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

BUCCI S.J., SCHOLZ F.G., GOLDSTEIN G., MEINZER F.C., HINOJOSA J.A. AND HOFFMANN W.A

TREE PHYSIOLOGY

Heron publishing

Lugar: Victoria Canada; Año: 2004 vol. 24 p. 1119 - 1127

0829-318X

The impact of nocturnal water loss and recharge of stem water storage on predawn disequilibrium between leaf and soil water potentials were studied in three dominant woody species of Neotropical savannas in central Brazil (Cerrado). Nocturnal water loss, measured with heat pulse sap flow systems, continued throughout the night during the dry season and contributed from 13 to 28 % of total daily transpiration. The magnitude of nocturnal transpiration-induced disequilibrium was assessed by comparing water potentials of freely transpiring and covered leaves. Leaf water potentials (YL) during the dry season were substantially less negative in covered (non-transpiring) leaves than in exposed transpiring leaves throughout the day and night. At predawn, differences in YL between covered and exposed leaves were about 0.4 MPa. The species-specific predawn YL of covered leaves was similar to the YL at 0 flow, obtained from the sap flow of freely transpiring plants versus YL relationships extrapolated to the x-axes (x-intercept at 0 flow). Stomatal conductance (gs) never dropped below 40 mmol m-2 s-1 at night in any of the species studied, and in some cases increased to levels as high as 100 mmol m-2 s-1 before the end of the dark period. In the three species studied, nocturnal gs increased linearly with decreasing air saturation deficits (D), but there were species-specific differences in the slopes of the nocturnal gs and D relationships. Withdrawal of water from and recharge of stem water storage compartments were assessed by monitoring diel fluctuations of stem diameter using electronic dendrometers. Stem water storage compartments tended to recharge faster when nocturnal transpiration of the entire plant was reduced by covering. Covered leaf Y did not stabilize in any of the plants before the end of the dark period, suggesting that water storages, even in covered plants, were not fully rehydrated at predawn. The expansion and contraction of stem diameters and the sap flow patterns adequately reflected, in all the trees, the dynamics of the water movement during utilization and recharge of stem water storages. This study showed that nighttime transpiration and recharge of internal water storage are two important processes contributing to predawn disequilibrium in water potential between leaves and soil in Neotropical savanna woody plants.