333 Effect of Two Irrigation Frequencies on Rose Flower Production and Quality C. Kittas, G. Dimokas, Ch. Lykas and N. Katsoulas University of Thessaly, School of Agricultural Sciences, Department of Agriculture, Crop Production and Rural Environment, Fytokou St., N. Ionia, GR-38446, Magnisia Greece Keywords: irrigation scheduling, transpiration, shoot bending, flower wet and dry matter, closed hydroponic system Abstract A better understanding of the effects of irrigation frequency on flower produc- tion and quality of rose plants can help to develop optimal irrigation systems. On this purpose experiments were conducted on a soilless rose crop (Rosa hybrida, cv. First Red) grown in a closed hydroponic system, in a greenhouse located on Eastern Greece. In order to study the effect of high and low irrigation frequency on rose crop flower production and quality, measurements of cut-flower fresh and dry weight, number of harvested flowers and flowering stem length, as well as measurements of microclimate variables were carried out. Irrigation scheduling was based on crop transpiration and irrigation was performed whenever accumulative solar radiation outside the greenhouse reached 470 kJ m -2 (high irrigation frequency) or 940 kJ m -2 (low irrigation frequency). The amount of water applied was 0.2 mm and 0.4 mm for high and low irrigation frequency, respectively. The results showed that irrigation frequency affected cut flower fresh and dry weight since the total values of cut flower fresh and dry weight measured at the end of the experimental period were 413 and 310 g m -2 for high and 120 and 89 g m -2 for low irrigation frequency, respectively. As far as the number of cut flowers is concerned, the results showed that the higher the irrigation frequency, the higher the production, since the total number of cut flowers measured at the end of the experimental period was 20.7 and 16.2 flowers per green- house m -2 for high and low irrigation frequencies, respectively. Finally, the results showed that the quality of rose flowering shoots was not affected by the irrigation frequency. In conclusion, it seems that the higher irrigation frequency improved the biomass production but did not affect the quality of harvested flowers. INTRODUCTION There is a world-wide increase in the use of hydroponic systems in horticulture. The main reasons for developing these systems are: the possibility of obtaining high yields, control of the growing conditions, and efficient use of fertilisers and water. However, many of these systems lack sophisticated equipment for proper irrigation scheduling. Optimal irrigation scheduling of greenhouse soilless crops is very important since it influences the rhizosphere environment, media water potential and salt accumula- tion, which in turn affects plant growth and photosynthesis and consequently crop production and quality (Lizarraga et al., 2001). Irrigation control involves the determination of both timing and quantity of water application. In the case of greenhouse crops, climate sensors located inside the greenhouse (temperature, humidity, CO 2 concentration) or outside (global radiation) are generally available and provide information that can be used not only for climate control, but also for irrigation scheduling. The most common and simple method used until now consists in estimating the crop transpiration by means of the radiation-based method (Morris et al., 1957; Stanhill and Scholte, 1974). The radiation method is considered by some authors the best way of scheduling irrigation in greenhouses (Abou Hadid et al., 1994). A solar integrator gives a starting signal to a water supply system after a previously set level of radiation is reached (De Graaf, 1988). An irrigation system controlled by a solar radiation method can satisfy the appropriate supply of nutrient solution to plants Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005