Effects of water stress and soil type on photosynthesis, leaf water potential and yield of olive trees (Olea europaea L. cv. Chemlali Sfax)

In Tunisia, olives are grown under severe rain-fed, arid conditions. To determine the behaviour of olive trees (cv. Chemlali Sfax) during the severe drought affecting Tunisian arid areas in 2002, a range of physiological parameters were investigated in three adjacent orchards. Two olive orchards were rain-fed, one located on a sandy soil, and the other on a sandy-loam clay soil. A third orchard was also located on sandy soil, but received remedial irrigation (415 mm of water per year; similar to 40% of olive evapotranspiration). Predawn leaf water potential (Psi(pd)) did not fall below -1.52 MPa for irrigated olive trees. However, a large decrease Psi(pd) was observed for rain-fed olive trees in the same period with.pd measured at about -3.2 MPa on sandy soil and -3.6 MPa on sandy-loam clay soil. At the same time, the minimal leaf water potential recorded at midday (Psi(min)) decreased to -4.15 MPa and -4.71 MPa in the rain-fed trees for sandy and sandy-loam clay soil, respectively. For irrigated trees, the Psi(min) was -1.95 MPa. These results were associated with relative water content, which varied from 80% for irrigated trees to 54 and 43.6%, respectively, for rain-fed trees and trees subjected to severe drought. In August, when the relative water content values were less than 50%, a progressive desiccation in the outer layer of canopy and death of terminal shoots were observed in trees, which grew on the sandy-loam clay soil. Furthermore, low soil water availability also affected (negatively) the net photosynthetic rate in rain-fed orchards (10.3 mu mol/ m(2). s for irrigated trees v. 5.3 mu mol/m(2). s in rain-fed trees on sandy soil) and stomatal conductance (98.5 mmol/m(2). s v. 69.3 mmol/m(2). s). However, it improved water use efficiency (7.6 v. 4.7 mu mol CO2/ mmol H2O), which increased by more than 50% in both groups of rainfed trees compared with the irrigated ones. We can conclude that olive trees respond to drought by showing significant changes in their physiological and biological mechanisms. These results also help our understanding of how olive trees cope with water stress in the field and how marginal soils can restrict growth and lower yields.