Seasonal variation in formation, structure, and chemical properties of phloem in Picea abies as studied by novel microtechniques
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Phloem production and structural development were interlinked with seasonal variation in the primary and secondary metabolites of phloem. Novel microtechniques provided new perspectives on understanding phloem structure and chemistry.
To gain new insights into phloem formation in Norway spruce (Picea abies), we monitored phloem cell production and seasonal variation in the primary and secondary metabolites of inner bark (non-structural carbohydrates and phenolic stilbene glucosides) during the 2012 growing season in southern and northern Finland. The structure of developing phloem was visualised in 3D by synchrotron X-ray microtomography. The chemical features of developing phloem tissues isolated by laser microdissection were analysed by chemical microanalysis. Within-year phloem formation was associated with seasonal changes in non-structural carbohydrates and phenolic extractive contents of inner bark. The onset of phloem cell production occurred in early and mid-May in southern and northern Finland, respectively. The maximal rate of phloem production and formation of a tangential band of axial phloem parenchyma occurred in mid-June, when total non-structural carbohydrates peaked (due to the high amount of starch). In contrast, soluble sugar content dropped during the most active growth period and increased in late summer and winter. The 3D visualisation showed that the new axial parenchyma clearly enlarged from June to August. Sub-cellular changes appeared to be associated with accumulation of stilbene glucosides and soluble sugars in the newest phloem. Stilbene glucosides also increased in inner bark during late summer and winter. Our findings may indicate that stilbene biosynthesis in older phloem predominantly occurs after the formation of the new band(s) of axial parenchyma. The complementary use of novel microtechniques provides new perspectives on the formation, structure, and chemistry of phloem.
KeywordsCarbohydrates Laser microdissection Microtomography Phloem parenchyma Phenolics Stilbene glucosides
Breast height (1.3 m)
Base of the living crown
Site at Kivalo, in northern Finland
Non-structural carbohydrates (defined here as soluble sugars + starch)
Site 1, Haapastensyrjä, in southern Finland
Site 2, Ruotsinkylä, in southern Finland
Synchrotron X-ray microtomography
We thank the staff of the Finnish Forest Research Institute, and several students and trainees, for assisting in field and laboratory work. We also thank Dr. P. Saranpää, Dr. H. Mäkinen, and Prof. B. Holmbom for their help designing of the study. We are grateful to the Electron Microscopy Unit of the Institute of Biotechnology, University of Helsinki, for providing laboratory facilities and help in sample preparation. We acknowledge the European Synchrotron Radiation Facility (ESRF) for providing beam time for the µCT measurements, and are grateful for P. Tafforeau and C. Soriano for their help in using beamline ID19. P. Ahvenainen, A. Kallonen, A. Meaney, and K. Pirkkalainen of the Department of Physics, University of Helsinki, provided valuable assistance in carrying out the µCT experiments. Prof. R. Serimaa is acknowledged for her help designing the study and participating in the µCT experiment. This study was carried out under the framework of the COST FP1106 network STReESS and funded by the post-doctoral research grant from the Academy of Finland (no. 250299). This work was also supported by JSPS KAKENHI Grant No. 26-04395 and JSPS post-doctoral Grant No. P14395.
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