Growth and chloroplast ultrastructure of two citrus rootstock seedlings inresponse to ammonium and nitrate nutrition

It is important to understand the differential response of citrus rootstock to various rates of nitrogen (N) forms in order to evaluate the tree respo nse to N availability under different production conditions. In this study, the effects of N sources (NH4-N or NO3-N), and rates (5, 15, 45, and 135 m g N L-1) on two citrus rootstock seedlings (Swingle citrumelo, SC; Cleopatr a mandarin, CM) growth (110 d) and N concentrations in various parts of see dling were investigated in a nutrient solution experiment. The effects of N nutrition on the chloroplast ultrastructure of leaves were examined at the end of the experiment. Rootstock and N sources significantly influenced th e growth of leaves, stems, and roots. The growth of all seedling parts of b oth rootstocks decreased with an increase in concentration of NH4-N in the solution. In contrast, there was a positive relationship between the seedli ng growth and the concentration of NO3-N in the nutrient solution with mark ed response observed as the NO3-N concentration increased from 5 to 15 mg L -1 The experiment demonstrated a distinct growth suppression effect with an increase hi concentration of NH4-N in the nutrient solution, particularly SC rootstock. Chloroplast ultra-structure of the leaves showed evidence of injury of the seedlings which received N entirely as NN4+ form, but the inj ury was not seen when the seedlings received N as the NO3-N form. The disru ption of chloroplast ultrastructure increased with increased rate of NH4-N. The most conspicuous characteristic of ammonium toxicity was the massive a ccumulation of strands granules and phytoferritin which is clearly an evide nce of NH3 toxicity. The results are important for understanding the implic ations of N source on seedling growth and chloroplast structure of citrus l eaves.