Localization of Photoperiodic Perception in Helianthus tuberosus

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Abstract
H. tuberosus was grown continuously on photoperiods of more than 18 hrs., under which conditions it did not form tubers. Tubers were formed if the plants were exposed to continuous short photoperiod (9 hrs.), exposed to an induction period of 16 short photoperiods, or if one leaf on a plant was exposed continuously to short photoperiod. They also formed tubers as the result of being grafted to a plant which was itself forming tubers as the result of exposure of all or one of its leaves to short photoperiod. They did not form tubers as the result of exposure of the terminal bud to short photoperiod. It is concluded, contrary to Zimmerman and Hitchcock''s deduction that the terminal bud is the locus for the perception of the initial photo-periodic stimulus, that the expanded leaves are the locus of such perception. It seems possible, in agreement with the above authors, that tuberization may be stimulated by some substance or substances with hormone-like properties.

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