Differential agro-physiological responses induced by partial root-zone drying irrigation in olive cultivars grown in semi-arid conditions

Olive cultivation is mostly practiced under Mediterranean agro-ecosystems with limited availability of water. With current climate change conditions, water scarcity will continue to challenge agricultural sustainability. Therefore, great emphasis is placed on irrigation management in arid regions with the aim of increasing water use efficiency. The present study was conducted during two growing seasons (2015 and 2016) consisting of a low yields year ('Off' year) and a high yields year ('On' year), respectively. The photosynthetic performance in terms of gas exchange and chlorophyll fluorescence, as well water status, vegetative growth and yield responses of three olive cultivars (Arbequina, Arbosana and Chetoui) grown under partial root zone drying (PRD) irrigation in semi-arid conditions of Tunisia were investigated. Olive trees were subjected to four irrigation treatments which corresponded to control irrigated at 100% of the crop evapotranspiration (100%ETC) on both sides of the root system and three PRD treatments. These later were irrigated on a 14-day alternating cycle on one side of the root system and received respectively 100%, 75% and 50% of the control (100%PRD, 75%PRD and 50%PRD). In 'Off' year shoot length was lower in PRD irrigated trees than in control trees, mainly for Arbequina and Chetoui cultivars and the lowest values were associated with 50%PRD treatment. However, in 'On' year shoot length increased slightly for all treatments, whereas remained lower under PRD irrigation treatments. Relating to water status, all cultivars under PRD treatments show a clear decrease in stem water potential (Psi(stem)) and relative water content (RWC). A clear difference in the regulation of leaf gas exchange parameters under different irrigated regimes was observed between the studied cultivars. Results indicated that the PRD induced a decrease in maximum efficiency of photosystem II (Fv/Fm), effective efficiency of PSII (Phi PSI), stomatal conductance (g(s)) and transpiration rate (E). Nevertheless, this decrease did not limit the overall photosynthesis process mainly in the two cultivars Arebquina and Chetoui; since assimilation rates and intrinsic water use (WUEI) still maintained under 50%PRD treatment independently of the decreased value of fluorescence parameters. Furthermore, reducing irrigation volumes by 25% (75% PRD) and 50% (50% PRD) compared to the control (100%ETC) increased oil yield and water productivity mainly for Arbequina cultivar without significant reductions on yield components. Based on agronomic and physiological adaptations to reduce water loss, it can be concluded that Arbequina is very promising for cultivation under PRD irrigation strategy compared to Arbosana and Chetoui.