REDUCING TRANSPIRATION BY AN EVAPORATIVE PAD TO DELAY SOLUTE ACCUMULATION IN CLOSED-LOOP FERTIGATION SYSTEMS

     A theoretical energy budget analysis of a greenhouse rose crop (Rosa hybrida cv Mercedes) served to define the cooling and transpiration reduction obtained by operating a wet pad as a function of outdoors humidity and the rate of ventilation.
Experiments were conducted with roses grown on artificial substrates irrigated either in a freely draining system (mean transpiration = 4.1 mm day^-1, EC = 1.9 to 2.1 dS m^-1) or a closed-loop system recirculation the nutrient solution (mean transpiration = 3.2 mm day^-1, EC = 1.9 to 4.5 dSm^-1). The wet pad treatment decreased transpiration (free drainage, mean transpiration = 3.1 mm day^-1, EC = 1.9 to 2.1 dS m^-1) and delayed the accumulation of solutes in the closed-loop system (closed loop, mean transpiration = 2.5 mm day^-1, EC maximum = 3.5 dS m^-1). Stomatal conductance of roses grown in the recirculated solution was lower than in the drained system.
The wet pad treatment mitigated leaf water potential drop due to high solute concentration of the closed-loop system during the hours of the day with high transpiration rate, but produced no measurable effect on stomatal conductance.
As predawn leaf water potential was not affected, the mid-day potential drop expresses a temporary stress response to the increased salinity of the nutrient solution.