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 similar to 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 mu mol 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 kbw 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.