Abstract
Xylem conductive capacity is a key determinant of plant hydraulic function and intimately linked to photosynthesis and productivity, but can be impeded by temporary or permanent conduit dysfunctions. Here we show that persistent xylem dysfunctions in unstressed plants are frequent in Alpine dwarf shrubs and occur in various but species-specific cross-sectional patterns. Combined synchrotron micro-computed tomography (micro-CT) imaging, xylem staining, and flow measurements in saturated samples of six widespread Ericaceae species evidence a high proportion (19%-50%) of hydraulically nonfunctional xylem areas in the absence of drought stress, with regular distribution of dysfunctions between or within growth rings. Dysfunctions were only partly reversible and reduced the specific hydraulic conductivity to 1.38 to 3.57 ×10-4 m2 s-1 MPa-1 . Decommission of inner growth rings was clearly related to stem age and a higher vulnerability to cavitation of older rings, while the high proportion of nonfunctional conduits in each annual ring needs further investigations. The lower the xylem fraction contributing to the transport function, the higher was the hydraulic efficiency of conducting xylem areas. Improved understanding of the functional lifespan of xylem elements and the prevalence and nature of dysfunctions is critical to correctly assess structure-function relationships and whole-plant hydraulic strategies.
Highlights
Efficient long‐distance water transport is paramount for survival and growth of land plants
The lower the xylem fraction contributing to the transport function, the higher was the hydraulic efficiency of conducting xylem areas
The observations suggest that persistent nonconducting xylem elements are widespread in the dwarf shrub growth form and occur in regular, stable, and species‐specific patterns including complete decommission of older growth rings, partial dysfunctions in each growth ring, and scattered occurrence of small inactive xylem areas
Summary
Efficient long‐distance water transport is paramount for survival and growth of land plants. There is only a rudimentary understanding of the functional status of xylem over time, the prevalence and nature of nonfunctional xylem conduits during periods without stress, and the impact of nonconductive xylem elements on plant transport capacity (compare Brodersen et al, 2019) Quantitative assessment of their proportion in the xylem tissue is highly desirable for a better assessment of structure–function relationships, for example in allometric scaling models (Petit & Anfodillo, 2009), and a better understanding of whole‐plant hydraulic strategies. Dwarf shrubs were chosen as study objects as they represent an important group of woody plants at high altitudes and latitudes, are adapted to high stress levels, and cover various ecological niches based on highly divergent physiological adaptations (including foliar, wood and root traits) Their low growth height enables to sample entire plants and to analyse the intact main stem, and to avoid cutting artefacts (Beikircher & Mayr, 2016; Sperry, 2013). The study should improve our knowledge of the prevalence, distribution, and nature of persistent nonfunctional xylem conduits during the growing season in plants that did not experience an extreme drought event or controlled dehydration, an important aspect for the plant hydraulic architecture
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