Interaction between Rootstocks and Scions 

• Scion-rootstock interaction studies had initially focused on graft vigor and stress tolerance.
• Rootstock-mediated effects on nearly all the fruit quality parameters are now established.
• The fruit quality effects of rootstocks are more in perennial crops like citrus than deciduous fruit trees.

Grafting of woody crops aims to manipulate scion for which appropriate rootstock selection is crucial. Rootstocks’ influence on several scion plant growth and health parameters was well-known. Research shows that fruit quality, thought to be a scion-based trait, is also affected by rootstocks. This article covers the fruit quality parameters changed by rootstocks and the extent of their importance in the rootstock-scion interaction.

Understanding the Impact of Rootstock-scion Interaction

Grafting is a well-developed technique in fruit trees, which combines a scion and a rootstock

to form a new plant with a blend of characteristics.

Scion-rootstock interactions are common and used in various fruits and vegetables to improve graft tree vigor, pest tolerance, nutrient uptake, and yield efficiency.

The choice of rootstock in grafting is crucial. It involves considering factors like vigor, compatibility with the chosen cultivar, adaptability to different soil types, yield, water conditions, and resistance to biotic and abiotic stresses. The aim is to improve overall plant health, productivity, and fruit quality while addressing challenges like diseases, pests, and adverse soil conditions like heavy metal pollution and salinity.

Table 1: “Rootstock effect on physical properties of ‘UFSun’ peach grown in Florida,” Shahkoomahally et al. 2020. (Image credits: DOI: 10.1002/fsn3.2005)

The mechanisms of rootstock-scion interaction are not fully understood. However, it is established that the interaction between rootstocks and scions happens through plant vascular systems- the phloem and xylem. Phloem sap contains nutrients and photosynthetates, like organic and inorganic compounds, hormones, and secondary metabolites, and is distributed to all parts of the plants, affecting its physiology.

Rootstock Influence on Fruit Quality

Research findings show that rootstock influence goes beyond influencing vegetative growth and health. Fruit quality, controlled by scion genotype, can also be manipulated by rootstocks. 

Rootstock-scion interaction changes fruit quality parameters such as size, shape, firmness, weight, dry matter content, soluble solids content (SSC), antioxidants, phenolics, and organic acids. As a result, rootstocks influence fruit flavor, color, and nutritional value. These effects have been reported for sweet cherries, peaches, citrus, grapes, etc.

Table 2: “Chemical attributes of ‘UFSun’ fruit produced on different rootstocks,” Shahkoomahally et al. 2020. (Image credits: DOI: 10.1002/fsn3.2005)

Rootstock Influence Mechanism

The role of rootstocks in fruit quality development is unclear and indirect. Rootstocks showing similar vigor but with different genotypes produce peaches of varying quality. Vigorous rootstocks generally reduce fruit quality, and dwarfing rootstocks result in better fruit quality by allocating more nutrients to fruits and better light penetration for photosynthesis.

The correlation among the fruit qualities also compounds our understanding of rootstock effects. For example, rootstock improvement of fruit size is negatively correlated with crop load. In citrus fruit, size is also negatively related to juice SSC. Enhanced vigor that produces more canopy and shading reduces color development and SSC accumulation.

Moreover, orchard management practices, like crop load management, could also potentially overshadow any effects from the rootstock on fruits. Fruit size are manipulated through hand and chemical thinning. When thinning reduces fruit numbers, the photosynthates produced by the fruit tree are partitioned among the fewer existing fruits, making them more prominent. 

Rootsock-mediated effects on fruit quality are not ubiquitous. Several studies have shown a wide split in the results of rootstocks between deciduous fruit crops and perennial citrus trees.

Perennial vs Deciduous Fruit Tree Effects

Rootstocks’ effects on fruit quality in deciduous crops are minor compared to citrus, where rootstocks significantly influence citrus fruit quality. This difference between perennials and deciduous crops is primarily due to the annual phenological cycle, fruit respiratory behavior, and canopy management techniques.

