Light photon flux density affects ethanol-mediated drought avoidance in cassava (Manihot esculenta Crantz)

Cassava is a globally important food source. Given the increasing frequency of climate change-induced drought, enhancing the drought resilience of cassava is paramount. Chemical priming can bolster tolerance to stress factors. We previously determined that pretreatment with low concentrations of ethanol enhances abiotic stress tolerance in Arabidopsis, tomato, and cassava. Nevertheless, the efficacy of ethanol treatment in complex natural settings remains to be fully explored. In this study, we assessed the impact of ethanol treatment on cassava under varying light photon flux densities (PFDs) and drought conditions. We observed that drought tolerance was enhanced by ethanol pretreatment at high (∼400 µmol photons m^-2 s^-1) and medium (∼60 µmol photons m^-2 s^-1) light PFDs but not under low light PFD (∼4 µmol photons m^-2 s^-1). Ethanol pretreatment under high and medium light PFDs promoted stomatal closure and drought avoidance, thereby preserving higher water content in plant tissues. Furthermore, ethanol pretreatment under these PFDs upregulated expressions of genes associated with ABA signaling and heat shock proteins (HSPs) relative to water pretreatment. In addition, starch accumulation in leaves was observed under all light PFDs with ethanol pretreatment. We hypothesize that ethanol pretreatment at light PFDs exceeding 60 µmol photons m^-2 s^-1 facilitates ethanol-mediated drought avoidance in cassava by activating at least three pathways: 1) ABA signaling, 2) protein folding-related response via triggering of the HSP/chaperone network, and 3) alterations in sugar and starch metabolism. Our findings support the application of optimal light PFDs to enhance the benefits of ethanol-induced drought avoidance in cassava.