Vegetative and reproductive behaviour of some olive tree varieties (Olea europaea L.) under deficit irrigation regimes in semi-arid conditions of Central Tunisia

Under semi-arid climate, olive trees suffer periodically from water shortage, which may reduce consistently the plant growth and fruit yield, depending on variety and how long the dry period is. To assure suitable growth and regular yields, complementary irrigation is nowadays applied to olive orchards, with however, variable water amounts which may not meet the crop water needs. The present work was carried out in order to study the behaviour of local and foreigner varieties like Chetoui, Chemlali, Coratina, Picholine and Manzanilla under climatic conditions of central Tunisia. Plants were planted in 2002 with a density of 204treeha⁻¹. Three irrigation treatments were applied during two consecutive growing seasons (2008–2009) with water amounts of 20% ETc, 50% ETc and 100% ETc, where ETc is the crop evapotranspiration. Growth parameters (length, metamer number and diameter) were measured regularly on representative shoots, which were previously tagged. Flowering and fruit set were monitored on the same trees. Yield, fruit characteristics and water use efficiency (ratio between yield/irrigation amounts) were determined for both seasons then for the overall growing cycle. Irrigation regime affects differently the shoot length and shoot thickness, flowering and fruit set, fruit characteristics (length, width and weight) and olive production depending mainly on trees’ fruit load, which was apparently the main factor controlling shoot growth dynamic. All varieties showed rhythmic growth and weak primary growth intensity during the On year, while primary growth was continuous with a higher intensity on the Off year. Basal diameter showed continuous growth. Linear relationship was found between basal diameter and length increment. Maximum productions recorded through the growing season cycle (an On year followed by Off year) were given by Picholine and Coratina irrigated with 100% ETc (23.0kgtree⁻¹ and 18.0kgtree⁻¹, respectively), by Manzanilla and Chetoui cultivated at 20% ETc (33.3kgtree⁻¹ and 27.1kgtree⁻¹) and finally by Chemlali at 50% ETc (26.4kgtree⁻¹). Most varieties showed higher water use efficiency at 20% ETc. To conclude we can say that, under the conditions of semi-arid climate of central Tunisia, with average annual rainfall of about 250mm, olive trees Chetoui, Chemlali, Coratina, Picholine and Manzanilla cultivated at 7m×7m spacing, can vegetate and produce properly with an irrigation amount ranging between 80mm and 200mm (20–50% ETc). The irrigation volume can be adjusted following the expected fruit crop load. Particularly, the smallest irrigation amount (80mm) is recommended for low fruit load years.