Nitrogen and Use by Plants

Plants utilize N (nitrate and ammonium) in a series of steps including uptake, assimilation, translocation and towards the end of the plants life cycle in recycling and remobilization
7
(Masclaux-Daubresse et al. 2009). Understanding this process provides insights into why [N] might vary from one plant structure to another, for example, a large shoot:root ratio might suggest greater assimilation of N to the above ground biomass than the below ground biomass. Sources of N can vary greatly due to environmental factors such as, precipitation, wind, soil type and pH and temperature. Within the roots of plants (the site of N uptake), two nitrate transport systems have been identified which operate together to draw up nitrate from soil solution and disperse it throughout the plant (Daniel-Vedele et al. 1998; Tsay et al. 2007). N assimilation involves the reduction of nitrate to ammonium and ammonium assimilation to amino acids. Nitrate reduction occurs in the cytosol of both shoots and roots and is catalysed by the enzyme, nitrate reductase, yielding nitrite (Meyer and Stitt, 2001). Nitrite is transferred to the chloroplast where it is reduced by the enzyme, nitrite reductase to ammonium (Masclaux-Daubresse et al. 2009). Ammonium is then assimilated in the chloroplast producing glutamate which is utilized in amino acid metabolism (Masclaux-Daubresse et al. 2009). N assimilation is focused on the nutrient demand of the whole plant and this is affected by external stimuli or stresses, as well as nutrient status (Masclaux-Daubresse et al. 2009). N remobilization is sensitive to natural variation – Walch-Liu and Forde (2008) demonstrated this in the plasticity of root development in Arabidopsis and Masclaux-Daubresse et al. (2009) showed an increase in root growth (up to 90% in some individuals) due to a localized supply of nitrates and suggested that such growth may have negative consequences for N uptake; if N is heavily utilized in root growth, then maybe other plant structures are experiencing N deficits. The allocation of N is another important factor determining crop growth and therefore yields.
N is a fundamental nutrient in a plants diet – it is a significant component in chlorophyll (the compound vital for photosynthesis), a primary constituent of amino acids (the building blocks for proteins), energy transfer compounds, such as ATP (adenosine triphosphate) require N to store and release energy from metabolism and is a key element in nucleic acids such as DNA, enabling plants to grow and reproduce (Cropnutrition, 2013).Therefore, N deficiency will swiftly inhibit growth. This is evident from simple observations on N-deficient plants, for example, chlorosis, a yellowing of the leaves (Taiz and Zeiger, 2010). Leaves experiencing severe N deficiency may become fully yellow (reducing photosynthesis due to a reduction in chlorophyll number) and fall off the plant. In younger leaves, such symptoms may not appear immediately as N can be mobilized from older leaves. Therefore, plants experiencing N starvation may have light green upper leaves and yellow or tan lower leaves (Taiz and Zeiger, 2010). As a plant
8
experiences a decline in N in their diet, their stems may become notably slender and woody also. This is a result of an accumulation of excess carbohydrates that cannot be employed in the synthesis of N containing compounds, such as amino acids (Taiz and Zeiger, 2010). These carbohydrates can be used up in anthocyanin synthesis. A build-up of this pigment results in a purple coloration on the stems, petioles and leaves of the plant (Taiz and Zeiger, 2010). Therefore, while it is clear that N is significant to plant nutrition, it is obvious to the naked eye when concentrations stray from the optimal requirement. Investigations into plant growth across a range of nitrogen concentrations will be able to identify an optimal concentration for maximum growth.

Tags: