How Rootstocks Affect Graft Hydraulic Balance

• Rootstocks of different genotypes have varying hydraulic conductance.
• Rootstock’s hydraulic conductivity will influence a scion’s water relations.
• Rootstocks influence a scion’s hydraulic conductance through its root system size and vigor, hydraulic signaling, and direct signaling.
• Dwarfing and drought tolerance are the two impacts of a rootstock’s hydraulic conductance on grafted trees.

Grafting scions to rootstocks is a standard technique in woody fruit trees to produce a cultivar with a new blend of features. The relationship between the rootstock and scion in a newly grafted tree is based on physiological attributes. One of the crucial characteristics is the grafted tree’s hydraulic properties, often called graft hydraulic balance. Learn more about how rootstocks can affect Graft Hydraulic Balance and its consequences for the grafted trees.

Rootstocks and Scions

Grafting is common in woody crop trees, as an appropriate rootstock can be a powerful crop management tool to improve growth, crop load, and yield efficiency. Rootstocks’ two effects on scions are their water relations and their vigor.

Rootstocks can determine the scions’ hydraulic properties or water relations, a concept integral to Graft Hydraulic Balance. They can restrict scion growth and increase yield efficiency. Hydraulic conductivity, a key component of graft hydraulic balance, is the ease with which water or fluids move through pore space, which in plants is through the xylem and phloem.

Rootstocks are a crucial tool that scientists use to breed food species with increased water use efficiency to ensure sustainable use of dwindling water resources. Optimizing water use and reducing leaf water loss would be possible by understanding rootstocks’ role in grafts.

The three ways rootstocks influence hydraulic balance in grafts are hydraulic signaling, chemical signaling, and rootstock vigor and size, see Figure 1. These three pathways influence stomatal conductance and transpiration, affecting a tree’s water relationship.

Figure 1: “An illustration of some of the main ways in which it is thought that rootstocks can affect shoot water relations and growth. These include direct chemical signaling through modification of xylem (or phloem) sap composition and hydraulic signaling resulting from alterations to xylem conductivity. Note that the hydraulic signaling loop also involves the feedback effects of altered canopy transpiration on soil water” Jones 2012. (Image credits: https://doi.org/10.1111/j.1469-8137.2012.04110.x)

Hydraulic Signaling

Hydraulic signaling results from alterations to xylem conductivity in rootstocks.

Rootstocks of different vigor have different water-transporting capacities. Less vigorous rootstocks have lower root mass, fewer feeder roots, smaller xylem vessels, and lower root hydraulic conductance. For example, in apples and peaches, smaller xylem vessels were responsible for lower water potentials during the afternoon.

When rootstocks change water availability to scions, they modify the root to shoot hydraulic signals. Less vigorous rootstocks will reduce water availability to scions, and this water deficit causes reduced tree growth.

It is assumed, though not yet proven, that when water availability reduces in tree species, it leads to smaller xylem vessels and higher vessel density in scions.

Direct Chemical Signaling

Rootstocks influence water balance by direct chemical signaling by altering levels of phytohormones like abscisic acid (ABA), cytokinins, and sap pH in the xylem and phloem. As a result, leaf and shoot growth and stomatal conductance are affected.

For example, ABA, usually produced in the roots, is supplied to leaves through the xylem and is crucial for many physiological functions, like stomatal closure, osmotic regulation, cuticular wax accumulation, etc., that can influence scion hydraulic properties.

ABA has a significant role in stomatal conductance during drought. The phytohormone is chiefly involved in hormonal signaling through increased concentrations in xylem sap to reduce stomatal conductance and transpiration during drought.

More vigorous rootstocks produce more cytokinins in the root tips, producing more vegetative growth and total stomatal conductance.

The factor controlling hormone bio-synthesis can differ among rootstocks. The control can emerge either from leaves or roots.

Root Vigor and Size

Root vigor, total length, and mass affect hydraulic relations in a grafted tree.

Dwarfing rootstocks have a less extensive root system than vigorous rootstocks. A smaller root system can explore less soil volumes, decreasing the water availability for the tree.

Specific root architecture parameters of rootstocks have vital consequences for hydraulic properties. The total root length area (RLA) can have varying hydraulic conductance between the soil-root interface and the rhizosphere. However, root length density and diameter are less relevant in influencing hydraulic conductance.

Even in low-moderate water deficit conditions, more root-length areas can produce higher stomatal conductance and transpiration. The biosynthesis of ABA is controlled by root system architecture; as the root length area increases, ABA production falls, leading to more stomatal conductance and transpiration.

Altered Canopy Transpiration

Both hydraulic and chemical signaling influence rootstock-specific stomatal regulation in leaves.

Lower hydraulic conductance results in lower leaf water potential, which reduces transpiration. Hydraulic signaling that affects canopy transpiration also creates a feedback loop that reduces available soil water for rootstocks and the grafted plant.

