Researchers are uncovering the chill requirements of new and old pear cultivars including ANP-0131 (Deliza®), ANP-0118 (Lanya®), ANP-0534, and PremP009 (Piqa®Boo®). In doing so they hope to understand the suitability of these cultivars for the Goulburn Valley now and under future climates.
When selecting new varieties, growers consider many important factors including marketability, time to full production and suitability for their region. One important climate suitability factor to consider is ability to meet winter chilling requirements. If a cultivar is grown in a warmer climate than it requires, flowering can be sporadic and/or light and delayed.
Fruit set can be reduced, resulting in lower yields, particularly if flowering periods of pollinating species are, or become, misaligned. These flowering difficulties may present as the climate warms in areas that are currently suitable. Climate suitability now and into the future is important for productive longevity.
Fruit trees are dormant over winter, which provides protection of their sensitive tissues (e.g. shoots and flowers) against cold damage. Apples and pears enter dormancy in response to a lowering of temperatures in late autumn.
The length of the dormancy period is determined by a cultivar’s chill requirement, and the current best measure of chill is chill portions (CP). The optimal temperature range in which to accumulate CP is from 6°C to 8°C, with temperatures of less than -2°C having no chill effect and temperatures of more than 14°C reversing previously accumulated chill.
A cultivar’s suitability now and into the future can be determined by contrasting its chilling requirement with the amount of chill accumulated at a site.
Chill requirements of pear cultivars
A cultivar’s chill requirement can be estimated by harvesting cut shoots from the field at regular intervals through autumn and winter and then subjecting them to controlled environmental conditions (i.e. 25°C and continuous light) and monitoring when these shoots exhibit budburst. When 50 per cent or more of buds burst after two weeks in the controlled environment, the chilling requirement is deemed to be satisfied. The chill requirement is then calculated as the total chill accumulated in the field from 1 March up until the day the shoots were cut.
In 2016 shoots of three new pear cultivars (ANP-0118, ANP-0534 and PremP009) and the standard industry cultivar Williams were cut every week from 30 May at sites at Tatura and Tatura East in northern Victoria. This data was combined with previous pear chill data collected in separate experiments in 2009, 2014 and 2015.
The chill requirement of the different pear cultivars ranged from 36.7 to 83.3 CP. The relative order of cultivars from medium to high chill was ANP-0534, ANP-0118, ANP-0131, Packhams, Williams and PremP009. Interestingly, although PremP009 had the highest chill requirement, it flowered at a similar time to Williams in the field. This suggests that flowering in PremP009 either has a lower requirement for warm temperature in spring to reach budbreak or the buds entered dormancy earlier than Williams. As PremP009 is new to Tatura, this cultivar had few floral buds monitored, which may have influenced the results.
Pear cultivars best suited to future climates
All pear cultivars are mostly suitable for the Goulburn Valley, the major area for pear production in Australia, because the chill requirement range of pears is less than seasonal accumulation in the region. A notable exception was 2016, when chill accumulation in Tatura was borderline for the high chill requirement range of Williams and PremP009. Indeed, both Williams and PremP009 will be the first cultivars to be adversely affected by climate change due to their higher chill requirement while ANP-0118 and ANP-0131 are likely to show more consistent flowering and fruit production under warmer winters due to their lower chill requirement.
Choosing pears for warmer climates
For growers wanting to keep growing Williams and PremP0009 into the future, plant growth regulators could be used to promote bud break in seasons of insufficient chill and fine tune the timing of budburst of different cultivars and their pollinators. However, physiological mechanisms that these products influence and their ongoing effectiveness is currently unknown. Understanding why a large range in chilling requirements (about 20 CP) was found using traditional experimental methods needs greater exploration with future risk highly dependent on assuming either the lower or higher range of the chilling requirement.
More research to understand the underlying genetic mechanisms of dormancy and chill accumulation and how they interact with plant growth regulators is needed to identify management options that growers can use under future climate change to support pear production.
We also suggest growers use the Dynamic Model that relies on chill portions to measure chill in their orchard. The traditional Chilling Hours Model, which assumes chill accumulation occurs only within the 0°C to 7.2°C temperature range, has proven inaccurate for the measurement of winter chill.
A spreadsheet set-up for application of the Dynamic Model is available on the Horticultural Industry Network web site.
We thank The Faculty of Veterinary and Agricultural Sciences, the University of Melbourne, for providing funds through the Richard Nicholas Trust to conduct these experiments and Dr Michael Santhanam-Martin for project support. We thank Alex Turnbull and Maurice Silverstein for provision of plant materials. Finally, we appreciate the support of Fruit Growers Victoria, Bill Kirkland from Plant & Food Research, Andrew Maughan from Freshmax and Brett Ennis from Prevar in conducting this experiment.
About the authors
Dr Rebecca Darbyshire, NSW Department of Primary Industries, Wagga Wagga, Susanna Turpin, TDI Select Fruits, Tatura, and Dr Ian Goodwin, Agriculture Victoria, Tatura.