ANALYSIS OF HEAT AND WATER STRESS OF PLANTS IN GREENHOUSES

Article number

107_12

Pages

71 – 78

Language

Abstract

Most analysis and experiments on plant productivity in greenhouses emphasize the problems of suboptimal levels of light, of temperature and of carbon dioxide, as expected in cool seasons and northern latitudes.

We attempt here to analyze the greenhouse climate in the warm season as found in southern latitudes, when high levels of radiation, temperature and humidity occur at the same time.
In such conditions, cooling and ventilation are needed.
Nevertheless, the plants may unavoidably be exposed to extreme and unfavorable conditions.

We have, as an example, made calculations for the summer climate of Southern-Central Texas (latitude 30°N). It is clear, hot and humid, the latter due to the proximity of the Gulf of Mexico and prevailing southerly winds and it would seem almost impossible to make use of a plant enclosure during that time without heavy shading and ventilation.

Using a previously developed model for greenhouse energetics and plant behaviour, we have estimated heat stress, indicated by the canopy temperature, and water stress, indicated by the canopy water potential and leaf resistance.
The details of the model are given elsewhere (van Bavel and Sadler, 1979).

The analysis has been applied to four methods of greenhouse construction:

GLV: a greenhouse with a glass roof and thermostat-controlled, forced cross-ventilation at 100 air changes per hour.

GLC: the same as GLV, but with evaporative cooling pads with recirculating water in the intakes.
It was assumed that the system worked at 100% efficiency, that is, that the incoming air was at the wet bulb temperature of the outside air.

FRT: a fluid-roof greenhouse with an insulated storage tank for the filter fluid of 200 L per m² floor space, as described by van Bavel and Damagnez (1978). The ventilation system was identical to that under GLV.

FRG: the same as FRT, but with cooling of the fluid in the storage tank by groundwater, using a liquid-to-liquid heat exchanger.
The temperature of the groundwater in College Station, Texas is about 22°C.

The local summer climate is described by the following typical figures: