63 Olive Fruit and Root Growth on Different Irrigation Regimes in Central Italy E.M. Lodolini, F. Morini, S. Polverigiani and D. Neri Department of Environmental and Crop Science Università Politecnica delle Marche Via Brecce Bianche, 60131 Ancona Italy Keywords: pioneer and fibrous roots, pulp-to-pit ratio, RDI, ‘Frantoio’, ‘Leccino’ Abstract The influence of two different Regulated Deficit Irrigation (RDI) on root and fruit growth was studied in two Italian olive cultivars (‘Frantoio’ and ‘Leccino’). Trees were exposed to irrigation with 35 and 70% of the total evapotranspiration (ETc) whereas rainfed plants were used as control. Total yield per plant did not show any significant difference among the treatments, but single fruit and pulp-to-pit ratio showed increased fresh and dry weights on irrigated trees mainly due to mesocarp. Such increases were significant for irrigated treatments compared to control but no differences were found within the two RDI treatments. Water availability reduced summer drop of pioneer and especially of fibrous root growth compared to control. Higher amount of water induced a higher fibrous root growth, but no differences were observed for pioneer roots. The results showed that 35% ETc supply was sufficient to overcome a limited period of water shortage improving fruit and root system development. On the other hand additional water supply up to 70% ETc did not show visible advantages on the short time period and therefore does not seem to be recommendable in central Italy. INTRODUCTION Olive is traditionally cultivated in environments potentially subjected to periodical drought. In such conditions olive accentuates its natural tendency to alternate bearing, as canopy and root vegetative development may be negatively affected (Tognetti et al., 2006). Also olive oil quality in environments subjected to severe drought in the summer may be dramatically reduced. Therefore summer irrigation can be considered a tool to maintain a good vegetative balance in order to stabilize high quality fruit production. A rational irrigation management must use a minimal water volume and avoid water loss. To obtain this result, a keen understanding of irrigation influence on root and fruit growth is necessary. Gucci et al. (2007) reported that irrigation can increase the number of fruits per tree, the fruit yield (on both fresh and dry weight basis), the single fruit fresh weight, and the oil content in the mesocarp (on dry weight basis) at harvest and that no differences were found between fully and 46% of the total water requirement (Regulated Deficit Irrigation, RDI) irrigated trees. Leaves, fruits and root differ on sensitivity to water availability (Westgate and Boyer, 1985; Breda et al., 1993) and interact in a complex fashion (Comas, 2005). A combination of factors may limit root production in non-irrigated orchards in dry years, with soil impedance possibly physically restricting root production in dry soil layers and reducing photosynthesis eventually leading to limiting carbon availability for root growth (Richards, 1983). The present study was a whole plant approach focusing on RDI effect on fruit as well as on root development in two national spread olive cultivars subjected to RDI in central Italy. We hypothesized that fruit growth would benefit of a minimum irrigation supply during the most draught period. We also assumed that root (especially fibrous roots) growth rate would also be enhanced with a minimum irrigation supply, increasing significantly but less than proportionally with further addition. Proc. XXVIII th IHC – Olive Trends Symposium Eds.: J. Tous et al. Acta Hort. 924, ISHS 2011