Carbon and nitrogen partitioning in the freshwater submerged macrophyte <Emphasis Type="Italic">Vallisneria gigantea</Emphasis> in response to ultraviolet-B irradiance

The aim of this study was to determine the effect of ultraviolet-B (UV-B) exposures (0.55 and 1.1 W m⁻²) on the distributional variations of plant carbon and nitrogen content in both below- and aboveground parts in Vallisneria gigantea Graebner in laboratory conditions for a 3-month period. Plant biomass, total organic nitrogen, total organic carbon, lignin, water soluble carbohydrates and chlorophyll a and b were analysed and compared using repeated measures of analysis of variance (ANOVA) between UV-B-exposed and nonexposed treatments. A significant reduction (F _{(2, 15)} = 754.5, P < 0.001) was observed in leaf chlorophyll a concentrations at UV-B exposure levels. In the high UV-B irradiation group, a significant decrease (50.3%) was observed compared with the initial aboveground biomass. The total organic nitrogen content at both high and low UV-B exposure levels declined significantly by 25.6% and 24.3%, respectively, in aboveground samples, while significant increases of 39.6% and 40%, respectively, were observed in belowground tissues, compared with non-UV-B treatment groups. The partitioning of total organic carbon in the aboveground tissues was reflected by significant increase in lignin and water soluble carbohydrates in aboveground tissues under UV-B stress. However, total organic nitrogen demonstrated greater partitioning into the belowground tissues of V. gigantea. This study highlights the defense mechanisms of V. gigantea through changes in the percentage composition of carbon and nitrogen compounds with negative effects on nutrient regeneration, which can be accelerated in a system exposed to UV-B irradiation at or above biologically effective levels.