Temperature: Heating and Cooling
II. Using Temperature to Control Plant Growth
Temperatures can also be used to steer the plant towards a particular growth pattern. In the long cropping cycles typical of greenhouse production, tomatoes tend to cycle between being overly vegetative at the beginning of cropping (too much growth and too little fruiting) and later being overly generative (too little growth and excessive fruit loads). Where uniform production is desired, it is helpful to be able to moderate these swings in crop productivity. The following guidelines [29] are sometimes used by growers to balance plant growth, although the data on which they are based is more empirical than experimental in origin.
The first problem is to decide if the plant is growing well or is imbalanced. Characteristics to look for in a well-balanced, well-nourished plant are: a stem 1 cm thick at a point 15 cm below the growing point; dark green leaves, and large, closely spaced, readily setting flower clusters.
Low light and low transpiration tend to promote vegetative growth. In an overly vegetative plant, stems are thicker and fruitset and yields are low. Flowers appear far down from the top of the plants, open slowly and incompletely and are pale yellow. The uppermost leaves are flat, soft, long; light colored, and may have a somewhat mottled appearance. The cluster stem is thin and long. Fruit will be slow to develop, few in number, and may be misshapen.
To steer the plant to a more generative growth pattern using environmental manipulations, the difference between day and night temperatures can be increased by up to 5°C and temperatures reduced more quickly in the early evening when going from day to night set-points. Greenhouse temperatures should be raised, with heat directed at the third truss beneath new flowers. The relative humidity should be lowered and ventilation should be increased. Increasing transpiration reduces turgor pressure and inclines the plant to generative, rather than vegetative growth. CO2 enrichment also encourages generative growth.
Sunny, cold weather and high transpiration promote generative growth. In an overly generative plant, stems are thinner (indicating lack of carbohydrates), growth is slow, and trusses are short and horizontal. Dark yellow flowers appear immediately below the top of the plant, and open quickly. Although fruit are large, well shaped, and develop rapidly in an overly generative plant, over the long term, yields will be reduced because growth is reduced at the top. Leaves at the very top of a too generative plant develop slowly resulting in short, dark, strong leaves, which may be curled under.
To correct an overly generative plant, day temperatures are lowered to re-direct assimilate from the already-set fruit to the top of the plant and the developing trusses. Lowering night temperatures will slow down fruit ripening, prolonging the problem of too much assimilate going to the older fruit, so this is not recommended for an overly generative plant. Reducing transpiration through raising relative humidity or reducing ventilation also stimulates vegetative growth.