Hydraulic leaf conductance and cavitation vulnerability decrease with elevation in Phacelia secunda J.F. Gmel. (Boraginaceae)

Abstract

High mountain ecosystems are characterized by freezing temperatures. Freeze-thaw events can induce embolism in plant xylem. The
risk of embolism increases with the conduit diameter, because wider xylem vessels can contain larger amounts of dissolved gas that
after water freeze may form big bubbles producing cavitation. Thus smaller vessels could be advantageous in habitats prone to freezing
events. However, smaller conduits decrease hydraulic conductance. The leaf hydraulic conductance (Kf) is a measure of the efficiency
in the water transport across the leaf, and it is defined as the ratio between the water flux across the leaf and the difference of hydraulic
potential across the leaf. Apparently, Kf is a bottleneck for hydraulic conductance of the whole plant and probably also a crucial factor
in gas exchange. In this work we evaluated how the elevational origin of P. secunda can affect the hydraulic conductance of the leaf and
the cavitation vulnerability of the vessels. We postulate that plants of P. secunda from higher elevations, as exposed to higher frequency
of freezing events, have minor vessels diameter and lower Kf than plants from lower elevations. To test our hypothesis, we determined
under field conditions the leaf hydraulic conductance (Kf), in plants from an altitudinal gradient (1,600, 2,800 and 3,600 m) in the

central Chilean Andes. In addition, we analyzed cross sections of petiole and leaf lamina to evaluate the changes in the number, size
and frequency of xilematic vessels in order to determine a cavitation vulnerability index. We found a negative correlation between the
hydraulic conductance of the leaf and elevation in P. secunda plants. This is related to the smaller diameter of xylem vessels found in
plants higher altitude, which results in a reduced vulnerability to cavitation. These changes could be considered as adaptives, because at
high elevations freezing events are more frequent, and hence, there is a higher probability of cavitation than at low elevations. However,
this can negatively effect stomatal conductance and carbon assimilation, restricting the photosynthetic responses of these plants.