Summary:

The experiment was carried out to determine the effects of different irrigation water levels on yield and some quality parameters of lettuce (Lactuca sativa L. Var. Longifolia cv.) under greenhouse conditions. Irrigation water was applied through drip irrigation method with 7 days interval during the total growing season. Irrigation treatments consisted of six different water levels (I1: non-irrigation, I2: 25 %, I3: 50 %, I4: 75 %, I5: 100 % and I6: 125 % of cumulative evaporation measured from Class A pan). The amount of irrigation water ranged between 0 and 106.3 mm among the treatments. Evapotranspiration (ET) values varied from 69.1 to 158.5 mm for the treatments. The highest yield was obtained from the I5 followed by I4. A significant polynomial relation (R2=0.77) was obtained between the yield and irrigation water, and linear relation (R2=0.99) was obtained between the yield and ET. This indicated that when irrigation water increased, yield also increased to a certain point. However, when the amount of irrigation water exceeded the plant water requirement, lettuce yield decreased. Yield response factor (Ky) was determined as 1.97. Since Ky ˃ 1, lettuce was very sensitive to water deficiency. In addition, the highest water use efficiency (WUE) and irrigation water use efficiency (IWUE) values were calculated in the I5 with 0.60 t ha.mm-1 and 0.96 t ha.mm-1 respectively. The best plant quality parameters which were plant weight (239.11 g), plant height (26.30 cm), plant diameter (44.82 cm), root weight (17.31 g), root length (9.02 cm), root diameter (10.04 cm) and leaf number (49) were also obtained in I5. As a result, it was suggested that I5 can be the most appropriate irrigation water level for lettuce with higher WUE, IWUE, yield and quality under greenhouse conditions.