The Effect of Root-zone Nitrogen concentration on Corn

Nitrogen (N) is a major enzyme constituent making it a determining factor in plant yield and health. When a plant is deficient in N the shoot:root ratio typically decreases, chlorosis occurs, and growth is reduced. The objective of this study was to determine the level of nitrogen needed for optimal growth of corn. Plants were grown in containers and watered with 0, 0.3, 1.0, 3.0, and 10 mM N for 34 days. Chlorophyll content, fresh, and dry mass measurements were then taken. Chlorophyll content increased with increasing N. Fresh and dry weight also increased rapidly up to three mM N and increased gradually up to 10 mM N. The 3 mM treatment supplied enough N to nearly optimize growth. The chlorophyll produced above three mM N did not significantly increase growth. Introduction Nitrogen is the fourth most abundant element in plants and composes 1-5% of total plant dry matter (Hawksford et al., 2012). N is a major part of amino acids, proteins, enzymes, nucleic acids, cell walls, chlorophyll, phytohormones, and secondary metabolites (Hawksford et al., 2012; Schrader, 1984). Nitrogen is involved in both structural components and metabolic reactions making it a large factor in determining a plants yield and health. If too much N is added to the soil the excess N can be leached out of the rooting zone and contaminate ground water. When a plant has a sufficient amount of N most of its resources will be moved to the shoot. When N is deficient the plant will move more of its resources to the roots in order to explore more of the soil environment to find the N needed. This lowers the shoot to root biomass ratio (Anandacoomaraswamy, 2002). When the plant is not able to resupply itself with the needed N through increased root production it will start to hydrolyze nucleic acids, proteins, chlorophyll, and other N containing compounds leading to leaf senescence (Hortensteiner and Feller, 2002). Since chlorophyll contains N, the plant will also begin to be chlorotic. Plants naturally breakdown proteins and move them from the older plant parts to the younger ones, so chlorosis will usually appear first on the older leaves and then move to the younger leaves (Schrader, 1984). Around 75% of the N in mesophyll cells is located in the chloroplasts where that N is used mainly in the enzyme Rubisco, which plays a large role in photosynthesis (Peoples and Dalling, 1988). When Rubisco is broken down, the plants ability to photosynthesize decreases which inhibits growth (Hawksford et al., 2012). For these reasons chlorosis and stunting are the two main visual symptoms of N deficiency. Since N is largely found in chlorophyll, a chlorophyll meter can be used to estimate the amount of N in the plant. A chlorophyll meter measures the amount of N in the leaf by comparing 650 nm wavelenghts that relate to chlorophyll activity and 940 nm wavelengths that calibrate the instrument by helping it compensate for leaf thickness and water status (Hawkins et al., 2007; Rambo, 2010). Studies by Girardin et al. (1985), Wolfe et al. (1988), Lohry (1989),