Phloem structure and development in Illicium parviflorum, a basal angiosperm shrub

Recent studies in canopy-dominant trees revealed a structure-function scaling of the phloem. However, whether axial scaling is conserved in woody plants of the understory, the environments of most basal-grade angiosperms, remains mysterious. We used seedlings and adult plants of the shrub Illicium parviflorum to explore the anatomy and physiology of the phloem in their aerial parts, and possible changes through ontogeny. Adult plants maintain a similar proportion of phloem tissue across stem diameters, but scaling of conduit dimensions and number decreases the hydraulic resistance towards the base of the plant. Yet, the small sieve plate pores resulted in an overall higher sieve tube hydraulic resistance than has been reported in other woody angiosperms. Sieve elements scaled from minor to major leaf veins, but were shorter and narrower in petioles. The low carbon assimilation rates of seedlings and mature plants contrasted with a three-fold higher phloem sap velocity in seedlings, suggesting that phloem transport velocity is modulated through ontogeny. While the overall architecture of the phloem tissue in basal-angiosperm understory shrubs scales in a manner consistent with trees, modification of conduit connections may have allowed woody angiosperms to extend beyond their understory origins.