Rates of Transpiration in a Leafy Shoot

Introduction

Transpiration is the process by which water evaporates through pores called stomata in the leaves. Stomata open to absorb carbon dioxide for photosynthesis, which in turn causes water to evaporate. Carbon dioxide moves slowly into the leaves due to a shallow concentration gradient, but water evaporates at a faster rate into a dry atmosphere.  Consequently more water escapes than carbon dioxide is absorbed.  The transpiration rate is the amount of water evaporating over time. Photosynthesis requires carbon dioxide in order to provide energy to the plant, so loss of water is a necessary compromise. Transpiration results in forces that pull water through the xylem of the plant, which is an important conductor for water and nutrients. This is called the transpiration stream. The purpose of our experiment was to document the effects of external abiotic factors on the transpiration rates of plants, and to identify the structure of the plant, in particular the xylem and the transpiration stream.  As we were in a laboratory setting, we had to simulate the conditions of darkness and humidity. This had limitations. We also examined the transpiration stream microscopically in celery. So why is water so important to a plant? “Water is the most limiting abiotic (non-living) factor to plant growth and productivity, and a principal determinant of vegetation distributions worldwide. The importance of water to plants stems from its central role in growth and photosynthesis, and the distribution of organic and inorganic molecules”. (McElrone, et al. 2013) Water forms part of the protoplasm, acts as a solvent for nutrients, which it then carries from the soil to the plant tissues, and is used for photosynthesis together with carbon dioxide, to from glucose and oxygen. It maintains turgidity in the cells (thereby giving structure to the plant), and helps in cell division for growth.  Water is also important for germination of seeds and helps in the conversion of starch to sugar via hydrolysis.  This enables the plant to convert stored starch to glucose during times when it cannot undergo photosynthesis. (Scialdone, A. & Howard, M. 2015). Plants actually only retain 5% of the water they absorb and transpiration is the main contributing factor. As water evaporates, it causes negative pressure, which pulls the water through in a continuous stream. Transpiration is a one-way system against gravity, so relies on the forces of cohesion and adhesion of water in the xylem. The cellulose walls of the xylem are polar, which attracts the water molecules (adhesion) and water molecules have a primary attraction to each other (cohesion).  This is known as capillary action. (siyavula, 2018) The structure of the xylem is crucial. The walls of the xylem are strengthened by lignin, this gives them strength, and the cells in the xylem undergo a programmed death at maturity, which means they are hollow and able to transport the water. (McElrone, et al. 2013) The cells of the xylem are principally vessel members so water can flow freely from vessel to vessels. (Petruzzello, M. 2018). In arid and acid conditions, plants have adapted to their environment.  These methods are discussed later in this report.

Objective

1.  To measure the rates of transpiration in a leafy shoot, by varying the physical conditions.
2. To identify the xylem in plants.
3. To measure the rate of water transport in a celery stalk
4. To evaluate how plants adapt to arid and acid conditions.

Method.

Part A Measurement of Transpiration rates in a leafy shoot, (Kennedy, 2018),

1. A potometer was used to measure the transpiration rate in plants. (Image 1)
2. We cut off a shoot from a Jasmine plant and put the stem through the bung of the potometer.
3. We then filled the system with water, ensuring there were no air bubbles, (this will slow the transpiration stream).
4. The meniscus was noted and was checked every 2 minutes for 10 minutes
5. This was repeated twice for each of the tests, control, clear plastic bag to simulate humidity and a black plastic bag for darkness.
6. It was important to allow at least 10 minutes between each part of the experiment, in order the plant and in particular the stomata to reset.

Image 1.  The Potometer Set Up

Part B Rate of water/dye transport in a celery stalk.

1. We took a celery stalk and trimmed off the last 1cm.  We then put the stalk in eosin solution, in order to follow the dye.
2. After 5 minutes we removed the stalk form the dye and after sectioning from the top with a razor blade we observed how far the water plus dye had risen. We also examined the stalk microscopically, to identify the xylem vessels.

Results

Table 1.  Rate of transpiration in plants in different abiotic environments.

