EFFECT OF NH4+/NO3- RATIOS ON GROWTH AND NITROGEN UPTAKE BY ONIONS

The modelling of ion uptake by plants requires the measurement of kinetic and growth parameters under specific conditions. The objective of this study was to evaluate the effect of nine NH4+:NO3- ratios an onions (Allium cepa L.). Twenty-eight to 84 day-old onion plants were treated with NH4+:NO3- ratios ranging from 0 to 100% of each ionic species in one mM solutions in a growth chamber. Maximum N influx (I-max) was assessed using the N depletion method. Except at an early stage, ionic species did not influence significantly I-max the Michaelis constant (K-m) and the minimum concentration for net uptake (C-min). I-max for ammonium decreased from 101 to 59 pmole cm(-2)s(-1) while I-max for nitrate increased from 26 to 54 pmole cm(-2) s(-1) as the plant matured. On average, K-m and C-min values were 14.29 mu M, and 5.06 mu M for ammonium, and 11.90 mu M and 4.54 mu M for nitrate, respectively. In general, the effect of NH4+:NO3- ratios on root weight, shoot weight and total weight depended on plant age. At an early stage, maximum plant growth and N uptake were obtained with ammonium as the sole source of N. At later stages, maximum plant growth and N uptake were obtained as the proportion of nitrate increased in the nutrient solution. The was no apparent nutrient deficiency whatever NH4+:NO3- ratio was applied, although ammonium reduced the uptake of cations and increased the uptake of phosphorus.