Comparative axial widening of phloem and xylem conduits in small woody plants
Along the stem axis phloem’s sieve elements increase in diameter basally at rates comparable to those of xylem conduits and in agreement with principles of hydraulic optimization.
Plant physiology relies on the efficiency of the two long-distance transport systems of xylem and phloem. Xylem architecture comprises conduits of small dimensions towards the stem apex, where transpiration-induced tensions are the highest along the root-to-leaves hydraulic pathway, and widen basally to minimize the path length resistance to water flow. Instead, information on phloem anatomy and allometry is extremely scarce, although potentially relevant for the efficiency of sugar transportation. We measured the hydraulic diameter (Dh) of both xylem conduits and phloem sieve elements in parallel at different heights along the stem of a small tree of Picea abies, Fraxinus excelsior and Salix eleagnos. Dh increased from the stem apex to base in both xylem and phloem, with a higher scaling exponent (b) of sieve elements than that of tracheids in the conifer (0.19 vs. 0.14) and lower than that of vessels in the angiosperms (0.14–0.22 vs. 0.19–0.40). In addition, sieve elements were larger than tracheids in P. abies and narrower than angiosperms vessels at any height along the stem. In conclusion, axial conduit widening would seem to be a key feature of both xylem and phloem long-distance transport architectures.
KeywordsPhloem Xylem Widening Tapering Sieve tubes Long-distance transport
- Angeles G, Bond B, Boyer JS, Brodribb T, Brooks JR, Burns MJ, Cavender-Bares J, Clearwater M, Cochard H, Comstock J, Davis SD, Domec JC, Donovan L, Ewers F, Gartner B, Hacke U, Hinckley T, Holbrook NM, Jones HG, Kavanagh K, Law B, Lopez-Portillo J, Lovisolo C, Martin T, Martinez-Vilalta J, Mayr S, Meinzer FC, Melcher P, Mencuccini M, Mulkey S, Nardini A, Neufeld HS, Passioura J, Pockman WT, Pratt RB, Rambal S, Richter H, Sack L, Salleo S, Schubert A, Schulte P, Sparks JP, Sperry J, Teskey R, Tyree M (2004) The cohesion-tension theory. New Phytol 163:451–452CrossRefGoogle Scholar
- Bohonak AJ (2004) RMA: software for reduced major axis regression v.1.17. University of San DiegoGoogle Scholar
- Choat B, Jansen S, Brodribb TJ, Cochard H, Delzon S, Bhaskar R, Bucci SJ, Feild TS, Gleason SM, Hacke UG, Jacobsen AL, Lens F, Maherali H, Martínez-Vilalta J, Mayr S, Mencuccini M, Mitchell PJ, Nardini A, Pittermann J, Pratt RB, Sperry JS, Westboy M, Wright IJ, Zanne AE (2012) Global convergence in the vulnerability of forests to drought. Nature 491:752–755PubMedGoogle Scholar
- De Schepper V, De Swaef T, Bauweraerts I, Steppe K (2013) Phloem transport: a review of mechanisms and controls. J Exp Bot 64:4839–4850Google Scholar
- Ho LC, Nichols R (1975) The role of phloem transport in the translocation of sucrose along the stem of carnation cut flowers. Ann Bot 39:439–446Google Scholar
- Mencuccini M, Hölttä T, Martinez-Vilalta J (2011) Comparative criteria for models of the vascular transport systems of tall trees. In: Meinzer FC, Lachenbruch B, Dawson TE (eds) Size- and age-related changes in tree structure and function, vol 4. Springer, Netherlands, pp 309–339CrossRefGoogle Scholar
- Sokal RR, Rohlf FJ (1981) Biometry. WH Freeman, New YorkGoogle Scholar