Normal tuber formation in the potato (Solanum tuberosum L.) involves two closely related yet distinctly different processes: development of the rhizome at an underground node, followed by tuber formation at the subapical region of the rhizome (Booth, 1963). These processes in Solanum species, especially S. tuberosum subsp. andigena Hawkes, are controlled by environmental factors, primarily temperature and photoperiod, that regulate levels of endogenous growth substances. Rhizome growth is favored by long days (Chapman, 1958) and, when coupled with high temperatures, “noninducing conditions” are created that result in poor tuber development (Gregory, 1956; Slater, 1968). Noninducing conditions also result in elevated gibberellin levels in potato plants (Okazawa, 1959, 1960). Conversely, Gregory (1956) also noted that short days and low night temperatures (“inducing conditions”) promote tuberization, and Dostal (1945) and Madec (1963) provided evidence for a specific tuber-forming stimulus formed in leaves under inducing conditions.
Gibberellins have also been implicated in the tuberization process (Kumar and Wareing, 1972). For example, Okazawa (1959, 1960) observed that endogenous gibberellin content was highest in potato plants grown under noninducing conditions and decreased when plants were moved to those favoring tuberization. Hammes and Nel (1975) proposed that tuber formation is controlled by a balance between endogenous gibberellins and the tuber-forming stimulus. For tuberization to occur, gibberellin levels must decrease below some minimum value.
Bamberg and Hanneman (1991) reported identifying a gibberellin-deficient potato dwarf in their breeding progeny, characterized by dark-green rosette foliage with short internodes. Its dwarfism could be reversed by foliar gibberellin treatment. van den Berg et al. (1995) subsequently determined that this type of dwarfism was related to the plant's inability to convert Gibberellin A12 to Gibberellin A35, thus preventing formation of a physiologically active form.
In an attempt to resolve the nagging question of whether tubers form in response to increased “tuberizing hormone” levels under inducing conditions or are the result of a decrease in endogenous gibberellins, the following study was conducted. The effects of inductive conditions and certain growth regulators on the tuberization response of leaf-bud cuttings of a gibberellin-deficient potato dwarf and its normal sibling were evaluated as a means of assessing the role gibberellins play in controlling tuber formation.
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