Treating Washington Navel orange as a photoperiod
sensitive variety, it is nevertheless possible to suggest at least
three ways in which photoperiod leads to floral initiation.
One is that at 24°/19°C exposure to short days leads via the
phytochrome pigment to the postulated but elusive florigen;
exposure of plants to long days or to high temperatures
(30°/2S°C) does not lead to florigen production and hence
no flowers are formed. Secondly, it can be postulated that
long days at 24°/19°C and all photoperiods tested at
30°/2So
C lead to the formation ofspecific flower inhibiting
substances; in this approach all of the plants irrespective of
treatment are considered to be capable of flowering but
only those (plants at 8 or 12 hour photoperiods at 24°/19°C