n agriculture and horticulture drought stress is one of the major problems, causing major crop losses every year as well as loss of aesthetic value in ornamentals. In agriculture crop loss is due to reduced numbers of tillers, spikes and grains per plant and reduced grain weight (Farooq et al., 2009). With the global human population rapidly increasing, simultaneously as water scarcity increases, the loss of crop will be even more serious than before. The discovery and development of stress tolerant crops to avoid yield loss during water stress is therefore very important. In the greenhouse industry, energy saving for economic profit is important to be able, but it also affects the plants. To reduce the amount of energy needed for CO2 and heating in the greenhouses, energy-efficient semi-closed greenhouses can be used. In these greenhouses the ventilation is reduced to a minimum, which consequently results in increased relative air humidity inside. This increase in air humidity affects the plants in different ways and might result in plants that are less tolerant to water stress (Torre and Fjeld, 2001).