What Are the Benefits of High-Density Orchards?

• The dwarfing rootstocks that increase tree numbers in high-density orchards have a smaller and shallower root system.
• Several vital root traits like root length density (RLD), lower root number density (RND), and fewer and smaller fine roots are characteristics of dwarfing rootstocks.
• The root traits in dwarfing rootstocks lower the vegetative vigor but increase flower number and fruit weight in high-density orchards.

High-density orchard plantings improve fruit quality and production and offer several management and environmental advantages. Dwarfing rootstocks with less vegetative vigor are used to accommodate an increasing number of trees per acre. Knowing the root system characteristics of the grafts is necessary since they support the productive success of orchard trees.

High-Density Plantings for More Food Production

The need to grow more food with existing land resources and zero environmental impact is changing agricultural and horticultural practices. Specialty crops like fruits and vegetables that are nutrition-dense are no exception.

Earlier, the large canopy size and spherical architecture allowed only 500 trees per acre in low-density plantings.

Modern high-density plantings (HDPs) can accommodate over 2500 trees per acre. The trees are closely planted, pruned, and trained to form 2D or planar canopies, creating a uniform fruiting wall.

The HDPs improve crop production and save costs because they have high light, land, water, input, and labor use efficiency. The trees are smaller, more accessible, and safer to manage without equipment, and they need less labor. A higher leaf area index and more sunlight interception in the canopy increase photosynthesis, boost yields and fruit quality, and reduce diseases. Moreover, HDPs increase pollinator attraction, carbon sequestration, and habitat for biodiversity.

Increasing tree density was possible by using semi-dwarfing and dwarfing rootstocks for grafts.

Dwarfing Rootstocks

Above-ground vegetative parts of scions with desirable fruit characteristics are grafted to rootstocks that provide the stem base and root system. Rootstock is chosen from varieties that are locally adapted, disease resistant, resource-use efficient, and have specified vigor. Rootstocks determine the growth and vigor of grafts to produce vigorous, semi-dwarf, or dwarf trees.

Dwarfing rootstocks were used to change from 3-D canopies to 2-D fruit canopies in HPDs. The rootstocks are instrumental in producing shorter trees with less vegetative vigor to reduce investment in pruning and labor costs. Cultivars with less vegetative growth can direct their carbon resources to produce more flowers and early fruits. Dwarfing rootstocks were first introduced for apples but are now used for several tropical and temperate fruits.

Though HDPs are becoming common globally, not much information on the root systems of rootstocks is available. Since roots are underground, studies and information on these plant organs lag in all crops.

However, there is enough information to highlight the role of roots and the importance of rootstock-scion interaction in influencing all aspects of a graft’s performance.

Root System of Dwarfing Rootstocks

The root system is the primary organ of importance in a graft, and its morphology or architecture can affect the graft’s functioning. The critical root traits identified for grafts are the size of the root system, distribution, biomass, and fineroots.

Smaller Root System

Figure 1: “Dwarfing mechanism: contribution of scion-rootstock interactions toward fruit crop improvement,” Hayat et al. 2021. (Image credits: doi:10.48130/FruRes-2021-0003).

Studies show that the above-ground and below-ground architecture are correlated. The root systems of a dwarfing rootstock are smaller than other cultivars, see Figure 1. The ratio of above-ground vegetative parts and underground root systems is the same regardless of rootstocks and cultivar and will vary only with soil. Generally, tree roots comprise 25-35% of the total tree biomass in intensive orchards.

Dwarfing rootstocks have less root biomass than vigorous rootstocks and can support only a smaller above-ground biomass. Less root biomass changes leaf and fruit nutrition, flower number, flowering date, and fruit size. The root diameter of dwarfing rootstock is smaller, and, combined with a lower root length density, it produces less total root surface area.

More fine roots than coarse roots are present in dwarfing rootstocks.

Root Length

The root length density (RLD) and root number density are lower in HDP grafts than in vigorous rootstocks.

In apples, around 89% of the total root system length belonged to fine roots, and around 35% was in depths of 20 to 40 cm, see Figure 2. In this soil depth, the average fine root length was 7993.1cm, and the coarse root lengths was 1417.6cm.

There is less variation in fineroots length among rootstocks, but the coarse roots (> 3 mm) length varies significantly within dwarfing rootstocks.

Total root lengths can vary depending on rootstocks due to coarse root lengths.

