To find a suitable indicator for properly cold-treated tulip bulbs ( Tulipa gesneriana L. cv. Apeldoorn), the content of the endogenous free gibberellins (GAs) GA 1 , GA 4 , GA 9 , GA 24 , and GA 34 was investigated. GA levels were measured in the shoots and basal plates at the start and at the end of a complete cold treatment of 12 weeks at 5 °C by combined gas chromatography–mass spectrometry using deuterated internal standards. Bulbs stored at 17 °C for 12 weeks served as controls and the experiment was repeated three times. Before the cold treatment, GA 1 and GA 4 were the major occurring GAs in the shoots. After 12 weeks, GA 4 was the main GA component and the levels of GA 1 were low in precooled and nonprecooled bulb shoots. The levels of GA 4 , GA 9 , GA 24 , and GA 34 in precooled and nonprecooled bulb shoots and basal plates were similar. Hence, no direct correlation between cold-stimulated growth and a change in the endogenous GA status in shoots or basal plates was determined during the cold treatment. The free GA content in shoots or basal plates at the end of bulb storage cannot be used as a marker in a test for properly cold-treated `Apeldoorn' tulip bulbs.

To investigate the role of gibberellins (GAs) in the cold requirement of tulip (Tulipa gesneriana L. cv. Apeldoorn), bulbs were dry‐stored at 5°C or at 17°C for 12 weeks prior to planting at 20°C. Only precooled bulbs showed rapid sprout growth and developed a full‐grown flower. Endogenous GA levels were measured in sprouts and basal plates at the time of planting and in the second week after planting, by combined gas chromatography‐mass spectrometry using deuterated internal standards. GA4 was the major gibberellin. while GA1, GA9 and GA34 were present in lower amounts. At the time of planting, sprouts from non‐cooled bulbs contained significantly more GA4 and GA1, per sprout than those from precooled bulbs. Hence, there is no direct correlation between rapid sprout growth after planting and high GA levels at planting. In the second week after planting, floral stalks of precooled bulbs contained 2 to 3 times more GA4 and its metabolite GA34 per floral stalk and per g fresh weight than those of non‐cooled bulbs. The results are discussed with regard to the role of gibberellins in the cold‐induced floral stalk elongation of tulip.

The involvement of gibberellins in the regulation of stem elongation and flowering has been implicated in cold-requiring plants, including tulip (Tulipa gesneriana). To investigate their role in tulip, an inventory was made of GAs, including conjugated forms, in sprouts of cooled and noncooled bulbs. Using GC-MS, GA(4), GA(9), GA(12), GA(24), GA(34) and three GA-related compounds were detected. All detected GAs and GA-related compounds were found in the free, as well as in the conjugated form. They occurred in sprouts of both cold and noncold-treated bulbs.

The involvement of gibberellins (GAs) in the regulation of floral stalk elongation and flower development has been studied in tulip. The biological activity of GA4 and GA9, both endogenous in tulip bulb sprouts, and GA1, was tested in vitro on sprouts of cooled and non‐cooled tulip bulbs (Tulipa gesneriana L. cv. Apeldoorn), in the presence or absence of the GA biosynthesis inhibitor paclobutrazol. At early starting dates of incubation, floral stalks from both cooled and non‐cooled bulbs hardly showed any elongation in the absence of exogenous GA. Paclobutrazol had no effect on floral stalk elongation, and the response to GAs of sprouts from cooled bulbs was greater than that of sprouts from non‐cooled bulbs. At later starts of incubation, considerable floral stalk elongation occurred without GA application. Paclobutrazol inhibited this floral stalk elongation, and the growth of sprouts from both cooled and non‐cooled bulbs was stimulated by GA application. The effect of paclobutrazol was reversed by simultaneous application of GA4 or GA9. Application of GA with and without paclobutrazol resulted in the same elongation of the floral stalk, indicating the absence of substantial side effects of the inhibitor. The isolated sprouts did not develop a full‐grown flower without the addition of GA. GA4 was more effective than GA9 in stimulating this flower development. The results demonstrate that both sprouts from cooled and non‐cooled bulbs are responsive to exogenous GAs in vitro, and may be a site of GA biosynthesis.