Disentangling nature and nurture

Researchers aim to understand how the interaction of genetics and environment drives tree performance. By Anna Mouton.

An eight-year Hortgro-funded project in collaboration with Stellenbosch University and independent cultivar-evaluation company Provar is untangling the factors driving adaptability and stability in pome and stone fruit. Provar initiated the project.

The postgraduate students involved shared project updates at the 2023 Hortgro Science Research Showcase and a recent cultivar information day hosted by Provar. This article summarises key points from their presentations.

What is adaptability?

All living organisms have a genotype — their genes — and a phenotype — their observable characteristics. Observable characteristics result from gene expression, but the relationship is complicated because environmental factors modify the extent of gene expression.

For example, a tree may have the genes to produce red apples, but to express those genes, the tree must first bear fruit. When it does, the extent of gene expression for red colour development will be further influenced by environmental factors such as light exposure and night-time temperatures.

Interactions between gene expression and environmental conditions make it hard to rank cultivars. The fruit yields and quality in one season at one site may not — and often do not — translate into similar performance at another time or place.

“The adaptability of new cultivars is probably not foremost in the mind of all producers when they make planting decisions,” said Tristan Dorfling, Provar Project Manager: Pome Rootstocks.

“We plant most of our apples and pears in quite diverse growing regions with a lot of mountainous terrain and microclimates, so we see maladapted cultivars even though they might be planted in an area that has, for example, sufficient chilling.”

Broadly adapted cultivars have genes that allow them to interact positively with a wide range of environments. For example, bud break may be condensed regardless of the environment.

Stable cultivars have genes that interact less — whether positively or negatively — with the environment. Their performance tends to remain constant across environments and seasons. For example, the fruit of some apple cultivars develop an intense red colour, seemingly regardless of external factors.

Both performance and stability reflect the adaptability of a cultivar and can be used to quantify its ability to excel consistently under different conditions. Consistency is essential for sustainable fruit production.

Understanding the interactions

“Relying solely on trial and error for cultivar selection can introduce a high degree of risk into a grower’s business,” cautioned Dorfling. “If the chosen cultivars consistently underperform, the financial stability of the whole business operation could be jeopardised.”

He pointed out that most apple cultivars grown in South Africa are not locally bred and that there is no reason to expect them to be adapted to our conditions. The same holds for many other deciduous-fruit types. Local evaluation data are indispensable — but how can we account for those interactions between diverse genotypes and environments?

Researchers can tease out the interactions through multi-environment trials, explained Anika Kock, Provar Project Manager: Stone Fruit. “Multi-environment trials involve planting crops in various environments to see how the performance and stability of the genotypes change in different environments.”

Kock and her colleagues study the interactions between genotype and environment using sophisticated statistics that can provide insights into which measured variables drive adaptability and stability for key characteristics such as yield and fruit quality.

Many South African deciduous-fruit production areas already qualify as warm and will only become warmer due to climate change. “That is why studying adaptability is so important,” said Dorfling. “We need to know which genotypes are more and which are less flexible. Which ones are not able to adapt under these conditions?”

Adaptability research so far

The first apple, plum and cherry trees for the adaptability project were planted in 2019. Ten cultivars of each fruit type were established on three trial sites representing a range of environmental conditions. Pears were added in 2022.

Pome fruit and plum research are funded by Hortgro, while cherry adaptability research was jointly funded by the cherry industry and the Western Cape Department of Agriculture.

Dorfling, Kock and Chad van Wyk were the MSc students on the project in 2020–2021. They were succeeded by Cara du Toit, Portia Solomon, and Karel Vahrmeijer for 2022–2023. Simon Cook participated as part of an exchange project with KU Leuven, and Brend Spreeth joined in 2023.

While working on the project, the students intern at Provar. Provar employed Dorfling, Kock and Van Wyk after they completed their internships.

The students collect data on bud break, vegetative growth, bearing habit and complexity, fruit yield and quality, pruning, and temperature.

“The bud-break data can provide insights into the likelihood of the genotype portraying prolonged dormancy symptoms,” said Kock. “An example of one of the traits derived from the bud-break data is the bud-break period— a tree with a condensed bud-break period will be less likely to produce fruit of uneven size and mixed maturity.”

Vahrmeijer highlighted the value of data on bearing habits. “We intend to cluster genotypes by area-specific bearing habits and correlate this with branch-based yield data, ultimately revealing how environments influence the conversion of bearing positions into fruit.”

Data collection on pome fruit and plums will continue until at least 2025. Analysis of the enormous data set will help pinpoint the characteristics that best predict the performance and stability of a cultivar.

“At the end of the study, an adaptability index will be developed,” said Solomon. “The index will identify important traits that drive adaptability. These are key aspects for independent evaluation of new genotypes and will also enable evaluators to better match genotypes and environments when producers are planning new plantings.”

For those who missed the Hortgro Research Showcase, the presentations on the adaptability projects for pome fruit and stone fruit are available on the Hortgro YouTube channel.

Project participants

Project funder: Hortgro.

Project manager: Dr Iwan Labuschagne. Provar.

Academic leader: Dr Esmé Louw. Department of Horticultural Science. Stellenbosch University.

Project collaborator: Dr Nigel Cook. Prophyta.

Project statistician: Dr Mardé Booyse. Agrimetrics Unit. Agricultural Research Council.

Postgraduate students

Simon Cook (Japanese plums)

Tristan Dorfling (apples)

Chad van Wyk (cherries)

Anika Kock (Japanese plums)

Cara du Toit (apples)

Karel Vahrmeijer (cherries)

Portia Solomon (Japanese plums)

Brend Spreeth (pears)

Pro-Hort sites

Boland Landbouskool. Paarl.

Klipboschlaagte. Smuts Brothers Agri. Robertson.

La Plaisante Estate. Wolseley.

Lushof. Graaff Fruit. Warm Bokkeveld.

Nooitgedacht. Dutoit Agri. Koue Bokkeveld.

Oak Valley Estate. Elgin.