Leaf hydraulic conductance (Kleaf) and drought vulnerability in terms of leaf water potential inducing 50% loss of Kleaf (P50), were assessed in four genotypes of Coffea arabica L. We tested three hypotheses: (1) leaf P50 is lower in small leaves with higher vein densities; (2) lower P50 translates into lower Kleaf, limiting gas exchange rates and higher leaf mass per unit area (LMA); (3) P50 values are coordinated with symplastic drought tolerance. We found partial support for Hypotheses 1 and 3, but not for Hypothesis 2. Significant correlations existed among leaf size, vein network and drought resistance. Smaller leaves displayed higher major vein density, higher Kleaf and more negative P50. Kleaf was correlated with leaf gas exchange rates. A negative relationship was observed between Kleaf and LMA, whereas P50 was found to be positively correlated with LMA. Across coffee genotypes, reduced leaf surface area and increased vein density shifts P50 towards more negative values while not translating into higher LMA or lower Kleaf. Breeding crop varieties for both increased safety of the leaf hydraulic system towards drought-induced dysfunction and high gas exchange rates per unit of leaf area is probably a feasible target for future adaptation of crops to climate change scenarios.

Responses of leaf and shoot hydraulic conductance to light quality were examined on shoots of silver birch (Betula pendula), cut from lower (‘shade position’) and upper thirds of the crowns (‘sun position’) of trees growing in a natural temperate forest stand. Hydraulic conductances of leaf blades (K_lb), petioles (K_P) and branches (i.e. leafless stem; K_B) were determined using a high pressure flow meter in steady state mode. The shoots were exposed to photosynthetic photon flux density of 200–250 µmol m⁻² s⁻¹ using white, blue or red light. K_lb depended significantly on both light quality and canopy position (P < 0.001), K_B on canopy position (P < 0.001) and exposure time (P = 0.014), and none of the three factors had effect on K_P. The highest values of K_lb were recorded under the blue light (3.63 and 3.13 × 10⁻⁴ kg m⁻² MPa⁻¹ s⁻¹ for the sun and shade leaves, respectively), intermediate values under white light (3.37 and 2.46 × 10⁻⁴ kg m⁻² MPa⁻¹ s⁻¹, respectively) and lowest values under red light (2.83 and 2.02 × 10⁻⁴ kg m⁻² MPa⁻¹ s⁻¹, respectively). Light quality has an important impact on leaf hydraulic properties, independently of light intensity or of total light energy, and the specific light receptors involved in this response require identification. Given that natural canopy shade depletes blue and red light, K_lb may be decreased both by reduced fluence and shifts in light spectra, indicating the need for studies of the natural heterogeneity of K_lb within and under canopies, and its impacts on gas exchange.

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.

Effects of water deficit on plant water status, gas exchange and hydraulic conductance were investigated in Betula pendula under artificially manipulated air humidity in Eastern Estonia. The study was aimed to broaden an understanding of the ability of trees to acclimate with the increasing atmospheric humidity predicted for northern Europe. Rapidly-induced water deficit was imposed by dehydrating cut branches in open-air conditions; long-term water deficit was generated by seasonal drought.