Drought on Stomatal Aperture

Plants need both water and energy for survival. However, in C3 photosynthetic plants like Phaseolus vulgaris, these needs are balanced through the opening and closing of stomata (figure 1). This complex system responds to many factors such as CO2 concentration, light, and water availability. C3 is the most common type of photosynthesis among plants on earth, accounting for roughly 95% of plant species (Boretti & Florentine, 2019). C3 plants, including the Phaseolus vulgaris (figure 2), typically do not perform well in arid environments because they only photorespire with open stomata, leading to lower energy production during drought and a more careful balance.

This project takes a closer look at drought stress influences on stomatal aperture over varying durations of time. Understanding how plants like the Phaseolus vulgaris regulate water loss improves our understanding of plant physiology. This can help us use water more efficiently and more effectively under different environmental conditions, leading to a better agricultural output.

Hypothesis

If the duration of drought increases, then the change in stomatal aperture of Phaesolus vulgaris will be increasingly closed because of hormonal signalers like ABA that promote stomatal closure in response to drought stress.

The experimental design involves growing Phaseolus vulgaris plants to germination. All plants were watered thoroughly during this time. Next, samples of the first leaf of all Phaseolus vulgaris plants are collected, and their stomatal apertures are measured. Then, the plants will not be watered at all for increasing durations of time. Finally, stomatal apertures are measured and compared to those before. The predicted outcome is that as drought duration increases, stomatal aperture will decrease because Phaseolus vulgaris adapts to its dry environment by closing its stomata, thereby limiting water loss by transpiration.

Some materials used to grow the plants include the pots, soil, grow lights, and seeds. As well as tools to extract samples, such as the microscope, slides, tape, and clear nail polish.

Ideally, there would be five plant subjects for each duration-of-drought group. However, only fifteen of the thirty-five seeds germinated. So, there could only be three plants per duration of drought.

In order to maintain accurate results, some key variables such as the type of soil, the temperature of the environment, the volume of soil, and lighting must be kept constant. Soil was measured carefully, and from the same bag, the thermostat was closely monitored and always set to 18°C, and grow lights were always on for fifteen hours per day.

There is a strong positive linear relationship between the change in closed stomata of a Phaseolus vulgaris as a percent, and the duration of a drought (figure 7). The Phaseolus vulgaris plants in the 0-day drought group had the least change, a 0% increase. Whereas the Phaseolus vulgaris plants in the 6-day drought group had the largest increase in closed stomata, 19.717%. The equation of the line is y = 2.4536x + 2.788. This means that, on average, for every day of drought after 0 days, the stomatal closure of the Phaseolus vulgaris is increased by 2.788%. The Pearson coefficient (r) of this data is 0.9476, meaning that it is a strong, positive relationship. Example equation to find the change in the number of closed stomata as a percent after 4 days of drought:

There is also a strong positive linear relationship between the change in density of closed stomata of a Phaseolus vulgaris and the duration of drought (figure 8). The Phaseolus vulgaris plants in the 0-day drought group had the least change, with no increase in closed stomatal density. Whereas the Phaseolus vulgaris plants in the 8-day drought group had the largest increase in closed stomatal density, 27.2463 stomata/mm2. The equation for this relationship is y = 3.563x + 2.515. Example equation to find the change in number of closed stomata per mm2 after 4 days of drought:

The Pearson coefficient (r) of this data is 0.9347, meaning there is a strong positive relationship, but not as strong as measuring the change in stomatal aperture as a percent (r value is 0.0129, closer to 0 when measured as a density). Therefore, estimating the stomatal aperture of a Phaseolus vulgaris as a percent is more accurate than estimating the stomatal aperture as a density.

Discussion

This strong positive relationship (figure 7) between duration of drought and reduced stomatal aperture occurs because of the guard cells’ response to the abiotic stress of drought (Agurla et al. 2018). Drought causes stomatal closure because the guard cells lose turgor pressure due to the decrease in water availability outside the cell. Additionally, plants have evolved chemical mechanisms for closing stomata to prevent water loss. These chemicals promote the loss of turgidity in guard cells, which causes stomatal closure. These chemicals include abscisic acid, reactive oxygen species, and calcium ions (Ca+2) (Liu et al. 2022). These mechanisms cause stomatal closure in response to drought, which explains why, as the duration of drought increased, stomatal closure did as well. These results agree with the accepted scientific context. Other experimental and research results prove that plants under drought stress tend to close their stomata in response. They do this to prevent water loss through transpiration in legumes such as the Phaseolus vulgaris (Reynolds-Henne et al. 2010).

Conclusion

If the duration of drought increases, then the change in stomatal aperture of a Phaseolus vulgaris will be increasingly closed. This hypothesis has been confirmed by the strong positive relationship seen between closed stomata on the lower epidermis of the first true leaf of a Phaseolus vulgaris and the duration of a drought.

Throughout this investigation, there was one major source of error, with some additional negligable/minor errors. Most importantly, the grow lights did not cover all plants equally, leading to uneven distribution of light intensity and growth (figure 9). If redone, a large ceiling grow light would be best. In the future, ABA levels and K+/Ca2+ ion flux could be manipulated and tracked to improve stomatal regulatory efficiency. Another direction this could go is by adding multiple variables like heat and CO2 density to explore how the interactions of these stresses affect stomatal efficiency/regulation.

Thanks to...

Mrs. Marshalok

-For refining the research question and providing feedback for the whole project along the way.

-For providing access to a light microscope, and helping to efficiently identify open/closed stomata.

Mr. Pilot

-For providing feedback on error bars.

-For pulling us out of class to give us time to work on our project boards.

Emily Cross

-For helping to prepare the project for nationals by giving advice on application, citations, background information, and other nitty gritty details.

Articles

Agurla, S., Gahir, S., Munemasa, S., Murata, Y., & Raghavendra, A. S. (2018). Mechanism of Stomatal Closure in Plants Exposed to Drought and Cold Stress. Advances in Experimental Medicine and Biology, 1081, 215–232. https://doi.org/10.1007/978-981-13-1244-1_12

Boretti, A., & Florentine, S. (2019). Atmospheric CO2 Concentration and Other Limiting Factors in the Growth of C3 and C4 Plants. Plants, 8(4), 92. https://doi.org/10.3390/plants8040092

Liu, H., Song, S., Zhang, H., Li, Y., Niu, L., Zhang, J., & Wang, W. (2022). Signaling Transduction of ABA, ROS, and Ca2+ in Plant Stomatal Closure in Response to Drought. International Journal of Molecular Sciences, 23(23), 14824–14824. https://doi.org/10.3390/ijms232314824

Photosynthesis – Life’s Primary Energy Source - Lesson. (2019, October 22). TeachEngineering.org. https://www.teachengineering.org/lessons/view/cub_lifescience_lesson01

Reynolds-Henne, C. E., Langenegger, A., Mani, J., Schenk, N., Zumsteg, A., & Feller, U. (2010). Interactions between temperature, drought and stomatal opening in legumes. Environmental and Experimental Botany, 68(1), 37–43. https://doi.org/10.1016/j.envexpbot.2009.11.002

Image

Photosynthesis – Life’s Primary Energy Source - Lesson. (2019, October 22). TeachEngineering.org. https://www.teachengineering.org/lessons/view/cub_lifescience_lesson01