Phosphorus and Nitrogen Limitation on Plants Growth
Introduction
Plant growth and development rely on multiple environmental factors that include light intensity, temperature, water availability and vital minerals and nutrients. During the insufficiency of exogenous resources, plants maintain this imbalance through allocating the new biomass to that organ which specifically needs that insufficient nutrient (Hermans et al., 2006). Nutrients like nitrogen (N) and phosphorus (P) play significant roles in a plant life. They influence various metabolic factors of plant life through cooperating with each other such as in biomass distribution as well as regulating chlorophyll A and B levels.
The objective of this experiment is to understand that how various levels of Nitrogen and Phosphorus influence the plant growth. This experiment will help in understanding the applicable quantity of required nutrients essential for the plant development as well as how it is linked to the current environment issues. For the experiment purpose, Brassica rapa is selected because it has several unique features including a specific and centralized root system, small genome size and a short life span.
Impact of N and P deficiency on Plants
Nitrogen (N) is one of the most crucial key minerals that are extremely necessary for any form of life on earth (Butterbach-Bahl and Gundersen, 2011). The deficiency of these essential nutrients resulted in a retarded growth of plants because multiple metabolic reactions in the pants are hindered. N and P deficiency in plants result in accretion of carbohydrate in leaves, accumulation of Carbon in roots and an elevated root to shoot biomass ratio.
The deficiency of those nutrients obstructs the primary photosynthesis, glucose metabolism and carbohydrate distribution between the source and sink tissues. Moreover, Hermans and co-researchers have hypothesized that N and P deficiencies can change the carbohydrate metabolism in shoots that may result in elevated root to shoot biomass ratio changing the root morphology (Boussadia et al., 2010; Hermans et al., 2006). N deficit accumulates the excessive starch and sugar in the leaves. Nitrate extent on the leaves is negatively correlated with the carbon extent in the root. A detailed analysis of Arabidopsis microarray data exhibits that N scarcity induces several transcriptional alterations that may interfere with the metabolic pathways resulting in the sugar and starch accumulation in shoots with enhanced transportation of sugar towards the root. That is the main cause of increased R:S ratio. Additionally, it is stated that these changes may also be triggered by the accumulated sugar in leaves (Hermans et al., 2006).
Nature of N in the presence of P
Nitrogen is present in the plants in high amounts with other nutrient including Phosphorus (P), Potassium (K), Sodium (Na), Magnesium (Mg), and Calcium (Ca). In a tranquil atmosphere, N is treated as growth restraining nutrient due to its comparative lower availability (Menge et al., 2012). Evidence has verified this nature of N via analyzing its growth limiting impact of the growth and reproduction process of the numerous photosynthetic biota on earth’s ecosystems. Consequently, it may also impede the optimal performance, and other critical function that is necessary for survival, this phenomenon is termed as Nitrogen Limitation (Butterbach-Bahl and Gundersen, 2012; Elser et al., 2007).
Research by Naidoo (2009) showed that Avicennia marina mangroves plants enriched with P exhibit normal growth while in an N-limiting environment of marshes, N enrichment resulted in the resource allocation from roots towards shoots that yielded enhanced growth and productivity in the plants (Naidoo, 2009).
N in natural environment
The nitrogen limitation is caused due to the absolute shortage of N in the soil and the presence of N in an environment in a form which can not be accessible to the plant. Several atmospheric determinants play significant roles causing unavailability of N to the plants such as incorrect pH, water scarcity or pitiable root physiology. Now the alterations occurring in the environment due to biomass accumulation and excessive use of fertilizers have increased the quantity of accessible N to the plants (Menge, et al., 2012; Smith, 2007). Moreover, N cycle involves most of the microbial activities that include such as nitrification, denitrification, and anaerobic ammonium oxidation reactions (Vitousek et al., 2002; Doering et al., 1995).
Positive nutrient interactions in N and P
Though N is the most essential nutrient responsible for various plant metabolic activities it needs other minerals to work with such as P (Doering et al., 1995). The prominent quantities of two or more nutrients result in a positive interaction delivering the enhanced combined effects of those nutrients ("Phosphorous Interactions with Other Nutrients" 11). This interactive phenomenon facilitates balance nutrition to the plant impacting its growth, reproduction and other vital processes such as N with P interact positively (De Groot et al., 2003). Thus, the N limitation results in the deficiency of P through a depletion-driven limitation. On the other hand, the enriched N supply relative to P can result in the deficiency of P, and this occurs through either triggering P consumption in the atmosphere or by diminishing the Phosphorous-reserves. It is seen that use of N-enriched fertilizers increases the retrieval of P by the plants because N augments the P solubility in soil water as well as its P propinquity to the plant roots system. It is suggested that the N:P ratio is the effective indicator of nutrient limitation in upland ecosystems (Tessier and Raynal, 2003; "Phosphorous Interactions with Other Nutrients" 11).
P limitation occurs when P can not sufficiently fulfill its functions in plants such as healthy growth and reproduction (De Groot et al., 2003). The two main reasons for P limitation are its low profusion in the soil that inhibits its uptake from plants and easy adsorption by soil minerals (Elser et al., 2007; Conley et al., 2009; CropNutrition.com)
The literature has verified the effects of N and P multiple times when used together in comparison of their individual impacts on plants (Ågren et al., 2012). Thus, it is concluded that N limitation can also impact the P limitation by affecting its biochemical characteristics or upgrading its proximity to the plants (Ågren et al., 2012).
Why Brassica rapa as a chosen subject?
Brassica rapa species is selected for this particular experiment to analyze the mutual phenomenon of N and P limitation because Brassica rapa has specific root system with epidermal root hairs. This root physiology helps in nutrient absorption. Other specifications linked to Brassica rapa are its small size genome that is appropriate for analysis (Wang and Kole, 2015). Its genome is available along with all the required analytical tools. Moreover, it has a short lifespan of 35-40 days with a fast rate of development that will facilitate the evaluative analysis of experiment (Wendell and Pickard, 2007; Rakow, 2004).
The experiment helped in understanding the mutual independence and positive interaction between two nutrients, P and N concluding that N limitation influences the P limitation by either altering its biochemical distinctiveness or by making it more accessible to plant roots. In the presence of N, the plant used P more efficiently.
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