Effects of Triacontanol and Light on Stomatal and Photochemical Responses in Solanum lycopersicum L.

Triacontanol is a long-chain alcohol that is considered to be a plant growth promoter. Exogenous application of triacontanol increases dry and fresh weight, plant height, branching, stem and root length, yield, and other biochemical parameters; it also reduces the effect of different stressors. In this work, the effects of triacontanol on stomatal regulation and photochemical parameters in tomato plants exposed to three irradiance levels (100, 200, and 600 μmol m⁻² s⁻¹) were examined. Low and high irradiance levels caused a stress response in plants; photosystem II efficiency (ΦPSII) and photosynthetic-irradiance parameters were diminished. Triacontanol only reversed the negative effects of high irradiance, increasing photochemical response and photosynthetic-irradiance parameters. Moreover, a large population of low chloroplast number, chlorophyll content, and stomatal conductance were stimulated by triacontanol application. In relation to stomatal function, triacontanol had no effect on stomatal density; however, it affected stomatal size and morphology at the three irradiance levels evaluated. Bioassays in epidermal peels showed a direct effect on the stomatal aperture and an increase in aperture size in both light and dark conditions in triacontanol treatments. This is the first report of the role of triacontanol in the regulation of stomata and photochemical responses in relation to irradiance stress.