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Influence of Light and Temperature Environments Preceding and During the Dark Period on Floral Initiation in Pima Cotton (Gossypium barbadense L.)¹

Floral induction in American Pima cotton, (Gossypium barbadense L.), cultivars ‘Pima S-3’ and ‘Pima S-4’, occurred earlier (at lower node levels on the main stem) under short winter days than under long summer days when plants were grown under natural light and a controlled temperature program.

The intensities of red and far red light in natural sunlight were measured to determine if there were daily or seasonal changes in spectral quality that might be related to variations in floral initiation. Red light predominated during the day, but the balance shifted to a predominance of far red light between sunset and darkness. This relationship was not seasonal.

Total exclusion of all light from plants during the 1.5-hour period before sunset accelerated floral induction if the temperature during this period was above 30 C, but either had no effect or retarded induction if the temperature was below 30 C. Shortening or interrupting the natural dark period with low intensity lights of different spectral composition retarded induction. The intensity of blue light, but not red or far red light, appeared to be related to induction. The increase in the node number of the first fruiting branch was almost directly proportional to the time interval between sunset and the interruption of the dark period.

Floral induction was most sensitive to photoperiod and to high temperatures during sunset during the 2nd week of development after germination.

Key Words: Photoperiod • Flowering • Spectral quality • Red light • Far red light • Blue light

² Plant Physiologist, U. S. Western Cotton Research Laboratory, 4135 E. Broadway Road, Phoenix, Ariz. 85040.

Received for publication March 1, 1972.