Influence of ammonium and nitrate nutrition on plant growth and zinc accumulation by Indian mustard

The source of nitrogen (N) used in soil fertility practices affects plant growth, nutrient absorption, and the availability of nutrients. Consequently, the potential of plants to extract zinc (Zn) from soils may be increased by controlling the ratio of NH4+ to NO3- to maximize growth and Zn accumulation. The objectives of this research were to determine the effects of Zn supply and different molar ratios of NH4+ to NO3- on growth and Zn accumulation in Indian mustard (Brassica juncea Czern.). In a factorial experiment with solution culture, Indian mustard (accession 182921) was supplied with two concentrations of Zn (0.05 and 4.0 mg L-1) in combination with six N treatments with different molar percentage ratios of NH4+ to NO3- (0:100, 10:90, 20:80, 30:70, 40:60, and 50:50) for three weeks. Zinc supplied at 0.05 mg Zn L-1 represented a common concentration of Zn in solution culture, whereas 4.0 mg Zn L-1 was excessive for plant nutrition. If the supply of Zn in solution was excessive, plants developed symptoms of foliar chlorosis, which became severe if plants were supplied with 80% of N as NO3-. Supplying high proportions of NO3- in the nutrient medium stimulated Zn accumulation, whereas increasing proportions of NH4+ (up to 50% of the total N) enhanced shoot growth. The pH of nutrient solutions generally decreased with increasing proportion of NH4+ in solutions and with increased Zn supply. The Zn phytoextraction potential of Indian mustard was maximized, at about 15 mg Zn plant(-1), if plants received 10% of the total N as NH4+ and 90% as NO3-.