How do the different chemicals thin?

Understanding natural and assisted abscission. By Karen Theron.

Fruit growers use several chemical thinning compounds to control the number of fruit their trees produce — we discussed these in the preceding article. In this article, we explain how different chemical thinners work.

The abscission process

Abscission is how plants rid themselves of unwanted parts, like spent flowers and excess fruit. It helps to be aware of the natural process of abscission before trying to understand how chemical thinners work.

Abscission occurs in the abscission zone. For fruit, this zone lies between the stalk of the fruitlet and the short shoot — called the bourse — on which the flower cluster developed. Fruitlets drop when cell walls in the abscission zone break down. Breakdown is promoted by abscisic acid and inhibited by auxins. The interplay between these plant hormones is central to controlling fruit drop.

Ethylene plays a secondary role in abscission by reducing auxin levels and stimulating the breakdown of cell walls. In turn, auxins reduce the sensitivity of the abscission zone to ethylene. Other hormones that counteract abscission include cytokinins and gibberellins.

The relative levels of auxins to abscisic acid determine which fruitlets drop and which remain attached to the plant. The competition between fruitlets and the importance of each fruitlet to the plant is indicated by its sink strength. Strong sinks have high levels of auxins whereas weak sinks have low levels. Abscission is activated when auxin levels are low.

Primigenic dominance: king fruitlets rule

The terminal fruitlet in an apple cluster is called the king fruitlet. It usually sets first and dominates lower lateral fruitlets, causing many of these to drop. In contrast, the lowest lateral fruitlet in a cluster is dominant in pears. Lateral fruitlets have their own hierarchy. Dominated fruitlets grow more slowly and produce less auxin to prevent breakdown of their abscission zone.

The display of greater dominance by fruitlets that set first is called primigenic dominance. Vegetative growth can also exert dominance. Dominant fruitlets or shoots produce strong streams of auxins that inhibit the weaker streams from subsidiary fruitlets. This effect disappears as soon as the dominant fruitlet or shoot tip is removed.

The extent to which a fruitlet will dominate the rest of the cluster is influenced by:

1. the difference in time of fruit set. Older fruit, even by a few hours, are more dominant.
2. the number of seed per fruit. More seeds lead to greater dominance.
3. the proximity and vigour of vegetative shoots. Remember that shoots also compete for dominance.
4. the number of fruit per cluster.

As soon as a dominated fruitlet cannot produce enough auxin to maintain its abscission zone, it will drop.

So how do the different chemicals thin?

6-benzyladenine (6-BA)

6-Benzyladenine containing products (Bapsol, Exilis and MaxCel) stimulate vegetative growth, especially of the bourse shoots — shoots arising from the shoot within the flower cluster. Stronger shoots produce more auxins and may, with the help of 6-BA, outcompete fruitlets, causing them to drop. 6-BA is effective on cultivars with prominent bourse shoot development and less so on spur types.

1-naphthylacetic acid (NAA), 1-naphthaleneacetamide (NAD) and carbaryl

Fruitlets produce less auxin following application of NAA (PoMaxa), NAD (Amid-Thin W) and carbaryl (Carbaryl 850 WP, Karba 850 WP, Karbacure and Sevin XLR Plus). Auxin inhibits abscission, so less reaching the abscission zone will cause the fruitlet to drop. NAA and NAD are synthetic auxins, and it may also be that they block the transport of natural auxins. Blockage would contribute to less auxin moving to the abscission zone. However, there are still many questions and uncertainties around the mechanism of action of these three thinning compounds.

Metamitron

Metamitron (Nevis 150 SC) is a photosynthesis inhibitor. It increases competition among fruitlets by reducing available carbohydrates, causing more to drop. The efficacy of metamitron is increased by low irradiance (lowers photosynthesis); high temperatures, especially at night during bloom (increases respiration); high previous crop load (low carbohydrate reserves of tree); and strong shoot growth (increased competition for carbohydrates).

Ethephon and 1-aminocyclopropane-carboxylic acid (ACC)

Ethephon releases ethylene whereas ACC is converted to ethylene. Ethylene reduces auxin levels within fruitlets and inhibits movement of auxins to the abscission zone. ACC is currently being evaluated as a chemical thinner for apples.

S-abscisic acid (S-ABA)

S-ABA could be directly involved in abscission by stim­ulating enzymes in the abscission zone which cause cell walls to break down. S-ABA may also act by reduc­ing carbon availability. It is known that application of S-ABA results in stomatal closure, which results in a reduction of photosynthesis and therefore lower carbohydrate availability. Restricted carbohydrates result in stronger competition between fruitlets and shoots leading to increased fruit abscission. Registra­tion of S-ABA as a chemical thinner for pears is being explored.

Scorchers

So-called scorchers include ammonium thiosulphate and terbacil (a herbicide). They are applied during bloom and scorch the flower parts of open flowers, preventing fertilisation, seed formation and thus often fruit set. Fruit that have few or no seeds tend to abscise. Scorchers are most effective when trees have a condensed bloom period. There are no scorchers regis­tered for fruit thinning of apples and pears in South Africa.

The different chemical thinning compounds have different modes of action. The preceding article in this series discussed when each product should be used and what factors influence their efficacy. It is important to integrate all these aspects when selecting a product and deciding whether to incorporate more than one in a programme. Understanding their mode of action will assist in making an informed choice.

Image: Stimulation of vegetative shoots following 6-benzyladenine application.

Supplied by Gustav Lötze.