Leaf Initiation Correlates with Time of Maximal Sensitivity of the SDP Xanthium (Asteraceae) to Dark Interruption

Abstract

The hypothesis was tested that the well-known maximal sensitivity to a light break at or near the middle of the dark period of short-day plant Xanthium is correlated with a specific stage of leaf initiation. Samples were collected at various hours before and during noninductive 6-hr dark periods. Lengths of leaf primordia were calculated from serial transverse sections. The reproducible results confirmed that leaf initiation occurred at or near middark under the 18:6 hr light: dark growing conditions. The author suggests the working hypothesis that for a light break to be effective in nullifying the effect of a “long” night in photoperiodically sensitive plants, the light must react with a specific early stage of leaf initiation.