Repurposing Wastewater from the Cigarette Butt Recycling Process as a Growth Stimulant for Brachiaria ruziziensis Germain & Evrard In Vitro

In vitro methods rely on costly chemical inputs, such as synthetic nutrients, prompting us to search for sustainable alternatives. This study evaluated wastewater generated from the cigarette butt (CB) recycling process as a potential growth stimulant additive for in vitro plant cultivation. Seeds of Brachiaria ruziziensis Germain & Evrard were sown on agar media containing increasing CB wastewater concentrations from 0 to 25% v/v (CB0 to CB25, respectively) under controlled conditions. Germination was monitored over 10 days, and functional and physiological traits of shoot and root systems were assessed at the end. Responses were concentration-dependent and consistent with hormesis. Low concentrations, particularly CB2, enhanced germination (92.2% vs. ~67% in CB0), shoot elongation (~6 vs. 3.4 cm), and total biomass (~47 vs. ~33 mg fresh weight), while maintaining total chlorophyll and increasing carotenoids (147.8 vs. 103.3 µg g−1 FW) and chlorophyll a/b ratio (2.1 vs. 1.5). Contrarily, higher concentrations (≥CB10) reduced germination (47.6% at CB25), strongly inhibited root growth (0.5 cm at CB25), decreased total biomass (~19 mg at CB25), led to growth disorders, and reduced pigment stability. These inhibitory effects were associated with the accumulation of CB-derived compounds, including high nicotine levels and unbalanced nutrients. At low concentrations, coordinated root aerenchyma formation and modulation of stomatal density indicated anatomical plasticity under mild stress conditions, although their physiological significance remains to be clarified. Overall, CB recycling-derived wastewater can act as an effective growth stimulant for B. ruziziensis in vitro when applied at low concentrations, offering a potential alternative for plant biotechnology while contributing to waste valorization.