Thinning 101

Understanding natural fruit drop is essential for successful chemical thinning. By Anna Mouton.

Chemical thinning to remove surplus flowers and fruit is widely employed to improve fruit size and quality and reduce biennial bearing in pome fruit. New product registrations will hopefully soon offer similar benefits to stone-fruit growers.

Although chemical thinning is cost-effective, obtaining consistent results can be challenging. To explain why, this article explores the relationship between chemical and natural thinning and how environmental factors influence the outcome.

Competing for carbohydrates

Within a deciduous fruit tree, carbohydrates flow from sources to sinks. During the growing season, photosynthesising leaves are sources, and bark and roots are examples of sinks. During leafless periods, the roots and bark are sources, providing reserves to fuel flowering and early leaf development.

Sinks compete for carbohydrates, with larger and more metabolically active sinks usually winning. In general, fruit growth and shoot extension are the strongest sinks, followed by shoot thickening, root growth, and reserve storage.

Competition between sinks is especially heated during fruit set and early development because these processes coincide with shoot extension.

Shoots export carbohydrates only when they have enough leaves to supply their own needs and generate a surplus. For example, depending on their length and growth rate, apple shoots become carbohydrate exporters 17–24 days after bud break. Before that, they are sinks.

Long shoots need 10–12 fully developed leaves (up to 20 when they’re shaded) to support their own extension before they can lend a hand to fruit. Shorter shoots export carbohydrates earlier than longer ones, which is why spur leaves are so important for early fruit development.

Once shoots start exporting carbohydrates, they provide these to the growing fruit. The closer a source is to a sink, the better the source supplies that sink. Therefore, sufficient nearby leaves promote dry matter accumulation in fruit.

Besides contending with shoots, fruitlets also face competition from other fruitlets. Fruitlets that set first have an advantage due to superior size (bigger sinks are stronger) and hormone production (discussed below). When resources are scarce, late-setting fruitlets are likely to drop first.

In the abscission zone

Within the stalk of each fruit lies one or more cell layers called abscission zones. Fruit drop occurs when the walls of adjoining cells in the abscission zone separate.

Events in the abscission zone are hormonally determined. Ethylene tends to activate abscission, but the sensitivity of the abscission zone to ethylene is modulated by other hormones, notably auxins, which desensitise it to ethylene.

Fruitlets ramp up auxin (especially indole acetic acid) production shortly after set. This auxin moves down the fruit stalk and interacts with auxin streams from other fruitlets in the cluster as well as from shoot tips.

When auxin levels on the fruitlet’s side of the abscission zone are higher than on the tree side, abscission is prevented. But when the auxin stream beyond the abscission zone is stronger than the auxin levels trickling from the fruitlet, the abscission zone is activated, and the fruitlet drops.

Therefore, to survive, a fruitlet must produce sufficient auxin to dominate the auxin streams coming from other fruitlets and shoot tips. In general, setting earlier and having more seeds helps fruitlets hang in. The risk of drop rises with greater competition from other fruitlets and shoots, especially vigorous, nearby shoots such as bourse shoots.

In apples, the top flower in a cluster, called the king flower, opens first and is usually the most dominant. In pears, the lowest flower in the cluster is frequently the largest and opens first.

The lateral flowers in a cluster open in a set sequence, and those opening sooner and situated further from the king flower tend to be more dominant than those opening later and situated closer to the king flower.

Yet more hormones

As mentioned above, ethylene plays a role in abscission, but ethylene levels aren’t always directly correlated to fruit or leaf drop. However, when present, ethylene slows the flow of the auxin stream at the abscission zone by contributing to auxin breakdown and blocking auxin transport.

Gibberellins raise auxin levels. Hence, gibberellin sprays during flowering and gibberellins released by seeds enhance fruit set.

In addition to auxins and gibberellins, sink strength is affected by cytokinins and abscisic acid (ABA). High cytokinin and moderate ABA levels increase sink strength, while lower levels decrease sink strength.

Abscisic acid is a stress hormone. It triggers ethylene production and stimulates the enzymatic breakdown of cell walls in the abscission zone. Another of its effects is stomatal closure, which reduces photosynthesis and carbohydrate availability.

Cytokinins are important for cell division in fruit and other plant organs. Although cytokinins are mostly produced in the roots, small quantities also form in seeds. Cytokinin applications stimulate bourse shoot growth more than fruit growth, leading to fruit drop.

Fruitlets dropping out

Most pome- or stone-fruit flowers don’t develop into fruit. The majority of flowers and fruitlets drop within the first 3–4 weeks after full bloom. These are often either unfertilised or incompletely fertilised.

Fertilisation is necessary for seed formation. Seeds produce the auxins and gibberellins (especially GA₄ and GA₇) that suppress abscission and increase sink strength. Fruit with more seeds have a better chance of persisting.

Stone fruit only have one seed, so fertilisation is essential for fruitlet survival. Pears can set without seeds (parthenocarpy) because their ovaries produce high levels of auxins and gibberellins. Nonetheless, seeded pear fruitlets will outcompete unseeded fruitlets when both are present.

As discussed above, other factors also influence a fruitlet’s competitiveness. Additionally, anything that reduces carbohydrate availability in the tree escalates competition between sinks, whether shoots or fruits, and increases drop.