Annual phenological cycle

Carbohydrates, especially sugars, are crucial in determining fruit composition and flavor. The metabolism related to fruit flavor is species-specific and primarily under genetic control. Environmental factors also influence these events. Rootstocks may impact fruit quality through water relations and mineral nutrition. 

• Citrus fruits, maturing more slowly, provide a more extended timeframe for rootstocks to influence quality. Citrus trees’ have two vegetative flushes that compete with reproductive processes for carbohydrates. Since rootstock-mediated water relations are responsible for carbohydrate production, they also control citrus fruit quality. 
• In deciduous grafted fruit crops, stored carbohydrates support rapid growth in spring, and fruit development occurs over a shorter period, so rootstocks have a shorter period to influence fruit quality.

Fruit respiratory behavior

Rootstocks have a minor impact on climacteric fruit ripening, which involves increased respiration rate and ethylene production. 

Citrus, cherries, and grapes are non-climacteric fruits where rootstocks have more effect.

Canopy management

Canopy management is a widespread and essential practice for deciduous fruit crops, while for citrus trees, mechanical hedging and topping are often done primarily for orchard operations rather than directly impacting cropping or fruit quality.

For deciduous trees, canopy management seems to be intricately linked to optimizing factors that directly impact the quality and quantity of the fruit produced. Therefore, canopy management significantly influences deciduous fruit quality parameters like yield, fruit size, color, and sweetness due to the link between canopy management and light interception and use. 

Thus, environmental and management practices are more significant in fruit quality development in deciduous fruit crops than rootstocks. 

Incorporating Fruit Quality in Plant Breeding Research

Since the rootstock-mediated effects on fruit quality are indirect and minor, fruit quality has not often been considered while choosing rootstock. However, plant breeders are beginning to incorporate fruit quality effects of rootstock-scion interactions. Their research will need precise, non-destructive estimates of internal fruit chemical quality parameters like SSC, dry matter, titratable acids, and color. 

Modern fruit analysis devices from Felix Instruments Applied Science, like the portable F-750 Produce Quality Meter, can easily measure internal chemical quality parameters in real time in the field or laboratory. The fruit analysis meters rely on near-infrared spectroscopy and are trusted by scientists for accurate results, data logging, and the creation of new models. Such tools can be an asset to produce high-quality fresh produce that meets consumer requirements in local and international markets with suitable grafted plants. 

Sources

Kaleem, M. M., Nawaz, M. A., Alam, S. M., Ding, X., Cheng, J., & Bie, Z. (2023). Rootstock–scion interaction mediated impact on fruit quality attributes of thick-skinned melon during storage under different temperature regimes. Scientia Horticulturae, 312, 111823. https://doi.org/10.1016/j.scienta.2022.111823

Morales Alfaro, J., Bermejo, A., Navarro, P., Quinones, A., & Salvador, A. (2023). Effect of rootstock on Citrus fruit quality: A review. Food Reviews International, 39(5), 2835-2853.

Shahkoomahally, S., Chang, Y., Brecht, J.K., Chaparro, J.X., & Sarkhosh, A. (2021). Influence of rootstocks on fruit physical and chemical properties of peach cv. UFSun. Food Sci Nutr., 9: 401–413. https://doi.org/10.1002/fsn3.2005

Tietel, Z., Srivastava, S., Fait, A., Tel-Zur, N., Carmi, N., & Raveh, E. (2020) Impact of scion/rootstock reciprocal effects on metabolomics of fruit juice and phloem sap in grafted Citrus reticulata. PLoS ONE 15(1): e0227192. https://doi.org/10.1371/journal.pone.0227192

William S. Castle (1995) Rootstock as a fruit quality factor in citrus and

deciduous tree crops, New Zealand Journal of Crop and Horticultural Science, 23:4, 383-394, DOI: 10.1080/01140671.1995.9513914