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

Abstract

Summary In the sieve elements (SEs) of the phloem, carbohydrates are transported throughout the whole plant from their site of production to sites of consumption or storage. SE structure, especially of the pore‐rich end walls, has a direct effect on translocation efficiency. Differences in pore size and other features were interpreted as an evolutionary trend towards reduced hydraulic resistance. However, this has never been confirmed.Anatomical data of 447 species of woody angiosperms and gymnosperms were used for a phylogenetic analysis of end wall types, calculation of hydraulic resistance and correlation analysis with morphological and physiological variables. end wall types were defined according to pore arrangement: either grouped into a single area (simple) or into multiple areas along the end wall (compound).Convergent evolution of end wall types was demonstrated in woody angiosperms. In addition, an optimization of end wall resistance with plant height was discovered, but found to be independent of end wall type. While physiological factors also showed no correlation with end wall types, the number of sieve areas per end wall was found to scale with SE length.The results exclude the minimization of hydraulic resistance as evolutionary driver of different end wall types, contradicting this long‐standing assumption. Instead, end wall type might depend on SE length.

Computational models evaluating the impact of sieve plates and radial water exchange on phloem pressure gradients

“…Our model provides evidence to support the hypothesis that sieve plates are responsible for a major source of resistance in sieve tubes of young balsam poplar stems. In a recent study assessing sieve tube resistances across multiple families, Liesche et al () compared sieve plate and lumen flow properties in 447 angiosperm and gymnosperm species. In terms of absolute pressure drop across the sieve plate, our results lie on the upper end of values for sieve plates in angiosperms (Liesche et al, ).…”

“…In a recent study assessing sieve tube resistances across multiple families, Liesche et al () compared sieve plate and lumen flow properties in 447 angiosperm and gymnosperm species. In terms of absolute pressure drop across the sieve plate, our results lie on the upper end of values for sieve plates in angiosperms (Liesche et al, ). Therefore, for poplar stems, our results suggest that sieve plates contribute substantially to sieve tube resistance in comparison with other species.…”

“…We found that the most important sieve plate traits that impact plate resistance were total pore area and shape. Previous methodologies take into account the variability of pore radii in their models (Liesche et al, ; Savage et al, ) but assume pores are always circular. However, our results indicate that assuming pore shape as being circular instead of using their actual shape may underestimate pore flow by as much as twofold in comparison with past work (e.g., Jensen, Mullendore, et al, ; Thompson & Holbrook, ).…”

“…lumen (Jensen, Mullendore, et al, 2012;Liesche, Pace, Xu, Li, & Chen, 2017;Savage et al, 2017). However, other examples show conductivity reductions due to plates of up to 88% (Thompson & Holbrook, 2003a).…”

Regulation of Sucrose Transporters and Phloem Loading in Response to Environmental Cues

“…However, a systematic analysis of which factors influence SUT expression is missing, even in the model plant Arabidopsis (Arabidopsis thaliana). Not knowing the extent to which plants rely on SUT regulation to control carbon partitioning is a major reason for our limited understanding of the control of this process (Ayre, 2011;Liesche et al, 2017).…”