Growth and chloroplast ultrastructure of two citrus rootstock seedlings in response 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 response to N availability under different production conditions. In this study, the effects of N sources (NH(4)-N or NO(3)-N), and rates (5, 15, 45, and 135 mg N L(-1)) on two citrus rootstock seedlings (Swingle citrumelo, SC; Cleopatra mandarin, CM) growth (110 d) and N concentrations in various parts of seedling 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 the growth of leaves, stems, and roots. The growth of all seedling parts of both rootstocks decreased with an increase in concentration of NH(4)-N in the solution. In contrast, there was a positive relationship between the seedling growth and the concentration of NO(3)-N in the nutrient solution with marked response observed as the NO(3)-N concentration increased from 5 to 15 mg L(-1) The experiment demonstrated a distinct growth suppression effect with an increase hi concentration of NH(4)-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 NN(4)(+) form, but the injury was not seen when the seedlings received N as the NO(3)-N form. The disruption of chloroplast ultrastructure increased with increased rate of NH(4)-N. The most conspicuous characteristic of ammonium toxicity was the massive accumulation of strands granules and phytoferritin which is clearly an evidence of NH(3) toxicity. The results are important for understanding the implications of N source on seedling growth and chloroplast structure of citrus leaves.