From this table we can see that the transpiration rate on the control was higher than the treatments. The second control results were not consistent and we can hypothesis that the result was compromised by factors such as insufficient time to settle, lack of light for carbon dioxide or other factors in the experiment.  The stomata of the plant could have taken time to open again. The clear plastic bag, simulating humidity lowered transpiration rate and the black plastic bag had the same affect.  It should be noted that the results were all lower on the second run of the experiment.  This could be a result of carbon dioxide requirements in the plant, or that the stomata were already closing.

Figure 1 :  Comparison of transpiration rates under different conditions

This clearly shows that the transpiration rate in the control on the first run was higher than the treatments.  This confirms our expectations that humidity and darkness reduce transpiration rates.

Table 2:  Movement of water through celery stalk

The results show that the water and dye travelled through the celery stalk to the mid rib of the leaves.  Capillary action enabled the continuous stream.

Image 1: celery stalk showing the red dye in the xylem.

The xylem vessels are shown by the red dye.  The diagram also shows the phloem, which via translocation, brings energy and nutrients to the plant from the sources to the sinks.

Discussion

The results confirm that the water travels through the plant in a transpiration stream at a consistent rate unless external factors reduce the rate.  We conducted the experiment during the day so the plant would be reacting to light. Daylight activates pumps in the guard cells that open channels to allow potassium ions to flow in. This reduces the water potential in the cells. As this water potential continues to decrease the guard cells become more turgid, which opens up the stomata for carbon dioxide intake. (Daszkowska-Golec, A. 2018) The osmotic advantage of potassium becomes less over the day.  At night the stomata close as water leaves the guard cells, and they become flaccid. Abscisic acid also triggers the loss of potassium ions from the guard cells. (Hewitson, J.et al, 2018). When stomata shut transpiration stops. Water remains in the stem and the leaf due to the adhesion and cohesion forces, meaning the plant doesn’t lose structure. Loss of water creates negative pressure, which combined with cohesion and adhesion in the xylem, causes a continuous transpiration stream. Cohesion forces are created by the strong attraction of water molecules to each other and adhesion forces result from the attraction of the water molecules to the polar cell walls in the xylem. (Mclaughlin, D.1988)

The results demonstrated continuous movement of water in the potometer and the movement of the dye in the celery stalk.  We had removed the roots and therefore the water uptake was directly from the water in the potometer, prompted by transpiration (water loss).  Water would normally be absorbed through the roots. The roots have root hairs, which allow the root to increase its surface area and the water crosses the epidermis to the endodermis, where the Casparian strip, (a band of waterproof materials) directs the water to the xylem. There were limitations to this.  The potometer is a method that “provides an indirect measurement of the transpiration rate by measuring the absorption of water not the amount of water vapour given off directly”. (Siyavula, 2018) This suggests that though we can see the absorption rate, we are not actually measuring the amount of water being evaporated.  The results of part A of the experiment (table 1 and figure 1), correlate to what we would expect to see.  The first control experiment was higher than the other experiments as there were no known environmental influences, and in light we would expect the stomata to open for carbon dioxide.  However, we should note that we were in a lab with air conditioning, and the light source would produce a slower transpiration rate than outside. (Richter, C. 2018) There was a high probability of high levels of carbon dioxide in the lab setting, due to human respiration. (Miller J et al, 2010). High carbon dioxide would make it easier to absorb down its concentration gradient. This could slow the transpiration rate. The second control was actually a lot lower, which may be because we had not set up the experiment correctly, or that the 10 minutes was not sufficient for the plant to reset (stomata). The second experiment using the plastic bag was to simulate humid conditions.  Humidity is the measure of water molecules in the air.  When humidity is high the water potential gradient between the inside of the leaf and the atmosphere is low, thereby lowering the transpiration rate. Our results confirm this. The limitation of this was that we could not actually measure the amount of vapour in the bag, and it would take time for humid conditions to affect the transpiration rate. In addition the rate of carbon dioxide would be low in the bag.  The black plastic bag was used to simulate darkness. We would expect the rate of transpiration to slow in darkness. This is because the solutes will move out of the guard cells and water will follow.  This makes the guard cells flaccid and causes the stomata to close. Our results confirmed this (Siyavula, 2018). There would also be limited carbon dioxide. There are other factors that could have affected the transpiration rate, Wind changes the water potential by removing the water that forms a layer on the outside of the leaf, and temperature affects the movement of water molecules and the rate of evaporation. We could have improved our results by not using air conditioning (which would have impacted transpiration rates), by doing the tests in different orders, the control followed by test 1 and then test 2 and then repeated, rather than the 2 controls followed by test 1 twice and test 2 twice.  This would have given us a better picture of the spread of results.  The potometer itself was not easy to set up and this definitely affected our second control results.  The experiment could have been conducted outside of the lab using natural conditions over a period of time. In order to appreciate the mechanisms of water transport, the visual aspect of the celery experiment was important.  Under the microscope we could see that the dye was visible in the xylem vessels (image 1), and could follow its movement through the xylem. We could estimate that the dye would reach the leaves within an hour.  The unbroken dye stream showed the uninterrupted transpiration stream through the xylem vessels.  We could also see the detail of the xylem, which is the reason celery is so tough.  The movement through the vessels was a visual demonstration of the cohesion and adhesion of capillary action.

