ENVIRONMENTAL CONTROL OF GLASSHOUSES
Factors of the aerial environment which will be touched on in this paper are air temperature, relative humidity and carbon dioxide content. Ait movement must also be an important factor but this is not normally under any appreciable degree of control.
There are also several factors comprising the environment of the roots, namely soil temperature, moisture regime, nutrition, aeration and volume of soil, and some of these may be controlled.
There are two problems in the control of any environmental factor, firstly to obtain control at a chosen level and secondly to choose that level at the optimum value for the crop. This choice often requires knowledge which has not yet been acquired and so experiments may be needed to give the required information. The best experiments can only be done if all the important environmental factors which can be controlled are under control; for example if the effect of temperature on tomatoes is being investigated the air temperature by both day and night, that is both by heating and ventilation, must be automatically controlled and in addition it is desirable for watering and nutrition to be standardised at near the optimum levels. Figure 1 shows the effect of daytime and nighttime air temperature on the growth of young tomato plants. For plants grown at the same-night-time temperature higher day temperature gives greatly increased rate of growth as indicated by the height of the plants. On the other hand, for plants grown at the same day temperature a higher night temperature gives only slightly increased growth rate.
The effect of air temperature on the setting of tomato fruit can be illustrated by the following experiences.
With day temperature 62° fahrenheit and night temperature 56° fahrenheit, there are large flower trusses with very poor setting. When the night-time air temperature is increased to 62° fahrenheit the setting is still poor; also chimera can be seen on the plants. If the night temperature remains at 56° fahrenheit but day temperature is increased to 68° fahrenheit the setting is very good. This shows that fruit setting is influenced by day temperature and not by that at night and an adequate temperature by day is 68° fahrenheit (20° centigrade).
Figure 2 shows the effect of day and night temperature after planting on the proportion of plants with part of the stem sterile because of chimera. Night-time air temperature has no apparent effect, but that by day shows a clear relationship, with increased day temperature giving reduced chimera.
The foregoing examples illustrate (1) the importance of distinguishing between day temperature and night temperature (2) the greater importance of air temperature by day than by night, and hence (3) the importance of controlling daytime air temperature by means of automatic ventilation.
In the above experiments the plants were presumably responding to their own temperature rather than that of the air. Thus under different conditions of radiation exchange the apparent response of air temperature might be somewhat different, since the discrepancy between