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Abstract

Nitrogen is vital for tree growth. Recent declines in Swedish forest productivity and a growing interest in forest fertilisation highlight the urgent need to understand the mechanisms governing tree nitrogen uptake and utilisation. While it is well established that nitrogen availability influences the quantity, structure, and chemistry of wood, less is known about the effects of different nitrogen forms, the underlying molecular mechanisms, and the short-term nitrogen responses in the developing xylem tissues of forest trees.

To investigate the effects of nitrogen, I developed a strategy to systematically vary both the sources and levels of nitrogen supplied to hybrid aspen (Populus tremula × P. tremuloides) trees. In addition, a near-infrared (NIR) spectroscopy method was established to characterise the wood’s physical and chemical properties. Controlled nitrogen additions showed that increased nitrogen availability enhances biomass accumulation and radial expansion of xylem cells, while decreasing lignin content and wood density. Among the nitrogen sources tested, nitrate was unique in that it significantly stimulated shoot biomass accumulation and increased the proportion of p-hydroxyphenyl units (H-type lignin) in the wood.

Dynamic, time-resolved transcriptome analyses revealed that nitrate is transported in the xylem sap of hybrid aspen stems and taken up into differentiating xylem tissues, rapidly inducing the expression of genes related to xylem cell expansion. Functional assays indicated that nitrate promotes xylem cell expansion via the transcription factor CYTOKININ RESPONSE FACTOR 4 (CRF4). A population-wide analysis of nitrogen transport and metabolism-related gene expression suggested that nitrogen uptake into differentiating xylem tissues has to be tightly regulated, since excessive nitrogen retrieval from the xylem sap seemed to suppress radial tree growth.

Altogether, these studies elucidate the mechanisms by which nitrogen—and nitrate in particular—regulates wood formation in Populus trees. They also provide advanced imaging and molecular tools that can be used to optimise nitrogen use and promote forest tree growth.

Keywords

cambial growth; wood formation; nitrogen responses; nitrate signalling; wood characterisation; NIR imaging; single cell RNA sequencing

Published in

Acta Universitatis Agriculturae Sueciae
2025, number: 2025:88
Publisher: Swedish University of Agricultural Sciences

SLU Authors

UKÄ Subject classification

Forest Science

Publication identifier

  • DOI: https://doi.org/10.54612/a.86bpiki3tb
  • ISBN: 978-91-8124-072-6
  • eISBN: 978-91-8124-118-1

Permanent link to this page (URI)

https://res.slu.se/id/publ/142489