Abstract
In greenhouse production of a number of vegetable and ornamental plant species, a short diurnal temperature drop in the end of the night or in the morning is commonly used to reduce stem elongation as an alternative to chemical growth retardants. Experiments were carried out to quantify the effects of a temperature drop at different times of the day and night on growth and photosynthetic activity of young cucumber plants. During 6 days plants were exposed daily to a temperature of 10 °C for 2 h at the beginning, in the middle and at the end of the night and day periods. The results have shown that plant response to drop may be qualitatively different in the light and darkness. While strongest effects of drop are observed when it is given in the daytime, for practical application in greenhouses it is more appropriate to reduce temperature at night. However, it may not be strictly necessary for cucumber seedlings to apply drop at the end of the night as it was stated in the literature. Thus, our results may cast doubt on the following statements: (a) temperature drops are not effective when delivered at other times of the day or night (except before sunrise), (b) optimal time for drop effects depends on the daily dynamics of stem and petiole elongation rate. It is rather drop itself is capable of modifying the dynamics of plant growth in the daily cycle.
Highlights
Plant growth retardants are widely used as tools for height control in order to obtain more compact plants
The greatest retardation of the leaf petiole growth was noted when DROP had been given in the middle and at the end of the day, while the smallest inhibition occurred when it had been given in the middle and at the end of the night (N2 and N3)
There were no significant differences between leaf area of plants treated by DROP at the beginning, in the middle or at the end of the day as well as between treatments at different times in the night
Summary
Plant growth retardants are widely used as tools for height control in order to obtain more compact plants. It has long been known that stem elongation can be controlled by temperature treatments rather than by the use of chemical growth retardants (Moe & Heins, 1990). There is evidence that a temperature drop in the middle of the night had small effect on plant height and petiole length in Euphorbia pulcherrima (Moe et al, 1992) and Begonia x hiemalis (Moe & Mortensen, 1992; Grindal & Moe, 1994). In our previous studies we have shown that a temperature drop at the beginning, middle or end of the night was effective in reducing plant height and petiole length in cucumber (Sysoeva et al, 1997, 1999). It is suggested that a temperature drop at the beginning of the day is much more effective than later during the day in Euphorbia pulcherrima (Ueber & Hendriks, 1992) and Pelargonium (Ueber & Hendriks, 1995), and that a temperature decrease at any time during photoperiod may be effective in inhibiting stem elongation as a temperature decrease at the beginning of the day in Fuchsia, Antirrhinum majus L., Petunia and Salvia splendens F. (Erwin & Heins, 1995)
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