EFFECTS OF NO3-, NH4+ AND UREA NUTRITION ON ENDOGENOUS LEVELS OF IAA AND 4 CYTOKININS IN 2 EPIPHYTIC BROMELIADS

The long-term effects of different nitrogen sources on the endogenous IAA and cytokinin levels in two bromeliad species were investigated. I n nature, Vriesea philippocoburgii is a tank-forming epiphytic bromeli ad which uses the tank water reservoir as a substitute for soil, where as Tillandsia pohliana is a tankless atmospheric epiphytic species. A culture was established from seeds germinated in aseptic condictions, and the plantlets were grown for 6 months in a modified Knudson medium to which was added 8 mol m(-3) of nitrogen in the form of NO3-, NH4or urea. The hormonal contents of the bromeliad shoots were determined by means of high-performance liquid chromatography (HPLC), coupled to an enzyme-linked immunosorbent assay (ELISA) for indole-3-acetic acid (IAA), isopentenyladenine (iP), isopentenyladenosine ([9R]iP), zeatin (Z) and zeatin riboside ([9R]Z). Nitrogen supplied in the form of ure a gave the highest values of fresh and dry weights for both species, a nd this was positively correlated to IAA levels. The cytokinin pattern s showed that isopentenyladenosine was the predominant form for both s pecies in all samples. However, urea induced the highest level of this riboside form and also the highest level of total cytokinins for V. p hilippocoburgii, while NH4+ had the same effect on the atmospheric spe cies. These results are discussed in terms of the different growth hab its of these two species in nature. It is suggested that urea may be a n important source of nitrogen often found inside the tank of V. phili ppocoburgii. NO3- treatment increased the IAA/Cks balance, mainly for V. philippocoburgii, while urea and NH4+ shifted this ratio in favour of cytokinins, thus apparently inhibiting root development in both spe cies.