Role of aquaporin activity in regulating deep and shallow root hydraulic conductance during extreme drought

Abstract

Key message Deep root hydraulic conductance is upregulated during severe drought and is associated with upregulation in aquaporin activity. Abstract In 2011, Texas experienced the worst single-year drought in its recorded history and, based on tree-ring data, likely its worst in the past millennium. In the Edwards Plateau of Texas, rainfall was 58 % lower and the mean daily maximum temperatures were[5°C higher than long-term means in June through September, resulting in extensive tree mortality. To better understand the balance of deep and shallow root functioning for water supply, we measured root hydraulic conductance (K R ) in deep (*20 m) and shallow (5-10 cm) roots of Quercus fusiformis at four time points in the field in 2011. Deep roots of Q. fusiformis obtained water from a perennial underground (18-20 m) stream that was present even during the drought. As the drought progressed, deep root K R increased 2.6-fold from early season values and shallow root K R decreased by 50 % between April and September. Inhibitor studies revealed that aquaporin contribution to K R increased in deep roots and decreased in shallow roots as the drought progressed. Deep root aquaporin activity was upregulated during peak drought, likely driven by increased summer evaporative demand and the need to compensate for declining shallow root K R . A whole-tree hydraulic transport model predicted that trees with greater proportions of deep roots would have as much as five times greater transpiration during drought periods and could sustain transpiration during droughts without experiencing total hydraulic failure. Our results suggest that trees shift their dependence on deep roots versus shallow roots during drought periods, and that upregulation of aquaporin activity accounts for at least part of this increase.

Co‐occurring woody species have diverse hydraulic strategies and mortality rates during an extreme drought

“…We parameterized TREES with available site‐specific measurements. We used half‐hourly meteorological data (Lampasas 2.7 ENE, TX, USA, 30 km from Colorado Bend Field site and 153 km from Menard field site; both sites experience similar climate and water stress; see Johnson et al, and Figure S1) for the period April 1 to September 30, 2011, which was the peak of drought severity. Among the key parameters in Table S1, the hydraulic, gas exchange, and leaf area index parameters were from observations made at the study site.…”

“…Under current and near‐future climatic conditions, we can expect warmer temperatures, greater vapour pressure deficits (VPD LA; McDowell & Allen, ) and more frequent and severe droughts (Allen, Breshears, & McDowell, ; Diffenbaugh, Swain, & Touma, ; Polley, Briske, Wolter, Bailey, & Brown, ; Trenberth et al, ) than experienced in the recent past. In fact, current work suggests recent droughts in the United States are the most severe of the past ~1,200 years (Johnson, Sherrard, Domec, & Jackson, ; Griffin & Anchukaitis, ).…”

Leaf hydraulic parameters are more plastic in species that experience a wider range of leaf water potentials

“…This site has karst topography with shallow soils (typically less than 20 cm) with underlying fractured limestone (see Auken, Ford, Stein, & Stein, ; Brown, Stafford, Shaw‐Faulkner, & Grubbs, for more detailed site descriptions). The area is considered semi‐arid (based on the methods of Zomer et al., ; Zomer, Trabucco, Bossio, van Straaten, & Verchot, ) and has a mean annual temperature of 19.5°C and a mean annual precipitation of 702 mm, which is unevenly distributed in many years with the majority occurring in the spring, fall, and winter months (see Johnson, Sherrard, Domec, & Jackson, for example). The ecosystem is characterized by open savanna‐like woodlands, and the dominant woody species are Diospyros texana Scheele, Juniperus ashei J. Buchholz, Prosopis glandulosa Torr.…”

Soil water potential determines the presence of hydraulic lift of Populus euphratica Olivier across growing seasons in an arid desert region

“…Transpiration was considered as the main energy source for water absorption from soils to plant roots, as well as for water transportation across plant as no energy required (Zimmermann 1984;Kramer, Boyer 1995). However, in some particular cases, it was reported that energy-consuming aquaporin's might play an important role in water transportation between roots and their adjust soils (Prieto et al 2012;Johnson et al 2014). Specifically, plants spent their energy on active transport of water when they suffered from the extreme environment, e.g.…”

“…Specifically, plants spent their energy on active transport of water when they suffered from the extreme environment, e.g. the storage water in plants was insufficient to meet their metabolic activities, or plants urgently needed to lift water from groundwater into shallow soils via plant stems to absorb minerals (McElrone et al 2007;Prieto et al 2012;Johnson et al 2014). Within the HL process, it may be not enough water potential difference to lift water from deep soil layers or groundwater to shallow soil layers due to transpiration shutoff at night (Prieto et al 2012).…”