What’s the blooming difference?

By Rebecca Darbyshire and Heidi Parkes

Winter and spring temperatures and their interactions, seem to influence when apple trees flower – researchers are uncovering what is really going on.

Successful flowering and pollination is a critical and sensitive part of the fruit production cycle. The timing of flowering is one important aspect within this phase. This dictates cross-pollinating varieties and influences frost risk. The timing of flowering is dependent on temperature with both winter and spring conditions playing a role. With future shifts in temperature, driven by climate change, the timing of flowering may be modified. Such changes could lead to undesirable outcomes such as variety mismatch, whereby cross-pollinating varieties flower out of sync and reduce pollination success and fruit set.

Gaining a greater understanding of flowering timing under historical and future Australian climates is one component of a University of Melbourne project ‘Crossing the threshold: adaptation tipping points for Australian fruit trees’, which is funded by the Australian Government.

As part of this project, and an associated HAL-funded project ‘Understanding apple and pear production systems in a changing climate’, a team of researchers have taken observations of flowering timing for pome fruit and cherry at Manjimup (WA), Applethorpe (QLD), Shepparton (VIC) and Huon (TAS).

Figure 1. Timing of mean 80% flowering for Cripps Pink apple. The ranges are the absolute minimum and maximum values observed. No range indicates all trees reached full bloom on the same observation day.

This spread of study sites has provided unique insight into different flowering patterns across Australia’s growing regions. A summary for ‘Cripps Pink’ apple for spring 2013 is plotted in Figure 1. These values represent when approximately 80% of all the flower buds on the tree have reached or surpassed full bloom. For 2013, trees in Applethorpe flowered first followed by Shepparton, Huon and then Manjimup. The trees at Manjimup flowered substantially later than all the other sites; 37 days later than Applethorpe, 29 days later than Shepparton and 23 days later than Huon.

When interpreting these initial results it may appear that in some circumstances warming may in fact delay flowering. For instance, if climate change warms Shepparton’s climate to be more like Manjimup, flowering timing would be delayed, not accelerated.

This may seem counterintuitive and indeed this notable delay in flowering at Manjimup compared with the other sites was unexpected.

Let’s consider these results using two common theories that relate temperature to flowering timing.

    1. Timing of bloom is driven solely by spring temperatures

Using this concept it is assumed that spring temperatures alone dictate flowering timing with warmer temperatures accelerating bloom timing. As such, warmer locations are expected to flower before cooler locations. For the 2013 observations, Manjimup should have flowered after Applethorpe and before Shepparton based on September and October temperatures (Table 1). Not after both Shepparton and Huon as was found.

    2. Timing of bloom is driven by the sequential effects of winter chill and spring temperatures

This approach assumes a specific amount of winter chill (related to cold winter temperatures) must be accumulated prior to any response to warm spring temperatures. That is, there is a full switch from being in ‘winter mode’ to ‘spring mode’.

Through this framework a possible explanation of these observations is that the chill requirement for ‘Cripps Pink’ was met very late at Manjimup, delaying when spring temperatures could act to prompt growth (note that ‘Cripps Pink’ chilling requirement is unknown). Considering accumulated winter chill across the study sites, Manjimup did accumulate noticeably less chill, meaning it would take longer to reach the chill requirement. Even so, under this scenario it would still be expected that Manjimup would flower before Huon due the persistently colder Tasmanian conditions compared with Manjimup.

“Osi Tabing takes flowering measurements on the Applethorpe research station, Queensland, to help get to the bottom of how spring and winter temperatures influence blossom timing.”

Neither of these commonly used approaches satisfactorily explain our observations. Emerging research from California proposes another theory which may better capture the process. This new concept suggests that there is a more nuanced relationship between winter and spring conditions, with the level of cold winter temperatures experienced able to modify the amount of warm spring temperatures needed to prompt flowering. Specifically, additional winter chill reduces the amount of spring warming needed for flower development. Initial interpretation using this method of our observations is promising.

Outputs from the final analyses should provide a better representation of the relationship between flowering timing and temperature. This will assist both current and future management decisions, allowing for better matching of new plantings and their pollinisers to growing areas and provide greater confidence in estimated future climate impacts and suitable adaptation options.

About the authors

Rebecca Darbyshire is a Research Fellow at the University of Melbourne and Heidi Parkes is a Horticulturist with Queensland’s Department of Agriculture, Fisheries and Forestry. For more information contact Rebecca on 03 5833 5909 or [email protected] and Heidi on 07 4681 6126 or [email protected].