Locus and Physiology of Photoperiodic Perception in Plants

Abstract

Photoperiodic responses of intact plants whose bases and tops have been exposed to contrasted length of daylight, indicate that the stimulus is not normally transmitted from the exposed area. 1 Flowering is usually restricted to the zone receiving the reproductive photoperiod. More recently, however, by the use of reciprocal grafts of photoperiodically sensitized plants, it has become possible to demonstrate stimulus transfer from one part to another across the graft union. 2 Opaque boxes and cloth bags have also been employed in studies on the interrelationships of contrasted photoperiods on adjacent parts. 1 , 2 , 3

The graft technique is not wholly satisfactory in studies of this sort, however, because it is difficult, entails delay and post-operative care, causes considerable mortality and involves the extraneous stimulus of severe traumatism. The results of whip, cleft and bud grafts vary greatly. 4 , 5 The use of small bags or boxes impairs gas exchange and creates temperature differences between the exposed and shaded regions, thereby introducing possibility of error since the developmental temperatures of apical meristems also influence the reproductive behavior of many plants. Bagging is tedious and apt to involve injury whenever a large number of multistemmed plants is employed.

In order to minimize these difficulties and at the same time employ contrasted photoperiodic exposure on contiguous parts, plants were grown to a convenient height under vegetative light conditions on opposite sides of a thin, opaque panel having a long, 2-inch horizontal slit. Tops of plants were trained through the slit to various desired lengths from both sides of the panel, and one side was then given long day and the other short day illumination. Long photo-periods were provided by use of automatically controlled electric lights hung from pulleys.