The stem xylem of Patagonian shrubs operates far from the point of catastrophic dysfunction and is additionally protected from drought‐induced embolism by leaves and roots

Abstract

Hydraulic architecture was studied in shrub species differing in rooting depth in a cold desert in Southern Argentina. All species exhibited strong hydraulic segmentation between leaves, stems and roots with leaves being the most vulnerable part of the hydraulic pathway. Two types of safety margins describing the degree of conservation of the hydraulic integrity were used: the difference between minimum stem or leaf water potential (Y) and the Y at which stem or leaf hydraulic function was reduced by 50% (Y -Y50), and the difference between leaf and stem Y50. Leaf Y50 -stem Y50 increased with decreasing rooting depth. Large diurnal decreases in rootspecific hydraulic conductivity suggested high root vulnerability to embolism across all species. Although stem Y50 became more negative with decreasing species-specific Ysoil and minimum stem Y, leaf Y50 was independent of Y and minimum leaf Y. Species with embolism-resistant stems also had higher maximum stem hydraulic conductivity. Safety margins for stems were >2.1 MPa, whereas those for leaves were negative or only slightly positive. Leaves acted as safety valves to protect the integrity of the upstream hydraulic pathway, whereas embolism in lateral roots may help to decouple portions of the plant from the impact of drier soil layers.

Herb Hydraulics: Inter- and Intraspecific Variation in Three Ranunculus Species

“…Although some of these species also had very vulnerable leaves at P k50 of 21 MPa or greater (Hao et al, 2008;Chen et al, 2009;Blackman et al, 2012;Guyot et al, 2012), the majority of previously studied species exhibited P k50 of 21.5 MPa or less and down to 24.3 MPa (Sack and Holbrook, 2006;Blackman et al, 2010;Johnson et al, 2012;Nardini et al, 2012;Bucci et al, 2013). Accordingly, stems were typically also more vulnerable to drought-induced hydraulic failure in herbs (21 to 23.8 MPa; Kocacinar and Sage, 2003;Rosenthal et al, 2010;Tixier et al, 2013;Nolf et al, 2014) Leaf hydraulic analysis in our study showed that all three Ranunculus spp.…”

Divergent Hydraulic Safety Strategies in Three Co-occurring Anacardiaceae Tree Species in a Chinese Savanna

“…First, the differences in the minimum Ψ leaf or Ψ stem values and the values at which 50% of the hydraulic conductance was lost (P50 leaf or P50 stem ) were defined as the HSMs in the leaves and stems, respectively (Bucci et al, 2013): HSM ( leaf ) = minimum Ψ leaf − P50 leaf ; HSM (stem) = minimum Ψ stem − P50 stem . The minimum values of Ψ leaf and Ψ stem occurred at midday during the early dry season for L. coromandelica , and during the late dry season for P. weinmanniifolia and T. paniculata (Table 1).…”

“…Hydraulic safety margins (HSMs) reflect the degree of hydraulic conservatism of a plant or a given organ (Meinzer et al, 2009; Bucci et al, 2013; Johnson et al, 2016). HSM has been most commonly calculated as the difference in water potential between the minimum value experienced in the field and the value at either 50% loss of xylem hydraulic conductivity (P50 stem ) or at 50% loss of leaf hydraulic conductance (P50 leaf ) (Meinzer et al, 2009).…”

Wood Anatomy and Plant Hydraulics in a Changing Climate

“…9.5). Additional relationships between hydraulic traits and wood density include a positive correlation with resistance to embolism (Bucci et al 2013;Hacke et al 2001a;Ogasa et al 2013;Pratt et al 2007), a negative correlation with sapwood capacitance (Meinzer et al 2008b;Scholz et al 2011), and a negative correlation with minimum leaf water potential and leaf water potential at turgor loss (Meinzer 2003;Meinzer et al 2008b). …”

“…A hydraulic safety margin can be defined as the difference between a given point along a plant organ's xylem vulnerability curve (e.g., P 50 ) and the organ's normal daily minimum xylem pressure determined by stomatal regulation of transpiration (Brodribb et al 2003;Bucci et al 2013;Jones and Sutherland 1991;Meinzer et al 2009;Sparks and Black 1999;Tyree and Sperry 1988). Multispecies plots of branch P 50 against minimum branch water potential (an estimate of xylem pressure) reveal that safety margins increase as minimum water potential becomes more negative (Pockman and Sperry 2000) and that the tracheid-bearing conifers tend to sustain more negative branch water potentials and maintain larger hydraulic safety margins than the Fig.…”