Root morphometry and tolerance of Canna indica in floating treatment wetlands for Cr(III) and Cr(VI) removal

MUFARREGE, M.M.; DI LUCA, G.A.; HADAD, H.R.; MAINE, M A.; NOCETTI, E.; CAMPAGNOLI, M.A.

Brugges

Simposio; Symposium on Wetland Pollutant Dynamics and Control, WETPOL.; 2023

Floating treatment wetlands (FTWs) consist of a buoyant artificial medium, which facilitates rootdevelopment of plants in the water column. Macrophytes are an essential component of FTWs. Plants possess mechanisms to modulate their element acquisition in response to their environment availability and demand during their life cycle It is of vital importance to know in depth about their tolerance and adaptability to survive in an environment different from their natural habitat. This work aimed to evaluate the changes in root morphometry and tolerance of Canna indica plants and the implication in the removal efficiency of Cr(III) and Cr(VI) in a FTW.Plants were collected from an unpolluted pond near Santa Fe City, Argentina. The plants were pruned for their transport to the greenhouse. Plastic reactors (70 L) containing 4 Kg of sediment and 38 L of tap water were installed outdoors under a semi-transparent plastic roof. FTWs consist of a plastic net and have a surface area of 0.10 m2. Buoyancy was provided by a PVC frame. Each raft had a total of 4 plants. The rafts were designed to allow roots (hanging in the water column) and rhizomes to remain in the water while aerial parts emerge. After plant acclimatization (about 30 days), fifteen reactors were treated with Cr(III) or Cr(VI) solutions and three were used as biological controls (CB: with FTW, without the addition of experimental solution). Treatments were 5 and 10 mg L-1 of Cr(III), and 5 and 10 mg L-1 of Cr(VI). Also, a biological control (BC) with FTW without Cr addition was used. Water was sampled periodically. The experiment lasted 35 days. Cr concentrations in leaves, roots, rhizomes, and sediment were determined at the end of the experiment. Chlorophyll a concentration was determined at the beginning and the endof the experiment. For the study of the internal morphology of roots; segments close to the base of the roots were extracted. The cross-sectional areas of roots, steel, and metaxylematic vessels were measured, and the number of vessels was counted. Both Cr species were efficiently removed from water in all treatments. The roots were the main accumulator organ of this metal and there was translocation to the aerial parts of the plants, being significantly higher in the case of Cr(VI) 10 ppm. A decrease in biomass and chlorophyll concentration was determined in Cr(VI) 10 ppm due to the translocation of this metal. In the Cr(VI) 5 and 10 ppm treatments,the plants showed symptoms of chlorosis. In the histological sections analyzed in the different treatments, significant changes were observed in the morphology of the roots with respect to the BC. The crosssectional areas of roots were significantly higher in 5 and 10 mg L-1 Cr(III) treatments. Cr(VI) treatments presented the largest vessel area and the lowest vessel number. Plasticity morphology is an important mechanism for the plant to tolerate harmful conditions. Root modifications allowed C. indica to tolerate the conditions to which it was exposed, without affecting the removal efficiency of the two added Cr species.