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

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.

Photoperiod- and temperature-mediated control of phenology in trees - a molecular perspective.

Contents 511 I. 511 II. 512 III. 513 IV. 513 V. 517 VI. 517 VII. 521 VIII. 521 Acknowledgements 521 References 521 SUMMARY: Trees growing in boreal and temperate regions synchronize their growth with seasonal climatic changes in adaptive responses that are essential for their survival. These trees cease growth before the winter and establish a dormant state during which growth cessation is maintained by repression of responses to growth-promotive signals. Reactivation of growth in the spring follows the release from dormancy promoted by prolonged exposure to low temperature during the winter. The timing of the key events and regulation of the molecular programs associated with the key stages of the annual growth cycle are controlled by two main environmental cues: photoperiod and temperature. Recently, key components mediating photoperiodic control of growth cessation and bud set have been identified, and striking similarities have been observed in signaling pathways controlling growth cessation in trees and floral transition in Arabidopsis. Although less well understood, the regulation of bud dormancy and bud burst may involve cell-cell communication and chromatin remodeling. Here, we discuss current knowledge of the molecular-level regulation of the annual growth cycle of woody trees in temperate and boreal regions, and identify key questions that need to be addressed in the future.

Scanning Electron Microscopy of the Phloem.

In vascular plants, sugars are transported through the phloem tissue from areas of production, the leaves, to heterotrophic organs, where they are needed for growth and storage. Inside the phloem, transport takes place in specialized cells called sieve elements. Sieve elements are connected end-to-end by sieve plates to form a sieve tube. Sieve plates have small perforations called sieve pores. Transport of sugars is pushed through the tubes, plates, and pores by osmotic potential differences in the plant. Physical constraints govern the speed and volume of sugar flow through this tube system. Understanding the phloem requires precise anatomical measurements to model the effect of sieve element physical parameters on flow. Presented is a detailed method to prepare phloem tissue for scanning electron microscopy to obtain large quantities of high-resolution data of the plants sugar transport tissue.