LATERAL SHOOT FORMATION IN APPLE IN THE FIRST YEAR AFTER BUDDING AS AFFECTED BY AIR HUMIDITY AND SOIL TEMPERATURE

Under completely controlled conditions the effect of air humidity (two levels, 50% and 90%) in combination with soil temperature (12 and 22°C) on vegetative development of apple trees (cvs. ‘Rode Boskoop’ and ‘Elstar’) in the first season after budding was evaluated. Air was 20°C throughout. Irrespective of root temperature in both cultivars, shoot growth was much better at high than at low humidity. In ‘Boskoop’ at both humidities, growth at the soil temperature of 22°C exceeded that at 12°C, but in ‘Elstar’ root temperature was ineffective at low humidity. The effects on total growth were mainly in the number and length of lateral shoots; terminal growth was barely influenced by treatments. Soil temperature affected the distribution of the lateral shoots along the main axis. In both cultivars at 12°C most laterals were found in the range of 21 to 30 nodes counted from the graft union against in the 11 to 20 range at 22°C. Air humidity did not influence lateral shoot distribution. Therefore, the distance between the graft union and the most basal lateral shoot was distinctly smaller at the higher soil temperature. An attempt was made to explain these data in the scope of current hypotheses on apical dominance.