Annual Variation of Photosynthesis of the Olive Tree Under Different Watering Conditions and Related to Chlorophyll Accumulation

Abstract

Olive trees were irrigated since planted either with drinking or with brackish water. The amount of irrigation applied covered 50%, 75% and 100% of potential evapotranspiration (PET at Tunis: 1400 mm; mean rainfall 4–50 mm). The results show that the olive tree has high photosynthetic capacity (compared with other species) but that it is rather affected by extreme watering conditions and by salinity. Excess of water (compared with needs) induces the same effects as lack of water or salinity and decreases photosynthetic capacity and chlorophyll accumulation. Nevertheless the effects of excess of water and salinity are not additiva Lack of water reduces photosynthesis more than does salinity. Also the effect of reduced irrigation on stomatal density is different of that of salinity.

The Olive tree possesses a seasonnal rythm of photosynthesis, depending mostly on temperature variation. Therefore assimilation drops during winter and grows quickly in spring with a small decrease by budbreak and after that a quick increase that coincides with fruit growth. The annual rhythm of chlorophyll accumulation does not always coincide with CO₂ assimilation variations, however reduction of photosynthesis by salinity and low temperatures is partially linked with variations in the amount of chlorophyll, the variations of chlorophyll a being the most important.