SOME ASPECTS OF WATER-RELATIONS IN MACADAMIA

Changes in water relations of young potted macadamia (Macadamia integrifolia, Maiden and Betche) trees during gradual onset of stress were studied in a glasshouse. Critical levels of water status beyond which irreversible damage occurs were determined. Relative water content (RWC, R² = 0.87**), and leaf (_L, R² = 0.92**) and xylem (_x R² = 0.78**), water potentials all declined exponentially during the first 2 weeks to levels of ca 50% RWC and of less than -4.0 MPa. The decline in Osmotic potential (, R² = 0.53**) was linear, but variable. _L was highly correlated with _x (R² = 0.96**) and with RWC (R² = 0.87**). Stomatal conductance declined sharply between -1.0 to -1.5 MPa by 80%, indicating sensitivity of stomata to water stress. Soft, young leaves wilted at _L of less than -2.4 MPa but mature hardened leaves did not develop visible stress symptoms until _L of -4.1 MPa was reached, although physiological damage may occur before wilting. Visible stress symptoms coincided with zero turgor (P = 0). At less than -5.0 MPa, leaves suffered irreversible damage. These responses to developing water deficits are being applied to bearing macadamia trees in a through-draining lysimeter in which water is withheld at different phenological stages. Since macadamias appear to tolerate moderate levels of water stress, an attempt is being made to identify the critical stage or stages at which such deficits adversely affect yield and quality of nuts. From this, deficit irrigation strategies which maintain yield and quality may be developed.