Articles
INTENSITY EFFECTS OF A TEMPERATURE DROP ON PELARGONIUMS
This may be explained by the intensity, the duration and the time of the temperature drop, the day length and interactions with other climatic factors.
As investigations with pelargoniums and poinsettias have shown, the effect of a temperature drop is largely governed by the intensity of the drop.
It was also found that an intensive and short duration temperature drop resulted in higher growth retardation than a moderate temperature drop over a longer period with the same sum of temperature reduction.
Whereas the effects of a moderate temperature drop may be explained by the lowering of the mean day temperature, it is not possible to explain the effects of an intensive temperature drop by this mechanism.
It is thus assumed that after-effects occur when a critical temperature threshold is exceeded.
The critical temperature range for the investigated pelargonium variety is about 2 – 6 °C.
In addition to intensity and duration, the effect of a temperature drop is also modified by the point in time at which it occurs.
After exceeding the species specific critical temperature threshold, cooling treatments applied to pelargoniums at the beginning of the light phase resulted in greater retardation of stem growth than later in the day.
Parallel investigations performed in growth chambers gave similar results to investigations under natural light conditions.