Dwarfing rootstocks and water stress

Promising preliminary results from a Hortgro-funded study show that dwarfing rootstocks are not more susceptible to water stress than conventional rootstocks. By Engela Duvenage.

Initial findings from a three-year study run counter to the views of some producers that dwarfing rootstocks used in high-density plantings are more sensitive to dry spells than conventional rootstocks. Dwarfing rootstocks assessed in this study showed similar recovery after drought stress compared to more vigorous rootstocks.

“The risk is probably not as high as we thought it would be,” notes project leader Prof. Stephanie Midgley, Climate Change and Risk Scientist at the Western Cape Department of Agriculture.

Midgley says that information about the water use of specific cultivars, and especially of newly available rootstocks, empower apple growers to make decisions that increase the resilience of their farming operation. This is important in light of climate change and the likelihood of increased periods of water stress resulting from irrigation water restrictions.

The study was conducted in a full-bearing Rosy Glow orchard established in 2010 on sandy soils in the Witzenberg Valley as part of a rootstock-evaluation trial. Drought-stress treatments were performed during the summers of 2018 and 2020 on (in order of increasing vigour) M.9 Nic29, MM.109 with an M.9 interstem, G.222, and M.793 rootstocks.

In the first year of the trial, half the trees received no water for two three-week periods, separated by a month of standard irrigation. The other half received standard irrigation. As this had little impact on the trees, the trial was repeated for a second year, and water was withheld for substantially longer.

A water deficit trial was also conducted under more controlled conditions in 2019 using potted one-year-old Rosy Glow trees at the Welgevallen Experimental Farm on the Stellenbosch University campus. Five rootstocks were evaluated: M.9 Nic29, G.202, G.222, M.7, and M.793.

In the orchard, different rootstocks can potentially give rise to smaller or bigger root systems, but in pots the root volumes of different rootstocks are all the same. By conducting trials in the orchard and in pots, researchers can tease out whether the response of rootstocks to water deficits is physiological or simply due to differences in root volume.

In both 2018 and 2019 it was found that daily and seasonal water use was strongly influenced by rootstock vigour, even when accounting for differences in canopy leaf area. M.9 Nic29 and M.793 showed the best recovery from water deficit. Trees on these rootstocks slightly increased transpiration when again irrigated.

Midgley says that even though trees on more dwarfing rootstocks used less water, none of the trees in the experiment showed signs of significant stress or reduced their water use in the 2018 field trial. However, data analysis for the final year of the project has not yet been completed — these results are expected to show greater differences between rootstocks, as a more severe water deficit was applied.

Image: An apple tree reveals all its secrets to science.

Supplied by Stephanie Midgley | Western Cape Department of Agriculture.