In you protocol, you make the H2DCFDA and DAF-2DA stock solution...

The two probes are soluble in organic solvents such as DMSO and will be stable for at least six months in these solvents if stored at -20C, but are sparingly soluble in aqueous buffers. For use, the stock solution can be immediately diluted with buffer to achieve the desired concentration or pH, but these working solutions are not stable and not recommend storing for more than one day. So, we use the two different solvent for stock solution and the working sdolution. To exclude the effect of DMSO on the stomatal aperture or the fluorescence intensities, we make control treatments involved the addition of DMSO at the same concentration in the working solution but not involved the fluorescent probe, the results showed that the DMSO at the concentration in working solution had no effect either on the stomatal aperture or the fluorescence.