Can dwarfing rootstocks cope with drought?

A Hortgro-funded project assessed the response of new and popular apple rootstocks to deficit irrigation. By Anna Mouton.

Dwarfing rootstocks are quite literally the foundation of high-density apple orchards. They encourage earlier bearing that supports the rapid attainment of full production and high cumulative yields. But can they withstand South African conditions?

“After the big drought of 2015–2017, the industry was worried that higher density orchards on more dwarfing rootstocks might be more negatively affected,” says Dr Stephanie Midgley, Climate Change and Risk Assessment Scientist at the Western Cape Department of Agriculture.

Midgley led a recently completed project evaluating water stress in potted trees and full-bearing orchards. Most of the research was conducted by Dr Lindsay Muchena for her PhD.

Muchena explains that more dwarfing rootstocks tend to produce smaller and shallower root systems and smaller canopies than more vigorous rootstocks. In addition, rootstocks affect the graft union and rootstock’s hydraulic conductivity — how easily water flows from roots to shoots.

“It was expected that dwarfing rootstocks would result in higher resistance to water flow such that the scion will be exposed to more stress than trees on a vigorous rootstock,” says Muchena.

Different rootstocks also produce different levels of plant hormones like abscisic acid. Abscisic acid generated by roots under water stress stimulates responses like stomatal closure in leaves. Research has shown that some rootstocks produce more abscisic acid than others.

The genetics of a rootstock will determine which strategies it uses when faced with drought. For example, some rootstocks — like M.9 — close their stomata to survive, whereas others grow a larger root system.

Midgley and Muchena compared how five apple rootstocks cope when exposed to deficit irrigation to address growers’ concerns that dwarfing rootstocks might be more susceptible to water stress. They also assessed the water use of well-watered trees on the different rootstocks.

Drought stress in the field

Studies of full-bearing trees were conducted in the Koue Bokkeveld in a Rosy Glow orchard originally laid out to evaluate rootstocks. As these evaluations had been completed, the trees were available to Midgley and Muchena.

“It was a nice opportunity to use the existing trial trees,” recalls Midgley. “It didn’t matter if we stressed them. In a normal orchard, farmers wouldn’t be very keen to conduct drought trials.”

The trees were established in 2010 at 4.0 m x 1.25 m on deep sandy soil. The rootstocks included in the water-deficit trials were the semi-vigorous M.793, the semi-dwarfing G.222, M.7 and MM.109 with an M.9 EMLA interstem, and the dwarfing M.9 RN.29. M.9 RN.29 is usually sold under the trade name M.9 Nic29.

Trees were assigned to either a well-watered or a water-deficit treatment. The well-watered trees received the same irrigation as the rest of the orchard block. Midgley and Muchena consulted with irrigation adviser Louis Reynolds of Fruitful Crop Advice and farm manager Johan Visser of Dutoit Agri on appropriately implementing the water-deficit treatment.

During the 2017–2018 season, the water-deficit treatment consisted of two approximately three-week periods of no irrigation. The first was from 20 January to 8 February, and the second was from 8–30 March. No significant rain fell during this time.

Data on environmental conditions and tree physiology were collected, and transpiration, crop water stress, leaf gas exchange, water potential, hydraulic properties, root growth, yield, fruit quality, and water productivity were determined.

In addition to assessing rootstock effects on tree water stress, Midgley collaborated with Dr Elke Crouch, Postharvest Physiology Research Chair in Deciduous Fruit at Stellenbosch University, to investigate fruit quality after storage.

“We didn’t get very clear results in the first season, so we gave the trees extra stress in the second season,” says Midgley. During the 2019–2020 season, the water-deficit treatment consisted of nearly ten weeks without irrigation, from 17 January to 23 March. There was again no significant rain during the water-deficit treatment.

The potted version

In 2018–2019, Midgley and Muchena ran a pot trial at Welgevallen Research Farm, Stellenbosch, using one- and two-year-old Rosy Glow trees. The rootstocks tested were the dwarfing M.9 RN.29, the semi-dwarfing G.202, G.222, and M.7, and the semi-vigorous M.793.

“It really takes a lot to stress a mature full-bearing apple tree,” notes Midgley. “Obviously, in a pot-limited environment with small trees, you’ll get a much stronger drought response.”

The water-deficit treatment consisted of two approximately ten-day periods of no irrigation. The first was from 23 February to 3 March, and the second was from 23 March to 1 April. Almost no rain fell during this time.

Similar data on environmental conditions and tree physiology were collected as for the field trial, keeping in mind that the potted trees were not yet bearing. Dry-mass accumulation and root:shoot ratios were determined at the end of the trial.

What did we learn?

Physiological measurements — such as leaf water potentials and stomatal gas exchange — showed that the irrigation deficits successfully stressed the trees in the first season of the field trials. However, the response of the different rootstocks surprised the researchers.

