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EFFECTS OF INTENSITY, DURATION AND TIMING OF A TEMPERATURE DROP ON THE GROWTH AND FLOWERING OF EUPHORBIA PULCHERRIMA WILLD. EX KLOTZSCH
A moderate temperature drop from 24 to 16°C reduced plant height by between 5 and 25% depending on duration.
No significant effects on flowering were obtained.
Even with an 8 hour drop, the visible bud stage was only delayed by about 4 days.
In contrast, a large temperature drop from 24 to 8°C resulted in much greater height control.
Even a 2 hour drop reduced stem elongation by about 50 %. This effect was increased to 75 % by a drop lasting longer than 4 hours but was also accompanied by leaf chlorosis and a very strong delay in flowering.
No effects of timing (position effect) were shown with a moderate temperature drop, but with a large temperature drop, plants were more sensitive to a drop after sunrise than to one later in the day.
The effects of a moderate drop followed the basic temperature curve for poinsettia and could be explained by a reduction in DT or DMT. In contrast, the effects of a large, brief temperature drop were much greater than were given by a moderate, long duration drop with the same ‘drop sum’, and cannot be explained on the basis of DT or DMT reduction.
These large temperature drops seem to exceed the integrating range for temperature response and induce after-effects, which cannot be compensated for by a following period of higher temperature.