Oaks [beech family (Fagaceae)] are globally iconic trees, prized for their contributions as keystone species, their landscape value, and their strong, rot-resistant wood. Despite their importance, many species of oaks are under threat globally from a range of issues, such as habitat loss, competition from invasive species, and attacks from pests and diseases (Kramer and Pence, 2012; Oldfield and Eastwood, 2007). Of the 175 oak species able to be fully evaluated for the International Union for Conservation of Nature’s Red List of Oaks (Oldfield and Eastwood, 2007), roughly 45% were categorized as critically endangered, endangered, vulnerable, or near threatened. Conservation of this genus is challenging because oak acorns do not survive the methods used for long-term seed banking, an important plant conservation method (Bonner, 2003). One tool to support conservation efforts is tissue culture (in vitro culture or micropropagation) (Hartmann et al., 2002; Pence, 2010). Tissue culture can be used to maintain conservation germplasm collections in cultivation (ex situ) for intermediate-term storage (Guerrant et al., 2004; Pence, 2011), increase the number of individuals for conservation both outside and inside native habitat (in situ) (Guerrant et al., 2004; Pence, 2011), and supply shoot tips or somatic embryos for cryopreservation (Hartmann et al., 2002; Pence, 2010, 2011). Tissue culture methods are also needed for the recovery of zygotic embryo axes from cryopreservation, another potential conservation method for oaks (Xia et al., 2014).
Oak tissue culture research has predominantly focused on a handful of economically important, rather than threatened, species. These include english oak [Quercus robur (Chalupa, 1988; Vieitez et al., 1994), northern red oak [Q. rubra (Vengadesan and Pijut, 2009; Vieitez et al., 1993)], and cork oak [Q. suber (Manzanera and Pardos, 1990; Romano et al., 1992)]. Only three studies were found investigating tissue culture propagation of threatened oak species, conducted by Kramer and Pence (2012) studying maple-leaved oak (Q. acerifolia), arkansas oak, boynton sand post oak, and georgia oak (Q. georgiana); Kartsonas and Papafotiou (2007) studying dwarf trojan oak (Q. euboica); and Tamta et al. (2008) studying brown oak (Q. semecarpifolia).
In addition, there have been fewer reports on culturing oaks from mature material than from juvenile material as it tends to be more difficult to establish and grow in culture (Kartsonas and Papafotiou, 2007; Vieitez et al., 1993). Despite having better establishment in tissue culture, juvenile material is not always feasible to obtain, especially from threatened oak species. With few trees to produce them, the availability of acorns from threatened species for seedling material can be low. This is a particular challenge because seedlings from threatened species are also needed for propagation and population reinforcement, limiting the amount available to divert to tissue culture (Guerrant et al., 2004; Kramer and Pence, 2012). Furthermore, verification of seedling parentage may be hard to determine because oak species hybridize easily (Kramer and Pence, 2012; Oldfield and Eastwood, 2007). Depending on the circumstances, the basal branches may or may not be present on individual plants. Acquiring stump sprouts is not practical when there are only a limited number of individuals available because the primary method of production of these sprouts is to cut a tree down to the stump (Wendling et al., 2014). Wounding or removal of root material for induction of adventitious shoots could injure and harm the plant by creating entry for pests and pathogens, and depleting the tree’s carbohydrate storage (Kramer and Kozlowski, 2012). Therefore, determining, a reliable method of tissue culture involving mature-phase material would be useful in overcoming the challenges of obtaining juvenile-phase plant material from threatened species.
With ≈500 oak species in the world (Oldfield and Eastwood, 2007), it would be valuable to predict oak tissue culture responses by taxonomic group and establish protocols that could be generalized across related species. The first objective was to determine which of the two commonly used media in the tissue culture of beech family species, Lloyd and McCown WP medium (Lloyd and McCown, 1980) or GD medium (Gresshoff and Doy, 1972), was more effective in the establishment, growth, and multiplication of material from mature phase material from a range of oak species. The second objective of this study was to determine whether there was a pattern in tissue culture responses (growth, survival, and contamination) by species or section. The third objective was to determine whether there was a tissue culture response pattern by the moisture level of natural habitat (mesic or xeric) or young leaf texture (rugose and hairy or smooth and glabrous).
