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1. Normalization criteria determine the interpretation of nitrogen effects on the root hydraulics of pine seedlings

   https://doi.org/10.1093/treephys/tpaa068  

   Toca, Andrei; Villar-Salvador, Pedro; Oliet, Juan A; Jacobs, Douglass F (June 2020, Tree Physiology)

   Mencuccini, Maurizio (Ed.)

   Abstract Plant hydraulics is key for plant survival and growth because it is linked to gas exchange and drought resistance. Although the environment influences plant hydraulics, there is no clear consensus on the effect of nitrogen (N) supply, which may be, in part, due to different hydraulic conductance normalization criteria and studied species. The objective of this study was to compare the variation of root hydraulic properties using several normalization criteria in four pine species in response to three contrasting N fertilization regimes. We studied four closely related, yet ecologically distinct species: Pinus nigra J.F. Arnold, Pinus pinaster Ait., Pinus pinea L. and Pinus halepensis Mill. Root hydraulic conductance (Kh) was measured with a high-pressure flow meter, and values were normalized by total leaf area (leaf specific conductance, Kl), xylem cross-section area (xylem specific conductance, Ks), total root area (root specific conductance, Kr) and the area of fine roots (fine root specific conductance, Kfr). Controlling for organ size differences allowed comparison of the hydraulic efficiency of roots to supply or absorb water among fertilization treatments and species. The effect of N on the root hydraulic efficiency depended on the normalization criteria. Increasing N availability reduced Kl and Ks, but increased Kh, Kr and especially Kfr. The positive effect of N on Kr and Kfr was positively related to seedling relative growth rate and was also consistent with published results at the interspecific level, whereby plant hydraulics is positively linked to photosynthesis and transpiration rate and fast growth. In contrast, normalization by leaf area and xylem cross-sectional area (Kl and Ks) reflected opposite responses to Kr and Kfr. This indicates that the normalization criteria determine the interpretation of the effect of N on plant hydraulics, which can limit species and treatment comparisons. 

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2. Aquaporin regulation in roots controls plant hydraulic conductance, stomatal conductance, and leaf water potential in Pinus radiata under water stress

   https://doi.org/10.1111/pce.13460  

   Rodríguez‐Gamir, Juan; Xue, Jianming; Clearwater, Michael J.; Meason, Dean F.; Clinton, Peter W.; Domec, Jean‐Christophe (November 2018, Plant, Cell & Environment)

   Abstract Stomatal regulation is crucial for forest species performance and survival on drought‐prone sites. We investigated the regulation of root and shoot hydraulics in threePinus radiataclones exposed to drought stress and its coordination with stomatal conductance (g_s) and leaf water potential (Ψ_leaf). All clones experienced a substantial decrease in root‐specific root hydraulic conductance (K_root‐r) in response to the water stress, but leaf‐specific shoot hydraulic conductance (K_shoot‐l) did not change in any of the clones. The reduction inK_root‐rcaused a decrease in leaf‐specific whole‐plant hydraulic conductance (K_plant‐l). Among clones, the larger the decrease inK_plant‐l, the more stomata closed in response to drought. Rewatering resulted in a quick recovery ofK_root‐randg_s. Our results demonstrated that the reduction inK_plant‐l, attributed to a down regulation of aquaporin activity in roots, was linked to the isohydric stomatal behaviour, resulting in a nearly constant Ψ_leafas water stress started. We concluded that higherK_plant‐lis associated with water stress resistance by sustaining a less negative Ψ_leafand delaying stomatal closure. 

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3. Coordinated decline of leaf hydraulic and stomatal conductances under drought is not linked to leaf xylem embolism for different grapevine cultivars

   https://doi.org/10.1093/jxb/eraa392  

   Albuquerque, Caetano; Scoffoni, Christine; Brodersen, Craig R; Buckley, Thomas N; Sack, Lawren; McElrone, Andrew J (December 2020, Journal of Experimental Botany)

   Lawson, Tracy (Ed.)

