The Chilling Optimum of Idaho and Arizona Ponderosa Pine Buds
Ponderosa pine (Pinus ponderosa) seedlings from Idaho (var. ponderosa) and Arizona (var. scopulorum) grown in a container nursery received optimum chilling [2,010 hr (84 days) of temperatures below 5°C]. While seedlings were in the greenhouse, days required for 50% of the population to break bud were similar for both seed sources and decreased inverse exponentially from 74 to 23 days as chilling hours accumulated to the optimum. When subsequently placed into either refrigerated or frozen storage, Idaho seedlings broke bud significantly faster than Arizona seedlings when returned to favorable environmental conditions for growth. All seedlings removed from refrigerated storage broke bud faster, were less cold tolerant, and therefore less quiescent than seedlings that had been frozen.
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