ROOTSTOCK EFFECTS ON WATER RELATIONS OF PEACH TREE UNDER CONTRASTING WATERING CONDITIONS

Water status of trees is affected by evaporative demand, soil water availability and hydraulic conductance of the root system. The objective of this work is to test the possibility of improving water status of peach trees by the choice of the rootstock for variable levels of deficit irrigation. A field experiment was carried out in 2006 and 2008 at the experimental station of INAT-Mornag (36°41’N, 10°15’E) in northern Tunisia with the season-maturing peach cultivar Royal Glory grafted on GF677 and Cadaman rootstocks. Tree water status was characterized by measurements of predawn (Ψ_PD) and midday (Ψ_m) leaf water potentials and stem water potential (Ψ_stem).
Trees were subjected to differential deficit irrigation conditions during the two seasons. Total applied water during the irrigation season (April-September) was in the range 30-60% ETc considered as severe-mild deficit level. Water supply indices taken as ratios between cumulated water supply by precipitation and irrigation (P+I) and evapotranspiration (ET_o) were defined to represent watering conditions of trees over different periods of time: seasonal (K_s), 30-days (K₃₀), 15-days (K₁₅), and 7-days (K₇). During the two years, seasonal water supply indices (Ks) which reflect long term soil water availability, decreased from 1.2-1.3 to 0.4-0.55 between the beginning and the end of the irrigation season. K₇ and K₁₅ representing short term conditions, varied from 0.5 to 0 in 2006 and from 0.8 to 0.15 in 2008 showing better watering conditions during the latter year. Values of tree water potential of the considered scion-rootstock combinations were variable under these levels of deficit. Under mild deficit, better tree water status was observed for GF677, with lowest Ψ_PD and Ψ_stem values of -0.84 and -1.81 MPa respectively. For Cadaman, the values were respectively 0.10 and 0.13 MPa lower compared to GF677 rootstock. Under severe deficit conditions, values of water potential Ψ_PD and Ψ_stem dropped under -1.0 and -2.0 MPa respectively without significant difference between the two rootstocks. It appears that rootstock could be used as a mean to improve significantly soil water use of mid-season cultivars for mild stressed conditions. Investigation of relationships between water supply indices and tree water potential showed a high correlation between Ψ_stem and K_s which could be adopted as good indicator of tree water status over the growing season and used for irrigation scheduling.