By Ute Albrecht
There have been numerous reports of malformed roots in field-grown citrus trees recently. The rootstock propagation method is often suspected as the culprit. Before drawing quick conclusions, it is important to recognize that there are many different factors that can influence the root structure of a citrus tree aside from the propagation method. These include the genetic background of the rootstock, nursery management, handling of the trees during and after transplant, field management, soil environment, and other biotic and abiotic factors. This article explains some of the many factors that can determine the root structure of field-grown citrus trees.
ROOTSTOCK GENETICS
The root structure is determined in some part by the genetic background of the selected rootstock. This may manifest in obvious ways such as rooting depth, root thickness and length, as well as the number, arrangement and proportion of structural and fine roots. It can also manifest in more subtle ways such as different root growth rates, differences in fine root structure and chemistry, or differences in size and arrangement of the water and nutrient-conducting (vessel) elements of the sap wood.
For example, fine roots from rootstocks with pummelo parentage (e.g., US-802 and sour orange) tend to be coarser than roots from some of the mandarin × trifoliate orange rootstocks (e.g., US-897 and US-942). These rootstocks also differ considerably in their vessel element arrangement and metabolite profile. Although subtle, such differences can affect water and nutrient transport and have a profound influence on the vigor and other properties of the grafted tree.
ROOTSTOCK PROPAGATION METHOD
Historically, citrus rootstocks have been propagated from seeds. Besides being convenient, seed propagation enables the production of genetically identical (“true-to-type”) plants because of the special phenomenon of nucellar embryony. Nucellar embryos arise from the maternal tissue and are therefore genetically identical to the mother plant.
Zygotic embryos, in contrast, combine genetic information from different parents and produce “off types,” which must be rogued in the nursery. In the past, rootstocks have been selected for this trait. This, however, slowed the breeding process and prevented the use of other (non-nucellar) rootstocks with potentially superior traits. Consequently, some of the newer rootstocks (e.g., US-1279, US-1281, US-1282 and US SuperSour 1) that were released faster than in the past do not produce nucellar seedlings and need to be propagated by other (vegetative) methods, such as cuttings or tissue culture. Using tissue culture, rootstocks can be produced in large quantities uniformly and year-round, eliminating the dependency on seeds.
Citrus rootstocks that are grown from seeds typically develop one well-formed taproot from which multiple smaller lateral roots arise. In contrast, rootstocks that are produced by cuttings or tissue culture produce multiple adventitious roots that are smaller in diameter but also contain numerous finer lateral roots. These differences in the root structure are notable throughout the nursery stage (see citrusindustry.net/2018/06/13/alternatives-rootstock-propagation-seed-differences-concerns-expectations).
FIELD ROOT SYSTEMS
The main functions of roots are anchorage, structural support, and the uptake of water and nutrients. Root systems are plastic and shaped by the local physical and chemical soil environment. The soil environment can vary considerably with differences in soil layers (horizons), hard pans, debris (e.g., tree stumps and roots from previous plantings), compaction, aeration, water content and fertility. Combined, these factors can have a tremendous impact on root growth, root structure and the symmetry of the root system. One factor that is often neglected but can greatly affect the root structure, and ultimately tree performance, is handling and transplanting in the nursery and in the field.
In the modern citrus nursery, trees are grown in plastic containers which restrict root growth horizontally and vertically. Once roots reach the bottom of the nursery container, they begin to circle. Circling will continue in the field unless these roots are trimmed off to encourage new growth. If not removed, circling roots may girdle the root trunk or other roots and inhibit tree growth.
Damage can occur during transplanting from rough handling, leaving roots exposed to the sun for extended periods before planting, addition of dry fertilizer to the planting hole, or providing inadequate contact with the surrounding soil. It is important to inspect roots before planting. An inferior root system before field planting will likely cause long-term problems.
Once trees are planted, they begin to produce new roots for structural support and forage for water and nutrients. There are different types of root systems such as tap root, lateral and heart root systems. Most trees have a lateral root system, even if starting out with a tap root system. Lateral root systems usually lack a deep tap root, extend much beyond the canopy dripline and are restricted to the upper areas of the soil.
University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers have excavated numerous citrus trees to study the structures of seed, cuttings and tissue culture propagated rootstocks during the nursery stage and the first two years in the field. They found that regardless of how the rootstocks had been propagated, the root systems were lateral and very shallow. None of the excavated root systems had a distinctive tap root, even those from seed propagation. The adventitious root system of cuttings and tissue culture propagated rootstocks was clearly distinct from the tap root system throughout the nursery stage.
However, this was not true after two years of field growth. This is because after planting, trees form new roots from the root stump for support and exploration. These new roots form most of the root system of the mature tree. Interestingly, it was found that in a bedded grove, more roots grow toward the bedside than toward the swale side. Roots on the bedside were also longer and thinner than roots on the swale side. This reinforces the importance of the soil environment.
Unfortunately, researchers also found root systems with numerous pot-bound roots, girdling roots or other abnormalities. Some of these malformations may have occurred because of the restrictions of the nursery container and improper transplanting; others likely occurred because of site-specific variations. Although the most recently excavated root systems are still being analyzed, preliminary data shows no evidence that cuttings and tissue culture propagation caused more malformations than seed propagation.
CONCLUSIONS
At this time, no evidence has been found that cuttings and tissue culture propagation negatively influence tree growth and productivity. However, this assumes that the trees coming from the nursery are healthy and have a well-formed root system.
Inspecting trees carefully before planting is recommended. A tree with an inferior root system will not produce a healthy tree in the field. Equally important is site preparation and taking care during and after planting to prevent root damage and malformations to ensure trees will be productive for many years to come.
Acknowledgment: This project is supported with funds from the Citrus Research and Development Foundation and the U.S. Department of Agriculture National Institute of Food and Agriculture.
Ute Albrecht (ualbrecht@ufl.edu) is an assistant professor at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.
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