Secondary phloem diversity and evolution in Bignonieae (Bignoniaceae)

Abstract

The secondary phloem of Bignonieae is extremely diverse, with sister lineages exhibiting distinct phloem anatomies derived from contrasting patterns of evolution in fibre abundance. Fibre abundance in the tribe has diversified in correlation with sieve tube arrangement, sieve tube morphology, number of companion cells and parenchyma type. The results challenge long-standing hypotheses regarding general trends in cell abundance and morphological cell evolution within the phloem, and demonstrate the need to expand studies in phloem anatomy both at a narrow taxonomic scale and at a broad one, such as to families and orders.

Height‐related scaling of phloem anatomy and the evolution of sieve element end wall types in woody plants

“…It has been proposed that a change from compound to simple end walls parallels the trends for tracheary elements in the xylem (Bailey, 1953;Carlquist, 1975). However, studies that characterized the bark and wood structure of many species of a specific family did not support this hypothesis (Donghua & Xinzeng, 1993;Kotina & Oskolski, 2010;Pace et al, 2015). Mullendore et al (2010), upon not finding a correlation of sieve pore size with transport velocity in herbaceous angiosperms, suggested that the ability for plugging the pores could be a critical factor in a plant's reaction to stress.…”

“…Only mature trees that reached the typical maximum height were sampled. The procedure for sampling and analysis resembles the procedure followed in previous investigations of SE end wall anatomy (MacDaniels, 1918;Esau & Cheadle, 1959;Pace et al, 2015). A 1 9 2 cm piece of bark was cut from the stem at breast height (1.3 m) using a sharp knife, immediately fixed in 70% formalinacetic acid alcohol (Berlyn & Miksche, 1976) and subsequently stored in 70% ethanol.…”

“…A correlation would indicate that the parameter was a potential driver for the development of a certain end wall type in a given species. Earlier analyses have focused solely on the relationship of end wall type to other characteristics of the phloem and/or bark tissues (Zahur, 1959;Den Outer, 1983;Chavan, 1986;Pace et al, 2015). Mathematical modeling based on phloem anatomy suggested that trees use the same mechanism of osmotically driven pressure flow for phloem transport as herbaceous plants (Jensen et al, 2012a).…”

“…In Bignoniaceae, collapse of the SEs has never been recorded in the phloem of fibrous species, whose SEs bear compound end walls, not even in the nonconducting phloem. By contrast, of the species featuring scarce fibers, whose SEs have simple or compound end walls with few sieve areas, most experience total collapse of their SEs in the nonconducting phloem (Pace et al, 2015). This shows that the mechanical tissue type can be of high significance for phloem physiology and a potential evolutionary driver towards multiseason usage of SEs.…”

“…In contrast to SE end wall type, the mechanical tissue type appeared evolutionary-conserved. It should be noted, however, that we analyzed only the mechanical tissue type and not mechanical tissue abundance, which was implicated in SE stability in the Bignonieae (Pace et al, 2015).…”