BackgroundThe taxonomy and systematics of Salix subgenus Salix s.l. is difficult. The reliability and evolutionary implications of two important morphological characters (number of stamens, and morphology of bud scales) used in subgeneric classification within Salix remain untested, and a disjunct Old–New World distribution pattern of a main clade of subgenus Salix s.l., revealed by a previous study, lacks a reasonable explanation. To study these questions, we conducted phylogenetic analyses based on 4,688 bp of sequence data from four plastid (rbcL, trnD–T, matK, and atpB–rbcL) and two nuclear markers (ETS and ITS) covering all subgenera of Salix, and all sections of subgenus Salix s.l.ResultsSubgenus Salix came out as para- or polyphyletic in both nrDNA and plastid trees. The plastid phylogeny successfully resolved relationships among the major clades of Salix, but resolution within subgenus Salix s.l. remained low. Nevertheless, three monophyletic groups were identifiable in subgenus Salix s.l.: the ‘main clade’ of subgenus Salix s.l., with New and Old World species being reciprocally monophyletic; the section Triandroides clade; and the subgenus Pleuradenia clade. While nrDNA regions showed higher resolution within subgenus Salix s.l., they failed to resolve subgeneric relationships. Extensive, statistically significant gene-tree incongruence was detected across nrDNA–plastid as well as nrDNA ETS–ITS phylogenies, suggesting reticulate evolution or hybridization within the group. The results were supported by network analyses. Ancestral-state reconstructions indicated that multiple stamens and free bud scales represent the plesiomorphic states within Salix, and that several significant shifts in stamen number and bud scale morphology have occurred.ConclusionsSubgenus Salix s.l. is not monophyletic, and the evolutionary history of the subgenus has involved multiple reticulation events that may mainly be due to hybridization. The delimitation of subgenus Salix s.l. should be redefined by excluding section Triandrae and subgenus Pleuradenia from it. The evolutionary lability of bud-scale morphology and stamen number means that these characters are unreliable bases for classification. The disjunct Old–New World distribution of subgenus Salix s.l. appears to be linked to the profound climatic cooling during the Tertiary, which cut off gene exchange between New and Old World lineages.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0311-7) contains supplementary material, which is available to authorized users.

A titania layer with ordered nanostructures is expected to be of high photocatalytic activity due mainly to its high specific surface area. In the present work, large-area films with ordered titania nanorods were deposited on titanium substrates through a solution approach. The nanorods, with the phase composition of a mixture of anatase and rutile, grew on top of a condensed anatase interlayer along mainly the rutile [001]-axis. The photocatalytic activity was evaluated by decomposing rhodamine B in water and compared with the general sol-gel derived titania films and a commercial DP-25 titania coating. It is found that the as-deposited titania nanorods exhibited extremely high initial photocatalytic activity but declined to a poor value after the consumption of beneficial oxidative peroxo complexes coordinated to Ti(IV). A subsequent thermal treatment eliminated such complexes but at the same time improved the crystallinity of the titania nanorods. The photocatalytic activity of the thermally treated titania nanorods was stable and significantly higher than that of the sol-gel derived film and commercial DP-25 coating.

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