Micropropagation, a technique used to rapidly increase available stock of new cultivars, can be used for propagation of apple rootstocks (Webster and Jones, 1989). Micropropagation is also useful for production of stock plants with improved rooting ability (Quamme and Hogue, 1994; Webster and Jones, 1991). For apple, micropropagation is inefficient in the proliferation stage (Aklan et al., 1997; Dobránszki and Teixeira da Silva, 2010; Isutsa et al., 1998; Pua et al., 1983; Webster and Jones, 1989). Efficiency of proliferation requires a rapid increase in the number of new shoots that elongate sufficiently to transfer to the rooting stage. For some woody plant taxa or genotypes, in vitro shoot growth is stunted with poor axillary branching and shoot tips that fail to elongate (Geng et al., 2012; Moreira da Silva and Debergh, 1997; Naik et al., 1999; Tao and Sugiura, 1992; Webster and Jones, 1989).
Some otherwise promising rootstocks have limited commercial use because propagation is difficult (Nelson, 1976; Quamme and Hogue, 1994). For example, poor shoot proliferation and insufficient shoot elongation occur with ‘Ottawa 3’ (Ott.3), ‘Malling 9’ (M.9), ‘Malling 26’ (M.26), ‘Budagovsky 9’ (B.9), and ‘P 2’ (Aklan et al., 1997; Lane and McDougald, 1982; Pua et al., 1983; Webster and Jones, 1991). New rootstocks in the Geneva series such as G.41 have been selected for reduced suckering and lack of burr knots, but with the undesirable consequence of being difficult to propagate (Robinson et al., 2011). Previous research on micropropagation of the Geneva series focused on rooting and establishment of viable explants (Isutsa et al., 1998), but methods to enhance shoot proliferation are necessary to decrease the time for rapid propagation of newly released cultivars.
Axillary shoot proliferation and elongation can be enhanced by altering the spectral light quality of in vitro plantlets (Chee and Pool, 1989; Economou and Read, 1987). Compared with white light, blue light increases in vitro shoot number in Amelanchier (Behrouz and Lineberger, 1981) and plum (Baraldi et al., 1988), but decreases shoot number and shoot length in Spiraea and apple (Norton et al., 1988a, 1988b). Red light increases the length of axillary shoots in apple (Muleo and Morini, 2006, 2008), plum (Muleo and Thomas, 1997), Azorina vidalii (Moreira da Silva and Debergh, 1997), and Vaccinium corymbosum (Noė et al., 1998), but not in grape (Chee and Pool, 1989). The inconsistent responses may be the result of variation in physiology among taxa or in conditions among experiments (Baraldi et al., 1988; Economou and Read, 1987; Norton et al., 1988a). At low intensity, blue light has little effect on shoot growth of tobacco, but at higher intensity, it enhances shoot growth compared with red or yellow light (Seibert et al., 1975). At low intensity, red light produces the greatest proliferation rate, but under higher intensity, red light does not differ from white or blue light in its effect on shoot proliferation (Baraldi et al., 1988). Altering light quality can be an efficient method for improving shoot proliferation, because it does not increase the duration of the propagation cycle, unlike prolonged subculturing (Webster and Jones, 1991).
Few studies have measured variation in response to light quality among apple rootstock cultivars. In MM.106 apple, culturing under red or blue light reduces in vitro shoot number compared with white or green light, but red light increases shoot length (Muleo and Morini, 2006). In contrast, shoot number and shoot length of the M.9 apple are increased by red light relative to blue, yellow, green, white, or far-red light (Muleo and Morini, 2008).
GA3, added to proliferation media in combination with cytokinin and auxin, improves shoot proliferation in apple (Pua et al., 1983), but responses may vary with explant condition (Elliot, 1972). Light quality can affect phytohormone balance in developing shoots, with red and blue light altering endogenous auxin and gibberellins (Baraldi et al., 1995). We hypothesized that light quality may alter responses of apple rootstocks to exogenous GA3.
The purpose of this study was to test the effect of blue and red light on in vitro shoot growth of three apple rootstocks, B.9, G.30, and G.41, which vary in their ease of propagation. Additionally, we tested the effect of blue and red light in combination with GA3 on in vitro shoot growth of G.30.
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