Bulbing responses of two cultivars of red tropical onions to photoperiod, light integral and temperature under controlled growth conditions

The effects of photoperiod, light intensity and temperature on bulb formati on and bulb structure of two tropical onion cultivars were investigated. Fr om an initial experiment it was observed that the number of true scales and sheath scales differed significantly between the cultivars 'Red Creole' an d 'Agrifound Dark Red'. When these two cultivars were given 11, 12 and 13 h photoperiod treatments, it was found that both cultivars needed at least 1 2 h photoperiod for bulb formation. Modify the R/FR ratio from 1.22 to 1.16 in the final hour of the 11 h light period did not induce bulbing. The 13 h photoperiod increased the number of true scales and decreased the number of sheath scales compared with the 12 h photoperiod in both cultivars but t otal scale + leaf sheath numbers remained nearly constant. When onion plant s were grown under 0%, 25%, 50% and 75% shading treatments, (12 h photoperi od), only plants receiving 0% and 25% shading bulbed. Low light intensity d ecreased the number of true scales and increased the number of sheath scale s. Four temperature regimes were compared in a growth room experiment. Plan ts under the 29-34 degrees C treatment bulbed within two weeks and matured within six weeks. However, plants receiving the 25-30 degrees C treatment d elayed bulb initiation more than those plants receiving 17-22 degrees C and 21-26 degrees C treatments. At the lowest temperature, bigger bulbs with t hick necks were produced. This may be due to changes in bulb structure sinc e at low temperature, the number of sheath scales was increased, however th e number of true scales remained relatively constant in both cultivars. Dor mant leaf initials decreased with decreasing temperature while the number o f secondary meristems significantly increased. The results suggest that 'Re d Creole' was more responsive to shorter photoperiods, bulbing earlier than 'Agrifound Dark Red'. There was no significant difference in time to bulbi ng in response to temperature between the two cultivars if measured by bulb ing ratio however there were differences in bulb structure which suggested that 'Red Creole' bulbed earlier. These effects may be due to the breeding histories of the two cultivars. It is suggested that studying bulb structur e may provide a useful method of interpreting onion bulbing responses.