Research into the drought tolerance and diversity of apple rootstocks is an important element in making apple cultivation more climate-resilient in the long term, not just in the Lower Rhine region.
Apple tree propagation by grafting
For a true-to-type propagation of apple trees, an apple rootstock – root with trunk piece – is grafted with a scion – a one-year-old shoot – i.e. joined together in order to grow together. In grafted apple trees, the rootstock determines the vigour of the root system and the final size of the tree, while the scion determines the fruit characteristics and is also decisive for the vigour of the crown.
‘Unlike in intensive fruit growing with its bush trees, i.e. trees with compact and narrow growth, there are only a few existing rootstock varieties in the strong-growing area, in the so-called semi- and high-stem cultivation. The trunk height, i.e. the height between the ground and the lowest crown branch, is 120 cm for half trunks and 180 cm for tall trunks. In intensive fruit growing today, mostly low trunks or spindle trees – bush trees with a trunk height of between 40 and 60 cm – are grown on a low-growing rootstock. In agroforestry tree strips, different tree forms – from low to tall trunks – can be of interest to farms and can be used, so vigorous rootstocks are also attractive. While we still have a larger selection of apple rootstocks in the low-growing and medium-growing areas, there are currently only two options for seedling rootstocks in the high-growing area: Bittenfelder and Antonowka,’ says Anna-Lea Ortmann, PhD student at the Agroforestry Living Laboratory and responsible for the research project.
‘Climate change is now causing more problems with the existing rootstocks. A wider range of rootstocks, which would enable a selection that is more adapted to the location and climate change, would also make sense in terms of risk diversification.’ This is where her research project steps in.
2024: Greenhouse drought stress experiment
The research project started in February 2024 in the Tropical Greenhouse with Study and Showpiece Gardens at Rhine-Waal University of Applied Sciences (HSRW). A total of 264 annual plants from six different apple rootstock varieties, ranging from strong to weak-growing, were planted in standard pots and air pots, in equal numbers. A drought stress experiment was conducted from June to July 2024: While one half of the rootstocks was irrigated regularly, the other half was irrigated at a low level and thus placed under drought stress in order to collect data on various drought stress indicators. In September 2024, all the trees were moved from the greenhouse to the adjacent study and showpiece gardens. There, they were acclimatised to outdoor conditions and irrigated via precipitation.
2025: Grafting and tree nursery
At the beginning of the new year, the rootstocks were removed from the pots, freed from the substrate and washed. For the first time since February 2024, the root system was examined in detail and samples were taken. 256, i.e. around 97% of the apple rootstocks remained from the drought stress experiment. The rootstocks, now two years old, were grafted by Jürgen Opschroef, Agroforestry Living Laboratory cooperation partner of LIKK e.V. (Landschaftspflege im Kreis Kleve) and Anna-Lea Ortmann.
They opted for the grafting technique of copulation with counter-tongue: the trunk of the apple rootstock was cut at an angle. In addition, a second cut was made to form the so-called counter-tongue, so that the ends of the rootstock and the graft could be inserted into each other. The joint was connected with Parafilm, a stretchable and sealing sealing film. The top ends of the scions were also cut and their length shortened to a total of around 10-15 cm, leaving three buds on the shoot. Finally, the tips of the scions were dipped in a special tree resin to protect the cut created by the shortening from drying out.
The cultivars selected for grafting the apple rootstocks were Zabergäurenette and Rheinischer Winterrambur. Due to the planned repetitions in the field trial, only two varieties were chosen. ‘We decided on two old varieties. The Zabergäurenette is considered robust. I first learnt about it from Hans-Joachim Bannier, a well-known pomologist, and was impressed by its taste,’ says Anna-Lea. ‘The Rheinische Winterrambur is a common old variety here on the Lower Rhine in orchards and gardens. The Berkhöfel organic farm, a cooperation partner of our Agroforestry Living Laboratory, has had good experiences with the trees and fruit.’
The grafted apple rootstocks were then replanted at Berkhöfel’s tree nursery. If everything goes well, the rootstocks will grow together with the scions from the start of the growing season and the onset of sap flow. A rating will be carried out in summer. Anna-Lea: ‘Rating means that we carry out a qualitative assessment of the apple trees. For example, we check the growth, the health of the foliage and the growth height.’
Winter 2025/26: Transplanting into agroforestry areas on the Lower Rhine
This November, the apple trees are to be transferred to the two cooperation partners Berkhöfel and Jürgen Janssen. Whilst one location with a loamy soil offers the optimum conditions for the trees, the trees at the second location will encounter a sandy, rather poor soil. This will allow further research into the development of the trees in the field and valuable insights to be gained.
Kommentare