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Molecular gas constitutes the dominant mass component of protoplanetarydiscs. To date, these sources have not been studied comprehensively at thelongest far-infrared and shortest submillimetre wavelengths. This paperpresents Herschel SPIRE FTS spectroscopic observations toward 18 protoplanetarydiscs, covering the entire 450-1540 GHz (666-195 $\mu$m) range at R~400-1300.The spectra reveal clear detections of the dust continuum and, in six targets,a significant amount of spectral line emission primarily attributable to$^{12}$CO rotational lines. Other targets exhibit little to no detectablespectral lines. Low signal-to-noise detections also include signatures from$^{13}$CO, [CI] and HCN. For completeness, we present upper limits ofnon-detected lines in all targets, including low-energy transitions of H2O andCH$^+$ molecules. The ten $^{12}$CO lines that fall within the SPIRE FTS bandstrace energy levels of ~50-500 K. Combined with lower and higher energy linesfrom the literature, we compare the CO rotational line energy distribution withdetailed physical-chemical models, for sources where these are available andpublished. Our 13CO line detections in the disc around Herbig Be star HD 100546exceed, by factors of ~10-30, the values predicted by a model that matches awealth of other observational constraints, including the SPIRE $^{12}$COladder. To explain the observed $^{12}$CO/$^{13}$CO ratio, it may be necessaryto consider the combined effects of optical depth and isotope selective(photo)chemical processes. Considering the full sample of 18 objects, we findthat the strongest line emission is observed in discs around Herbig Ae/Bestars, although not all show line emission. In addition, two of the six T Tauriobjects exhibit detectable $^{12}$CO lines in the SPIRE range.

Signatures of warm carbon monoxide in protoplanetary discs observed with Herschel SPIRE