Heat unit model for classifying the environmentally controlled period during ecodormancy

Abstract

Ecodormancy is a state in which the growth of a tree is inhibited by external environmental conditions. In this study, the concept of temperature range-based growth inhibition during the ecodormancy period was applied to estimate the heat requirements of the Niitaka oriental pear. Hourly temperature and flowering data from 12 regions of South Korea from 1999 to 2014 were analyzed, and the results were validated by flowering data from 2015–2017. Temperature ranges equal to 7 °C or higher had a heat accumulation effect that advanced flowering, whereas temperatures below 6.0 °C delayed flowering and negated the heat accumulation effect. In particular, 1 h at −2.0 to 6.0 °C and below −2.0 °C negated the heat accumulation effects of 1 and 8 h at temperatures of 7.0 °C or higher, respectively. Due to these negating effects, a ‘limited heat accumulation period’ occurred that maintained the heat accumulation values at 0. Furthermore, this period was consistent with the basic concept of ecodormancy because it indicated that the heat effect required for bud burst and flowering could not be achieved under low-temperature environmental conditions. The ‘limited heat accumulation period’ exempted the heat requirements from being confounded by errors in the estimation of the ecodormancy starting point, even though this point varied by up to 19 days depending on the chilling requirement model. The developed model was validated in terms of the consistency of the heat requirements and the accuracy of the flowering prediction. The heat requirement of Niitaka pear from 1999 to 2014 was 334.0 heat units (Hus), and the values were similar in 2015, 2016 and 2017 when this was not inputted into the model. The developed model presented errors of 2.5–2.8 days between the actual and estimated flowering dates.