Condition of Young Japanese Knotweed (Reynoutria japonica Houtt.) Offshoots in Response to Microwave Radiation of Their Rhizomes

Invasive Japanese knotweed is one of Poland’s most aggressive and difficult-to-control species. New methods of control are of utmost importance. This study aimed to analyze young knotweed offshoots’ regeneration and physiological condition after treatment of their rhizomes with microwaves of high frequency. The fresh rhizomes were microwaved in the laboratory once for different times (from 5 to 60 s, interval 5 s). Next, in the greenhouse, the growth of offshoots and their biochemical state (spectrophotometry of leaves) were recorded in three measurement series. It was shown that the microwave treatment for 35 s directly destroys knotweed rhizomes of a diameter of 1–3 cm, so it does not produce new offsprings. The treatment times from 5–25 s delay offspring regeneration and growth and development of leaves. The leaves initiate the mechanisms of protection against microwave stress in offshoots grown from the rhizomes exposed to microwaves for 10 s (increase in the anthocyanin index) and the 5 or 20 s treatments (higher values of the photochemical reflectance index). Based on the effective dose (ED50) analysis, the threshold of harmful effects of microwaves on the growth and development of Reynoutria japonica rhizomes was assumed to be 17–19 s.