Dormancy and rest-breaking basics

Rest-breaking agents are indispensable in mild winter regions, but their limitations must be heeded. By Jenny Underhill.

Deciduous fruit trees evolved in Northern-hemisphere regions with very cold winters. These trees naturally go dormant at the onset of cold weather as an adaptation to survive potentially life-threatening temperatures and conserve water and energy. They shed their leaves, stop growing, and protect vulnerable meristems (undifferentiated plant cells that enable growth) by enclosing them in hard, scaly buds.

Dormant trees require sufficient cold exposure (measured in chill units) to resume normal growth in spring. This adaptation ensures that trees do not start growing during brief warm spells in late autumn and winter, which could prove lethal if extreme cold returns. The chill requirement varies among species and cultivars.

Consequences of insufficient winter chill

If deciduous fruit trees do not receive sufficient chill units, they may take a long time to become dormant, and their exit from dormancy is disrupted.

“The biggest problem is that fewer buds break than normal, and the period of bud break is delayed and extended,” says Dr Esmé Louw, a researcher in the Department of Horticultural Science at the University of Stellenbosch. “Ideally, bud break should be a two-week sprint of condensed flowering.”

When bud break is drawn out, pollination is asynchronous, resulting in a range of fruit sizes and mixed maturity that carries through until harvest. “Many horticultural practices — such as foliar applications or fruit thinning — are based on fruit size. Consequently, these interventions are off-target,” explains Louw.

Winter chill also strongly influences tree architecture. When a tree receives adequate chill, the terminal bud is released from dormancy first, and the tree develops a strong leader. However, when chill accumulation is incomplete, the buds below the terminal bud become autonomous and form new leaders, creating a bush rather than a tree. As a result, the tree may never fill its allotted space.

What is artificial rest-breaking?

Rest-breaking chemicals are used to artificially initiate, increase, and synchronise bud break in trees that do not receive sufficient winter chill to overcome dormancy fully. The chemicals also mitigate mixed fruit maturity and basal dominance.

“When teaching my fourth-year horticulture students, I use a simple analogy: Imagine partying all night, only falling asleep at 4:00 AM, and having your alarm go off at 6:00 AM. Just as a cup of coffee or a cold shower forces you awake, chemical rest-breaking agents force trees that did not experience enough winter chill to wake up and develop strong, uniform growth,” explains Louw.

Most deciduous fruit production areas in South Africa have mild winters, and chemical rest-breaking is a standard practice. The two most common rest-breaking agents are hydrogen cyanamide and mineral oil, applied separately or in combination. They are typically sprayed at bud swell.

Both hydrogen cyanamide and mineral oil are under pressure from local activists and European markets. “We’re currently on a quest to find alternative rest-breaking agents that are at least equally effective or hopefully better,” says Louw.

How do rest-breaking agents work?

Effective rest-breaking agents place trees under severe oxidative stress, in which cells accumulate harmful reactive oxygen species, such as hydrogen peroxide. Oxidative stress triggers physiological changes in dormant buds, thereby promoting respiration, breaking dormancy, and initiating growth.

Hydrogen cyanamide inhibits the catalase enzyme, which converts harmful hydrogen peroxide into harmless water and oxygen. Without catalase, hydrogen peroxide accumulates, leading to oxidative stress.

Mineral oil has a different mechanism. When sprayed onto trees, the oil coats branches and buds with a film that limits the diffusion of oxygen, carbon dioxide, and water vapour. The tree suffocates temporarily, accumulates reactive oxygen species, and experiences oxidative stress.

“The increased concentration of reactive oxygen species signals to the tree that there is an emergency, forcing it to react,” explains Louw.

According to Louw, the problem with hydrogen cyanamide is that it works best at levels that are close to lethal, and achieving the optimal dose can be tricky.

“If the stimulus is too weak, the trees stay in a groggy slumber, leading to low and protracted bud break,” she says. “But if the stimulus is too strong, you don’t just wake the tree — you can damage and kill it.”

Rest-breaking agents must be applied at the right time, under favourable weather conditions (during and after treatment), and at the correct dose. Other factors to consider include cultivar sensitivity, tree age, and general orchard health.

Limitations of rest-breaking agents

Louw emphasises that there is a limit to how well you can mitigate the effects of inadequate winter chill through rest-breaking agents alone. “Nothing is as good for deciduous fruit trees as enough chill,” she says, “and rest-breaking agents are more effective the more chill accumulation the tree experiences.”

When selecting cultivars, it is important to match their chill requirement to the chill expected for the site. Because tree architecture relies on sufficient chill, interventions such as pruning, bending, and chemical treatments are required in the establishment years to ensure the orchard reaches its fruiting potential.

“Managing insufficient chill is a challenge because it is not a problem that goes away,” says Louw. If anything, she thinks it is becoming more relevant.

“I don’t have to pitch my research on dormancy and rest-breaking because climate change is doing it for me,” she says. “Even growers in the Northern hemisphere, where winter chill used to be a non-issue, are turning to South Africans for advice because we are among the front runners in artificial dormancy release and rest-breaking.”