Monitoring sap flow as indicator of transpiration and water status of an experimental olive tree orchard

The relationship between water loss indicators and water status constraints in rainfed olive trees was studied under different environmental conditions and different levels of soil water status to assess drought adaptation. The experiment was carried out in an experimental olive tree orchard in southern Italy, from 2002 to 2007. Whole-plant water use was determined using a xylem sap flow method (compensation heat-pulse technique). Ancillary ecophysiological parameters, including stomatal conductance and water potentials (and stem radius variation for 2006 and 2007), were periodically measured. During the experimental period, from June to September, predawn leaf water potential decreased and this was associated with increasing soil moisture deficit, while the difference between predawn and midday leaf water potentials increased, suggesting anisohydric regulation of plant water potential. Olive trees exhibited a tight stomatal control over transpiration. The mean daily transpiration rate, canopy conductance and decoupling coefficient were lowest in 2007, the driest year. In 2007, the transpiration rate of the olive tree orchard averaged 1.01 and 1.16 mm d-1 in rainfed and well watered plants, respectively. These results support the idea that olive trees had conservative water use strategies. Soil-to-leaf hydraulic conductance declined in response to decreasing predawn leaf water potential, explaining increasing stomatal control of transpiration under progressive soil drying. Whereas the investigated olive trees did not experience severe water stress, an increased frequency of exceptionally warm and dry summers may have a more serious impact than a single event and would give resistant genotypes a competitive advantage in the long run.