GAS-EXCHANGE AND WATER RELATIONS IN FIELD-GROWN SAUVIGNON BLANC GRAPEVINES

Two post-veraison irrigation levels, 1.0 and 3.5 mm/day [low (L) and c ontrol (C) treatments, respectively] were applied in an eight-year-old Sauvignon blanc vineyard. Simultaneous measurements of gas exchange p arameters and leaf water potential (LWP) were made on the same leaves four weeks after veraison. Midday LWP at veraison was -1.2 MPa. A sign ificant difference between treatments developed from veraison to mid-A ugust. Midday LWPs in mid-August were -2 MPa and -1.4 MPa for the L an d the C treatments, respectively. Photosynthesis rate (A) was found to decline with LWP with no differences between treatments. The similar ratio of stomatal conductance (g(s)) to A in both treatments suggests that water stress did not affect the photosynthetic apparatus. However a temporary decrease in non-stomatal conductance was detected below L WPs of -1.5 MPa. Similar g(s)-LWP relationships in both treatments ind icates that osmotic adjustment was not responsible for the differences measured between the two irrigation treatments. A direct effect of va por pressure gradient on g(s) was indicated.