Effects of heat treatment, timing of heat treatment, and gibberellin A4/7 on flowering in potted Picea abies grafts

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Abstract
Potted grafts, made from scions of more than 50‐year‐old Norway spruce trees (Picea abies (L.) Karst.), were induced to flower in several experiments by heat‐ and gibberellin A4/7 (GA) treatments in an acrylic greenhouse at Biri nursery in Norway. The grafts were treated when they were 1.5–3 m tall, 4–8 years after grafting. GA alone (spray and injection) enhanced female flowering, especially in combination with heat. Heat alone was ineffective; few grafts formed seed cone buds. Low amounts of GA (0.5–2 mg per graft) could enhance female flowering in previously treated grafts (retreatment), but only a slight response to 2 mg GA was found among previously untreated grafts. Heat alone induced profuse male flowering, GA + heat did not further enhance the male response, and GA alone did not induce formation of pollen cone buds. The highest male scores were found for early and long heat treatments (LHT) which started when shoots were elongated to 25–60% of final length and ended at least one week after termination of shoot elongation. Late and short heat treatment (SHT), starting from 86–95% elongation and ending on the same date as LHT, gave significantly fewer pollen cone buds than LHT in each experiment. If heat treatments started early (20–30% elongation), but terminated before or at time of shoot growth cessation (95–100%), no pollen cones were formed. GA could be phytotoxic, especially after spraying, but also when high amounts of GA were injected (40 mg). Good female flowering and low levels of needle damage were found when 2–10 mg GA was injected. The possibility of regulating sex expression in Norway spruce is discussed.