relative addition rate technique as a tool for alleviating ammonium toxicity in cucumber (cucumis sativus l., cv. styx) plants

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.