Figure 2: “Length and depth distribution of total root system on various dwarfing apple rootstocks in granny Smith apple trees,” Gjamovski et al. 2018. (Image credits: http://dx.doi.org/10.20903%2Fcsnmbs.masa.2018.39.1.120)

Shallow Root System

The dwarfing rootstocks are also shallower than vigorous rootstocks. In temperate regions, roots of intensive rootstocks are found in soil depths of 25-100 cm. Most fine roots are found between 20-40 cm in depth, while most coarse roots, in terms of weight, were found in the soil layer 0-20 cm in apple rootstocks, see Figure 2.

Information on the root system’s length and mass distribution is crucial to planning the horizontal distances and vertical heights in an orchard.

Root Distribution

Horizontal root distribution depends on species and tree age. In the early years of an orchard, there can be fewer inter-row lateral roots. The young roots face competition from grass roots for nutrition and water and possible allelopathy effects. In peaches, most roots grow within a row. However, more roots are seen growing laterally in peaches over the years. In cherry rootstocks, the lateral root growth is more evenly distributed, with more in-between row growth and less within the rows. Controlling or reducing meadow growth increases lateral root growth in young trees.

Lower Hydraulic conductance

Dwarfing rootstocks have smaller root diameters. The roots’ xylem vessels are smaller and restrict water flow, lowering water conductance to the above-ground parts.

Fewer Fine Roots

Fineroots (<2 mm) are responsible for water and nutrient absorption from soils and are crucial for tree growth and development.

Root count: Though the proportion of fineroots is higher than coarse roots, dwarfing rootstocks have fewer fineroots than standard rootstocks.

Fineroot diameter: Fineroot diameter is less in dwarfing rootstocks than in vigorous ones.

Lifespan: The lifespan of fine roots in dwarfing rootstocks is shorter than more vigorous rootstocks in the same soil conditions. Within a cultivar, the lifespan of fine roots differs depending on the season and fruiting stages. Fineroot production and mortality can vary between fruit-bearing and nonbearing trees. In spring and summer, fine roots survive for less than 80 days, but fine roots have a lifespan of over 81 days when produced at other times.

More Studies

As HDPs become more popular because of their many benefits, it will be necessary to research the root systems, as several root attributes like total root length differ based on cultivars or distribution patterns differ based on species. Moreover, studies on fine roots in dwarfing rootstocks have just begun. Minirhizotron systems used with permanent root tubes can be ideal for multi-year studies. The CID Bio-Science Inc. offers two scanners, the CI-600 In-Situ Root Imager, and the CI-602 Narrow Gauge Root Imager, which can help scientists develop more sustainable fruit production systems.

Sources

An, H., Luo, F., Wu, T., et al. (2017). Dwarfing effect of apple rootstocks is intimately associated with low number of fine roots. HortScience, 52(4), 503-512.

 

Anthony, B. (2022, Jan 12). Can High Density Orchards Yield More Crop per Drop? Retrieved from https://sustainability.colostate.edu/humannature/can-high-density-orchards-yield-more-crop/

 

Black, B.L., Drost, D., Lindstrom, T. et al. (2010). A comparison of root distribution patterns among Prunus rootstocks. J. Amer. Pomol. Soc. 64:52–60.

 

Gjamovski, V., Kiprijanovski, M., & Arsov, T. (2018). Distribution of root system at apple cv. Granny Smith grafted on different dwarfing rootstocks. Contributions, Section of Natural, Mathematical and Biotechnical Sciences, 39(1), 69-74.

 

Gregory, P.J., Atkinson, C.J., Bengough, A. G., et al. (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., Iqbal, S., Coulibaly, D., et al. (2021). An insight into dwarfing mechanism: contribution of scion-rootstock interactions toward fruit crop improvement. Fruit Research 1: 3. DOI:10.48130/FruRes-2021-0003.

 

De Silva, H.N., Hall, A.J., Tustin, D.S. & Gandar, P.W. (1999). Analysis of distribution of root

Length density of apple trees on different dwarfing rootstocks. Ann. Bot. (Lond.) 83: 335–345.

 

Zhang, Z., Li, M., Yao, J., et al. (2021). Root architecture characteristics of differing size-controlling rootstocks and the influence on the growth of ‘Red Fuji’ apple trees. Scientia Horticulturae, 281, 109959.