For fruitlets surviving the first drop, the second hurdle is the November drop (which can occur in December). At this point, the developing embryo is vulnerable to abortion, particularly when competition between fruitlets in a cluster and between fruitlets and shoots is fierce.

As with the first drop, smaller fruitlets and those with fewer seeds are most likely to drop. In most seasons, orchards would ideally already be sufficiently thinned by November, and the second drop would be minor.

Factors that enhance thinning

Anything that accelerates respiration and depresses photosynthesis will reduce carbohydrate availability and promote thinning. High temperatures, especially at night, raise respiration rates. Overcast weather limits photosynthesis, as does anything that induces stomatal closure, including extreme heat or restricted water.

Exceptionally cold weather (below 10 °C) may also slow tree metabolism, and very wet soils impact root function. Both may lead to more fruit drop.

Within the tree, shading hinders photosynthesis in affected leaves and fosters shoot rather than fruit growth. Those shoots then compete with fruitlets, because extending shoots and developing fruits are equally strong sinks.

In addition to these mechanisms, research suggests that shading alters auxin and gibberellin dynamics, thereby directly enhancing fruit drop.

Bourse shoots can be friends or foes to fruitlets. Bourse shoots are primary carbohydrate sources for fruitlets in the same clusters. Nonetheless, overly vigorous bourse shoots can become sinks, generate strong auxin streams, outcompete fruitlets, and cause fruit drop.

How chemical thinners thin

The chemical thinners registered in South Africa amplify natural processes leading to fruit drop, often by reducing the carbohydrates available to developing fruitlets through suppressing photosynthesis or increasing competition.

Metamitron inhibits photosynthesis directly. Abscisic acid inhibits it indirectly by closing stomata. Abscisic acid also increases cell wall breakdown in the abscission zone. Both metamitron and ABA are discussed in detail later in this issue.

Products containing the cytokinin 6-benzyladenine (6-BA) with or without gibberellins boost shoot growth. Stronger shoots, especially bourse shoots, generate strong auxin streams and compete with fruitlets for carbohydrates. Furthermore, 6-BA can increase respiration and, therefore, carbohydrate use.

Cytokinins also enhance fruit growth. Therefore, despite heightened competition and fruitlet drop, those fruit that survive are bigger than they would otherwise have been. This effect is independent of a lighter crop load.

1-naphthylacetic acid (NAA), 1-naphthaleneacetamide (NAD), and carbaryl all reduce auxin production and transport by and from fruitlets. The former two compounds are synthetic auxins, whereas carbaryl is an organic insecticide. Lower auxin on the fruitlet side relative to the tree side of the abscission zone leads to drop.

Ethephon releases ethylene, whereas 1-aminocyclopropane-1-carboxylic acid (ACC) is an ethylene precursor. As discussed above, ethylene increases fruit drop. A separate article in this issue covers ACC in detail.

It is possible to reduce the crop load by applying flower scorchers such as ammonium thiosulphate, but the response is unpredictable.

Using chemical thinners

There are very few South African registrations of chemical thinners for stone fruit, so the following discussion focuses on pome fruit.

Chemical thinning usually coincides with the first natural fruit drop, occurring within about a month of full bloom. The three most common application windows are petal drop to 6 mm fruitlet size, 8–12 mm fruitlet size, and 15–20 mm fruitlet size.

In general, earlier thinning tends to be more effective than later thinning because smaller fruit drop more readily, and the sooner the crop is reduced, the greater the benefits for remaining fruit and return bloom.

For the earliest window (petal drop to 6 mm), registered options include metamitron, NAA, NAD, 6-BA, and 6-BA in combination with gibberellins (the latter for apples only). Some products can be combined in tank mixes or in thinning programmes (sequential applications).

For the 8–12-mm window, options include 6-BA, metamitron, NAA, and ABA (the latter for pears only).

By the 15–20-mm window, fruit are usually far less susceptible to chemical thinning, except when using ACC, which is specifically registered for apples in this window.

Carbaryl registrations cover the entire period from petal fall to 15 mm, but applications are more effective in the earlier windows.

Thinning programmes must take cultivar differences, tree age, orchard history, microclimate, and season into account. For apples, Granny Smith, Starking and Top Red are easy to thin, Braeburn and Cripps Pink are difficult, and Fuji and Cripps Red are very difficult.

Anything that increases stress and decreases vigour encourages fruit drop. Weak spurs thin more easily than strong spurs, and young trees (up to 5 years) thin more easily than older trees because the young trees have fewer reserves.

For most compounds, thinning is optimal at 20–27 °C. Overcast weather promotes thinning and sunny weather reduces it, due to the effect on photosynthesis. Cool weather can retard shoot growth and, therefore, thinning by compounds such as 6-BA. Conversely, thinning can be enhanced by very hot weather.

Most thinners need slow-drying conditions to allow uptake. Metamitron is an exception, as application to wet leaves can lead to over-thinning.

The details of application rates, timing, combination with other actives, and cultivar effects differ between products, even when they have the same active ingredient, so growers should read labels carefully and consult their technical advisers when planning thinning programmes.