UV-B and far-red light shape morphological and phytochemical responses in kratom

Kratom (Mitragyna speciosa) has gained increasing interest due to its unique leaf alkaloids with pharmacological potential. Comparisons of climate conditions in Florida production areas with those in the plant's native habitat in Southeast Asia indicate a need to better understand how light quality, particularly ultraviolet (UV)-B radiation, influences plant growth and alkaloid biosynthesis. This study evaluated the effects of far-red (FR) light supplementation and end-of-day (EOD) UV-B exposure on plant growth, photosynthetic performance, and leaf alkaloid content of kratom.

Plants were grown under controlled-environment conditions under five light treatments: white (W) light alone; W plus 1 or 2 h of EOD UV-B exposure; and W supplemented with FR light combined with 1 or 2 h of EOD UV-B exposure. Total photosynthetic photon flux density (PPFD) was maintained at 300 µmol·m^-2·s^-1, with additional supplemental FR and EOD UV-B provided at 90 and 0.3 µmol·m^-2·s^-1, respectively. Plant growth, photosynthesis, and alkaloid content were analyzed.

Low-dose EOD UV-B had minimal effects on plant architecture and leaf growth, but slightly reduced leaf biomass and photosynthetic capacity. Extending EOD UV-B exposure from 1 to 2 h reduced the stem caliper by 20%, branching by 14%, and new leaf area by 14%. Supplementation with FR at 30% of total PPFD induced a shade-avoidance response, increasing plant height by 58% to 76% while reducing leaf number, photosynthesis, and biomass. Corynantheidine concentration in leaves increased by 175% under W plus 1 h of EOD UV-B, whereas speciociliatine, mitraciliatine, and isopaynantheine decreased in aging leaves.

Light-quality-induced stress can modify alkaloid accumulation in kratom, and these responses must be carefully balanced against reductions in growth and biomass. These findings improve understanding of environmental regulation of kratom alkaloid biosynthesis and provide a basis for designing lighting strategies to optimize both biomass production and the accumulation of desirable alkaloids.