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 (NO(3)(-):NH(4)(+) 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 NO(3)(-) as a sole nitrogen source. C. fimbriatum did not accumulate NO(3)(-) 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 NH(4)(+) contents and higher soluble sugar contents than the NH(4)(+)-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 NH(4)(+) tissue contents was augmented by increasing NH(4)(+) in the medium and by the presence of glutamine. Our results strongly suggest that organic nitrogen and NH(4)(+) are probably the most important nitrogen sources to C. fimbriatum plants.     

Direct root tip conversion ofCatasetum into protocorm-like bodies. Effects of auxin and cytokinin

Root apex conversion ofCatasetum fimbriatum into protocorm-like bodies (PLBs) can occur in the absence of any added plant growth regulator. The presence of exogenous auxins in media drastically reduced the number of PLBs formed; on the other hand the concentrations of these auxins used greatly increased the process of callus formation. No effect on the mean number of root tip conversions into PLBs was observed with chlorogenic acid. However, this process was significantly increased in one of the concentrations used of p-coumaric acid. BA did not have any effect on callus formation, but caused marked acceleration in the process of root tip conversion and on the mean number of PLBs formed. PLB formation observed in the absence of any exogenous growth substance seemed to reflect a disruption in the interactions between the excised roots and the rest of the plants. The presence of light decreased the process of conversion.     

Propagation,growth, and carbohydrates of <Emphasis Type="Italic">Dendrobium</Emphasis> Second Love (Orchidaceae) <Emphasis Type="Italic">in vitro</Emphasis> as affected by sucrose,light, and dark

In general, plant material grown in vitro has low photosynthetic ability to achieve positive carbon balances. Therefore, a continuous supply of carbohydrates from the culture medium is required, and sucrose has been the most commonly used carbon source. In this paper, we investigate the effects of different sucrose concentrations and the presence and absence of light on the endogenous levels of soluble carbohydrates and starch as well as on the proliferation and growth of Dendrobium Second Love (Orchidaceae) in vitro. The possibility of using etiolated stem segments as a means for micropropagating this hybrid was also verified. The results obtained indicated that the presence and absence of light and the sucrose concentrations used influenced the amounts of soluble carbohydrates and starch and the proliferation of D. Second Love shoots and roots. An increase in sucrose concentration caused a progressive increase in the amounts of total carbohydrates and starch. Under both light conditions, sucrose was the main sugar found in the shoots followed by glucose and fructose. The addition of sucrose to the culture medium up to 2% and 4% was advantageous to the number of shoots produced per explant and the root longitudinal growth in the presence and absence of light, respectively. Shoot and root dry matter and the number of roots formed per explant increased as sucrose concentration was raised up to 6% in both light treatments. The use of dark-grown shoot segments proved to be a useful and reliable alternative for the micropropagation of this hybrid.     

In vitro flowering of Dendrobium

Dendrobium plants are the most frequently used orchids to study the process of in vitro flowering. Among the various environmental cues and plant growth regulators employed thus far, cytokinins have shown the most prominent effects on the transition from vegetative to reproductive stage. A clear correlation between flowering establishment and enhancement of endogenous cytokinins has been observed. However, the experimental utilization of a large amount of different Dendrobium species and hybrids, combined with a large diversity of variable media types and media compositions make interpretation of flowering control in the in vitro environment difficult. The combination of a specific model Dendrobium in vitro regeneration system or protocol with a simple, but well defined culture medium, could contributed, to some degree, to reaching a clearer consensus about the physiological and molecular mechanisms controlling the Dendrobium flowering phase change.