Silber, A., Institute of Soils, Water and Environmental Sciences, Agricultural Research Organization, The Volcani Centre, Bet Dagan 50250, Israel
Israeli, Y., Northern RandD, P.O. Box 90000, Rosh Pina 12100, Israel
Levi, M., Northern RandD, P.O. Box 90000, Rosh Pina 12100, Israel
Keinan, A., Northern RandD, P.O. Box 90000, Rosh Pina 12100, Israel
Shapira, O., Northern RandD, P.O. Box 90000, Rosh Pina 12100, Israel
Chudi, G., Northern RandD, P.O. Box 90000, Rosh Pina 12100, Israel
Golan, A., Northern RandD, P.O. Box 90000, Rosh Pina 12100, Israel
Noy, M., Shaham, Extension Service, Ministry of Agriculture, P.O. Box 28, Bet Dagan 50250, Israel
Levkovitch, I., Institute of Soils, Water and Environmental Sciences, Agricultural Research Organization, The Volcani Centre, Bet Dagan 50250, Israel
Assouline, S., Institute of Soils, Water and Environmental Sciences, Agricultural Research Organization, The Volcani Centre, Bet Dagan 50250, Israel

The performance of 'Hass' avocado trees grown in lysimeters under different irrigation regimes obtained by manipulating drip irrigation frequency and root volume was examined. The experimental design comprised six treatments (3×2) with three irrigation frequencies and two container volumes (100- and 200-L). The three irrigation frequencies were: pulsed irrigation (10-20min every 30min) throughout the day (Irg1), one daily irrigation event beginning at night and terminated in the morning every day (Irg2) and one irrigation event every two days (Irg3). Irrigation management induced significant differences in water availability in the root zone and subsequently, the diurnal and periodic water uptake. Water uptake of trees in the Irg1 treatment closely followed changes in the meteorological conditions (air temperature, pan evaporation and vapour pressure deficit) while that of trees in the Irg2 and Irg3 treatments was depressed by intermediate and severe water stress, respectively. The experimental treatments had little effect on the vegetative growth, flowering or fruit-set processes. However there were significant treatment differences on fruitlet abscission and accordingly, on fruit yield. Black spots initiated from the seed became apparent on some of the fruits at the beginning of June, and about two weeks later an intensive abscission of fruitlet begun that ended at the beginning of July. The abscission was more intense in the 100-L than the 200-L containers, in the following order: Irg3>Irg2≥Irg1 regardless of the container volume. Net CO 2 assimilation during periods of fruit growth decreased in trees exposed to moderate or severe water stress (Irg2 and Irg3, respectively) and therefore, it is plausible that fruitlet abscission resulted from carbohydrate stress. Improvement of water and nutrient availability, especially in periods where the activity of the root system was weak as a result of low carbohydrate supply presumably played a dominant role in seeds or fruit function. © 2011 Elsevier B.V.