Getting to the root of salt tolerance

Which stone-fruit rootstock is best for brackish conditions? By Anna Mouton.

A new project funded by Hortgro will determine the tolerance of ten stone-fruit rootstocks to brackish conditions. The research is being carried out by independent cultivar-evaluation company Provar. The research team is led by geneticist Dr Iwan Labuschagne and includes horticulturist Werner Truter as project manager and stone-fruit evaluator Carl Hörstmann. Provar is also drawing on the expertise of well-known rootstock specialist Dr Piet Stassen.

Salinisation is the accumulation of salts in soil. The salts are present as ions — commonly sodium and chloride but also magnesium and calcium. High salt levels can harm plants and impact soil structure. Soil and water with elevated salt concentrations are called brackish.

Salinisation and drought make up the one-two punch of climate change. Rain is necessary to flush salts out of the soil and to increase flows in rivers that carry the salts away. During dry periods salts collect unchecked while producers irrigate less and are often forced to rely on poor-quality water that adds even more salt to their soil. Truter points out that brackish soils have become a fact of life for many growers.

Does salinisation spell the end of a stone-fruit orchard? Truter doesn’t think so. “There are ways to minimise the impact of brackish conditions. If you start with a rootstock that’s better adapted to brackish conditions and you apply the correct management, it’ll be far better than starting with a sensitive rootstock and then trying to manage that.”

Putting rootstocks to the test

Stone-fruit rootstocks in general are sensitive to brackish conditions. However, previous research conducted by Stassen and others indicated that some stone-fruit rootstocks — such as Marianna — are better able to cope with salinisation than others. Truter explains that breeders have been selecting certain rootstocks for adaptation to brackish conditions. The new project will compare the performance of these more recent rootstocks to conventional rootstocks.

Ten rootstocks are included in the trial, and all are grafted with African Delight. The young trees are due to be planted in the coming spring. “It was challenging to find a suitable site,” says Truter. “It can’t be too brackish, or your trees will die. But it can’t have so little brackishness that there’s no effect on the trees. There’s a fine balance.”

The trial site is in Bonnievale in the Little Karoo. It is sufficiently brackish that Truter expects significant performance losses in the absence of appropriate management practices. “The trees won’t die but they’ll struggle. Their growth will probably be reduced, and they’ll never reach optimum yields.”

Tree growth rate will be evaluated from the start by recording variables such as height and trunk diameter. Fruit number and yield will be added once the trees begin to crop. Fruit will be harvested during the last two years of the trial and samples subjected to a full maturity index and quality analysis.

“The rootstock trial in the orchard will run for a minimum of six years,” says Truter. “At the end of the trial we hope to be able to quantify the effect of brackish soils on vegetative growth and production of the different rootstocks.”

The potted version

The same ten rootstocks that are being trialled in the orchard will also be tested under controlled conditions in containers. Rootstocks will be exposed to several levels of brackishness ranging from irrigation with purified water to severe salinisation.

The potted trees will not be grafted but left to develop their natural crown. Truter explains that the rootstock and the scion interact. The question is whether rootstocks under controlled conditions show similar behaviour to rootstocks grafted with scions and planted in an orchard.

Trees in containers will be grown for one season and then analysed destructively to determine properties such as the fresh and dry mass of the root system and the crown. The containerised trials will be repeated on a second batch of trees.

“We also aim to establish a protocol that can be used by people who are breeding new rootstocks,” says Truter. “So if they want to evaluate whether a rootstock is adapted to brackish conditions, they can use a method that’s been shown to work.”

Rootstock choice can make or break an orchard. Truter believes that their rootstock trials will help growers identify the rootstocks that are best adapted to their conditions. “It’ll give the industry a bigger picture of all the aspects that you should consider before planting a rootstock.”

Table

Rootstocks that are included in this trial and their licence holders.

What are people saying about this research?

Charl Stander | Franschhoek Marketing

“There are many new rootstocks, and we know very little about some of them. They were developed in places with different conditions to ours and they were never tested for tolerance to brackish conditions. We know some rootstocks break their dormancy later and some have a greater chill-requirement. Some prefer light soils whereas others prefer heavy soils. But we know almost nothing about adaptation to brackish conditions. We need this research so that growers can make the best long-term decisions. It will be especially useful to growers in the Klein Karoo because that area is more prone to salinisation.”