Abstract
During the night, plant water loss can occur either through the roots, as hydraulic redistribution (HR), or through the leaves via the stoma, as nocturnal transpiration (E n), which was methodologically difficult to separate from stem refilling (R e). While HR and E n have been reported across a range of species, ecosystem, and climate zone, there is little understanding on the interactions between E n and/or R e and HR. As water movement at night occurs via gradients of water potential, it is expected that during periods of high atmospheric vapor pressure deficit (VPD), water loss via E n will override water loss via HR. To test this hypothesis, sap flow in stems and roots of Populus euphratica Oliv. trees, growing in a riparian zone in a hyperarid climate, was measured once in a year. Nocturnal stem sap flow was separated into E n and R e using the “forecasted refilling” method. Substantial nocturnal sap flow (38% of 24‐hr flux on average) was observed and positively correlated with VPD; however, the strength of the correlation was lower (R 2 = .55) than diurnal sap flow (E d) (R 2 = .72), suggesting that nocturnal stem sap flow was attributed to both water loss through the canopy and replenishment of water in stem tissues. Partitioning of nocturnal sap flow shows that R e constituted approximately 80%, and E n ~20%, of nocturnal sap flow. The amount of root sap flow attributed to redistribution was negatively related to E d (R 2 = .69) and the amount of acropetally sap flow in stems, R e (R 2 = .41) and E n (R 2 = .14). It was suggested that the magnitude of HR is more strongly depressed by R e that was recharge to the water loss via E d than by E n. It was consistent with whole‐tree water balance theory, that the nighttime upward sap flow to xylem, stem refilling and transpiration, may depress hydraulic redistribution of roots.
Highlights
Plant water movement can occur via two major pathways: (1) the roots, where the movement of water is from different parts of the root zone with large gradients in water status [hydraulic redistribution (HR)] (Burgess, Adams, Turner, & Ong, 1998; Nadezhdina et al, 2010) or (2) the leaves, nocturnal transpiration (En) from the canopy via the stoma (Dawson et al, 2007; Howard, van Iersel, Richards, & Donovan, 2009)
Substantial nighttime Vs was observed and increased with vapor pressure deficit (VPD) but with low determination coefficient (R2 = .55), which suggests that the nocturnal Vs should be attributed to both of En and Re
We estimated that En accounted for approximately 7.5% of 24-hr sap flow, compared to Re of 29.7%
Summary
Plant water movement can occur via two major pathways: (1) the roots, where the movement of water is from different parts of the root zone with large gradients in water status [hydraulic redistribution (HR)] (Burgess, Adams, Turner, & Ong, 1998; Nadezhdina et al, 2010) or (2) the leaves, nocturnal transpiration (En) from the canopy via the stoma (Dawson et al, 2007; Howard, van Iersel, Richards, & Donovan, 2009). En may result from increased stomatal conductance caused by a combination of high temperature, low humidity, and high soil water availability These conditions are often typical to riparian forests in the hyperarid climate where soil water availability remains high because of trees’ access to shallow groundwater throughout the summer (Yu et al, 2016). We evaluate the contributions of these different sources of sap flow in this species by (1) separating nocturnal sap flow into En and Re and (2) determining the relationship between HR and En and/or Re of P. euphratica through in situ measurement in a hyperarid climate in NW China
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