Photoperiodic Induction as Influenced by Environmental Factors

Abstract

1. The leaves of Xanthium plants are the locus of photoperiodic induction. Induced leaves, when grafted to vegetative plants, continue to supply a stimulus for flower initiation when exposed to continuous long photoperiod. 2. If during each 24-hour period, Xanthium plants are exposed to 5-hour dark periods at 40⚬ F. alternating with 9-hour photoperiods at approximately 70⚬, then six to eight such consecutive cycles of dark period and photoperiod are required for photoperiodic induction. A treatment with four such cycles produces a residual effect which lasts more than 24 hours. 3. In these experiments, Biloxi soybeans flowered only after exposure to consecutive long dark periods, each alternating with a short photoperiod. Plants did not flower, regardless of the number of long dark periods they received, unless at least three of the dark periods were in consecutive order. 4. Soybeans exposed to an induction period of three or more consecutive long dark periods, and which produced as a result a certain number of flower primordia, would not produce new primordia subsequently unless they were again exposed to another induction period of three or more long dark periods. 5. The length of the critical dark period for Xanthium is not constant. It decreases slightly in length with increasing age of the plant and varies slightly with variations in humidity. If plants are shaded during their photoperiods their critical dark periods increase slightly in length. 6. Variations in temperature greatly affect the length of the critical dark period, that of plants at 70⚬ F. being approximately 81/3 hours while that of plants at 40⚬ is approximately 11 hours.