Some studies report that less vigorous rootstocks have smaller stomata. Less hydraulic conductance due to rootstocks and subsequent water stress could result in dwarfing stomata. However, it is unclear if this results from genotype or hydraulic conductance.

Chemical signaling by ABA directly controls stomatal conductance and transpiration. ABA also has localized effects on hydraulic conductance and signaling in rootstocks and influences leaf water status, showing an interaction between the two signaling pathways.

Benefits for the Scion

The hydraulics of rootstock–scion combinations have several beneficial consequences for the grafted plant. These include dwarfing and water use strategies.

Dwarfing

Dwarfing rootstocks have a low xylem-to-phloem ratio and lower hydraulic conductivity to scions. Therefore, scions have less water uptake and experience a water deficit that limits their shoot growth and increases yield efficiency.

The xylem also gets distorted at the grafting site, creating an axial resistance to water flowing upwards to the scion. However, concentrations of major foliar nutrients are not restricted.

Though most studies agree that water relations control by rootstocks causes dwarfing, the role of growth regulators in alerted signaling due to specific rootstocks could also be responsible for dwarfing.

Drought tolerance

As shown in apples, peaches, and grapefruits, rootstocks can be critical in scion response to drought.

Rootstocks can be selected or bred to improve water use efficiency and drought tolerance of the scion, thereby optimizing graft hydraulic balance. However, the effects of rootstocks on water relations, a part of Graft Hydraulic Balance, will also impact scion vigor.

Though dwarfing rootstocks can improve water use efficiency by using less water, in some cases in apples, a vigorous rootstock could extract more water from the soil during drought to make the graft drought tolerant.

Drought signaling uses chemical and hydraulic signals, but their relative importance is not yet established. ABA and chemical signaling is crucial in drought conditions to directly reduce stomatal conductance. At the same time, hydraulic signaling influences stomatal conductance by controlling entire organs or the plant.

Scientists are trying to identify the genes in rootstocks that control scion water relations to inform the choice of rootstocks to improve drought tolerance in rootstocks.

Identifying Suitable Rootstocks

We still don’t understand the response of the different phenotypes-dwarfing, semi-dwarfing, or vigorous rootstocks, and their effects on fruit yield and quality in different soil water conditions to generalize the choice of suitable rootstocks. To get a better picture, more research on rootstock interactions in the rhizosphere and the impact on stomatal conductance and transpiration is needed.

Precision tools capable of non-destructive analysis and estimation, like those supplied by CID Bio Science Inc., would be valuable assets in further studies. The CI-340 Handheld Photosynthesis System can measure transpiration and stomatal conductance in real-time, and the minirhizotrons CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager are ideal for long-term root experiments in identifying rootstocks for future-proofing fruit production.

Sources

Bristow, S. T., Hernandez-Espinoza, L. H., Bonarota, M.-S., & Barrios-Masias, F. H. (2021). Tomato rootstocks mediate plant-water relations and leaf nutrient profiles of a common scion under suboptimal soil temperatures. Frontiers in Plant Science, 11. https://doi.org/10.3389/fpls.2020.618488

Else, M. A., Taylor, J. M., Young, S., & Atkinson, C. J. (2018). The effect of the Graft Union on hormonal and ionic signalling between rootstocks and scions of Grafted Apple (Malus pumila L. Mill.). Environmental and Experimental Botany, 156, 325–336. https://doi.org/10.1016/j.envexpbot.2018.07.013

Gregory, P. J., Atkinson, C. J., Bengough, A. G., Else, M. A., Fernández-Fernández, F., Harrison, R. J., & Schmidt, S. (2013). Contributions of roots and rootstocks to sustainable, intensified crop production. Journal of Experimental Botany, 64(5), 1209–1222. https://doi.org/10.1093/jxb/ers385

Hayat, F., Li, J., Iqbal, S., et al. (2023). Hormonal Interactions Underlying Rootstock-Induced Vigor Control in Horticultural Crops. Appl. Sci.13, 1237. https://doi.org/10.3390/app13031237

Jones, H. G. (2012). How do rootstocks control shoot water relations? New Phytologist, 194(2), 301–303. https://doi.org/10.1111/j.1469-8137.2012.04110.x

Peccoux, A., Loveys, B., Zhu, J., Gambetta, G. A., Delrot, S., Vivin, P., Schultz, H. R., Ollat, N., & Dai, Z. (2017). Dissecting the rootstock control of scion transpiration using model-assisted analyses in grapevine. Tree Physiology, 38(7), 1026–1040. https://doi.org/10.1093/treephys/tpx153

Xu, H., Ediger, D., Singh, A., & Pagliocchini, C. (2021). Rootstock–Scion Hydraulic Balance Influenced Scion Vigor and Yield Efficiency of Malus domestica cv. Honeycrisp on Eight Rootstocks. Horticulturae, 7, 99. https://doi.org/10.3390/horticulturae7050099