Investigations on the cause and control of alternate bearing of Unshu orange trees. III.

Effect of fruiting in the previous season on the flower formation

Abstract

1. The alternate bearing habit of an Unshu orange orchard (33 years old in 1929) was investigated on the basis of yield records since 1929. In this orchard, where fruit thining and pruning had not been practised, yield per 10 a. was about 6, 000kg in on years and 1, 000 to 2, 000kg in off years. This fluctuation in yield had increased year after year.
2. Annual variations in the yield, percentage of terminal shoots, ratio of blossom-bud to node number, and percentage of secondary shoots sprouting from the fruiting and non-fruiting branches testified the alternate bearing tendency statistically.
3. Annual variation of fruit number per unit volume (m³) of tree crown was also indicative of alternate bearing habit.
4. The most desirable ratio of fruiting shoots to spring flush which gave sufficient yield every year was confirmed as 40 to 45 per cent. However, the alternate bearing trees put forth large number of fruiting shoots and small number of spring flush in an on year, while in an off year these trees had large number of spring flush with a correspondingly small number of fruiting shoots. This pattern of growth in trees induces the tendency of alternate bearing.
5. Percentage of flower bud in on and off years varied in the range between 47.1 and 16.9 per cent. In some cases the fruiting shoot in off year produced flower buds in the next year.
6. Alternate bearing habit is the expression of ill balanced number of fruiting shoots to the total number of shoots produced. Excessive bearing of flowers and development of fruits make the tree difficult to form the flower buds owing to the lack of florigen accumulation. In excessive fruit bearing tree, even spring flush could not produce the flower in the next year. This shows the remarkable alternate bearing habit due to ill balanced ratio between the fruiting shoot and spring flush of the past year.