Water relations, physiological behavior and antioxidant defence mechanism of olive plants subjected to different irrigation regimes

The effects of variable water supply were investigated in 50-year-old Olea europaea (cv “Koroneiki”) L. trees grown in an experimental orchard located in northwest Crete. The trees were subjected to four irrigation treatments: rainfed (RF), full irrigation (FI) with 100% of the crop evapotranspiration (Etc.) level and two deficit irrigation treatments, FRDI-fully irrigated, that received no irrigation water from July 12 to August 18 and DRDI-irrigated with 60% of ETc level, that received no irrigation water from July 12 to August 18. The effects of watering regime on soil moisture content, plant water status, gas exchange characteristics, chlorophyll content, antioxidant enzyme activities, i.e. superoxide dismutase (SOD; EC 1.15.1.1), catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APX; EC 1.11.1.11) and guaiacol peroxidase (GPX; EC 1.11.1.7) and on malondialdehyde (MDA) levels were evaluated. There were no marked differences between FI and FRDI treatments, except at the end of the no-watering period. On the contrary, stem water potential (Ψ) was affected by the level of water supply, leading to reduced stomatal conductance (gs) and carbon assimilation rate (Pn) in DRDI treatment. DRDI plants exhibited a close relation between photosynthetic rate (Pn) and stomatal conductance (gs), suggesting that stomatal closure is the dominant limitation to photosynthesis. Chlorophyll degradation and enhanced synthesis of carotenoids assisted plant to maintain its physiological processes under the water deficit and high light conditions. Olive tree was able to restore stem water potential (Ψ) after rewatering, and the recovery of leaf water status was coupled with an up-regulation of leaf gas exchange characteristics in DRDI plants. Leaves grown under rainfed conditions revealed signs of oxidative stress as indicated by the increased levels of lipid peroxidation, whereas irrigation mitigated the oxidative damage at leaf level in DRDI plants. Superoxide dismutase, ascorbate peroxidase and guaiacol peroxidase activity was increased in DRDI leaves, while catalase activity was inhibited. On the contrary, the lower expression of the enzymatic antioxidant system under rainfed conditions did not allow a better protection against a more pronounced oxidative stress.

► Evaluate water saving techniques in olive orchards located in Mediterranean areas.
► Determine the defence mechanism of olive trees against drought and high light stress.
► Clarify the possible occurrence of non-stomatal limitations to photosynthesis.