Document Type : Research Article
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Abstract
Abstract
In this study different proportions of NH4+ and NO3- in the growth medium on plant growth were investigated in hydroponic greenhouse experiments. The objectives of these studies were to investigate if ammonium (NH4+) stress in cucumber, a species sensitive to NH4+ toxicity, can be alleviated by matching the nitrogen supply rate to the plant nitrogen demand. Ammonium was applied at a relative addition rate of R=0.15 day-1 or R=0.25 day-1. For comparison, plants were also grown with NO3- as N source, as well as at constant concentrations of 1 and 4 mM NH4+. The fresh weight of plants grown with NH4+ as sole N source at R=0.15 day-1 was similar to that of NO3--fed plants, while at the high NH4+ supply rate of 0.25 day-1 a small growth reduction occurred. When available as a constant concentration, NH4+ decreased plant growth at 4 mM. Root:shoot ratio was lower in NH4+-fed plants compared to NO3--fed plants, and the NH4+-fed plants had more branched roots. Calcium concentration decreased in the roots, stems and leaves of NH4+-fed plants compared to NO3--fed plants. There were no differences in leaf chlorophyll fluorescence at the various N treatments. It is concluded that even a putative NH4+ sensitive species such as cucumber can tolerate NH4+ if the supply rate matches the N demand of the plants.
Keywords: Nitrogen, Ammonium toxicity, Nnitrate, Relative addition rate, Hydroponics, Cucumber
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