Plant adaptations to arid and acid conditions.

In an arid environment plants will lack water and energy.  This causes plants to close their stomata,which not only reduces water loss but also carbon dioxide and nutrients. The plant can reduce transpiration by shedding leaves or rolling leaves to reduce the surface area. It may also have thick waxy cuticles to act as a barrier to evaporation, or a reduced number of stomata. (Freeman, J.M, 2018) Plants that can keep the stomata open partially can minimize the water loss and still allow carbon dioxide intake. (Waggoner and Zelitch, 2018) Another adaptation is sclerophylly, where plants form hard leaves that will not suffer permanent damage from wilting. (Basu, S, et al 2018) Succulents like cacti, store water in their leaves.  They also have to absorb large quantities of water in short periods, when rain is available so they have a shallow root system to assist. In addition the plants often have thorns or spiny surfaces that discourage thirsty animals from eating them. They can also use a system called Crassulacean Acid Metabolism. This allows them to store the carbon dioxide they take in during the night in the form of a chemical called malic acid, which is converted back to carbon dioxide during the day. (Dr. Biology, 2014) The eucalyptus has long leaves, which reduces the surface area and has its stomata located underneath the leaves where tiny hairs decrease air movement. (Robb, A. 2018) Plants also try and actively gain water from the environment.  The Australian mulga angle their leaves so water runs to the ground and the roots can absorb it. One of the problems of lost water is loss of turgor.  This can be adapted to via osmotic adjustment. This is the process of reducing water potential in cells by increasing solutes and therefore what water is available will go to those cells. (Basu et al., 2018)

Acid conditions can also have a detrimental effect on plants.  Acidity in soils comes from hydrogen ions in the soil solution. A low pH correlates to a high number of hydrogen ions. Many processes contribute to the formation of acid soils including rainfall, fertilizer use, plant root activity and pollutants such as acid rain. Acidic soil affects ion uptake. One aspect of a drop in soil pH, is that aluminium becomes soluble, which slows down root growth, and therefore gives less access to water and nutrients. In very acid soils, all the major plant nutrients may be unavailable, or only available in insufficient quantities. (Gazey, P. 2018) “Some plants can cope with nutrient deficiency in acidic soils via modifications to their root morphologies and in their nutrient uptakes and metabolisms” (Hammond et al. 2004).  Plants increase root surface area in order to access nutrients, or explore new areas including growing up out of the soil (gravitropism). The roots can also become concentrated where there are more nutrients. In addition Mycorrhizal fungi form a symbiotic relationship with plant roots where fungi provide plants with increased nutrient uptake in exchange for carbon derived from photosynthesis. (Kluber et al. 2018)

Conclusion

Transpiration is a vital process in plants.  The evaporation of water, which is a byproduct of carbon dioxide intake, causes water to move from the roots to the leaves in a continuous stream.  This carries nutrients from the soil to the parts of the plant that require them.  It moves against gravity through forces of negative pressure, cohesion and adhesion.  The structure of xylem facilitates this transport.  Transpiration rates will be lower in times of abiotic stress.  Humidity, heat, wind, darkness and drought will all affect the rates.  Plants have adapted ways to cope in arid and acid conditions in order to survive. 

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