IntroductionThe olive tree (Olea europaea L.) is a sclerophyllous species of the Mediterranean area which is one of the most typical and is important for the rural economy, local heritage and the environment. Olives are mainly cultivated in arid and semi-arid regions where plants are frequently exposed extremely high temperatures and scarcity of water. In fact, as for many Mediterranean species adapted to semi-arid climates (Lo Gullo and Salleo, 1988), the olive tree is able to tolerate the low availability of water in soil by means of morphological, physiological and biochemical adaptations acquired in response to periods of water shortage often lasting throughout the spring-summer period (Connor and Fereres, 2005). Most studies on the physiology of olive trees have focused on the responses of this species to stress under both controlled (Tombesi et al., 1984; Bongi et al., 1987; Angelopoulos et al., 1996; Dichio et al., 1997) and field condition (Larsen et al., 1989; Michelakis et al., 1996; Moreno et al., 1996; Fernandez et al., 1997; Fernández and Moreno, 1999; Moriana et al., 2002). The most relevant mechanisms against drought stress are the regulation of stomata closure and transpiration (Moreno et al., 1996; Nogues, and Baker, 2000), the regulation of gas exchange (Moriana et al., 2002), a very developed osmotic adjustment (Chartzoulakis et al., 1999), the regulation of the antioxidant system (Bacelar et al., 2007), the appearance of leaf anatomical alterations (Chartzoulakis et al., 1999), and the ability of extracting water from the soil due to a deep root system (Fernandez et al., 1997) and to a high water potential gradient between canopy and root system (Tombesi et al., 1986). Olive trees are confirmed to be efficient soil water users, thanks to their xylem sap transport and the ability to maintain significant rates of gas exchange even during drought stress (Tognetti et al., 2004). At Wadi Mashash, Negev Desert, BGU’s experimental desert farm, we are seeking to grow an olive tree orchard without high-tech irrigation and only through the collection of the desert’s natural rainfall. During certain rain, q special event occurs in which flash floods run down the desert hills into wadis. These waters are conveyed through dirt channels and funneled into basins in which the trees are planted. Trees and intercrops can only be planted after a winter flood. But for three years in a row, there were no floods in Wadi Mashash, preventing plant intercrop. Thus, we decided to use this unexpected condition to consider the strategies by which olive trees adapt to extreme drought and to see the responses of plants after rewatering by artificial flooding some experimental plots. Research objectivesThe aim of the research is to study the physiological responses of olive plants, such as: photosynthesis, stomatal conductance, sap flow transpiration and the root dynamics during a water deficit and rewatering. Achievements 1. Physiological responses of plantsThe measurements were happened around 10-12 am local time, the light intensity varied between 1500-2000 µE m?2s?1 and leaf temperature between 28-40 °C from March to August. Before rewatering date 19 June, all experimental trees were suffered from drought stress and showed very low value of photosynthetic rate and stomatal conductance even the B plot tree was looking like almost dead without doing any photosynthesis (Fig. 1a,b). After adding water into plot D,E,F, the big differences in assimilation rates between wetted and controls were shown clearly. The photosynthetic rates were greatly increasing over first months and reached maximum value of 20 ?mol CO2/m-2s at 2 weeks after watering. Then the increase changes were slowly declined due to less water available in soil but still higher than the values of controls. Exceptionally, on 2 August which was extrememly hot day, the temperature was 40 0C, the photosynthetic rates were suddenly droped off because of closing stomata as the stomatal conductances were very low in wetted plots. Figure 1. Effects of drought stress and rewatering on (a) photosynthesis rate (?mol CO2/m-2s), (b) stomatal conductance (mmol/m-2s) of olive trees. (A-B-C were control plots, D-E-F were wet plots, rewatering date was 19 June)). 2. Granier sap flow measurement2.1 Determination of ?Tmax Determination of ?Tmax is fundamental for the calculation of sap flux density which is necessary to calculate the value of the whole tree sap flow. ?Tmax is influenced by the thermal properties of the wood surrounding the heated probe. ?Tmax for dry wood is usually higher than for wet wood. During the development of a severe soil water deficit, there could be a drift in daily values of ?Tmax. This drift in ?Tmax could occur during both drying and rewetting phases. According to figure 2, there was great drift of ?Tmax in E and F plots in contrast to D plot which can be explained by the D tree instantly uptaked and transpired water after wetting plots, but the E and F tree needed time to consider adapting with new environment. When less water was available in soil, the ?Tmax value were increasing especially from 56 days after watering in all plots. Figure 2. Determination of ?Tmax over a period from 19 June to 26 September 2017. When there is a period of water stress, ?Tmax should be determined by breaking the whole period into drying and wetting period and applying Granier’s double regression method to each of the periods. 2.2 Calculation of whole tree sap flowThe Granier probe only measures the sap flux density along a 20mm x 3.14 x 1 mm2 cylinder. To extrapolate this measurement to the whole cross-sectional area of the sapwood, total sap flow of the tree is calculated as the product of sap flux density and the area of the cross section of the sapwood at the level of the heated probe. The most ideal case is when the depth of the sap-wood is equal to the length of the probe, where the cross section is the area of the annulus of the sapwood (Lu et al., 2004). Figure 3. The sapwood depth is equal to the probe length (=2cm).