3. Results
Time until flowering varied from 48 to 59 days in the different cultivars (Table 1). The different growth parameters
as well as the keeping quality and amount of powdery mildew also varied between the cultivars. Amber
and Absolute Hit were the most mildew-sensitive cultivars. Although the magnitude of the effects of PFD, LP
and the timing of the lighting period (DL) on growth, flowering and keeping quality varied to some extent between
the cultivars (illustrated by the standard errors) in principle the effects were the same in all cultivars.
Therefore, the mean effects on the six cultivars are presented. Increasing the LP from 16 to 24 h∙day−1 decreased
time until flowering from 58 to 53 days at 100 μmol∙m−2∙s−1 PFD and from 54 to 45 days at 150 μmol∙m−2∙s−1
PFD (Table 2). Total number of flowers (buds included) increased 20% to 30% by increasing the PFD from 100
to 150 μmol∙m−2∙s−1 PFD. Increasing the LP from 16 to 20 h∙day−1 increased the total number of flowers by 40 to
50% , while a further increase in LP had only a small effect (Table 2). Plant dry weight increased about 25%
when LP increased from 16 to 20 h∙day−1, while a further increase in LP had less effect. Increasing the PFD
from 100 to 150 μmol∙m−2∙s−1 (50% increase in PAR) caused a 30% to 40% increase in the mean growth rate
until flowering. Increasing the DL from 16 to 20 h∙day−1 (25% increase in PAR) caused an increase in the rate of
30 to 40%, while a further increase to 24 h∙day−1 only slightly (