Assessing environmental controls over conductances through the soil–plant–atmosphere continuum in an experimental olive tree plantation of southern Italy

Constraints on plant water status and water use indicators were studied in orchard grown Olea europaea L. trees, cv. Nocellara del Belice, in order to assess productivity and drought adaptation. The study was conducted on 16-year-old grafted plants, grown under different water treatments for 14 years in an experimental site plantation in southern Italy. Water treatments were: a non-irrigated, rainfed, control (T0) and three treatments that received seasonal water amount equivalent to 33 and 66% of crop evapotranspiration (ETC) from the beginning of pit hardening to early fruit veraison (respectively T33 and T66), and 100% of ETC throughout the irrigation season (T100). During 2006 and 2007 growing seasons, plants were continuously monitored by automatic point dendrometers measuring stem radius variation and whole-plant water use was determined using a xylem sap flow method (compensation heat-pulse technique). Additional ecophysiological parameters, such as stomatal conductance and water potentials were periodically measured, as well as vegetative development. Predawn leaf water potential decreased in rainfed trees 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, but insufficient to prevent loss of hydraulic conductance under severe drought stress. The stem radial increment and sap flux did not differ consistently between water treatments in the mild year (2006), while irrigated trees had higher water use than rainfed trees in the dry year (2007). However, plants growing under rainfed conditions showed a small increase in maximum daily shrinkage during drought periods compared to irrigated ones, although no marked differences were recorded between irrigated treatments. The mean daily transpiration rate, canopy conductance and decoupling coefficient were higher in 2006 than in 2007, the driest year, only, and in fully irrigated rather than rainfed plants. These results support the idea that rainfed trees had more conservative water use strategies than irrigated trees, and that deficit-irrigated trees acclimated somewhat functionally and structurally to long-term partial watering. Based on these observations, we can argue that loss of hydraulic conductance is an important mechanism for increasing stomatal control of transpiration under progressive soil drying. Another view implies that the increasing difference between soil and leaf water potential during summer induced stomatal closure and minimized the risk of a collapse of the conductive system, decreasing transpiration and reducing hydraulic conductance.