The nitrogen absorption by corn occurs differently in different stages of this plant development, studies on the best periods of application and the forms of the nutrient uptake by the plant are extremely important for better management of this culture, and understanding about the influence of physiological characteristics with the absorption of this nutrient may contribute to a better use of nutrients in corn. Thus, the objective of this study is to assess the physiological responses of maize, depending on the time of urea application in topdressing, and its relation to the absorption capacity, use of fertilizer and seed yield. Two experiments will be developed under field conditions in the experimental area which belongs to CENA / USP. In the first experiment, the corn physiological characteristics will be evaluated according to the period of urea fertilizer application in topdressing, being related to crop yield and fertilizer. In the second experiment the physiological characteristics related to the leaf absorption of volatilized ammonia will be evaluated at each time of urea fertilizer application. In both experiments, the nitrogen application treatment in topdressing will be held at different periods of the study, which will be the phenological stages V4, V6, V8, V10 and V12. The physiological characteristics to be evaluated in the experiments are: net photosynthetic rate, substomatal CO2 concentration, atmospheric CO2 concentration, stomatal conductance, transpiration and flux density of active photosynthetic radiation. The measurement of these variables will be held in fully developed leaf of each plant through a plant gas exchange analyzer (IRGA, mod. Lci-DC, ADC, Hoddesdon, UK), whose readings will be taken between 9 and 11 a. m.. Along with these assessments, the content will also be determined by indirect chlorophyll index (SPAD). Evaluations will be performed from the time the plants reach the development stage according to the following form: V4 + Control, V4 + V6 + Control, V4 + V6 + V8 + Control, V4 + V6 + V8 + V10 + Control, V4 + V6 + V8 + V10 + V12 + Control. The two experiments will be repeated twice in each season of production in 2011/2012 and 2012/2013 seasons. It is expected that in this study the physiological variables of the corn crop related to nitrogen uptake via roots and leaves are found at different stages of this culture development.
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