Evaluating nitrogen use by two co-occurring vascular epiphytes that differ in mode of photosynthesis

Background/Question/Methods

Nitrogen (N) availability and use are of central importance to plant success in natural ecosystems. Due to microbial processes and N cycling and conversion in soils, the main source of N for terrestrial plants is typically NO₃^-. Vascular epiphytes, however, with their unique position in forest canopies, are exposed to different sources of mineral nutrition, and must be adapted to optimize uptake of nitrogen that is available. Our study investigated N uptake by two species of vascular epiphytes, Anthurium schlectendalii, a C₃ plant, and Aechmea bracteata, a CAM plant. For one month prior to the nitrogen treatments, the plants were given only deionized water. Subsequently, we used a simple factorial design to investigate effects of N treatment on initial and relative growth rates. Plants were supplied with either nitrate [Ca(NO₃)₂] or ammonium [NH₄Cl] as their source of N. Plants were first grown under low light conditions, and halfway through the experiment they were moved to high light conditions. Relative growth rates were measured for both treatments and both light conditions. Leaf chlorophyll content was evaluated along with leaf protein and nitrogen concentrations.

Results/Conclusions

The CAM epiphyte, Aechmea bractata, had a significantly greater response when grown with ammonium versus nitrate. Plants grown with NH₄⁺ were relatively greener (50 ± 5, relative units) than plants grown with NO₃^- (8 ± 2, relative units). Aechmea also had a significantly higher growth rate when grown with NH₄⁺ than NO₃^- (20 ± 2 cm/yr vs. 10 ± 1 cm/yr, respectively). In contrast, the C₃ epiphyte, Anthurium schlectendalii, did not differ in greenness between the two treatments. Mean growth rate (cm/yr) of Anthurium was significantly greater in high light conditions than low light conditions when grown with both NH₄⁺ and NO₃^-. Anthropogenic influences (e.g. nitrogen fertilization and burning of fossil fuels) are affecting N cycling in ecosystems worldwide. An understanding of the basic physiological requirements of these plants within an ecological framework will be an important component in implementation of effective restoration and conservation strategies.