An experiment was conducted to determine the optimum water requirement of drip irrigated tomatoes cultivated under greenhouse conditions. The study was conducted at the Agricultural Research Center, Ahwaz, Iran from February to June 2003. One and three irrigations per day at 100, 80, 60 and 40% of crop evapotranspiration (ETc), based on the Penman-Monteith (PM) method, were tested for their effects on crop growth, crop yield, and water use efficiency (WUE). Tomato (Lyeopersicon esculentum var. Troy 489) plants were grown in a poly-net greenhouse. The results showed that the amount of water applied significantly affected crop growth, yield and WUE, while irrigating once or three times per day did not have a significantly affect on crop performance. Drip irrigation at 80% of ET, provided the maximum crop yields and WUE suggesting that the optimum water requirement for this variety of tomato is nearly 80% of the ETc. Based on this, the water requirement for tomatoes in a tropical greenhouse can recommended at 5.2-6.4 mm day(-1) or 0.37-0.46 L plant(-1) day(-1). The ratio of calculated ET, based on the climatic parameters inside to outside the greenhouse was 60-85%.
