Water stress of a greenhouse crop results in a lower-than-normal transpiration and photosynthesis rates, which affects the humidity and CO 2 concentration of the greenhouse air. This change may be detected and used for stress diagnosis if the signal-to-noise ratio is sufficiently large. Under midday conditions, when water stress is likely to occur, the greenhouse is often ventilated, resulting in a rather small signal-to-noise ratio. It is suggested here that the signal level may be increased by exciting the system, namely by suspending the ventilation for a short period of time. The transient response of temperature, humidity and CO 2 concentration to the abrupt change of ventilation may be monitored, analysed and compared to a modelled normal response for the same greenhouse with an unstressed crop. A significant discrepancy between the two would indicate that the crop is stressed.
Lawn, serving as a crop substitute, has been submitted to several irrigation-drought sequences in a small experimental greenhouse. Each day, in the early afternoon, ventilation was suspended for three periods of 30 min each. Differences in the response of CO 2 concentration enabled the detection of all stress periods as soon as the rate of transpiration decreased to below normal. The humidity response was slightly less sensitive, and the temperature response was the least sensitive. Water-stress detection via the excitation method was considerably better than detection based on observations at full ventilation.