The July 2013 installment of “Citrus Nursery Source” highlighted the likely methods of propagation for new promising rootstocks in light of disease challenges and the need to produce a large number of trees in a short period of time. Since that article, discussion has ensued among nurseries and growers about the pros and cons of rootstock production via tissue culture (TC). Rumors abound about inconsistent vigor, poor root health, susceptibility to windstorms, etc., though little research has been done in this area. In an effort to substantiate or dispel these concerns, we reached out to folks with expertise and experience in this field.
Letters From Spain
Dr. Francisco Llatser (AVASA) confirms that though TC has been used extensively in Spain (for approximately a decade), TC production of rootstocks was only recently authorized for use on rootstocks that do not produce enough seed. This was initially done to accommodate Dr. Juan Forner’s citrus tristeza virus-tolerant FA 5 rootstock.
Dr. Mireia Bordas (Agromillora Research, S.L.), who operates in several production areas, including the U.S., has produced 260 million trees by micropropagation over 27 years, including plants of 400 species and varieties. Dr. Bordas offered, “Similar issues regarding tree vigor, quality of roots, and susceptibility to wind were raised at the beginning of the development of the TC technology in other fruit trees. After 30 years of experience and a large number of plants micropropagated and planted in fields, all of these concerns have been addressed. In fact, uniformity and consistency in field performance are among the main advantages of TC plants. In citrus sps., micropropagation guarantees the genetic identity, avoiding tetraploids or zygotic embryos that could escape selection in the nursery.”
Duarte Nursery has produced about 900,000 citrus trees on TC rootstocks with few problems reported by growers. Their experience and observations indicate no difference in tree vigor or root health. There was at least one grower with J-roots but this has been an issue related to transplantation into larger pots and it would be inappropriately identified as an issue related to TC production. Duarte also reports equal vigor on the nursery bench between trees produced from seed and those from TC. Though they are not aware of any TC citrus trees being challenged with severe winds, nut varieties produced from TC have performed well through high spring winds.
Dr. Graham Barry offered a key point for consideration. “If the nursery trees are of similar stem diameter at the time of planting, then there will be relatively little difference in tree performance. However, one of the goals of using TC instead of seedlings is to speed up the process of tree propagations and, as a result, its conceivable that tissue cultured trees would be smaller at the time of planting.”
It is critical to make a fair comparison of TC trees vs. seedlings. When this issue was discussed with Florida growers, one grower suggested he might consider low cost structures to house TC trees for growth prior to planting in the field. This would assure better survival in the field and still maintain production at the nursery level.
Applicable Florida Observations
In his early work at the Citrus Research and Education Center (CREC), Dr. Bill Castle, UF/IFAS, compared growth of seedlings vs. cuttings. These observations have taken on renewed importance as trees from rooted cuttings and TC behave very similarly. He observed no difference in tree performance, root health, or wind anchorage. This remained true throughout the trial. Furthermore, Dr. Castle emphasized that in the Flatwoods, the tap-root — usually present on seedling rootstocks — serves no real purpose because of the shallow soil. Even distribution of roots remains the key.
Managing The Fundamentals
Though the foundations of TC production are in science, there are a number of other factors that seem to influence results. Several labs could use the same mother stock, but the resulting plantlets may vary in quality and performance. Following a prescribed laboratory protocol is important, understanding it is another. Tony Fortier of Phoenix Agrotech identifies the following fundamental management issues in production of TC plants:
- Source and quality of mother-stock (the most important)
- How long the plant material is run through the lab
- Level and types of plant growth regulators at each stage (speed does not always translate to field performance)
- Type of nitrogen source and quantity and overall media composition
- Ability to identify the right type of cells and callus to work with once initiated in the lab (microscopy)
Phillip Rucks of Phillip Rucks Citrus Nursery emphasizes similar factors as his TC lab places great emphasis on the custom composition of media and the establishment of specific protocols for each variety. This involves keen knowledge of the process and product as well as the investment of time in the process. This is not a one-size-fits-all business.
Based on input from those knowledgeable in the science and art of TC plant production, nurseries and growers can have confidence in the science of TC production. Like conventional propagations, performance may vary depending on the management of the process. Labs that have the expertise and experience will develop trees with similar vigor and performance of standard trees. Nursery practices after receipt of TC liners from the lab also will vary and have some bearing on the final product. One thing is certain, this process provides hope of acreage replacement on a large scale and is another important tool in the box.
In addition to those mentioned in the article, the author also would like to thank Dr. Javier Castillon (Duarte Nursery) and Dr. Fred Gmitter (UF/IFAS, CREC) for their insights.