Horticultural Attributes and Root Architectures of Field-grown ‘Valencia’ Trees Grafted on Different Rootstocks Propagated by Seed, Cuttings, and Tissue Culture

Abstract

Huanglongbing (HLB) is a devastating disease of citrus that is found in most citrus production areas around the world. The bacterium associated with HLB resides in and damages the phloem, restricting the movement of photosynthates throughout the plant and leading to tree decline. Considerable root loss can be observed in affected trees even when few disease symptoms are visible aboveground. Root traits can substantially influence tree performance and use of superior rootstocks is one strategy to manage tree health and reduce production losses in a disease-endemic environment. Citrus rootstocks are typically propagated by seed, but due to the increased demand for some of the best-performing cultivars, propagation by other methods is being used to overcome seed shortages. In this research, differences in root architecture and root growth of six different rootstocks propagated by seed, cuttings, and tissue culture, and their influence on the grafted ‘Valencia’ (Citrus sinensis) scion were investigated. A field trial was established in southwest Florida in 2017. Trees were evaluated for their performance during the first 2 years after planting and a subset of trees was excavated for detailed analysis of root architectures and biomass distribution. Significant differences among propagation methods were found for the rootstock trunk diameter and the lateral (structural) root length, which were largest in seed-propagated rootstocks. Most of the other horticultural and root architectural traits were not significantly influenced by the rootstock-propagation method; however, many of the measured variables were significantly influenced by the rootstock cultivar regardless of the propagation method. The results showed that rootstocks propagated by cuttings and tissue culture were similar to seed-propagated rootstocks in their influence on the grafted tree during the early years of growth in the field.

Different Sweet Orange‒Rootstock Combinations Infected by Candidatus Liberibacter asiaticus under Greenhouse Conditions: Effects on the Roots

“…The largest increase in root length measured with the minirhizotron system occurred from September to October after the transplant of trees into larger pots, and from March to July of the following year, which corresponded to the most active time of leaf and shoot growth. Minirhizotron field studies by Pokhrel et al (2021) found the largest net increase in live root length between June and July after the spring leaf flush. Bevington and Castle (1985) suggested that root growth follows shoot elongation and is stimulated by increases in soil temperature when water is not a limitation.…”

“…A typical citrus tree root system under present Florida growing conditions is composed mainly of lateral (structural) roots that provide anchorage, and fibrous roots that absorb water and nutrients (Pokhrel et al, 2021). The efficiency of the root system, especially the fibrous roots, can enhance the performance of the tree under disease stress.…”

“…Several studies demonstrated substantial differences in citrus root system architectures among different rootstocks (Castle and Youtsey, 1977;Pokhrel et al, 2021;Savage et al, 1945). It is also well documented that the rootstock cultivar can have a strong influence on the vigor of a grafted citrus tree (Forner-Giner et al, 2014;Girardi and Murão Filho, 2006).…”

SuperSour: A New Strategy for Breeding Superior Citrus Rootstocks

“…Historically, there was concern about whether the characteristics and health of rootstocks propagated by stem cuttings and tissue culture would be as good as that of rootstocks propagated by seed. But rootstocks propagated by all three methods (seed, cuttings, and tissue culture) have been studied intensively, with clear indication that nursery and field performance are primarily determined by the genetic traits of the rootstock cultivar, and only minimally affected by propagation method (Albrecht et al, 2017(Albrecht et al, , 2020Pokhrel et al, 2021). In one current field trial with Valencia scion and four rootstock clones, a comparison of the rootstocks propagated by nucellar seedlings, stem cuttings, and tissue culture (Figure 1) demonstrated a significant effect of rootstock clone on young tree growth and canopy health, but no significant effect from rootstock propagation type (Table 2).…”

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“…Under this pressure for replanting and resetting HLB-affected groves, there is a clear concern among citrus nursery operators and growers on seed availability of highly demanded rootstocks in Florida. Currently, seed propagation remains the preferred method of rootstock production in commercial citrus nurseries (Albrecht et al, 2020;Pokhrel et al, 2021), in part because there is no evidence on the vertical transmission of viable cells of the HLB pathogen "Candidatus Liberibacter spp." through seeds to citrus seedlings (Belasque Júnior et al, 2009;Bagio et al, 2020), and also because the extended belief that different root architectures arising from vegetative propagation will result in trees of inferior quality as compared to those from seeds (Albrecht et al, 2020); as a result, over 80% of the licensed citrus nurseries prefer uniform liners from seeds rather than from tissue culture sources (Chaires, 2017).…”