The fundamentals of fruit set

What does it take to get from blossom to bin? By Anna Mouton. Technical input by Prof. Karen Theron, Chair in Applied Preharvest Deciduous Fruit Research at Stellenbosch University.

Fruit set is that hopeful moment when the base of a blossom begins to swell. This is the first visible indicator of successful fertilisation and fruit development. But how did we get here? And what can be done to ensure that the tiny fruitlets prosper? In this article we explore the many variables that determine the destiny of a blossom — will it become a fruit in your bin or end as mulch on the orchard floor?

Pollination

Pollination is the transfer of pollen from the anthers onto the stigma. The pollen can be from the same tree or a different tree of the same or a different cultivar. Peaches and nectarines are usually self-fertile — trees can pollinate their own blossoms or those of other trees of the same cultivar. Most pome fruit and Japanese plums are self-incompatible — these trees require pollen from a different cultivar.

How does a tree recognise that pollen is from a different cultivar? Compatibility is determined by a group of genes known as S-locus genes. The S-locus genes have different versions that are called S-alleles — alleles are just variations of a gene. Different S-alleles code for different proteins in the pollen as well as in the stigma and the style. These proteins allow the identification of self or other.

Sometimes different cultivars will share the same S-alleles and cannot cross-pollinate each other. This is more likely to occur in cultivars that are closely related so closely-related cultivars are not ideal cross-pollinators. Examples of related cultivars that are poor cross-pollinators are Cripps Pink and Cripps Red.

Cultivars which are derived from mutations have the same S-alleles as and are incompatible with their parent cultivars. Royal Beaut is an example — it arose as a sport of Royal Gala and will therefore not pollinate Royal Gala.

Tables listing S-alleles exist for many cultivars and can be used to inform planting choices.

Triploid cultivars produce abnormal pollen so are poor cross-pollinators for other cultivars. Jonagold is an example of a triploid cultivar.

Cross-pollination also depends on overlapping flowering periods. Compatible pollen is of no use if not present during the time that a flower is receptive. Keep in mind that cultivars with different chill requirements may show inconsistent overlap in their flowering periods. Combine cultivars with similar chill requirements for the best results. Consider planting two cross-pollinators for cultivars that have short effective pollination periods — more on pollination period later.

Compatible cultivars flowering at the same time is not enough. Pollen needs an Uber and that usually means bees. Honeybees are the most commonly used pollinators of commercial pome- and stone-fruit orchards. Effective pollination depends upon enough bees and favourable environmental conditions. Bees — like the rest of us — slow down in extreme temperatures or high winds. They also perform less well under nets.

Fertilisation

Pollination is only the first step toward fruit set. The pollen still has a long way to go after landing on the stigma of a flower. A pollen grain produces a pollen tube that grows down the style to reach the ovule. The rate of growth is temperature dependent with the optimal range differing for different cultivars.

A single stigma can receive many pollen grains, and these will compete to reach the ovule. Pollen tubes grow faster when there are many contestants — another reason to ensure an ample pollen supply. The ovule contains two female nuclei. One female nucleus is the egg, and the other is the polar nucleus which will give rise to the endosperm. The endosperm is that part of the seed that nourishes the embryo.

The winning pollen grain produces two male nuclei of which one fertilises the egg and the other the polar nucleus. This process whereby two male nuclei fertilise two female nuclei is called double fertilisation. Incomplete fertilisation occurs when either the egg or the polar nucleus is not fertilised and most often ends in fruit drop.

Fertilisation is a race against the clock. Female nuclei form very late during flower development — sometimes after the flowers have already opened — and have a limited life span. The male nuclei have to reach the ovule before the female nuclei die. Can the pollen tube get there in time?

Effective pollination period is the term used to describe the window of opportunity for fertilisation. It equals the difference in the life span of the ovule and the time it takes the pollen tube to grow to it. A stronger ovule and faster pollen tube growth will yield a longer effective pollination period. A weaker ovule and slower pollen tube growth will make for a shorter effective pollination period.

A long effective pollination period obviously increases the probability of successful fertilisation and fruit set. Pollination period is partly determined by genetics. This is why a cultivar such as Doyenné du Comice can have an effective pollination period of as little as one day while the period is up to ten days in Conference. A Doyenné du Comice orchard therefore requires a much higher ratio of cross-pollinators than one of Conference.

The effective pollination period of most pome- and stone-fruit cultivars has not been determined but is probably in the region of four to five days at normal spring temperatures.

Factors affecting fertilisation success

High-quality flowers set better because their ovules live longer. They also have more cells in their base which results in larger fruit. So what is needed for high-quality flowers?

Flower buds need resources from bud formation onward. Trees that run short of reserves will have less to spend on their flowers and are more likely to produce short-lived pollen and ovules. Such trees are candidates for poor fruit set. Managing trees so that they have ample reserves can translate into effective pollination periods that are as much as three times longer than in trees with low reserves.

