How it works and why it matters. By Anna Mouton.
The ancestors of the apples we grow today probably came from some freezing mountain on the borders of Kazakhstan — not from the Garden of Eden. These trees evolved winter dormancy to survive extreme cold. Many deciduous plants become dormant as an adaptation to seasonally hostile environments. But what happens when we grow them in regions with different climates?
All stone and pome fruits produced commercially in South Africa are deciduous trees from the Northern hemisphere. They all undergo winter dormancy and require winter chill to a greater or lesser extent. Understanding how winter dormancy works — and what happens when it doesn’t work — is key to managing these crops. Read on for a digest of what you need to know.
What is dormancy?
Dormancy is a temporary cessation of growth of any part of a plant that has a meristem. A meristem is a group of cells that can divide to form new tissues and organs. Deciduous trees protect the dormant meristems in their shoots by enclosing them in buds. Different buds, on the same tree, at the same time, can experience different levels of dormancy.
The lack of growth of deciduous fruit trees during winter is called endodormancy and is caused by growth-inhibiting factors within the bud itself — it is triggered and lifted by specific cues within the bud. A bud that is endodormant will not resume growth unless it has received the correct stimulus to lift the endodormancy.
Cold is often the cue for buds to become endodormant. Further cold is essential to lift that bud out of endodormancy. Transient warmer temperatures will not stimulate growth in a bud that is endodormant. This strategy protects plants — not starting growth during a brief warm spell prevents subsequent cold damage to their tender new flowers and leaves.
The process of winter dormancy
Cooler weather in autumn prompts stone- and pome-fruit trees to start preparing for the potentially life-threatening cold of winter. Shorter days may also play a role in inducing dormancy in stone fruit. The trees stop growing and start storing the reserves that will both sustain them during winter and power them up in spring. They form budscales to protect their sensitive shoot meristems from low temperatures.
The relatively sudden onset of cold weather marks a clear start to winter dormancy for stone- and pome-fruit trees in their native habitats. There, dormancy is considered in the offing as soon as the trees lose their leaves. Our warmer autumns and winters don’t send as clear a signal — the trees take a long time to settle into their winter rest. Some never even get as far as shedding all their leaves. This makes it hard to judge whether trees are fully dormant or still only dozing off.
Trees that are fully endodormant need a certain amount of cold — measured in chill units — before their buds become responsive to environmental triggers for growth. The optimal temperature for chill accumulation in apple trees is 7.2°C. No chill accumulation occurs at temperatures above 16.5°C and none occurs below freezing. Higher temperatures may reverse the effects of chilling that has already occurred — more on that later.
Chill requirement varies between species and cultivars. Apples mostly have a higher chilling requirement than stone fruit other than cherries.
Chill accumulation starts when trees are fully endodormant. Low temperatures before this contribute nothing to fulfilling chill requirement. This is a problem when trying to measure how many chill units a tree has accumulated. In mild regions, you can never be sure when the tree is fully endodormant and stacking up those units.
Buds can resume growth as soon as temperatures become favourable after their chill requirement has been met. Bud-break and flowering are promoted by warmth. A hot spring is key to kick-starting trees that haven’t had enough winter rest. Trees that received sufficient chill will get moving even in a cool spring.
Why trees need to chill
The deciduous fruit trees that we cultivate in South Africa can survive without winter chill. The trees will continue to cycle through periods of growth and dormancy, regardless of environmental temperatures, but the process will be very slow. Without management interventions, such trees will not be able to exhibit their natural growth habit, because their buds never synchronise.
In the milder stone- and pome-fruit production areas of South Africa, there is usually enough cold in autumn and winter for trees to become fully endodormant but not always enough for all buds to fully exit endodormancy.
Normal winter dormancy that is broken by sufficient chill has the effect of synchronising all the buds on the tree. In an apple tree that received sufficient chill, the terminal bud is dominant, and the growth potential of buds decreases from tip to base along the shoot. Bud-break in trees that completed a normal winter dormancy is strong and condensed.
In warmer areas, some buds will have progressed further out of endodormancy than others by the time environmental conditions again favour growth. The buds don’t get the benefit of a reset.
Bud-break after insufficient winter chill tends to be delayed and protracted. Fewer buds break than normal. Buds that do break quickly establish dominance over slower buds. This has implications for both fruit set and tree architecture.
Lack of winter chill may negatively affect flower differentiation resulting in flowers that may be abnormal and drop at different stages of development.
The main short-term impacts of insufficient winter chill are reduced fruit production and mixed maturities in the following season. Fruit production is lower due to fewer flower buds of poorer quality. But dominance by structures that develop from buds that break early also suppresses later-setting fruit and may increase fruit-drop. Those fruit that make it to harvest will cause headaches by maturing at different times. Increased post-harvest disorders are sure to follow.
Trees with protracted and uneven bud-break are harder to manage because fruit are not at the same stage of development at the same time. How do you decide when to apply treatments such as chemical thinners?
Insufficient winter chill also has long-term effects on tree productivity and architecture. Reduced bud-break leads to bare shoots and fewer spurs — thus reducing the potential sites for fruiting. When fewer buds break, the resulting lateral shoots grow more strongly, which diminishes the dominance of the leader, and reduces its growth. The tree develops basal dominance and struggles to fill its allotted space.
Ultimately, fruit growers are farming with light. Optimal productivity requires tree heights to be 80%–100% of row widths, so as to harvest as much light as possible. Trees that fail to reach this height equate to lost income.
The different chill requirements, and response to insufficient chill, of different cultivars can lead to non-overlapping flowering periods of cross-pollinators. This may have contributed to the poor fruit set in certain Japanese plums in recent years.
Weathering warm winters
The climate in many South African fruit-growing areas doesn’t provide enough cold for some stone- and many pome-fruit cultivars. Producers can overcome some of the problems with bud-break by using rest-breaking agents. These chemicals are indispensable in many areas and likely will become more so in future.
Rest-breaking agents act by stressing the tree — the tree experiences a threat to its survival and responds by leaping awake. The risk is that strong rest-breaking treatments can damage a tree and it is not always easy to judge what the most effective application rate for a given season might be.
Producers also manage the effects of warm winters on tree architecture by cultural practices such as pruning and bending. The additional measures applied in warmer areas increase the cost of production. The result could be lower profitability when growing trees in areas with insufficient chill.
The most promising solution for warmer areas is the breeding of low-chill cultivars. The chill requirement of pome- and stone-fruit trees is heritable — it can be used as the basis for selection in a breeding programme. Low-chill cultivars of some fruit types are already available. But not all can compete with established cultivars when it comes to fruit quality and storability.
The problem of insufficient chill will become worse as climate change progresses. Producers will need to keep a careful eye on winter chill when selecting cultivars and managing orchards. We discuss the different models for estimating chill units — and their limitations — in the next article in this series.