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plants grow towards the light (particularly young seedlings), a process known as phototropism. This can result in lopsided plants and it is important to ensure, as far as possible, that light is uniform from all directions. Placing a reflecting surface behind a pot (on a window sill for example) can help to reduce bending towards the outside light. A less well-known response is found in the seedlings of some tropical climbers, such as the Swiss cheese plant {Monstera deliciosa), which grow towards the relative darkness of the nearest tree. Temperature
There are also some interesting responses to temperature. A higher temperature during the day has been found to be associated with longer stems, while a lower day temperature results in a more compact and sturdier growth habit. For some time it was widely assumed that the effect was due to the difference between day and night temperature (DIF). A day temperature lower than the night temperature ( - D I F ) would result in shorter plants, while a day temperature higher than the night temperature ( + DIF) would result in taller plants. However, recent work at Horticulture Research International has shown that the effect on elongation is due to the absolute day and night temperatures, rather than to the difference between them. There was a strong positive effect on increasing height with an
Science and the Garden
increase in day temperature in tomato (Lycopersicon esculentum) and chrysanthemum, while night temperature had little or no effect (Fig. 8.2). In many cases (e.g. chrysanthemum, poinsettia {Euphorbia pulcherrima), tomato and Fuchsia spp.) the effect of lowering the day temperature was greatest in the early part of the day. For example, in both tomato and poinsettia, stem length was decreased by dropping the day temperature from 20 to 12°C for four hours at the beginning of each day (this is now known commercially as DROP): lowering the temperature later in the day had much less effect. Unfortunately, this is not always the case and some plants (e.g. Petunia spp.) are more sensitive later in the day when it is more difficult to lower the temperature. Because gibberellin-deficient mutants do not show a response to a drop in temperature, it is suggested that the dwarfing effect of lowering the temperature may be associated with a decrease in the production of gibberellin by the plant and, if so, it would be comparable with the effects of genetic dwarfing (Chapter 3) and growth retardant chemicals (see below). Although not practicable out of doors, lowering the temperature for part of each day, especially in the early part of the day, is feasible in the glasshouse and has been suggested as an alternative to costly chemical sprays.
Fig. 8.2 The effects of day temperature (DT) and night temperature (NT) on stem elongation in tomato and chrysanthemum. Lowering the day temperature has a much greater effect to reduce height than lowering the night temperature. (From F.A. Langton and K.E. Cockshull, Scientia Horticulturae, 69, 229-37, with permission from Elsevier Science, Amsterdam.)