Pacific Northwest Research Station

Title: Processes preventing nocturnal equilibration between leaf and soil water potential in tropical savanna woody species.

Author: Bucci, Sandra; Scholz, Fabian G.; Goldstein, Guillermo; Meinzer, Frederick C.; Hinojosa Jose A.; Hoffman, William A.; Franco, Augusto C.

Date: 2004

Source: Tree Physiology. 24: 1119-1127

Description: The impact of nocturnal water loss and recharge of stem water storage on predawn disequilibrium between leaf (Ψ_L) and soil (Ψ _S) water potentials was studied in three dominant tropical savanna woody species in central Brazil (Cerrado). Sap flow continued throughout the night during the dry season and contributed from 13 to 28% of total daily transpiration. During the dry season, Ψ_L was substantially less negative in covered transpiring leaves, throughout the day and night than in exposed leaves. Before dawn differences in Ψ_L between covered and exposed leaves were about 0.4 MPa. When relationships between sap flow and Ψ_L of exposed leaves were extrapolated to zero flow, the resulting values of Ψ_L (a proxy of weighted mean soil water potential) in two of the species were similar to predawn values of covered leaves. Consistent with substantial noctural sap flow stomatal conductance (g_s) never dropped below 40 mmol m^-2s^-1 at night, and in some cases, rose to as much as 100 mmol m^-2s^-1 before the end of the dark period. Nocturnal g_s decreased linearly with increasing air saturation deficit (D), but there were species-specific differences in the slopes of the relationships between nocturnal g_s and D. Withdrawal and recharge of water from stem storage compartments were assessed by monitoring diel fluctuations of stem diameter with electronic dendrometers. Stem water storage compartments tended to recharge faster when nocturnal transpiration was reduced by covering the entire plant. Water potential of covered leaves did not stabilize in any of the plants before the end of the dark period, suggesting that, even in covered plants, water storage tissues were not fully rehydrated by dawn. Patterns of sap flow and expansion and contraction of stems reflected the dynamics of water movement during utilization and recharge of stem water storage tissues. This study showed that nightime transpiration and recharge of internal water storage contribute to predawn disequilibrium in water potential between leaves and soil in neotropical savanna woody plants.

Keywords: capacitance, nocturnal transpiration, say flow, soil plant-atmosphere continuum, stomata

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Citation

Bucci, Sandra; Scholz, Fabian G.; Goldstein, Guillermo; Meinzer, Frederick C.; Hinojosa Jose A.; Hoffman, William A.; Franco, Augusto C.  2004.  Processes preventing nocturnal equilibration between leaf and soil water potential in tropical savanna woody species..   Tree Physiology. 24: 1119-1127