Fate of cyromazine applied in nutrient solution to a gerbera (Gerbera jamesonii) crop grown in a closed hydroponic system

Cyromazine was applied at two doses (375 and 750 mg l-1) via the nutrient solution in a winter crop of gerbera (Gerbera jamesonii) grown on pumice in a closed-loop system. The persistence of cyromazine in the system and its uptake and distribution within the plant were studied for a period of 121 days by regularly collecting and analysing samples of nutrient solution, drainage solution, substrate, roots, leaves, and flowers. The residues of cyromazine in leaf were further studied in a summer experiment which lasted 50 days. The concentration of cyromazine fluctuated within a relatively high range in both the drainage solution and the nutrient solution supplied to the plants during the first day after its application but declined progressively thereafter. Nevertheless, 37 days after application, mean concentrations of 6.2 and 10.9 mg l-1 cyromazine were found in the drainage solution at low and high doses, respectively. The corresponding cyromazine concentrations 121 days after its application were 0.02 and 0.03 mg l-1. Similar values were measured also in the nutrient solution. The adsorption of cyromazine by the pumice was lower than 0.02 mg kg-1. The half-life time of cyromazine in the closed system was 9.65 days. In the winter experiment, the highest cyromazine concentrations were determined 17 days after application in root and 56 days in leaf and flower, regardless of the dose, while on day 121 the leaf concentrations were 52 and 62 mg kg-1 fresh weight (f. wt) at the low and the high dose, respectively. In the summer experiment, the leaf concentration of cyromazine reached a maximum (232 mg kg-1 f. wt) 14 days after its application and decreased only slightly on day 24, but dropped rapidly to 66 mg kg-1 f. wt on day 50. The above results indicate that cyromazine applied via the nutrient solution may persist for more than 120 days in closed hydroponic systems, while its maximum concentration in the plant tissues occurs several days after application.