Introduction
Climate change is predicted to increase the occurrence of extreme
drought events, altering the water available for plant growth (Dai 2013;
IPCC 2014). Mechanistic
understanding of plant growth and survival, as well as changes in
community assembly and species distribution in response to changes in
water availability can be understood through physiological ‘response’
traits directly linked to plant water economics (also termed ‘hydraulic
traits’; Engelbrecht et al. 2007; Suding et al. 2008;
Reich 2014; Griffin-Nolan et al. 2018). Unfortunately, those
physiological traits have been mostly omitted in large-scale species
distribution and community-scale trait studies because their measurement
is time- and labor-intensive, and unfeasible under field conditions (for
reviews see Bartlett et al. 2012b; Griffin-Nolan et al.2018). To this end, different morphological response traits (e.g.,
specific leaf area) have been used as proxies. However, the relationship
between these proxies and precipitation (as a dominant measure of water
availability for plants) on the community scale is weak, highlighting
the need for a more careful trait selection (for review see
Griffin-Nolan et al. 2018).
A turning point could arise from a recent development of a fast and
feasible osmometry method to determine leaf osmotic potential at turgor
loss point (πtlp), one of the key physiological response
traits. The osmometry method, originally developed on woody species
(Bartlett et al. 2012a) has been subsequently validated for
herbaceous species (Griffin-Nolan et al. 2019; Májeková et
al. 2019), and thus can be applied across a wide range of functional
types. Leaves maintaining a more negative πtlp remain
turgid at lower soil water potentials allowing them to maintain critical
physiological processes (growth and photosynthetic assimilation of
CO2) under drier conditions (Scholander et al.1965; Schulze et al. 1987; Kubiske & Abrams 1990; Kramer &
Boyer 1995; Koide et al. 2000; Mitchell et al. 2008;
Bartlett et al. 2012b). This leaf-level dehydration tolerance
(sensu Volaire 2018) trait can be scaled-up to a whole-plant
drought tolerance (sensu Noy-Meir 1973), a link well documented
in woody species (Lenz et al. 2006; Baltzer et al. 2008;
Bartlett et al. 2012b, 2016b). It is important to note that we
are referring to drought tolerance as the ability of plants to maintain
growth under decreased water availability (sensu Noy-Meir 1973),
as opposed to drought avoidance and drought escape).
πtlp also clearly relates to other leaf-level hydraulic
traits, such as leaf water potential at 50% loss of hydraulic and
stomatal conductance (Bartlett et al. 2016a; Farrell et
al. 2017; Griffin-Nolan et al. 2019).
A question that remains open is
the degree of coordination between πtlp as the trait
mechanistically linked to drought tolerance and functional traits
commonly used as proxies for drought tolerance in trait-based and global
change ecology. Coordination among physiological and morphological
aspects of plant’s phenotype on both leaf and whole-plant level is
important for understanding the mechanisms governing plant’s responses
to water availability and their adaptive nature. To this end, we have
selected four traits spanning a spectrum of ‘hard’ to ‘soft’ traits on
the leaf- and whole-plant level: intrinsic water-use efficiency (iWUE)
captured by carbon stable isotope δ13C, leaf dry
matter content (LDMC), specific leaf area (SLA, inverse of leaf mass per
area, LMA) and plant height.
Water use efficiency represents a ratio of net CO2assimilation to stomatal conductance (Farquhar et al. 1989; Seibtet al. 2008). If water is the main limiting factor, iWUE should
increase with decreasing water availability (Araya et al. 2010).
The carbon stable isotope δ13C in leaves has been
often used as a surrogate of iWUE (Farquhar et al. 1989; Seibtet al. 2008). Leaf δ13C is an integrated,
long-term measure of the ratio between internal and ambient
CO2 concentrations
(Ci/Ca) that reflects the stomatal
conductance to CO2 and thus also drought-induced
stomatal closure. This is because plants preferentially assimilate
lighter 12C and increasingly use heavier13C when CO2 (and thus the abundant
lighter isotope) is less available. Such a situation typically occurs in
leaves with closed stomata under drought stress (Farquhar et al.1989).
Specific leaf area (SLA; inverse of leaf mass per area, LMA), has been
the most commonly used proxy in relation with water availability
gradients, especially in large-scale studies along very long gradients
(Westoby 1998; Westoby et al. 2002; Wright et al. 2004;
Díaz et al. 2016; Griffin-Nolan et al. 2018). It also
represents the second axis of the global spectrum of form and function
(Díaz et al. 2016), an important trait of the leaf economics
spectrum (Wright et al. 2004), as well as
a prominent trait for assessment
of plant strategies (Westoby 1998; Pierce et al. 2013). Decrease
in SLA and increase in leaf dry matter content (LDMC) under decreasing
water availability can be explained by the decrease in leaf expansion
rates achieved by formation of smaller cells and/or tighter packed cells
with less air space in between and/or thicker cell walls in order to
reduce water requirements under drought (Garnier et al. 2001;
Poorter et al. 2009). Though LDMC and SLA are often considered
interchangeable (e.g. Pierce et al. 2013), when leaf thickness as
the third player is being considered (Vile et al. 2005), their
relationship is hyperbolic (Garnier et al. 2001; Vendraminiet al. 2002), potentially resulting in SLA and LDMC diverging in
their response to the stress considered (e.g. Hodgson et al.2011).
Plant height is the leading dimension of the global spectrum of form and
function (Díaz et al. 2016), a prominent trait defining plant
strategies (Westoby 1998; Pierce et al. 2013) and as Westobyet al. (2002) states: “[…] the quantitative trait
that has been adopted by virtually everyone doing comparative plant
ecology”. A decrease in plant height along gradients of decreasing
water availability relies on two assumptions that act together in the
field conditions. First is the simple premise that plant growth and
biomass production are reduced under water limitation (Schulze et
al. 1987). Second is the ‘hydraulic limitation hypothesis’ (Ryanet al. 2006), which states that with increasing height increases
also the difficulty to supply leaves with water and the risk of
embolism, leading to stomata closure, decrease in photosynthesis, and
less carbon available for growth.
An important factor to be considered when investigating the coordination
among different traits possibly related to the same function (i.e. water
availability) is whether such a relationship is biophysically based, and
thus transferable to all plant species, or whether it is reflecting the
adaptations typical for a certain plant functional type (PFT) or a
phylogenetic clade. If two traits are related in the same way within
different PFTs, or the relation holds when accounting for phylogenetic
relatedness between species, it would suggest that whenever there was an
evolutionary change in one trait in one direction, it coincided with an
evolutionary change of a second trait in the same direction due to
adaptations to the same selection pressure (‘selective correlation’;
Stebbins 1950; Felsenstein 2004).
Here, we screened 122 temperate
grassland species from different plant functional types (forbs and
graminoids) across a broad range of water availability conditions in
European temperate grasslands (from dry through mesic to wet
grasslands). We assessed the relationships between πtlpand other traits related to plant drought tolerance on the leaf and the
whole-plant level. We hypothesized that higher physiological leaf-level
dehydration tolerance (more negative πtlp) will be
coordinated with (1) physiological trait represented by higher intrinsic
water use efficiency (iWUE) measured as δ13C; (2)
lower values of the specific leaf area (SLA) associated with less
acquisitive species within the leaf economics spectrum; (3) leaf-level
morphological trait represented by higher leaf dry matter content
(LDMC); (4) plant stature represented by smaller maximum vegetative
height. We further investigated whether the aforementioned relationships
are biophysically-based, and therefore general, or driven by the
differences inherent in the major plant functional types and/or inherent
in different phylogenetic clades.