Bonner, F. 2003 Care and collection of acorns: A practical guide for seed collectors and nursery managers. U.S. Dept. Agr. For. Serv. Natl. Seed Lab., Dry Branch, GA
Botanic Gardens Conservation International 2014 PlantSearch database. 20 Apr. 2015. <http://www.bgci.org/plant_search.php>
Chalupa, V. 1988 Large scale micropropagation of Quercus robur L. using adenine-type cytokinins and thidiazuron to stimulate shoot proliferation Biol. Plant. 30 414 421
Gamborg, O.L. & Phillips, G. 2013 Plant cell, tissue and organ culture: Fundamental methods. Springer, Berlin, Germany
Guerrant, E.O., Havens, K. & Maunder, M. 2004 Ex situ plant conservation: Supporting species survival in the wild. Vol. 3. Island Press, Washington, DC
Hartmann, H.T., Kester, D.E., Davies, F.T. Jr & Geneve, R.L. 2002 Plant propagation – Principles and practices. 7th ed. Prentice Hall, Upper Saddle River, NJ
Kartsonas, E. & Papafotiou, M. 2007 Mother plant age and seasonal influence on in vitro propagation of Quercus euboica Pap., an endemic, rare and endangered oak species of Greece Plant Cell Tissue Organ Cult. 90 111 116
Kramer, P. & Kozlowski, T.T. 2012 Physiology of woody plants. Academic Press, New York, NY
Lloyd, G. & McCown, B. 1980 Commercially-feasible micropropagation of mountain laurel, Kalmia latifolia, by use of shoot-tip culture Comb. Proc. Intl. Plant Propagators’ Soc. 30 421 427
Murashige, T. & Skoog, F. 1962 A revised medium for rapid growth and bio assays with tobacco tissue cultures Physiol. Plant. 15 473 497
Nixon, K. 2006 Global and neotropical distribution and diversity of oak (genus Quercus) and oak forests, p. 3–13. In: M.M. Caldwell, G. Heldmaier, R.B. Jackson, O.L. Lange, H.A. Mooney, E.-D. Schulze, and U. Sommer (eds.). Ecology and conservation of neotropical montane oak forests. Springer, Berlin, Germany
Oldfield, S. & Eastwood, A. 2007 The red list of oaks. Fauna Flora Intl., Cambridge, UK
Pence, V.C. 2011 Evaluating the costs for the in vitro propagation and preservation of endangered plants In Vitro Cell. Dev. Biol. Plant 47 176 187
Ragazzi, A., Moricca, S., Capretti, P., Dellavalle, I. & Turco, E. 2003 Differences in composition of endophytic mycobiota in twigs and leaves of healthy and declining Quercus species in Italy For. Pathol. 33 31 38
Romano, A., Martins-Loucao, M. & Noronha, C. 1992 Influence of growth regulators on shoot proliferation in Quercus suber L Ann. Bot. (Lond.) 70 531 536
Tamta, S., Palni, L.M.S., Purohit, V.K. & Nandi, S.K. 2008 In vitro propagation of brown oak (Quercus semecarpifolia Sm.) from seedling explants In Vitro Cell. Dev. Biol. Plant 44 136 141
Vengadesan, G. & Pijut, P.M. 2009 In vitro propagation of northern red oak (Quercus rubra L.) In Vitro Cell. Dev. Biol. Plant 45 474 482
Vieitez, A.M., Ballester, A., Amo-Marco, J. & Sanchez, M.C. 1994 Forced flushing of branch segments as a method for obtaining reactive explants of mature Quercus robur trees for micropropagation Plant Cell Tissue Organ Cult. 37 287 295
Vieitez, A.M., Pintos, F., San-Jose, M.C. & Ballester, A. 1993 In vitro shoot proliferation determined by explant orientation of juvenile and mature Quercus rubra L Tree Physiol. 12 107 117
Wada, S., Niedez, R.P., DeNoma, J. & Reed, B.M. 2013 Mesos components (CaCl2, MgSO4, KH2PO4) are critical for improving pear micropropagation In Vitro Cell. Dev. Biol. Plant 49 356 365
Wendling, I., Trueman, S.J. & Xavier, A. 2014 Maturation and related aspects in clonal forestry - Part II: Reinvigoration, rejuvenation and juvenility maintenance New For. 45 473 486
Xia, K., Hill, L.M., Li, D.Z. & Walters, C. 2014 Factors affecting stress tolerance in recalcitrant embryonic axes from seeds of four Quercus (Fagaceae) species native to the USA or China Ann. Bot. 114 1747 1759
Yang, G. & Read, P.E. 1992 Pre-forcing treatments influence bud break and shoot elongation in forced woody species J. Environ. Hort. 10 101 103