Experimental evidence supporting the concept of light-mediated modulation of stem hydraulic conductance
2010
Sellin, Arne | Õunapuu, Eele | Karusion, Annika
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 (Kstem) 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 Kstem. Both canopy position (long-term effect) and current photosynthetic photon flux density (PPFD; short-term effect) had a significant impact (P < 0.001) on Kstem measured in shoots taken from the lower (shade shoots) and upper (sun shoots) third of the crowns of approximately 25-year-old trees growing in a natural forest stand. The shade shoots responded more sensitively to light manipulation: Kstem increased by 51% in shade shoots and 26% in sun shoots when PPFD increased from 70 to 330 micromoll m-2 s-1. In 4-year-old trees growing in a dense experimental plantation, Kstem, specific conductivity of branch-wood (kbw) and potassium ion concentration ([K+]) in xylem sap varied in accordance with canopy position (P < 0.001). Both Kstem and kbw increased considerably with light availability, increasing within the tree crowns from bottom to top; there was a strong relationship between mean values of Kstem and [K+] in hydraulically sampled branches.
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