Unravelling roots of lianas: a case study in Sapindaceae

Background and Aims Roots are key in the evolution of plants, being in charge of critical functions, such as water and nutrient uptake and anchorage of the plant body. Stems of lianescent Sapindaceae conform to the anatomical patterns typical of climbing plants, having cambial variants in their stems and vessel dimorphism in their wood. The roots of these lianas, however, are largely unexplored, so we do not know whether the plant habit has as strong an impact on their anatomy as on the anatomy of their stems. Our aim was, therefore, to thoroughly explore the anatomy of liana roots, underground organs under selective pressure completely different from that experienced by the stems. Methods We studied mature roots of 14 species belonging to five of the six genera currently recognized in the lianoid tribe Paullinieae (Sapindaceae) using traditional methods for macro- and microscopic analyses, as well as micro-computed tomography (micro-CT) techniques. Key Results Roots were shown to be strongly shaped by the lianescent habit in Paullinieae, exhibiting traits of the lianescent vascular syndrome in terms of both wood and overall anatomy. The only way to distinguish root from stem in secondary growth is by the exarch protoxylem position in the roots, as opposed to the endarch position typical of the stems. The most conspicuous trait of the lianescent vascular syndrome, which is the presence of vessel dimorphism, is evident in all roots, and we hypothesize that it helps to create an efficient, safe pathway for water conduction from this organ towards the stems. Other anatomical features present were parenchyma bands, present in the wood of almost all of the analysed species, except for Thinouia and Urvillea, where parenchyma-like fibre bands alternating with ordinary fibres are present. The majority of the roots showed no cambial variants. However, lobed roots were found in Urvillea rufescens and phloem wedges were observed in Serjania lethalis and Serjania caracasana. Neo-formed peripheral vascular strands and cylinders were common in mature roots of Serjania caracasana, and vascular connections were found uniting the peripheral and central vascular cylinders through phloem wedges, as revealed by anatomical and micro-CT analyses. The vascular connections likely represent another key mechanism to create a network that increases the area of vascular tissue and contributes as an additional conduction pathway within these thick roots. Conclusions Some traits from the lianescent vascular syndrome, such as vessel dimorphism, are present in the roots of lianescent Sapindaceae, while others, such as cambial variants common in the stems, are largely absent.