Nocturnal transpiration prevents leaf to soil water potential equilibration in tropical savanna woody species

BUCCI SANDRA J., SCHOLZ FABIÁN G., GOLDSTEIN GUILLERMO, MEINZER FREDERICK C., HINOJOSA JOSÉ, HOFFMANN WILLIAM.

Edinburgo

Conferencia; International Conference Tropical Savannas & Seasonally dry forest; 2003

Nocturnal water loss is generally not studied, in non-CAM plants, because it is assumed that stomata are closed at night. However, under certain environmental conditions nocturnal transpiration can be relatively high. Nocturnal temperatures are high and relative humidity tend to be low in tropical savannas, particularly during the dry season, and consequently the vapor pressure difference between leaf and air could result in large nocturnal water loss if stomata are not completely closed. The main objective of this study was to evaluate the occurrence and magnitude of nocturnal transpiration in some dominant woody species of savannas in central Brazil (Cerrado), during the dry season. Sap flow, stomatal conductance (gs) and leaf water potentials (YL) were measured in tree growing in a Cerrado site. Field manipulations were performed to restrict transpiration in whole plants. Leaf water potentials for covered (non-transpiring) leaves were less negative throughout the day than for freely transpiring leaves. At nighttime, YL differences between exposed and covered leaves ranged from 0.1 to 0.5 MPa, suggesting that predawn YL was more negative than bulk soil Y estimated by predawn YL of freely transpiring leaves. All species maintained their stomata partially open at night. Nighttime gs represented about 30% of daily total gs. Sap flow in freely transpiring trees was positive (water moves from roots to leaves) at nighttime, but preventing water loss decreased substantially the magnitude of nighttime water loss. Sap flow in covered trees tended to be negative (water moves from leaves to soil) at night. Predawn disequilibrium between plant and soil Y, during the dry season, was largely the result of nocturnal water use.