A four-year study was conducted on young Olea europaea L. trees to investigate the long-term effect of deficit irrigation on some physiological process, in particular gas exchange and fruit growth, productive response and oil yield and oil quality, of young trees grown in the most suitable part of Tuscany for olive growing during the onset of fruit production.
Water was supplied by subsurface drip irrigation to satisfy fully (FI), or partially (DI, SI) of tree water needs during the summer. Tree growth was reduced by deficit irrigation, whereas return bloom was not. Fruit yield of DI trees was 68% that of FI ones, but the fruit yield efficiency of FI and DI treatments was similar. Fruit set and the number of fruits of FI trees were similar to those of DI trees and significantly higher than in SI trees. No significant differences in fruit fresh weight were found between FI and DI trees. The oil yield and oil yield efficiency of the deficit irrigation treatment were 82% and 109% that of fully-irrigated trees, respectively.
Irrigation affected fruit growth and final fruit size. Fresh weights of whole fruit and mesocarp were similar between fully- (FI) and deficit-irrigated (DI) trees. The flesh/pit ratio of DI trees was 13-25% (DW) higher than that of FI-trees in three out of the four years, but similar when expressed as FW. In the last two years of the study the highest values of the flesh/pit ratio were measured in trees with PLWP ranging between -0.8 and -1.5 MPa.
The irrigation regime had negligible effects on the free acidity, peroxide value, and fatty acid composition of VOO, but strongly influenced its phenolic concentration. Trees with high water status yielded oils with lower concentrations of total phenols and O-diphenols with respect to oils from severely stressed trees. The concentrations of secoiridoids, like the dialdehydic form of decarboxymethyl elenolic acid linked to (3,4-dihydroxyphenyl)ethanol (3, 4-DHPEA-EDA), the isomer of the oleuropein aglycon (3, 4-DHPEA-EA) and the dialdehydic form of decarboxymethyl elenolic acid linked to (p-hydroxyphenyl)ethanol (p-HPEA-EDA), were lower in FI than in SI treatments. The concentrations of lignans (+)-1-acetoxipinoresinol and (+)-1-pinoresinol were unaffected by the irrigation regime.
Gas exchange parameters were markedly influenced by the irrigation regimes. The highest values of net assimilation (A) and stomatal conductance (gs) were measured in fully irrigated trees. As the water status showed increasing water deficit and A and gs according to a negative exponential model in both years. Moreover, the increase in A and gs upon relief of water deficit lagged behind the recovery in pre-dawn leaf water potential (PLWP). There was a tight correlation between PLWP and MLWP and between PLWP and SWP.