Electron transfer from excited dye molecules (chlorophyll or fluorescein) to a semiconductor is demonstrated by photoaction and photoluminescence spectra on field-effect transistors consisting of dye-sensitized individual SnO2 nanowires. The photoaction spectrum shows a much better resolution for nanowires non-covalently functionalized with dye molecules than for dyes deposited on SnO2 nanoparticle-films. Possible reasons for the deviation between the photoaction spectra and ordinary optical absorption spectra as well as for the current-tail appearing along the falling edge are addressed. In dye-sensitized nanowires, electron transfer from photo-excited dyes to nanowires is analyzed by comparing gate-voltage dependences in photoaction and photoluminescence spectra. The importance of this study is in the understanding of electron injection and recombination provided, as well as the performance optimization of nanowire-based dye-sensitized solar cells.