Stomatal Spacing in plant leaves
Abstract
Most plants abide by the “one cell rule” which defines that the stomata in plants usually occurs by itself within the mesophyll layer separated by at least one epidermal/pavement cell on the leaf’s epidermis (Papanatsiou, 2016). Since stomates are the doorways to gaseous exchange in the plant leaves, allowing them to breath (transpire) and undertake photosynthesis, I undertook an experiment to highlight how stomatal structure, leaf physiology and gaseous exchanges significantly affect the unique stomatal frequencies and distribution observed in different plants. Stomatal spacing in the leaf is directly dependent upon an individual plant’s environmental requirements.
Main Text of Review
General: Stomatal Structure
The term is usually used collectively to refer to the entire stomatal complex, consisting of the paired guard cells and the pore itself, which is referred to as the stomatal aperture. Each side of the stoma is an individual cell, with the generic eukaryotic structure of a cell with membrane bound nucleus and organelles. with the modifications present on plants, namely chloroplasts and a cellulose cell wall surrounding the stomatal opening (Shtein, 2017)
The ability of the stoma to open and close is essential to its function, relying on the 2 guard cells surrounding the stomatal opening and the subsidiary cell which enclose and support the guard cells.
Photosynthesis, plant water transport (xylem) and gas exchange are regulated by stomatal function which is important in the functioning of plants. As such frequency and distribution of the stoma will significantly affect plant functionality under different circumstances.
Physiology
The way plants expand to capture sunlight to photosynthesise and produce their food, is accomplished through the surface area of the leaf. Since the growth and reproduction of plants relies upon their physiological adaptations to the environment (Wang, 2019). The physiology of leaves on a tree is the result of the long term evolutionary and ecological history of the plant, modified by the ecosystems of the plant’s limiting factors (Micol, 2003).
As a plant organ the leaf has maximised its ability to work as a structural unit that the plant to produce oxygen and glucose, feeding the plant. The leaf organ is primarily composed of an epidermal layer of skin tissue, the mesophyllic layer, which functions as the powerhouse that converts light energy into chemical glucose (6CO2 + 6H2O → C6H12O6 + 6O2), and the plant veins which transport this chemical food throughout the cell.
The stoma occurs on the epidermal layer providing a direct doorway for gaseous exchange from inside and outside the plant, to allow the photosynthetic reaction to take place by absorbing necessary gases and allowing the plant to transpire. That is why the stomatal distribution is important: it defines how the plant facilitates this reaction.
Capacity for Gaseous Exchange
Stoma, as pores in the epidermis, have their aperture controlled by the guard cells because its ability to open and close directly controls its capacity for gaseous exchange. It is through this function that the gradients of osmosis and diffusion into and out of the plant are controlled. Most importantly these stomatic pores allow the leaves to facilitate the entrance of CO2 for use as a reactant in the photosynthetic process by which they obtain the process that provides them with their food (Boyer, 1985). The stoma is also crucial in releasing the unnecessary product (during the day) of oxygen. Whilst these processes are necessary, unfortunately it also means that the high-water vapor content of the plant (water also being a reactant in the photosynthetic process) escapes to the outside through this aperture. It can be beneficial to release excess water vapour but often this water vapour escapes when it is still required. That is why it is crucial that the stoma is able to close as well as open. This is also why the frequency and distribution of the stomates significantly affects its diffusive gradient. Like most living organisms’ plants do, in fact also respire (C6H12O6 + 6O2 → 6CO2 + 6H2O), occurring during the night. The stoma
performs much the same function here as it did before, the only difference in this function being that the roles of the reactants and products, and hence what needs to be let in and let out, is reversed.
Stomatal Structure of Leaves in Different Plants:
The frequency and distribution of stoma on leaf surfaces varies widely among different species of plants. This frequency not only varies amongst different species but varies on different surfaces of the same plant (Hong, 2018).
Terrestrial plants generally possess thousands of stomates on the surface of the leaf. In these land residing species, the stoma is on the underside of the leaf as an adaptation to minimise exposure to air current and heat. Unlike their land-dwelling counterparts, aquatic plants have these on the top side of the leaf (Bertolino, 2019).
The differing needs and unique physiological conditions for any given plant species will affect the structure and frequency with which the stoma will appear on the leaf. In general, Stomatal distancing and hence frequency depend on two things: the amount of carbon dioxide, and the amount of water in the surrounding atmosphere.
