Fruit trees are usually a grafted combination of rootstock and top, or scion, that provides the fruit quality desired on a rootstock that will grow well in specific environments and provide certain qualities to the total plant. In citrus, rootstocks are used for better fruit quality, controlled tree size, disease resistance, and tolerance of unfavorable environmental conditions. Researchers are looking for rootstocks resistant to citrus greening in the hope that they will provide some greening resistance in the scion.This search has produced some good rootstock candidates and some issues with propagating sufficient numbers.
Traditionally the rootstocks are grown from seeds that were not a product of a fertilized embryo, but a special type of seed many citrus produce that is genetically identical to the mother, called nucellar apomictic. Some of the newer rootstocks developed to tolerate citrus greening disease do not produce these apomictic seeds or cannot be produced in sufficient numbers to satisfy demand. Other vegetative propagation methods like tissue culture and rooted cuttings are being used to satisfy the demand for these new rootstocks. However, are the root systems of plants produced through these other types of propagation as good as seedling root systems? Seedlings usually produce a tap root while tissue culture and cuttings produce adventitious or lateral roots. Tap roots are thought to be the most desirable for anchoring the trees, while lateral roots are best for scavenging nutrients and water from the soil. Researchers at the University of Florida set out to determine the differences in the root systems with these different types of propagation.
The researchers compared root architecture and growth of seven different rootstock plants generated from seed, stem cuttings, or tissue culture in the greenhouse in the first 10 to 20 weeks of growth. When propagated plants reached about ten inches high, they measured the dry weight of the shoots, roots, and leaves as well as the number of primary and first order lateral roots, and total root length. Specific root length was calculated as the ratio of root length to dry weight of roots.
Plants generated from cuttings had 11-16% of their weight in root mass while tissue cultured plants had 16-29% and seedlings 21-30%. Cutting propagated plants had the most primary roots (7-10) followed by tissue cultured plants with 2-6 primary roots and seedling plants with one primary root. The total number of first order lateral roots was highest in cutting propagated plants (108-185), followed by 53-103 lateral roots for tissue cultured plants, and 43-78 lateral roots for seedling plants. Specific root length was also highest in cutting plants followed by tissue cultured and seedling plants. The authors felt that plants produced by cuttings and tissue culture may be better suited for superior growth because of the larger number and length of roots, however, that research is yet to be done.
The full article is: Influence of Propagation Method on Root Architecture and Other Traits of Young Citrus Rootstock Plants. 2017. HortScience 52(11):1569-1576. By U. Albrecht, M. Bordas, B. Lamb, B. Meyering, and K.D. Bowman.