Effect of Irrigation Scheduling on Gerbera Flower Yield and Quality

Better understanding of the effects of irrigation frequency and dose on flower production of gerbera plants (Gerbera jamesonii) can lead to optimal water management and crop yield. Measurements of greenhouse microclimate and production and quality characteristics of a gerbera crop were carried out under two irrigation frequency regimens in soilless cultivation in a greenhouse located in Arta, Greece. Irrigation scheduling was based on solar radiation and performed whenever accumulated solar radiation energy outside the greenhouse reached 1650 kJm -2 (high irrigation frequency (HIF)) or 3300 kJm -2 (low irrigation frequency (LIF)). The amount of water applied was 0.125 mm and 0.250 mm for HIF and LIF, respectively. Stem fresh weight, length and thickness, and number of harvested gerbera flowers were measured along with crop evapotranspiration, crop leaf area, and greenhouse microclimate variables. Measure- ments started 8 months after transplanting and lasted 90 days (May to July). Leaf area, fresh weight, harvested cut flowers, and the main quality characteristics of gerbera flowers (stem length and flower diameter) were unaffected by the irrigation frequency. In the framework of the experiment, simple formulas for calculation of leaf area index were developed. Finally, a first approach study of an alternative remote sensing irrigation control method using a reflectance index was made and the results are presented. Optimal irrigation scheduling could lead to higher water use efficiency, an objective of very high importance nowadays. Adequate supply of water and nutrients results in higher water and nutrient use efficiency, better pro- duction control, and avoidance of stress sit- uations (Raviv and Blom, 2001). Irrigation control involves determination of both timing and quantity of each watering event. Methods for irrigation scheduling are usually based on calculation of crop evapotranspiration com- monly performed by means of an energy balance method (Allen et al., 1998; Donatelli et al., 2006) and estimation of the couple frequency-dose. Automatic control of irrigation events in the greenhouse is usually based on solar ra- diation (Katsoulas et al., 2006; Stanhill and Scholte, 1974). A solar energy integrator transmits a switching-on signal to the water supply system after a preset level of energy is reached. An irrigation control system based on the main factor affecting crop evapotrans- piration in the greenhouse can supply nutrient solution to plants without unnecessary water and nutrient losses (Roh and Lee, 1996). However, this method implies the knowledge of several crop parameters, most of them varied in function to crop phenological stage. Gerbera plants, because of frequent removal of old or damaged leaves, exhibit a fluctuation of leaf area, something that implies extra difficulties when attempting to formulate any model for scheduling irrigation. An alternative, optimal irrigation control approach would also include real-time plant water status monitoring and crop water stress indices. A variety of indices have been used to express crop water status and control ir- rigation events. Leaf water potential and stomatal conductance provide evidence of water stress, but they cannot be measured on- line and accordingly cannot be incorporated in automated systems (Katerji et al., 1988). Cure et al. (1989) found that crop water stress induced changes in the daily patterns of