How Rootstocks Influence Apple Fruit Quality and Yield

• Besides influencing tree growth and yield of scion cultivars, rootstocks also affect fruit maturity in apples.
• The experiment found a climacteric pattern, where ethylene production, respiration rates, and oxygen consumption peaked after 5–7 days in fruits produced from all rootstocks.
• The 17 rootstocks have significant differences in influencing apples’ yield, maturation, and fruit quality, which growers should consider during harvesting and post-harvest stages.

The amount of influence that rootstocks have on scion performance can be wide-ranging. However, the effects are also specific to combinations of rootstock and scions. Scientists, therefore, have to experiment to find the ideal rootstocks for the targetted growth and yield parameters of scions. In one such study on ‘Aztec Fuji’ apples, scientists at the University of Idaho conducted a gas analysis to see how ethylene and respiration rates were affecting fruits produced from different apple rootstocks.

Rootstocks and Scions

Standard rootstocks are used for several scion cultivars of a fruit, as is also the case in apples.

Rootstocks can affect mortalities, plant size, yield efficiency (yield per tree to the trunk cross-sectional), and yield per hectare. Rootstocks will also influence scions’ internal biochemical and physiological processes, such as ethylene production during maturation and storage.

Ethylene production in climacteric fruits like apples regulates ripening. Due to enzymatic activity, just before visible signs of ripening develop, there is an increase in carbon dioxide production, called climacteric respiration.

Ethylene production can also modify fruit qualities of color, texture, sugar content, and volatile compounds. The color change is measured in terms of the degradation of chlorophyll. The index of absorbance difference (I_DA) is a common means of showing the color change in fruits as an indicator of ripeness.

Rootstocks’ influence on scion growth and yield parameters will differ depending on compatibility.

Testing Rootstocks for ‘Aztec Fuji’ Apples

There is limited information on how rootstocks affect ‘Aztec Fuji’ apples’ maturity. So a team of pomologists and horticulturists Mahdavi, Fallahi, and Fazio tested 17 well-known rootstocks. They wanted to find the one which produced the best fruit maturity, quality attributes, and yield. They also investigated the impact of the different rootstocks on I_DA, ethylene evolution, and respiration.

The scientists decided to limit the experiment to only one year, 2020, because all rootstocks produced a full crop in that year, and the yield was close to long-term averages. Based on their prior experience, the pomologists decided they could expect the parameters also tested to be a good representative of long-term averages.

Apples were grown and trained into tall spindles in the semi-arid fields of Parma Research and Extension Center, University of Idaho, Parma. The orchard cultural practices of fertigation, thinning, and spraying was similar for all rootstock apple trees. The 17 rootstocks tested were:

• Two clones of the Budagovsky series (B.9, B.10)
• Four Cornell-Geneva clones, Geneva 11 (G.11), G.41N, G.202N, and G.935N
• Seven unreleased Cornell-Geneva clones (CG. 3001), CG.2034, CG.4004, CG.4003, CG.4214, CG.5222, and CG.4814
• One clone from the Pillnitz series (Supp.3)
• Three clones from the Malling series, which served as controls, M.9Pajam2, M.9T337, and M.26EMLA

In October, thirty-six fruits were sampled from each tree to measure yield and quality parameters.

Measuring Ripening Gasses

Gas analysis was used to evaluate internal ripening processes. Four apples from each replication were collected and stored in perforated plastic bags at 0°C. The scientists measured ethylene evolution, respiration, and oxygen levels six times on alternate days, starting one week after harvest – seven days after harvest (DAH) to 17 DAH.

The scientists used The F-940 Store It! Gas Analyzer to measure the ripening gasses. Following the instructions given by the manufacturer of the tool, Felix Instruments – Applied Food Science, the researchers collected 30 mL of gas from inside the sealed apple bags using a syringe and suction tube at a flow rate of 80 mL per min. The team measured and recorded ethylene and CO2 produced, as well as amounts of oxygen used.

The index of absorbance difference (I_DA) was estimated using two wavelengths of light between 670 and 720 nm to measure the color change in the apples. A DA meter read the I_DA at three spots on each fruit.

See how the Felix Instruments Produce Quality Meter is being used to measure apple maturity at KU Leuven.

Typical Climacteric Pattern

Fruits from all the grafts showed a typical climacteric pattern.