“In our study, the trees on the M.9 RN.29 dwarfing rootstock didn’t appear to be more susceptible to water stress compared to the more vigorous rootstocks,” says Muchena. “We also saw an increase in water use after exposure to water stress, which could be linked to an increase in root development. In that way, M.9 RN.29 didn’t behave as we expected.”

Trees on the different rootstocks showed large differences in transpiration rates and water use under regular irrigation. Those on M.793 had the highest transpiration rate — about 25 litres per day — while those on M.9 RN.29 had the lowest transpiration rate — about 12 litres per day.

When the water deficit was applied, M.9 RN.29 still had the lowest transpiration rate — about 7 litres per day — while the other four rootstocks had rates of 14–17 litres per day.

Under well-watered conditions, Muchena calculated that the total seasonal transpiration per individual tree would be 4 255 litres for M.793, 3 171 litres for G.222, 3 131 litres for MM.109 with an M.9 EMLA interstem, and 1641 litres for M.9 RN.29.

Although the total yield per hectare was the highest for trees on MM.109 with an M.9 EMLA interstem and the lowest for M.9 RN.29, the projected water productivity for trees on M.9 RN.29 — 15.7 kg per m³ — is the highest when standard industry tree spacings are taken into account.

By comparison, the projected water productivity is 14.8 kg per m³ for MM.109 with an M.9 EMLA interstem, 14.1 kg per m³ for G.222, and 11.8 kg per m³ for M.793.

“After recalculating using the standard industry spacings, MM.109 with an interstem had the highest water use,” says Midgley. “It was unexpected — we don’t yet fully understand the responses involved.”

The role of hydraulic conductivity

The dwarfing effect of certain rootstocks is due to low hydraulic conductivity — the graft union is a choke point that restricts flow between the rootstock and the scion. Together with narrower xylem vessels in the rootstock, this explains why dwarfing rootstocks use less water.

When soil moisture is readily available and atmospheric conditions favour high transpiration rates, a tree on a semi-vigorous rootstock like M.793 can respond by increasing stomatal conductance because its vessels allow free and easy water movement from the roots to the leaves. When soil moisture levels drop, the tree closes its stomata to limit water loss.

In contrast, a tree on a rootstock like M.9 RN.29 has less variable transpiration rates because it struggles to change water flows from the roots to the leaves due to high hydraulic resistance. Instead, the tree will use internally stored water to help replenish losses.

“The literature shows that trees on the more dwarfing rootstocks have smaller and fewer xylem vessels,” notes Muchena. “The more vigorous rootstocks have bigger and more xylem vessels.”

The trials on potted trees showed that both M.793 and M.9 RN.29 were better able to maintain transpiration rates during and following the first water-deficit period but had greater reductions in transpiration rates during the second water-deficit period compared to the other rootstocks. Both M.793 and M.9 RN.29 also recovered better from the second water-deficit period.

It seems likely that M.793 normally uses a lot of water and copes with water stress by closing its stomata. The plumbing limitations of M.9 RN.29 always restrict its water use, forcing it to be conservative and to draw on internal water stores.

“The MM.109 with the M.9 interstem came out as a very intriguing rootstock combination physiologically,” says Midgley. “MM.109 is a vigorous rootstock, so you’re getting this interesting combination of a bigger root system with a dwarf interstem that behaves similar to dwarfing rootstocks in some respects but like a vigorous rootstock in other respects.”

In conclusion, the study established that dwarfing rootstocks are not more susceptible to drought stress than more vigorous rootstocks. The superior water productivity of trees on M.9 RN.29 is a bonus, according to Midgley.

“Even at the much higher densities for orchards on dwarfing rootstocks, you still get lower water use per hectare,” she says. “I think this is very good news for industry.”

When recalculated using the standard South African industry tree spacings, the seasonal — October to May — transpiration of well-watered trees on M.9 RN.29 would be 469 mm compared with 725 mm for G.222, 726 mm for M.793, and 895 mm for MM.109 with an M.9 interstem.

However, growers should note that these results are specific to the rootstocks tested. Other rootstocks might respond differently, especially in harsher environments than the Witzenberg Valley.

What are people saying about this research?

“In my view, the main lessons learnt from this project are:

• Dwarfing rootstocks are not necessarily more sensitive to drought.
• When water use was recalculated using standard industry tree spacings, M.9 still used less water per hectare than more vigorous rootstocks.
• The dwarfing rootstocks had better water productivity. Growers with little water will get more bang for their buck with dwarfing rootstocks.”

Wiehann Steyn. General Manager, Hortgro Science.

“I think this was a great project. We obtained good information on the drought tolerance of dwarfing rootstocks, which turned out not to be as sensitive to water stress as the industry feared. Knowing the potential performance of rootstocks is also foundational for future research.”

Willie Kotze. Technical adviser, Dutoit Agri.