Growth and nitrogen metabolism of Catasetum fimbriatum (orchidaceae) grown with different nitrogen sources

Catasetum fimbriatum is an epiphytic orchid from South America that has been used for 15 years as a model plant for metabolic and developmental studies in our laboratory. In this work, C. fimbriatum plants were aseptically grown with 6 mol m(-3) of either glutamine or inorganic nitrogen forms (NO3-:NH4+ ratios). The highest biomass accumulation was found in plants supplied with glutamine; no significant difference was observed in plants incubated in the presence of inorganic nitrogen sources. Nitrogen assimilation was limited in the presence NO3- as a sole nitrogen source. C. fimbriatum did not accumulate NO3- and very low rates of in vivo nitrate reductase activity were observed. Most nitrate reductase activity (70%) was detected in the 2 cm apical roots. Nitrate-treated plants exhibited relatively lower amounts of free amino-N, chlorophyll and free NH4+ contents and higher soluble sugar contents than the NH4+-treated plants. While shoot glutamine synthetase activity was only slightly affected by nitrogen sources, root glutamine synthetase activity was not modified by any nitrogen form. Glutamate dehydrogenase-NADH activity in shoot tissues was not influenced by any nitrogen source. However, the glutamate dehydrogenase-NADH activity in roots was enhanced when NH4+ tissue contents was augmented by increasing NH4+ in the medium and by the presence of glutamine. Our results strongly suggest that organic nitrogen and NH: are probably the most important nitrogen sources to C. fimbriatum plants. (C) 2000 Elsevier Science B.V. All rights reserved.