Shimshi, D., Agricultural Research Organization, Gilat Experiment Regional Station, Mobile Post Negev 2, Israel

An irrigation experiment was conducted on wheat in the northern Negev, Israel. The growing season rainfall was 198 mm; six irrigation treatments, ranging from 0 to 320 mm were applied at different stages of growth. The grain yields ranged from 1.20 to 5.84 t/ha. Stomatal aperture was evaluated by leaf permeability, as measured with a fast-reading viscous flow porometer. Other indices of soil-plant water status measured were: soil moisture with a neutron probe; leaf water potential with a pressure chamber; CO2 uptake with a 14CO2-pulse apparatus; and leaf water saturation deficit. For the penultimate and flag leaves, midday leaf permeability was highly correlated with the soil moisture in the upper 60-cm layer. CO2-uptake, however, remained constantly high (ca. 0.8 mg m-2s-1 = 29 mg dm-2h-1) throughout a wide range of leaf permeability, from 10 down to 2 porometer units (p. u.); below this value, it decreased linearly with leaf permeability. Therefore, the value of 2 p. u. was tentatively regarded as a critical value for judging the critical values of the other indices studied; these were estimated to be: leaf water potential, -1.57 MPa = -15.7 bars; leaf saturation deficit, 18,8% and soilmoisture, 12.6% representing a 83% depletion of the available moisture in the Gilat soil. The grain yield was highly negatively correlated with the duration of period when the soil moisture was below these critical values. The use of the porometer method for evaluating water stress is discussed. © 1979 Springer-Verlag.