Role of leaf hydraulic conductance in the regulation of stomatal conductance in almond and olive in response to water stress

Abstract

The decrease of stomatal conductance (gs) is one of the prime responses to water shortage and the main determinant of yield limitation in fruit trees. Understanding the mechanisms related to stomatal closure in response to imposed water stress is crucial for correct irrigation management. The loss of leaf hydraulic functioning is considered as one of the major factors triggering stomatal closure. Thus, we conducted an experiment to quantify the dehydration response of leaf hydraulic conductance (Kleaf) and its impact on gs in two Mediterranean fruit tree species, one deciduous (almond) and one evergreen (olive). Our hypothesis was that a higher Kleaf would be associated with a higher gs and that the reduction in Kleaf would predict the reduction in gs in both species. We measured Kleaf in olive and almond during a cycle of irrigation withholding. We also compared the results of two methods to measure Kleaf: dynamic rehydration kinetics and evaporative flux methods. In addition, determined gs, leaf water potential (Ψleaf), vein density, photosynthetic capacity and turgor loss point. Results showed that gs was higher in almond than in olive and so was Kleaf (Kmax = 4.70 and 3.42 mmol s(-1) MPa(-1) m(-2), in almond and olive, respectively) for Ψleaf > -1.2 MPa. At greater water stress levels than -1.2 MPa, however, Kleaf decreased exponentially, being similar for both species, while gs was still higher in almond than in olive. We conclude that although the Kleaf decrease with increasing water stress does not drive unequivocally the gs response to water stress, Kleaf is the variable most strongly related to the gs response to water stress, especially in olive. Other variables such as the increase in abscisic acid (ABA) may be playing an important role in gs regulation, although in our study the gs-ABA relationship did not show a clear pattern.

Why are leaves hydraulically vulnerable?

“…have applied rigorous refinements of previous methods to this problem. Importantly, they confirm the dramatic decline of K leaf before turgor loss point and, following Hernandez-Santana et al (2016), demonstrate similar vulnerability using two different methods on the same species. They also used a recently developed vacuum method based on measuring leaves with severed minor veins to measure the leaf xylem hydraulic vulnerability (Scoffoni and Sack, 2015), and determined the outside-xylem hydraulic vulnerability by subtraction.…”

Leaf rehydration capacity: Associations with other indices of drought tolerance and environment

“…Notably, the species in that study were far less drought tolerant than our Southern California species and thus may exhibit different stomatal behaviours. Further, the difference may arise from the estimation of water status at stomatal closure, which was alternatively measured in the dark (Burghardt & Riederer, ) or for illuminated leaves (this study); several studies have shown that stomata remain substantially open for illuminated leaves even to bulk leaf turgor loss (Guyot et al, ; Hernandez‐Santana, Rodriguez‐Dominguez, Fernández, & Diaz‐Espejo, ). Although our findings for loss of cell integrity or viability before stomatal closure necessitate reconsideration of some common ideas of stomatal protection, they are consistent with those of several recent studies that showed substantial reductions in leaf hydraulic conductance ( K leaf ) before stomata closed by 50% (Bartlett et al, ; Brodribb et al, ; Johnson, Woodruff, McCulloh, & Meinzer, ).…”

Declining root water transport drives stomatal closure in olive under moderate water stress

“…An explanation for the discrepancies between our findings regarding unchanged K shoot and those that report a decline in leaf hydraulic conductance at low levels of water stress in olive under field conditions (Torres‐Ruiz et al ., ; Hernandez‐Santana et al ., ) still needs to be elucidated. For instance, combinations of different hydraulic approaches, including leaf anatomical analyses, to quantify the outside‐xylem hydraulic conductance of the leaves and to localize where the main constrains to water flow occur, may help to make more progress on this question.…”