Sylleptic Shoot Formation in Young Apple Trees Exposed to Various Soil Temperature and Air Humidity Regimes in Three Successive Periods of the Growing Season

Under otherwise controlled conditions, the effect of two soil temperatures (12 and 22 °C) and two air humidities (50 and 90%) applied during three successive periods of 6 weeks (starting at the beginning of the growing season) on sylleptic shoot production of young apple trees was evaluated during the first year of growth. The treatment effects were mainly reflected on sylleptic growth; the growth of the main shoot was much less influenced. In Period I sylleptic growth tended to be reduced at 12 °C. In Period II it responded greatly to the soil temperature in Period I resulting in a much stronger sylleptic growth at 12 than at 22 °C. However, the actual soil temperature in Period II was not of any importance. In Period III growth activity was low and there were no treatment effects. High humidity favoured sylleptic growth in Period I at the soil temperature of 22 °C but not at 12 °C, but in Period II the reverse was found. Humidity did not affect syllepsis in Period III. The various treatments greatly affected the distribution of sylleptic shoots along the main shoot. The ability to grow out into sylleptic shoots was restricted to buds in a certain stage of development. The results are discussed in terms of a negative feed back mechanism between shoot and root growth which controls sylleptic growth.