From blossom to apple

Understanding why some flowers set and others drop. By Grethe Bestbier.

Many questions regarding fruit production were raised during the past season. With heat waves, warmer autumn temperatures and drought conditions, growers were uncertain what to expect, especially in terms of fruit set. According to Prof. Karen Theron, Chair in Applied Preharvest Deciduous Fruit Research at Stellenbosch University, countless factors — plant and environmental — affect fruit set.

In the most basic sense, when there is a rapid increase in cell division in fertilised flowers, it is said that the fruitlet or flower has set. An unfertilised flower will not show that rapid increase in cell division, and will drop before forming a fruit.

According to Theron, to understand all the factors that control fruit set, we first have to know why certain flowers drop.

For most trees, there is a rapid first drop of flowers and even small fruitlets until about four weeks after full bloom, and a less severe second drop in November. Flowers lost during the first drop are either entirely unfertilised due to pollination problems, incompletely fertilised or outcompeted by other flowers or shoots.

The second drop varies in intensity and is usually due to competition. By this stage, there are fruit of different sizes on the tree. The fruitlets that drop will generally be dull in colour and smaller than others, with yellow stems. The stage of seed development inside the fruitlet also plays a role, explained Theron. “There is internal competition. If you end up with fruit with fewer seeds, they will tend to drop.”

Factors determining fruit set

Fruit set is determined by pollination, fertilisation, and competition. A flower needs to be pollinated to set — it requires overlapping flowering of a cross-pollinator and bee activity to transfer pollen. Both of these aspects are influenced by climate. An overlap in bloom is heavily dependent on winter and early spring climate, while bees become inactive in cold, windy, or cloudy conditions.

Once pollinated, a flower must be fertilised. Successful fertilisation depends on good flower quality, a long effective pollination period and a receptive stigma. In turn, all of these are influenced by various factors.

Firstly, fertilisation is influenced by flower quality, with high quality flowers being more prone to fertilisation due to a longer ovule lifespan. Quality depends on bearing position and the process of flower differentiation. In terms of bearing position, in apples the king or central flower outranks the lateral flowers. Differentiation, on the other hand, is influenced by stress factors such as excessive cropping, high autumn temperatures and drought, which all lead to a weaker flower.

Secondly, a longer effective pollination period is very important for successful fertilisation. The effective pollination period is the difference between the lifespan of the ovule and the time that the pollen tube needs to grow down to the ovule. Therefore, a longer-lived ovule or a more rapid pollen-tube growth rate, will result in a longer effective pollination period, and increased chances of fertilisation.

An ovule’s lifespan is dependent on flower quality, spring temperature — high temperature is a red flag — and cultivar. For pollen-tube growth, temperature also plays a significant role – the higher the temperature, the faster the tube grows.

“Now, you have a problem. Your ovule degenerates quickly at high temperatures, but your pollen tube grows faster. However, faster growth of pollen is not enough to cancel the negative effect the high temperature has on the ovule lifespan. In total, it means that high spring temperatures have a negative effect,” explained Theron.

Research from Europe has found that spring temperatures above 17 degrees Celsius are detrimental.

The receptivity of the stigma also plays a role in the effective pollination period. When spring temperatures are very high, especially if combined with low relative humidity, the stigma dries out faster, which can become a limiting factor.

The final consideration in fruit set is competition. Fruit-fruit competition takes place in the cluster, where the king flower dominates the laterals and is more likely to set. “The degree of dominance of a flower is determined by when it sets — so the earlier it opens, the better its chances are of becoming a dominant fruit and staying in the cluster — and by seed number — more seeds, higher chance of set,” said Theron.

Theron concluded by reminding the audience that fruit-shoot competition tends to be overlooked. It is increased by high night temperatures, heat waves during the day and low light levels, resulting in a reduced set.