   Abstract Drought decreases water transport capacity of leaves and limits gas exchange, which involves reduced leaf leaf hydraulic conductance (Kleaf) in both the xylem and outside-xylem pathways. Some literature suggests that grapevines are hyper-susceptible to drought-induced xylem embolism. We combined Kleaf and gas exchange measurements, micro-computed tomography of intact leaves, and spatially explicit modeling of the outside-xylem pathways to evaluate the role of vein embolism and Kleaf in the responses of two different grapevine cultivars to drought. Cabernet Sauvignon and Chardonnay exhibited similar vulnerabilities of Kleaf and gs to dehydration, decreasing substantially prior to leaf xylem embolism. Kleaf and gs decreased by 80% for both cultivars by Ψ leaf approximately –0.7 MPa and –1.2 MPa, respectively, while leaf xylem embolism initiated around Ψ leaf = –1.25 MPa in the midribs and little to no embolism was detected in minor veins even under severe dehydration for both cultivars. Modeling results indicated that reduced membrane permeability associated with a Casparian-like band in the leaf vein bundle sheath would explain declines in Kleaf of both cultivars. We conclude that during moderate water stress, changes in the outside-xylem pathways, rather than xylem embolism, are responsible for reduced Kleaf and gs. Understanding this mechanism could help to ensure adequate carbon capture and crop performance under drought. 

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4. How megadrought causes extensive mortality in a deep‐rooted shrub species normally resistant to drought‐induced dieback: The role of a biotic mortality agent

   https://doi.org/10.1111/pce.14768  

   Aguirre, Natalie M.; Ochoa, Marissa E.; Holmlund, Helen I.; Palmeri, Gabriella N.; Lancaster, Emily R.; Gilderman, Gina S.; Taylor, Shaquetta R.; Sauer, Kaitlyn E.; Borges, Adriana J.; Lamb, Avery N. D.; et al (November 2023, Plant, Cell & Environment)

   Abstract Southern California experienced unprecedented megadrought between 2012 and 2018. During this time,Malosma laurina, a chaparral species normally resilient to single‐year intense drought, developed extensive mortality exceeding 60% throughout low‐elevation coastal populations of the Santa Monica Mountains. We assessed the physiological mechanisms by which the advent of megadrought predisposedM. laurinato extensive shoot dieback and whole‐plant death. We found that hydraulic conductance of stem xylem (K_{s, native}) was reduced seven to 11‐fold in dieback adult and resprout branches, respectively. Staining of stem xylem vessels revealed that dieback plants experienced 68% solid‐blockage, explaining the reduction in water transport. Following Koch's postulates, persistent isolation of a microorganism in stem xylem of dieback plants but not healthy controls indicated that the causative agent of xylem blockage was an opportunistic endophytic fungus,Botryosphaeria dothidea. We inoculated healthyM. laurinasaplings with fungal isolates and compared hyphal elongation rates under well‐watered, water‐deficit, and carbon‐deficit treatments. Relative to controls, we found that both water deficit and carbon‐deficit increased hyphal extension rates and the incidence of shoot dieback. 

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5. The dynamic multi‐functionality of leaf water transport outside the xylem

   https://doi.org/10.1111/nph.19069  

   Scoffoni, Christine; Albuquerque, Caetano; Buckley, Thomas N.; Sack, Lawren (June 2023, New Phytologist)

   Summary A surge of papers have reported low leaf vulnerability to xylem embolism during drought. Here, we focus on the less studied, and more sensitive, outside‐xylem leaf hydraulic responses to multiple internal and external conditions. Studies of 34 species have resolved substantial vulnerability to dehydration of the outside‐xylem pathways, and studies of leaf hydraulic responses to light also implicate dynamic outside‐xylem responses. Detailed experiments suggest these dynamic responses arise at least in part from strong control of radial water movement across the vein bundle sheath. While leaf xylem vulnerability may influence leaf and plant survival during extreme drought, outside‐xylem dynamic responses are important for the control and resilience of water transport and leaf water status for gas exchange and growth. 

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