Note Light as a factor affecting the concentration of simple organics in the traps of aquatic carnivorous Utricularia species

Abstract

Rootless aquatic carnivorous Utricularia plants exude significant amounts of photosynthates into the trap fluid, where they have been shown to support complex microbial commensal communities. Using ion chro- matography, the composition of four groups of easily metabolised, carbon-rich organic compounds (sugars, sugar alcohols, amino acids and organic acids) was investigated in trap fluid collected from three aquatic Utricularia spe- cies, U. vulgaris, U. reflexa and U. stygia, for different trap ages, irradiance levels during plant growth and for expo- sure to periods of darkness. The total sum of the concentrations of the four groups of organic compounds in the trap fluid ranged within 14 - 42 mg l -1 in greenhouse-grown U. vulgaris, compared to 9.0 -14 mg l -1 in U. reflexa . The concentrations of organic compounds were significantly higher in younger traps than in the older traps of U. vulgar- is grown at high irradiance. Within the same trap age categories in U. vulgaris, the group concentrations of sugars, organic acids, and total sums of analysed compounds were significantly higher in plants growing at high irradiance when compared with those grown in the shade. Dark exposure of cut traps for 1- 2 d significantly decreased the concentrations of sugars and organic acids in the fluid. The total sum of organic compounds in traps of U. stygia grown outdoors (78.3 ± 19.2 mg l -1 ) was much higher than that in 'middle aged' traps of U. vulgaris (49.2 ± 4.2 mg l -1 ), grown under the same conditions. It may be concluded that the concentrations of organic compounds in the trap fluid of aquatic Utricularia are species specific, subject to rapid turnover and depend significantly on various endogenous (trap age) or exogenous factors (water chemistry, irradiance).