Plants in low carbon dioxide environments will have more stoma in order to ensure survival by providing access to a necessary reactant for the photosynthetic process by which they obtain their food. Similarly, if water is not a limiting factor, and is abundant, then the plant will also have more clustered and frequent stomatic occurrences because they do not need to stop water escaping the leaf, and to ensure easy access to the surrounding hydrogen ions in the water (Lawlor, 2015) Therefore it can be concluded that plants that do not need to conserve fluid will have more clustered stomates (and vice versa) and that, similarly, plants that need do not need to maximize CO2 intake will also have less stomatal frequency, with greater distancing between the stomates.
Conclusion
Ultimately, the frequency, structure and distribution of the stoma is a physiological trait of the plant which is adapted to suit the specific environmental needs of each individual plant species, and maximise the availability of the reactants readily present for use by the plant, in order to undergo photosynthesis and produce its food. In future, a topic of interest could be to explore how stomate distribution adapts to being in a low CO2 environment (requiring high stomates) whilst simultaneously living with low water content availability (requiring low stomates). It can be concluded that distancing and frequency of stomates on the plant, is directly linked to the availability of its reactants, necessary for its survival.
References
Main Text
1. Maria Papanatsiou, Anna Amtmann, Michael R. Blatt Plant Physiology Sep 2016, Stomatal Spacing Safeguards Stomatal Dynamics by Facilitating Guard Cell Ion Transport Independent of the Epidermal Solute Reservoir, 172 (1) 254263; DOI: 10.1104/pp.16.00850 http://www.plantphysiol.org/content/172/1/254
2. Shtein, I., Shelef, Y., Marom, Z., Zelinger, E., Schwartz, A., Popper, Z. A., Bar-On, B., & Harpaz-Saad, S. (2017). Stomatal cell wall composition: distinctive structural patterns associated with different phylogenetic groups. Annals of botany, 119(6), 1021–1033. https://doi.org/10.1093/aob/mcw275
3. Ferry, R J. “Stomata, Subsidiary Cells, and Implications.” MIOS Journal, vol. 9 iss. 3, Mar. 2008, pp. 9–16.
4. Miskin, K.E., Rasmusson, D.C. and Moss, D.N. (1972), Inheritance and Physiological Effects of Stomatal Frequency in Barley1. Crop Science, 12: 780–783 cropsci1972.0011183X001200060019x. doi:10.2135/cropsci1972.0011183X001200060019x
5. Ji Hua Wang, Yan Fei Cai, Shi Feng Li, Shi Bao Zhang. (2019) Differences in leaf physiological and morphological traits between Camellia japonica and Camellia reticulata, Creative Commons license, Published to Science Direct. https://doi.org/10.1016/j.pld.2020.01.002
6. José Luis Micol, Sarah Hake Plant Physiology Feb 2003, “The Development of Plant Leaves” 131 (2) 389–394; DOI: 10.1104/pp.015347
7. Boyer, J. S. (1985). Stomata and Gaseous exchange. Delaware, Newark: University of Delaware.
8. Hong T, Lin H, He D (2018) Characteristics and correlations of leaf stomata in different Aleurites montana provenances. PLOS ONE 13(12): e0208899.https://doi.org/10.1371/journal.pone.0208899
9. Bertolino, Lígia & Caine, Robert & Gray, Julie. (2019). Impact of Stomatal Density and Morphology on Water-Use Efficiency in a Changing World. Frontiers in Plant Science. 10. 10.3389/fpls.2019.00225.
10. Lawlor, David. (2015). Re: Why do plants with more access to water need more stomata? Retrieved from: https://www.researchgate.net/post/Why_do_plants_with_more_access_to_water_need_more_stomata/554a8055d3df3e134a8b465b/citation/download.
Figures
All figures (exc. 4) are original, with credit given to all image inferences.
1. Tranquada, George. (2013). Evolution and Environmental Degradation of Superhydrophobic Aspen and Black Locust Leaf Surfaces. https://www.researchgate.net/figure/General-Leaf-Cross-Section-Diagram-Sadava-et-al-2007_fig9_284663917 *
2. Stephen Hill (2019), “Stoma Open and Closed” Science Learning Hub — Pokapū Akoranga Pūtaiao: https://www.sciencelearn.org.nz/images/3889-stoma-open-and-closed
3. Anne Marie VanDerZanden, (2008), Environmental Factors Affecting Plant Growth Accessed on the 3rd of May from: https://extension.oregonstate.edu/gardening/techniques/environmental-factors-affecting-plant-growth
4. Bertolino, Lígia & Caine, Robert & Gray, Julie. (2019). Impact of Stomatal Density and Morphology on Water-Use Efficiency in a Changing World. Frontiers in Plant Science. 10. 10.3389/fpls.2019.00225. **
Notes
* Directly adapted from (Bertolino et al, 2019)
