CONTROL OF PLANT MORPHOGENESIS WITHOUT GROWTH RETARDANTS.

Control of plant height (internode length) and other plant characteristics by means of negative DIF (difference between day and night temperature) or 2-hour temperature drops at different times of the 24-hour growth cycle have been investigated in some greenhouse crops. The experiments were carried out in a phytotron with accurate control of the temperature treatments. The different temperature treatments were given during propagation and/or further growth of the plants. In general, short-day plant (SDP) such as poinsettia was sensitive to temperature drop during the last period of the night and just after dawn. Also tomato and greenhouse cucumber plants responded to a 2-hour temperature drop in the morning. With a temperature drop of 10 or 9°C the plant height at transplanting was reduced by 24% in cucumber and 38% in tomato, respectively. Thus a temperature drop was an effective way to control plant height in poinsettia and in young plants of tomato and cucumber without the use of growth retardants.

However, experiments with some pot herbs (Melissa officinalis L. and Ocimum basilicum L.) and bedding plants were not much affected by a 2-hour drop in the morning. But increasing length of the cool temperature treatment (more like DIF), resulted in efficient height control. It is concluded that negative DIF has a stronger effect on plant morphogenesis than a short temperature drop.

The impact of the length of the photoperiod (dark period) in relation to temperature drop and DIF on plant morphogenesis will be discussed.