The influence of light on the growth of watercress (Nasturtium officinale R. Br.)
In stream ecosystems, the growth of aquatic primary producers is affected by spatial and temporal variations in the riparian canopy, which can influence the availability of light resources. Aquatic plants can acclimate to low light environments by employing a suite of morphological or physiological mechanisms to increase light capture or photosynthetic efficiency. Some species may also use alternate types of propagules to colonize environments with heterogeneous light environments. In a greenhouse experiment we examined the morphological and physiological response of watercress (Nasturtium officinale R. Br.) to a gradient of increasing light levels, which ranged from 7% ambient light to full sunlight. We also determined if watercress seedlings and vegetative fragments differed in their growth response to increasing light levels. Total biomass and root biomass of seedlings and vegetative fragments decreased with decreasing light levels. The difference in plant biomass across treatments was due to morphological changes in total canopy area and leaf area, both of which increased with decreasing light levels. Seedlings and vegetative fragments did not differ in their response to light availability, but vegetative fragments had higher final biomass as a result of higher initial biomass. Physiological acclimation to low light levels appears to be of secondary importance for watercress as the concentrations of total chlorophyll, chlorophyll a, chlorophyll b, and chlorophyll a:b did not differ among light levels or between seedlings and vegetative fragments. Seedlings and vegetative fragments grown under high light levels had a greater percentage of carbon and a lower percentage of nitrogen than plants grown under low light conditions. The results of this study indicate that watercress displays considerable morphological plasticity and acclimates to low light conditions primarily by increasing leaf area and canopy surface area. There is no evidence that the type of watercress propagule (seedling vs. vegetative fragment) imparts any growth advantage in low light environments and watercress grown from either type of propagule showed no differences in their morphological or physiological responses to varying light regimes.
KeywordsAquatic macrophytes Watercress Light availability Morphological plasticity Propagule Physiological plasticity
We are grateful to Scott Cooper for advice on statistical analyses and for conversations that improved our interpretations. The manuscript was greatly improved by comments from three anonymous reviewers. Financial support for this work was provided by a mentoring grant from Susan and Bruce Worster awarded to BG and JS; the Santa Barbara Coastal LTER (grant no. OCE99-82105) provided additional funding.
- Agusti, S., S. Enriquez, H. Frost-Christensen, K. Sand-Jensen & C. M. Duarte, 1994. Light harvesting among photosynthetic organisms. Functional Ecology 8: 273–279.Google Scholar
- Bloom, A. J., F. S. Chapin & H. A. Mooney, 1985. Resource limitation in plants-an economic analogy. Annual Review of Ecology and Systematics 16: 363–392.Google Scholar
- Boyd, C. E., 1970. Chemical analyses of some vascular aquatic plants. Archives Hydrobiologia 67: 78–85.Google Scholar
- Cronin, G. & D. M. Lodge, 2003. Effects of light and nutrient availability on the growth, allocation, carbon/nitrogen balance, phenolic chemistry, and resistance to herbivory of two freshwater macrophytes. Oecologia 137: 32–41.Google Scholar
- Jeffrey, S. W. & G. F. Humphrey, 1975. New spectrophotometric equations for determining chlorophylls A, B, C1 and C2 in higher plants, algae, and natural phytoplankton. Biochemie und Physiologie der Pflanzen 167: 191–194.Google Scholar
- Mielecki, M. & E. Pieczynska, 2005. Influence of fragmentation on the growth of Elodea canadensis Michx. in different light conditions. Polish Journal of Ecology 53: 155–164.Google Scholar
- Oshima, K., Y. Tang & I. Washitani, 1997. Spatial and seasonal patterns of microsite light availability in a remnant fragment of deciduous riparian forest and their implications in the conservation of Arisaema heterophyllum, a threatened plant species. Journal of Plant Research 110: 321–327.CrossRefGoogle Scholar
- Sabater, S., H. Guasch, I. Muñoz & A. Romani, 2006. Hydrology, light and the use of organic and inorganic materials as structuring factors of biological communities in Mediterranean streams. Limnetica 25: 335–348.Google Scholar
- Spence, D. H. N., R. M. Campbell & J. Chrystal, 1972. Specific leaf areas and zonation of freshwater macrophytes. Journal of Ecology 61: 317–328.Google Scholar