June 1970.Covers not scanned.Includes bibliographical references.1. The study was conducted to evaluate v various methods of determining when bulbing is initiated in the onion plant, Allium cepa L. Evaluation was on the basis of accuracy, reliability, and simplicity. Methods of determining when bulb initiation has taken place are necessary for further research on bulbing, to develop field modifications of bulb initiation and development, to facilitate selection of breeding material, and to develop methods of predicting bulb maturity. Forty-four cultivars were grown under several environmental conditions. Reduction in the maximum ratio (in a given plant) of the foliage leaf blade length to sheath length almost always preceded internal scale formation. Prediction of internal scale formation occurred in greenhouse grown plants when this ratio was 10 or less. The same value for the youngest visible leaf can be used for this purpose in field grown plants. Other less reliable indices were means of youngest visible and maximum leaf ratios and corresponding sheath lengths, external minimum leaf ratios, base/neck ratios, number of visible leaves and plant height. 2. Effects of planting dates on bulbing were studied by planting seeds of the variety 'White Portugal' in the field at weekly intervals, from April 15 to May 6, 1968. Plants four weeks apart in age began bulbing less than a week apart. The increased responsiveness to increasing daylength may have been due to increasing temperature or plant age. However, higher percentages of plants bulbed in earlier than in later plantings. 3. Effect of temperature on bulb initiation was demonstrated by plants of the hybrid 'B 2190 A x Colorado 6' grown in the greenhouse and outdoors. Results indicated that the photoperiodic stimulus was more effective at higher temperatures. 4. Incandescent light interruptions for an hour at midnight, continuously or for a period of two weeks when plants were 12 weeks old, induced early bulbing. Other plants received the two weeks of night interruption when they were 4, 6, 8 and 10 weeks old. Older plants responded better to night interruption. The hybrid 'B 219 0 A x Colorado 6' was used in this study. In another experiment, using the same hybrid, plants were exposed to 1) naturally prevailing daylength (14 to 13 hours), 2) nine hours of natural daylight, and 3) naturally prevailing daylength plus continuous night interruption for an hour with incandescent light at midnight. All plants exposed to night interruption bulbed; but no bulbing was found in the other treatments. Distinct morphological differences were observed between plants exposed to the three treatments. The night interruption treatment resulted in plants significantly higher in base/neck ratios, lower in youngest visible and maximum lea£ ratios and longer corresponding sheath lengths and fewer visible leaves than produced by the other two treatments. The tallest plants resulted from the first treatment. Longer photoperiods resulted in an increase in plant height, number of visible leaves and reductions in both youngest visible and maximum leaf ratios
