Crescimento, características fotossintéticas e compostos nitrogenados em plantas jovens de Inga macrophylla fertilizadas com nitrato e amônio

Abstract

The carbon assimilation depends essentially on the ability of plants to capture and use the primary recourses available, of which stand out the form and amount of nitrogen. Legumes species, when in symbioses with soil bacteria, fix atmospheric nitrogen (N2) and endow plants with higher content of this nutrient. However, the availability of mineral nitrogen in soil as nitrate and ammonium can reduce the biological nitrogen fixation process and bring on changes on transport of nitrogen compounds by xylem. Thus, information related to the use of different nitrogen forms by species of leguminous are important to better understand the use of nitrogen, the effect on photosynthesis, and consequently the growth and biomass gain. The objective of this research was to investigate the growth response, carbon assimilation, the sensitivity of the biological nitrogen fixation process and possible modifications of transported compounds by xylem in young plants nodulated of Inga macrophylla and subjected to fertilization with different forms of mineral nitrogen (nitrate and ammonium) at a concentration of 5 mM. During 150 days of experiment and at regular intervals the following variables were determined: relative and absolute growth rate in height and diameter; leaf morphology; nodulation; accumulation and allocation of dry mass; foliar and root nutrient content; pigment chloroplastid contents; chlorophyll a fluorescence; gas exchange; nitrate, ammonium and ureide accumulation in leaf, root, nodules and sap and the transport of nitrogen compounds via the xylem. In general, ammonium and nitrate fertilization promoted the highest relative and absolute growth rates in height compared with other treatments, while the fertilization with ammonium resulted in higher accumulation of total dry mass and investment in total leaf area and average leaf area. Leaf nitrogen content did not differ between the treatments, indicating the nitrogen contribution from biological fixation in non-fertilized plants. Regarding the physiological aspect, the fertilization with nitrate proportionate higher investment in Chl a, Chl b, Cx+c, Chltotal. Difference was not observed between photosynthesis values, as well as to the variables Fv/Fm, PIABS e PItot. Although, plants fertilized with ammonium exhibited stomatal limitation . The fertilization with nitrate favored the nodulation, while the ammonium partially inhibited the nodulation. In all the treatments the ureides were the main form of N transport by xylem sap. The analysis of amino acid profile by xylem sap indicated the asparagine (ASN) as the most prominent amino acid. In this way, it concludes that plants of Inga macrophylla has the ability to use the mineral nitrogen both as ammonium as nitrate form at a concentration of 5 mM favoring growth, besides have strategies to conciliate use of mineral nitrogen and maintain the N2 fixation.