Abstract
A number of dry-down/recovery experiments were performed in three different growing seasons to determine stomatal changes in fully-expanded leaves of olive (cvs. "Frantoio" and "Leccino") plants grown in containers with different watering regimes or by withholding irrigation for up to four weeks. The level of water deficit was assessed by measurements of pre-dawn leaf water potential, whereas gas exchange parameters were measured by infrared gas analysis (IRGA). Stomatal density and aperture were determined by Low Temperature Scanning Electron Microscopy (LTSEM). The water use efficiency of cv. "Leccino" was significantly higher than that of cv. "Frantoio" mainly because of higher leaf conductance of the latter cultivar under well-watered conditions. In plants subject to severe water deficit (pre-dawn leaf water potential less than -1.5 MPa) the differences in water use efficiency disappeared. A more marked decrease in leaf water potential was measured in the cv. "Frantoio" than in cv. "Leccino" during dry-down cycles. Both cultivars recovered equally well upon relief of stress. Low temperature scanning electron microscopy images showed that stomata of cv. "Frantoio" were larger and more open than those of cv. "Leccino", confirming results of gas exchange measurements by IRGA.