LIGHT-DEPENDENT TEMPERATURE PROGRAMMES FOR EARLY-SOWN TOMATOES
Despite the success achieved by this growing technique, we should not dismiss from our minds the possibility that plant performance might be even further improved. Light-dependent control of glasshouse temperatures might indeed lead to maximum rates of photosynthesis being approached throughout the day. There is some scientific evidence to justify this approach, e.g. Gaastra (1963), which seems especially relevant now that CO2 enrichment has become an established technique in early tomato production.
Testing the theory on the basis of crop production, however, is by no means straightforward since a number of basic difficulties are immediately apparent. Firstly, a single thermostat setting for the day temperature will not result in a constant temperature being maintained throughout the day if appreciable amounts of solar radiation enter the glasshouse. There will, inevitably, be some degree of temperature modulation with solar radiation levels since all glasshouses are, by nature, "heat traps". Secondly, without means of cooling by refrigeration the precise implementation of a light-dependent temperature programme could only be achieved by using ventilators to dissipate heat when lower temperatures were required. However, this procedure would lead to repeated depletions of the CO2 enrichment levels, seriously reducing photosynthesis to the detriment of the crop. There is, in any case, a danger that one temperature programme will affect ventilation periods, and consequently CO2 enrichment levels, differently from another programme, so that temperature and CO2 enrichment effects might be confounded. Despite these difficulties, an attempt is being made at the Glasshouse Crops