Measured Ethylene Pattern

Figure 1. “Amount of ethylene evolution (mL·kg·h1) in ‘Aztec Fuji’ apples on different rootstocks (7DAH to 17DAH),” Mahdavi et al., 2022. (Image credits: HortScience 57, 1; 10.21273/HORTSCI16253-21)

Measured ethylene levels increased with time in all rootstocks starting from 7 days after harvest (DAH), and reached their maximum by 15 DAH, as shown in Figure 1. The respiration rate reached its maximum levels by 13 DAH. So the climacteric respiration peaked before ethylene evolution, see Figure 2.

Ethylene evolution and respiration were high in fruits from trees on G.3001, Supp.3, and G.202, moderate in fruits from CG.4004, G.41N, CG.4214, and B.9, and low in fruits from M.9Pajam2, M.26EMLA, and G.11.

Measured Oxygen Pattern

Figure 2: “Amount of respiration (mL·kg·h−1) in ‘Aztec Fuji’ apples on different rootstocks (7DAH to 17DAH),” Mahdavi et al., 2022. (Image credits:HortScience 57, 1; 10.21273/HORTSCI16253-21)

Measured oxygen levels were the highest in the bags at the start of the experiment, 7 DAH, and decreased until the climacteric respiration peak was reached, as O₂ is used for respiration. Hence, M9.T337, with the slowest respiration rates, has the highest O₂ consumption. In contrast, G.41N and Supp.3, with the fastest respiration rates, recorded minimum oxygen levels.

O₂ levels began to rise after the peak as the respiration rate decreased. The scientists account for the increase due to photosynthesis that could still be occurring in the green fruit skin and produced O₂, see Figure 3.

Figure 3: “Amount of oxygen (%) in ‘Aztec Fuji’ apples on different rootstocks from (7DAH to 17DAH),” Mahdavi et al., 2022. (Image credits: HortScience 57, 1; 10.21273/HORTSCI16253-21)

Physiological Findings & Correlations

When results from all rootstocks are combined, I_DA values decreased as ethylene, respiration rates, and water core increased.

Fruits from G.11, M.9T337, M.9Pajam2, CG.2034, and M.26EMLA had the highest I_DA values (>1.10), while Supp.3, CG.3001, CG.5222, G.935N, CG.4814, G.41N, and G.202N lower IDA indices (<0.87).

There was a positive correlation between firmness and I_DA, so G.11 and CG.4003 recorded high I_DA and firmness at harvest. The rootstocks that produced the least apple firmness were CG.3001,  G.41N, G.935N, and CG.5222.

The color was also related to I_DA. Fruit from rootstock Supp.3, CG.4003, CG.4814, and B.10 had the highest color scores, and G.935N fruits had the lowest color scores of all rootstocks.

Though the difference in soluble solids content (SSC) was not significant, fruits from rootstocks Supp.3 and CG.4003 had the highest SSC, and M.9Pajam2, M.26EMLA, and M.9T337 had the least SSC.

M.9337, G.41N,  M.9Pajam2, and G.11 had higher starch degradation patterns, though many had less SSC. Therefore, the SSC content had no correlation to starch degradation patterns, suggesting that sugar is supplied to fruits from the conversion of starch and other sinks in the trees.

Yield per tree was the highest in CG.4004, G.41N, and M.26EMLA, and lowest in CG.4003.

The larger trees produced by G.41N also had a high yield, yet its yield efficiency fruits per tree to the trunk cross-section was not the best. B.9 and M.9T337 trees had the best yield efficiencies, and Supp.3 had minimum yield efficiencies.

Trees producing more numbers or amounts of apples showed less ethylene and CO2, fruit color, SSC, and firmness at harvest. Trees producing more apples also had bigger apples, which was unexpected. But larger or heavier fruits were softer.

Recommendation Based on the Study

Scientists chose the CG.4004 as the most suitable rootstock for ‘Aztec Fuji’ apples considering all the evaluated fruit yield and quality parameters.

As with any research, the study unearthed new questions. Scientists think it would be advisable to conduct more research to fine-tune the relationship between increased I_DA and higher ethylene and respiration rates. Then I_DA could estimate apple maturity to guide post-harvest decisions, such as shelf-life or shipping dates. I_DA, according to them, can also be used instead of firmness to judge apple quality as the two parameters are correlated.

Growers can use the different results of the 17 rootstocks on apple yield and quality, demonstrated by the study, to make management decisions during harvest and post-harvest, to increase their ROI and prevent food loss.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Read the original peer-reviewed paper for more details:

Mahdavi, S., Fallahi, E., & Fazio, G. (2022). The Influence of Rootstock on Fruit Ethylene, Respiration, Index of Absorbance Difference, Fruit Quality, and Production of ‘Aztec Fuji’ Apple under a Full-crop Condition. HortScience, 57(1), 1-9. 10.21273/HORTSCI16253-21