Aspects of vegetative and reproductive physiology of dwarf Pharbitis nil

Developmental and growth responses resulting from differences in the status of the endogenous rhythm, phytochrome, and gibberellin (GA). were analyzed in Pharbitis nil. The effects of these three components on floral induction, floral development, stem elongation and leaf expansion were assayed independently to determine if the action of each regulatory component is the same or different in growth and development processes. The status of the endogenous rhythm and/or phytochrome was effected by application of specific photoperiodic treatments known to influence these two components differently. In order to assay effects of both a change in phytochrome status and light perception during a light sensitive phase of the endogenous rhythm, responses were analyzed following exposure of plants to a diurnal (24 hour) short day with a light break at the eighth hour of darkness. In order to show the developmental regulation resulting only from light impingement on the endogenous rhythm, responses were analyzed following exposure of plants to a bidiurnal (48 hour) short day with a light break at the eighth hour of darkness. This light break is followed by a long dark period of sufficient length to allow phytochrome to become innocuous in reference to flowering. The influence of GA status was evaluated for each of the photoperiodic conditions above. The effects of GA were studied by comparing the growth and development of a GA deficient dwarf (strain Kidachi), to that of the dwarf treated with exogenous GA.,, and to that of a normal strain (Violet) which contains abundant endogenous GA.

Normal and dwarf Pharbitis exposed to diurnal long days and diurnal short days with a light break remained vegetative due to combined inhibition imposed by the phytochrome status and light impingement during the photophobe (light sensitive) phase of the endogenous rhythm. However, plants exposed to diurnal light break cycles exhibited development of axillary buds which did not occur in plants subjected to diurnal long days. Application of GAo caused increased stem elongation, increased leaf area, and axillary bud development of plants exposed to diurnal long days or diurnal light break cycles.

Both normal and dwarf Pharbitis subjected to diurnal or bidiurnal short days were induced to flower. Shoot apex analysis of dwarf plants exposed to bidiurnal short days revealed subapical elongation and development of axillary buds, characteristics found only on GA-treated plants from other photoperiods. Both exogenous and endogenous GA increased flowering, stem elongation, and leaf area of plants treated with diurnal or bidiurnal short days. Applied GA^ was sufficient to overcome the stem growth deficiency of the dwarf; however, flower production of the dwarf strain equaled that of the normal only if it received the extended dark period of a bidiurnal short day in conjunction with applied GA-. Therefore, the extended dark period must facilitate synthesis and/or utilization of some factor which enhances flowering.

Pharbitis exposed to bidiurnal short days with a light break were induced to flower; however, the level of flowering was repressed compared to plants that received short day treatments. The length of darkness following the light break was sufficient for phytochrome reversion; therefore, the repression of flowering was due to light effect during a light sensitive phase of the endogenous rhythm. GA^ applied in conjunction with bidiurnal light break cycles increased flowering, stem elongation, and leaf area, but had no effect on the size or shape of developing floral primordia.

These data show that the effects of the endogenous rhythm, phytochrome status, and GA, may be analyzed separately and in combination, and that these components affect development of Pharbitis nil in different ways.