Effects of urea on growth and photosynthetic metabolism of two aquatic plants (Cabomba caroliniana A. Gray and Elodea nuttallii (Planch.) H. St. John)

Abstract

Increasing urea import into freshwaters is known to have a negative impact on aquatic ecosystems, however, few data exist about the effects of urea on the physiology and ecology of aquatic plants. In the present study, two submersed aquatic plants, Cabomba caroliniana A. Gray and Elodea nuttallii (Planch.) H. St. John were cultured with urea-enriched solution for 16days and the effects of urea on the two species’ physiology were determined on day 6 and day 16, respectively. The growth and photosynthetic metabolism of the two species were promoted after 16days of treatment with 1 and 5mg urea-NL−1, as indicated by the increase of relative growth rates (RGR), chlorophyll synthesis and elevated rates of net photosynthesis (PN). However, 10mg urea-N L−1 markedly inhibited the growth of C. caroliniana and E. nuttallii by 14 and 28%, respectively. Moreover, obvious decline of photosystem II activity and photosynthetic rate in E. nuttallii were found after 6days of 10mg urea-N L−1 treatment, which appeared earlier than those in C. caroliniana on day 16. All the urea treatments did not cause any damage to photosystem I in the two species. In addition, malondialdehyde (MDA) significantly increased while soluble carbohydrates (SC) markedly decreased in 10mg urea-N L−1 treated two species. Moreover, the decrease of SC in C. caroliniana was not induced to the extent of that in E. nuttallii. In conclusion, high urea concentration in water column may hamper the growth of aquatic plants. Furthermore, E. nuttallii may be more sensitive and vulnerable to urea than C. caroliniana when they inhabit the eutrophic water with high urea availability.