Experimental evidence supporting the concept of light-mediated modulation of stem hydraulic conductance.
It is a well-described phenomenon that plant leaves respond to changes in light intensity and duration by adjusting leaf hydraulic efficiency, and there is current consensus that up- or down-regulation of water channels (aquaporins) in the plasma membrane of the bundle sheath and mesophyll cells play a central role in the underlying mechanisms. Recently, experimental evidence has been provided also for light-mediated changes of stem hydraulic conductance (K(stem)) in field-grown laurel plants. This effect was attributed to differences in potassium ion concentration of xylem sap as a function of light conditions. In the present article, we report evidence obtained in silver birch (Betula pendula Roth), supporting the concept of light-mediated modulation of K(stem). Both canopy position (long-term effect) and current photosynthetic photon flux density (PPFD; short-term effect) had a significant impact (P < 0.001) on K(stem) measured in shoots taken from the lower (shade shoots) and upper (sun shoots) third of the crowns of ∼25-year-old trees growing in a natural forest stand. The shade shoots responded more sensitively to light manipulation: K(stem) increased by 51% in shade shoots and 26% in sun shoots when PPFD increased from 70 to 330 μmol m⁻² s⁻¹. In 4-year-old trees growing in a dense experimental plantation, K(stem), specific conductivity of branch-wood (k(bw)) and potassium ion concentration ([K(+)]) in xylem sap varied in accordance with canopy position (P < 0.001). Both K(stem) and k(bw) increased considerably with light availability, increasing within the tree crowns from bottom to top; there was a strong relationship between mean values of K(stem) and [K(+)] in hydraulically sampled branches.
Institute of Ecology and Earth Sciences, University of Tartu, Lai 40, 51005 Tartu, Estonia. arne.sellin@ut.ee
Arne
A
Sellin
Eele
E
Õunapuu
Annika
A
Karusion
D029662:Betula / Q000502:physiology*; D017753:Ecosystem; D008027:Light; D018515:Plant Leaves / Q000502:physiology*; D018547:Plant Stems / Q000502:physiology*; D018526:Plant Transpiration / Q000502:physiology; D014867:Water / Q000378:metabolism*
D014867:Water