Pome- and stone-fruit trees start forming new buds shortly after full bloom. Pome-fruit trees form mixed buds that contain both leaves and flowers whereas stone-fruit buds will give rise to either leaves or flowers but never both. The tree commits buds to flower production during summer. In some cultivars this means that next year’s flowers are developing at the same time as the tree is ripening this year’s harvest.

Heavy cropping can affect flower development directly due to competition for resources. This is especially obvious in some late-fruiting cultivars, but heavy cropping can also reduce flower quality in earlier cultivars simply by depleting reserves. Managing crop load and giving trees lots of love — fertilisation and irrigation — post-harvest are two ways to maintain reserves.

Differentiating flowers that compete with a heavy crop may have only two-thirds as many cells available for future fruit development as normal flowers. Cell numbers impact directly on fruit size at maturity. This problem cannot be corrected by management interventions such as thinning and can persist for a second season.

Early leaf drop in autumn stops the tree from building up reserves so beware of low nitrogen levels and drought stress. Warm autumns and winters ramp up the metabolism of the tree at a time when it has no leaves to generate energy. These conditions may lead to poor flowering the following spring.

Bearing position can to some extent predict flower quality and effective pollination period. Flowers borne on downward-facing spurs are of inferior quality to those borne on upward-facing spurs.

Flower and pollen quality are necessary but not sufficient to ensure fruit set. Temperatures during spring play an important role. Extreme heat between bud break and full bloom can disrupt the formation of pollen and ovules and render flowers infertile. Hot weather and low humidity during full bloom impair pollination by drying out the stigmas of blossoms.

Higher temperatures do speed up the growth of the pollen tube but simultaneously reduce the lifespan of the ovule. Too warm and the ovule dies before fertilisation can occur. Too cold is also detrimental — the effective pollination period will shorten by approximately one day for every one degree drop beneath 17 °Celsius.

Temperatures that optimise the effective pollination period vary for different cultivars but 25 °Celsius is a good ballpark figure.

Fruit drop

Not every flower will develop into a fruit no matter how good the quality or ideal the conditions. More than half will typically drop within the first month after full bloom as either unpollinated flowers or tiny fruitlets. Fruitlets may drop because of fertilisation failure or due to competition.

Fruitlets compete for resources by producing growth regulators called auxins. The auxins from a fruitlet are exported in a stream through the stalk to encounter the auxin streams from other fruitlets as well as from vegetative shoots. A fruitlet will drop if it produces a weak auxin stream relative to the auxin production from the rest of the tree.

Apple trees usually have around five and pears around ten flowers per bud of which one — the king flower — is usually larger and opens earlier than the others. The king flower is likely to be pollinated first which means it has a head start over the rest of the cluster. It can produce more auxins and demand more resources from the tree. Auxin production by other flowers in the cluster will depend on when they are pollinated relative to their peers.

Apples and pears usually have the potential to form ten seeds per fruit. The actual number depends on factors such as pollination. A fruit with more seeds produces more auxin than a fruit with fewer seeds. This enables it to command more resources.

The combined effect of all this competition is that the tree will drop smaller fruitlets and those with fewer seeds.

Vegetative shoots also draw resources from the tree. A strong shoot near or in a fruit cluster can contribute to fruitlets dropping — in extreme cases all fruit in the cluster may be lost. The impact is worsened by low light levels and high temperatures because these conditions increase energy demand while reducing photosynthesis. Cold nights during spring help mitigate competition from shoots by reducing their energy demands.

Fruit may also succumb to a second or a third drop. The second drop typically occurs in November for apples, but the date will vary according to flowering time. Under certain conditions the second drop can be significant.

Competition contributes to the second drop, but incomplete fertilisation also plays a role. Remember the polar nucleus? Failure to fertilise the polar nucleus leaves the developing embryo without an endosperm to nourish it. Eventually the embryo runs out of resources and is aborted. Fruit without viable embryos usually drops. Stone fruit is at greater risk of lacking a viable embryo because each fruit only forms a single seed. Pome fruit containing few embryos is also likely to drop because a deficient fruit produces less auxin and is therefore unable to hold its own in a cluster.

A stone fruit faces the additional challenge of hardening its stone at the same time as the embryo is growing actively. Stone-hardening is so energy-intensive that fruit stops enlarging until the process is complete. This accounts for the typical S-shaped growth curve of stone fruit. Poor reserves or stress during stone-hardening will increase the magnitude of the second drop in stone fruit.

The third drop is called the hartseerval — dismal drop — in Afrikaans because it happens just before harvest. Mixed maturities due to extended flowering increase the chance of riper fruit dropping later in the season. Adjacent apples in a cluster can also literally push each other off the tree as they jostle for space. Cultivars with a tendency to short fruit stems are at greater risk especially in warmer areas. Examples include Fuji and Cripps Pink.

Some cultivars — especially of pears — can carry seedless fruit to maturity so long as the fruit faces very little competition for resources. Fruit development can be supported in such trees by application of growth regulators.

Understanding the factors that underpin fruit set help to explain why things sometimes go wrong — as was recently the case for certain plum cultivars. We look into the reasons for last season’s poor plum set in the next article in this issue.