Understanding the mechanisms of winter dormancy break in apple in the context of climate change

NIAB EMR: M. Else & J. Lecourt
University of Reading: P. Hadley

Background

Recent studies have shown climate change is already impacting the UK with measured warmer winter temperatures and a decrease in severity and incidence of frost events. At the National Fruit Collections (NFC) at Brogdale, recorded mean annual temperature has risen by over a degree since 1960 and the apple collection as a whole (comprising over 2,600 named accessions) is now flowering 17 days earlier than it was in the early 1960s. The UK Climate Prediction programme predicts major changes in the UK climate from ~2020 onwards with warmer temperatures in all seasons as well as more extreme events such as late frost and heatwaves. Even if late frost events decline in severity and incidence, early flowering due to warmer spring temperature increases the risk-window for flowers to be damaged by frost.

Over the same period, mean winter chill accumulation at Brogdale has decreased by a third. A combination of the predicted increase in heat accumulation and chill decline represents a major risk for fruit production, affecting dormancy with consequences on bud break and flowering. Dormancy is an essential protection of sensitive growing tissues from adverse winter conditions. In most temperate fruit trees, dormancy is triggered by low temperatures and released after a period of cold exposure (chill accumulation), leaving the bud in a quiescent state capable of resuming growth when conditions are more favourable. In apple trees, chilling requirement for bud dormancy depends on the variety whilst suboptimal winter chilling delays bud break and anthesis causing variability in fruit development rate, fruit size, harvest date and quality.

Synchronisation of flowering between pollinators and commercial varieties is also at risk. Too warm winter temperatures can also result in flower primordia abscission, decreasing yield. Together, these results highlight the need to understand both the detailed requirements for winter chill and the mechanisms by which dormancy is broken, by both genetic pre-disposition and physiology.

The classical chilling hour model predicts that each crop species and cultivar requires a certain amount of chilling hours (CH), i.e. hours of temperatures between 0 and 7.2°C, for dormancy release. This numeric quantity of CH is known as the chilling requirement (CR). These models have not been tested with future climate prediction and may have to be updated to obtain solid data to adapt future plantings to future climates. A long term climate change experiment established at Brogdale will generate data on the responses of apple varieties to future changes in winter chill accumulation. This will be combined with long-term phenological data recorded by NIAB EMR and NFC, Brogdale over the last 60 to 70 years and further data from specific controlled environment experiments conducted at the Crops and Environment Laboratory, University of Reading which will be used to test these models. The experimental work also will aim at identifying physiological control of dormancy processes in perennial fruit crops, which remain largely misunderstood. The results obtained may lead to mitigation and/or modification of orchard management to reduce the need for winter chill, leading to more climate resilient orchards.

Overall objectives

This project aims to obtain a better understanding of the physiological mechanism controlling dormancy and chilling requirement. It will combine a modelling and an experimental approach in order to develop new strategies for climate change mitigation in apple orchards and evaluate such strategies in controlled environment studies. This work contributes to increasing the sustainability of the UK apple production, by acquiring crucial knowledge to forecast the effects of climate change on yield and quality.

Approaches

Experiments will be designed to answer the following questions:

  1. What do chilling and forcing models predict for the flowering of a range of apple tree varieties in the future UK climate (2020-2050)?
  2. Are the outputs of these models verified by experimental and historic data?
  3. Are there physiological markers which explain varietal variability in response to lack of chilling?
  4. Which varieties are best adapted to future growing conditions?
  5. Can new mitigations strategies be developed from an improved physiological understanding of dormancy, endodormancy release and flowering?

Anticipated outcomes

The research outcomes will assist in the development of management strategies to cope with the lack of chill in apple production, integrating variety selection with orchard management based on a better understanding of the physiological causes of dormancy, endodormancy release and flowering. The student will use an original combination of ecophysiology and modelling techniques to answer the five research questions. This project will be an opportunity for the student to use various methods to answer scientific questions, to conduct experiments, to develop strong modelling and ecophysiological skills.