Introduction
a) Brassica rapa.
The brassica rapa is a plant species that belongs to the genus Brassica and is one of the 51 genera in the tribe brassicae of the crucifer family (also known as the mustard family) (Rakow, 2004, pg.1). Within this family, there are the other 37 different members that provide edible seeds, flowers, stems, buds, and leaves (Rakow, 2004, pg.1). Based on their morphological characteristics there are three distinct groups of B. rapa. These include the oleiferous (oil-type rape) such as the Canadian Polish rape, the leafy-type B. rapa that includes the chinensis group, and the rapiferous-type B. Rapa, which comprises of the turnips (Prakash and Hinata, 1980 as cited in Canadian Food Inspection Agency, 2014). In nature, these plants thrive in highland landscapes such as plateaus and mountain areas while a few are known to favor coastal lowlands. Like most terrestrial plants, the growth of B. rapa is influenced by several factors such as the environmental variables of moisture and temperature. Other factors include anthropogenic interference, changes in soil nutrient levels and biological factors of predation, pests, and diseases. While these factors have different levels of environmental tolerance, extreme changes in the physical environment can negatively affect their ability to tolerate and adapt to the changing environment. One such factor that affects the growth of B. rapa is the changes in the available Nitrogen (N) and phosphorous (P) nutrients.
b) Nitrogen and Phosphorous Limitation
Nitrogen (N) is one of the essential, and key mineral elements required and necessary for any form of life on earth (Butterbach-Bahl and Gundersen, n.d., pg.100). Elements are considered essential not only because they do form part of the plant, but in their absence or insufficient supply, the plant is hindered from completing a normal, natural cycle of its life. Together with Phosphorous (P), and other elements such as Potassium (K), Sodium (Na), Calcium (Ca) and Magnesium (Mg), nitrogen is usually present in large quantities ("Phosphorous Interactions with Other Nutrients," n.d.). It would, therefore, seem that nitrogen (N) 'fixers' should naturally be having a competitive advantage in extracting, fixing, and availing nitrogen to plants whenever the supply of this element become wanting. In most cases, however, this is not always the case. Available evidence shows that much of the time nitrogen in unperturbed ecosystems is usually the growth-limiting nutrient – the one nutrient that is always exhausted first due to its low proportional availability to the others – which limits the Net Primary Ecosystem Production (NPP) of the system in question (Menge, Hedin, and Pacala, 2012). In several studies, the supply of Nitrogen (N) has increasingly been implicated in limiting the growth as well as the reproduction of most photosynthetic biota on the earth’s freshwater, marine, and terrestrial ecosystems (Butterbach-Bahl and Gundersen, n.d.; Elser et al., 2007, pg.1135). Therefore, inadequate supply of Nitrogen (N) to the extent that it limits the growth, optimal performance, and other essential plant functions such as reproduction is what is called Nitrogen Limitation.
In natural ecosystems, nitrogen limitation is caused by two main factors; the absolute shortage of nitrogen in the soil (growing medium) and the presence of nitrogen in a form that is not available to the plant. In nature, the latter can be caused by several factors such as an excess of another nutrient, incorrect PH, poor plant root physiology, or shortage of water. In temperate ecosystems changes in the biogeochemistry of the nitrogen cycle has been identified as the major cause of nitrogen deficiency. In the recent past, these changes have been accentuated by the little limitation of biomass production by Nitrogen due increasing deposition of nitrogen into natural systems and intensive use of fertilizers in agricultural systems. The effects of increased availability of N in the temperate natural ecosystem include local changes in biosphere-atmosphere element exchange, eutrophication of natural systems, and acidification of water as well as soil following the deposition of reactive nitrogen (Nr). The resulting consequences have been the disturbance of temperate species composition and richness, carbon sequestration processes, and compromised quality of both surface and drinking water (Menge, Hedin, and Pacala, 2012). Like in other planetary ecosystems, nitrogen cycling – the N biogeochemical cycle – in temperate terrestrial landscapes is primarily driven by a combinational interplay of plant processes, microbiological activities, and physicochemical processes such as mineral leaching, erosion, volatilization, emission, or diffusion. All these processes lead to the availability of N for plant use at localized or generalized scales through the displacement of the mineral.
In contrast to Carbon (C) cycling, where the nutrient’s largest fluxes are associated with ecosystem’s net primary production, the N cycle is dominated by microbial processes such as Nitrification, denitrification, and anaerobic ammonium oxidation in the surrounding soils, sediments, and water bodies. (Doering et al., 1995). The N cycle also differs with the natural cycling of phosphorous in which the element is made available to the biosphere mainly through mineral weathering ("Phosphorous Interactions with Other Nutrients," n.d.). While N is the most recognized plant nutrient, it does not work alone in plant nutrition but rather interacts with other mineral nutrients such as Phosphorous (P) (Doering et al., 1995). Nutrient interaction occurs when the level of one nutrient influences the response of another. Positive interaction, therefore, happens when the combined influence of the nutrients exceeds the total of the individual nutrient influences ("Phosphorous Interactions with Other Nutrients", n.d.). In natural ecosystems, such positive nutrient interactions serve as the basis for the creation of a balanced plant nutrition program responsible for optimal plant growth, reproduction, and healthy development. For instance, N has been shown to interact positively with P for optimum plant growth (De Groot et al., 2014, pg.257).
