Shade effect on photosynthesis and photoinhibition in olive during drought and rewatering

Olive tree (Olea europaea L.) is commonly grown under environmental conditions characterised by water deficit, high temperatures and irradiance levels typical of Mediterranean semi-arid regions. Measurement of gas exchange, chlorophyll content, chlorophyll fluorescence and photoinhibition was carried out on two-year-old olive trees (cv. ‘Coratina’) subjected to a 21-day period of water deficit followed by 23 days of rewatering. At the beginning of the experiment, plants were divided in to two groups and subjected to different light regimes: exposed plants (EP) under a mean photosynthetically active radiation (PAR) at mid-day of 1800 μmol m⁻² s⁻¹ and shaded plants (SP) under a mean PAR of 1200 μmol m⁻² s⁻¹. The effect of drought and high irradiance levels caused a reduction of gas exchange and photosystem 2 (PSII) efficiency, in terms of quantum yield of PSII (Φ_PSII) both in EP and SP. Shading conditions allowed plants to maintain a high photosynthetic activity at low values of stomatal conductance, whereas in EP the reductions in photosynthetic efficiency and intrinsic water efficiency were due to non-stomatal components of photosynthesis. The decrease in photosynthetic activity and the increase of photoinhibition under drought were more marked in EP than in SP. Full sunlight caused in EP a higher non-photochemical quenching, whereas SP showed a better photochemical efficiency. The information here obtained can be important to understand the mechanisms by which olive plants can minimize photoinhibition when subjected to simultaneous abiotic stresses.