In Vitro Rejuvenation of Woody Species
Juvenility and phase change in woody plant species exert profound impacts on plant morphology and the ability of explants to be successfully propagated in vitro. Morphological characteristics such as leaf shape modifications, thorniness, and the inability
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1. Introduction Multiplication of plants by macropropagation methods such as cuttings is demonstrably easier with juvenile plants than with cuttings taken from mature plants (1). In this chapter, we examine the extrapolation of this concept to in vitro methods of mass propagation, especially with woody plant species. 1.1. What Is Juvenility?
Juvenility was described as early as 1900 by Goebel as a physiological condition that may be characterized by morphological characteristics such as leaf shape modifications, thorniness, and the inability of the plant to initiate flowers (2). Juvenility is currently defined as a plant in its vegetative state that is not capable of responding to flower induction stimuli that normally would result in flowering in
Maurizio Lambardi et al. (eds.), Protocols for Micropropagation of Selected Economically-Important Horticultural Plants, Methods in Molecular Biology, vol. 994, DOI 10.1007/978-1-62703-074-8_30, © Springer Science+Business Media New York 2013
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P.E. Read and C.M. Bavougian
an adult plant (3, 4). That is, a plant that is not physiologically mature and therefore incapable of flowering and sexual reproduction. For example, apple trees may not flower and bear fruit for several years following planting. Indeed, the juvenile period may be very long, as in the cases of English oak (Quercus robur) and silver fir (Abies alba) that exhibit a duration of juvenility of 25–30 years, while European beech (Fagus sylvatica) has an even longer juvenile period of 30–40 years. Many other woody plants have juvenile periods of 5–10 or more years, e.g., Scots pine (Pinus sylvestris), several birch species (Betula spp.), and fruit trees such as apple and pear (5). Hackett and colleagues (1, 6, 7) have used English ivy (Hedera helix) as a model to study juvenility and phase change. Morphological differences become readily apparent when this species changes from the juvenile phase to the adult stage: juvenile H. helix exhibit a prostrate growth habit, lobed leaves, and an inherent ability to produce adventitious roots from stems, including the internodes. It also does not flower. However, the adult form has entire leaf margins and erect stems; it may flower, but exhibits recalcitrance with regard to its ability to form adventitious roots. Many explanations have been offered for why cuttings from mature tissues are difficult to root; one explanation involves tissue maturation and formation of lignin in the cells (8), since there is a negative correlation between lignin build-up and the ability to form adventitious roots. Juvenile grapevines cannot flower or produce tendrils (a modified inflorescence) and have spiral (2/5) phyllotaxy as compared to distychous phyllotaxy in mature vines. Mature grapevines normally possess tendrils and exhibit the capacity to flower (9). Juvenile members of the Citrus genus often exhibit thorniness, which usually disappears with maturation. Excellent reviews of the phenomenon of juvenility have been presented by Schaffalitzky de Muckadell (10) and Sax (1
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