Consequently, the limitation of N can cause a deficiency of P through various mechanisms such as the depletion-driven limitation where accumulated Phosphorous losses occur during long-term soil and ecosystem development. Moreover, increased supply of N relative to P in ecosystems can cause a deficiency of P by either stimulating biological processes such as productivity that consume more of P or by transforming the ecosystem to supply less of P such as depleting the Phosphorous-reserves. For example, the influence of N on P is demonstrated by the application of high N-content fertilizers to the soil and plants respond with an increased uptake of P. This observation occurs because N increases both the solubility P in soil water and P proximity to plant roots system ("Phosphorous Interactions with Other Nutrients", n.d.)
As pointed out previously, nutrient limitation to ecosystem NPP is widespread in terrestrial ecosystems. However, N and P are the most common limiting elements in terrestrial ecosystems frequently limiting individually (Liebig’s law of the minimum (LLM) or in combination (multiple limitations hypothesis [MLH]) (Ågren, Wetterstedt, and Billberger, 2012, pg.953). Like in N, limitation of P occurs when the supply of P is not sufficient to meet healthy growth and support optimal performances of essential plant function such as reproduction. This shortage is because P like N is an essential nutrient both as a constituent part of the structure of key plants compounds and as biochemical catalysts responsible for converting numerous biochemical reactions in plants. It is primarily noted for its role in photosynthesis where it is used to capture and convert the sun’s energy into useful organic compounds (CropNutrition.com, n.d.; De Groot et al., 2013, pg.257). Phosphorous availability in plants is limited by two factors; its naturally low abundance in the soil in obtainable form and its ease of adsorption onto various soil minerals. In acidic soils, for instance, P can be adsorbed by the oxides of iron and aluminum (Fe2o3/Feo) and (Al2O3) (Elser et al., 2007, pg.1135). Compared to other regions, P deficiency is pronounced in tropical soils. Most soils in these zones are derived from volcanic materials that either contain allophanes or are highly weathered and siliceous. Allophanes are minerals with a very high phosphorous fixing capacity and contribute to the removal of P in the soils.
According to Elser et al. (2007), the geochemical processes such as mineralogical transformations and ecological factors like sequestration has been instrumental in influencing both the identity and nature of N and P in tropical ecosystems (pg.1135). Therefore, the less disturbed soils of the tropics are frequently P-limited due to the region’s greater age than the deeply glaciation-disturbed soils of the temperate lands (Menge, Hedin, and Pacala, 2012). Additionally, the tropics are limited in P could be attributed to the impact of the zone’s fire regimes as fire is known to volatilize existing N pools while leaving behind Phosphorous reserves (Hungate et al., 2013 as cited in Elser et al., 2007, pg.1135).
According to LLM, Phosphorous will limit the growth of plants when its supply is not adequate to meet the P requirement of a particular plant. However, in situations where several factors influence the growth of plants at the same time, the MLH nutrient assumptions takes precedence. This scenario is common in ecosystems where there is a stoichiometric relation in and differences between the nutrient components of the ecosystem (Ågren, Wetterstedt, and Billberger, 2012, pg.953). This difference then leads to the constraint of the development of the system. Multiple studies such as Elser et al. 2007 and Harpole et al. 2011 show that while N and P are essential elements for plant growth, their combined effect on plants have a much stronger effect than individual responses (Ågren, Wetterstedt, and Billberger, 2012, pg.953). Based on such findings, De Groot et al. (2013), concludes that ecosystems in most cases are frequently both N and P-dependent. As co-limiters therefore, N limitation can influence the limitation of P by either biochemically substituting (filling in) the influence of P on plants or by affecting the uptake of P by the plant (Ågren, Wetterstedt, and Billberger, 2012, pg.953).
c) Brassica rapa: Chosen Subject for experimentation
In this particular study, the Brassica rapa species is used as the experimental specimen because like most plants, its roots system are central to nutrient absorption. Through tiny structures called the root hairs located in the epidermis of root tissues, plants can absorb dissolved nutrients from the soil. Like in temperate zones and in tropics where N and P affect plant growth, these nutrients are also contributory factors in not only the growth but also in the healthy development of Brassica. Rapa Sp. While this experiment can use a variety of plants, Brassica rapa is chosen for various reasons. First, its small size genome is not only suitable for the study but is also readily available (Wang and Kole, 2015, pg.142). Secondly, the analysis tool and scientific models for studying this species are also available. Such availability of study tools and information makes the study of the plant much easier. Lastly, the plant has quite a short life cycle of about 35-40 days making it easier to be studied and analyzed (Wendell and Pickard, 2007). Because of their rapid rates of growth and development, Brassica rapa are ideal plant specimen for experiments such as this with strict time restraints. Besides, B. rapa plants are easy to take care of are small in size, have a high density of growth rates, and can, therefore, be easily grown with relative ease in controlled experiments.
