The effect of nitrogen and plant growth regulators on sylleptic and proleptic shoot development of 'Hass' avocado (Persea americana Mill.): Relationship to yield

Contribution of 'Hass' avocado (Persea americana Mill.) sylleptic and proleptic shoot modules (SSM and PSM) to canopy complexity and yield was quantified. With tree age, development of SSM from previous SSM decreased to 6% but development of PSM from previous PSM increased to 66%. Twenty-year-old 'Hass' avocado tree canopies were 75% PSM and 25% SSM. SSM produced more harvested fruit (P≤0.05). SSM and PSM models illustrated that cultural practices increasing SSM number would increase yield. Foliar-applied 6-benzyladenine plus gibberellic acid 4+7 (BA+GA4+7) increased SSM number (64% and 300%) but decreased PSM number (38% and 46%), increasing SSM from 16.67% to 50% of the canopy of 3-year-old trees. Virtual trees depicted the resulting increase in canopy complexity and productivity. BA+GA4+7 warrants further investigation as a tool to increase SSM and 'Hass' avocado yield. With the goal of increasing 'Hass' avocado yield and reducing potential nitrate pollution of groundwater, nitrogen (N) fertilization strategies targeting specific phenological stages were tested: 28 kg·ha-1 N in November, January, April, July, and August (control); 2x N (56 kg·ha-1) in November, April, August, or November and April; or 3x soil or foliar N (84 kg·ha -1) in April (N to total 140 kg·ha-1·yr -1 was applied in the same months as the control); and 1.6x soil N (45 kg·ha-1·yr-1) split in July and August. The 2x N in April, 1.6x N split in July and August and control trees had greater 4-year average yields (kilograms/tree) than trees receiving 2x N in November and April or 3x foliar N in April (P=0.0178). Trees receiving 2x N in April or 1.6x N split in July and August had greater 4-year cumulative yields (kilograms/tree) than trees receiving 2x N in November and April or 3x foliar N in April (P=0.0374). Applying 1.6x N split in July and August produced high yields of large size fruit with 68% less N. This N strategy is more cost-effective and has a lower potential to cause groundwater nitrate pollution than other N strategies tested. Time of N application is an important factor in sustaining total yield and yield of large size fruit of 'Hass' avocado.