 in their control, and a rate of 3.60x10-4 μL/min/mm2 when they added the light. This produced the largest difference of our experiment at 1.30x10-4 μL/min/mm2. The group that used both light and wind calculated a transpiration rate of 3.13x10-4 μL/min/mm2 for their control and 3.91x10-4 μL/min/mm2 after they added both light and wind. This showed a difference of 7.80x10-5 μL/min/mm2, larger than with wind alone but less than that showed with light alone.

Discussion

Our results showed that light and wind do increase transpiration rates in plants, but we were surprised to see that, while both light and wind increase transpiration, the combination of these two factors produced a greater rate than wind acting alone but still did not increase transpiration as much as light acting alone. This may be that the wind cooled the plant and thus counteracted the heat of the light, which we believed to be the primary reason why light would increase transpiration. The effect of light on transpiration was thus largely negated when acting in combination with wind.

These discoveries are of very real significance for agriculture, where plants must be cultivated and watered to maximize growth. Careful study of how weather affects plant transpiration, and thus water loss, can help farmers determine how much to water their crops to maximize growth without overwatering them.

Literature Cited

Raven, P. H., G. B. Johnson, J. B. Losos, S. R. Singer. 2002. Biology, Seventh Edition. McGraw Hill, Boston. 1250 pp.

Tartachnyk, I. I., and M. M. Blanke. "Photosynthesis and Transpiration of Tomato and CO2 Fluxes in a Greenhouse Under Changing Environmental Conditions in Winter." Annals of Applied Biology 150.2 (2007): 149-156. Academic Search Complete. EBSCO. Ellender Memorial Library, Thibodaux, LA. 14 July 2008.

Vodopich, Darrel S., and Randy Moore. 2002. Biology Laboratory Manual, Seventh Edition. McGraw Hill, Boston. 555pp.

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