Works Cited
Ågren, Göran I., J. Å. Wetterstedt, and Magnus F. Billberger. "Nutrient limitation on terrestrial plant growth - modeling the interaction between nitrogen and phosphorous." New Phytologist 194.4 (2012): 953-960. Web. 25 Feb. 2016. <http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2012.04116.x/pdf>.
Butterbach-Bahl, Klaus, and Per Gundersen. "Chapter 6: Nitrogen processes in terrestrial ecosystems." Nitrogen Processes in Terrestrial Ecosystems. Cambridge: Cambridge UP, 2011. Web. 25 Feb. 2016. <http://www.nine-esf.org/sites/nine-esf.org/files/ena_doc/ENA_pdfs/ENA_c6.pdf>.
Canadian Food Inspection Agency. The Biology of Brassica rapa L. N.p., 2014. Web. 25 Feb. 2016. <http://www.inspection.gc.ca/plants/plants-with-novel-traits/applicants/directive-94-08/biology-documents/brassica-rapa-l-/eng/1330965093062/1330987674945>.
CropNutrition.com. "Phosphorous." Crop Nutrition - Trusted Crop Nutrition Expertiseâ? The Mosaic Company, n.d. Web. 25 Feb. 2016. <http://www.cropnutrition.com/efu-phosphorus#overview>.
De Groot, Corine C., Leo F. Marcelis, Riki Van den Boogaard, Werner M. Kaiser, and Hans Lambers. "Interaction of nitrogen and phosphorous nutrition in determining growth." Plant and Soil 248.1/2 (2003): 257-268. Web. 25 Feb. 2016. <http://link.springer.com/article/10.1023%2FA%3A1022323215010#page-1>.
Doering, PH, CA Oviatt, BL Nowicki, EG Klos, and LW Reed. "Phosphorous and nitrogen limitation of primary production in a simulated estuarine gradient." Marine Ecology Progress Series 124 (1995): 271-287. Web. 25 Feb. 2016. <http://www.int-res.com/articles/meps/124/m124p271.pdf>.
Elser, James J., Matthew E. Bracken, Elsa E. Cleland, Daniel S. Gruner, W. S. Harpole, Helmut Hillebrand, Jacqueline T. Ngai, Eric W. Seabloom, Jonathan B. Shurin, and Jennifer E. Smith. "Global analysis of nitrogen and phosphorous limitation of primary producers in freshwater, marine and terrestrial ecosystems." Ecology Letters 10.12 (2007): 1135-1142. Web. 25 Feb. 2016. <http://www.cedarcreek.umn.edu/biblio/fulltext/j.1461-0248.2007.01113.x.pdf>.
Menge, Duncan N., Lars O. Hedin, and Stephen W. Pacala. "Nitrogen and Phosphorous Limitation over Long-Term Ecosystem Development in Terrestrial Ecosystems." PLoS ONE 7.8 (2012): e42045. Web. 25 Feb. 2016. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3411694/>.
"Phosphorous Interactions with Other Nutrients." Better Crops 83.1 (199): n. pag. Web. 25 Feb. 2016. <http://www.ipni.net/publication/bettercrops.nsf/0/C2C2F44210F81C21852579800082017F/$FILE/Better%20Crops%201999-1%20p11.pdf>.
Rakow, G. "Species Origin and Economic Importance of Brassica." Biotechnology in Agriculture and Forestry 54 (2004): 1-7. Web. 25 Feb. 2016. <http://www.springer.com/cda/content/document/cda_downloaddocument/9783540202646-c1.pdf?SGWID=0-0-45-115127-p23861264>.
Wang, Xiaowu, and Chittaranjan Kole. The Brassica Rapa Genome. 4th ed. Springer, 2015. Web. 25 Feb. 2016. <https://books.google.co.ke/books?id=JE2GCgAAQBAJ&pg=PA142&lpg=PA142&dq=El-Soda,+et+al.,+2014&source=bl&ots=R5jXV2Pxuo&sig=sC6gRGEXcl3uxO6z0oIUXncXwt8&hl=en&sa=X&redir_esc=y#v=onepage&q=El-Soda%2C%20et%20al.%2C%202014&f=false>.
Wendell, D. L., and D. Pickard. "Teaching Human Genetics with Mustard: Rapid Cycling Brassica rapa (Fast Plants Type) as a Model for Human Genetics in the Classroom Laboratory." Cell Biology Education 6.2 (2007): 179-185. Web. 25 Feb. 2016. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1885900/>.
