Oaks (Quercus L.) are among the most important hardwood species in North America. Unfortunately, due to difficulty in vegetative propagation, hybrids and unique types of oaks are rarely introduced into the nursery trade. Poor rooting ability among plant species has been attributed to the presence of growth inhibitors (Barlow et al., 1961; Ooyama, 1962), a lack of or imbalance of hormones or rooting cofactors (Hess, 1961; Raviv et al., 1986), and the presence of physical barriers (Beakbane, 1961; Edwards and Thomas, 1980). Several anatomical studies have suggested a correlation between difficulty in rooting and the presence of a pericyclic sclerenchyma layer (Beakbane, 1961, 1969; Ciampi and Gellini, 1958; Edwards and Thomas, 1980; Goodin, 1965). Beakbane (1969) and Edwards and Thomas (1980) suggested that the presence of a continuous sclerenchyma layer might act as a physiological barrier to adventitious root initiation or as a mechanical barrier to root emergence. Roots arising from the cambium and phloem regions in Vaccinium corymbosum L. hardwood cuttings were reportedly impeded by a continuous layer of lignified pericyclic fibers and by the epidermis (Mahlstede and Watson, 1952). In some instances, a developing primordium was found to bend at right angles before emerging through gaps in the sclerenchyma sheath. Similar observations were made in Dianthus caryophyllus L. (Stangler, 1956) and Coffea robusta L. Linden (Reańo, 1940), where root primordia grew down inside the fiber sheath and emerged through the base of the cutting. Beakbane (1961) found that rooting was inversely proportional to the percentage of fibers and the integrity of the sclerenchyma ring in pear (Pyrus communis L.), beech (Fagus sylvatica L.), and apple (Malus Mill.) rootstocks.
In contrast, Davies et al. (1982) were unable to find a relationship between the presence of a sclerenchyma sheath and root formation in mature and juvenile leaf bud cuttings of Ficus pumila L. They concluded that the differences in rooting capabilities were more related to the ease of root initiation rather than restrictions by the sclerenchyma tissue.
The density of occurrence of sclerenchymatous cells in the cortex of Populus tremuloides Michx. cuttings was found to increase with tree age (Hicks and Gladstone, 1971). When subject to favorable rooting conditions, cuttings from mature tree age classes failed to form root primordia, whereas those from juvenile root suckers succeeded. Lovell and White (1984) saw the prevention of an initial phase of cambial activation in mature Griselina littoralis Raoul. and Griselina lucida G. Forst. cuttings as a more likely factor accounting for poor rooting rather than the presence of a sclerenchyma sheath. William et al. (1984) found no correlation between stem sclerification and poor root formation, but instead associated poor rooting with the suberization of tissue in the cortex. Sachs et al. (1964) found no relationship between ease of rooting and the density and continuity of the sclerenchyma layer in stem cuttings of cherry (Prunus L.), pear, and olive (Olea europaea L.) cultivars. They noted substantial cell expansion and proliferation in the cortex, phloem, and cambium that were enough to cause breaks in the continuous sheath of sclerenchyma tissue.
Etiolation as a pretreatment has been useful in improving rooting in a number of woody plant species. Some anatomical changes associated with etiolation are delayed lignification of pericyclic cells (Priestley, 1926; Reid, 1923; Stoutemeyer, 1961), the presence of undifferentiated parenchyma in root initiation sites (Herman and Hess, 1963), and softer tissues, especially in the pith and fibers (Smith, 1924). Maynard and Bassuk (1992) reported a delay in sclereid formation by up to 3 months in Carpinus betulus L. that correlated well with increased rooting potential in etiolated cuttings.
In spite of results that seem to disqualify the sclerenchyma ring as a barrier to the emergence of adventitious roots, it is a known fact that where extensive sclerification takes place within the cortex, rooting generally does not occur (Maynard and Bassuk, 1996; White and Lovell, 1984). However, it is evident from the literature that the relationship between the sclerenchyma sheath and rooting ability in plants has, to a large extent, been based on visual impressions. One objective of this study was to quantify differences in the extent of fiber-free gaps in Q. bicolor and Q. macrocarpa as stems used to furnish cuttings aged over a 9-week period. Additional objectives included identifying the site(s) of primordia initiation and recording the progress of adventitious root formation with a view to establishing whether the sclerenchyma ring can be interpreted as a barrier to the initiation and emergence of adventitious roots.
Amissah, J.N. 2007 Effect of juvenility and etiolation on clonal propagation of Quercus species Ph.D. Diss Cornell University Ithaca, NY
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