The Components of Plant Tissue Culture Media II - Horticultural ...

126 The Components of Plant Tissue Culture Media II
promoted early somatic embryo maturation stages in
European silver fir (Schuller and Reuther, 1993).
When used instead of sucrose, it improved rooting of
Annona squamosa microshoots (Lemos and Blake,
1996). In addition, galactose has been found to
reduce or overcome hyperhydricity in shoot cultures
(Druart, 1988; see Volume 2). Fructose has also been
reported to be effective in preventing hyperhydricity
(Rugini et al., 1987).
There are some situations where fructose supports
growth just as well as sucrose or glucose (Steffen
et al., 1988) and occasionally it gives better results.
Some orchid species have been reported to grow
better on fructose than glucose (Ernst, 1967; Ernst
et al., 1971; Arditti, 1979). Fructose was the best
sugar for the production of adventitious shoots from
Glycine max cotyledonary nodes, especially if the
concentration of nutrient salts supplied was
inadequate (Wright et al., 1986). Shoot and leaf
growth and axillary shoot formation in Castanea
shoot cultures was stimulated when sucrose was
replaced by 30 g/l fructose. The growth of basal
callus was reduced and it was possible to propagate
from mature explants of C. crenata, although this was
not possible on the same medium supplemented with
sucrose (Chauvin and Salesses, 1988). However,
fructose was reported to be toxic to carrot tissue if, as
the sole source of carbon, it was autoclaved with
White (1943a) A medium. When filter sterilized,
fructose supported the growth of callus cultures
which had a final weight 70% of those grown on
sucrose (Pollard et al., 1961).
Sucrose in culture media is usually hydrolysed
totally, or partially, into the component
monosaccharides glucose and fructose (see below)
and so it is logical to compare the efficacy of
combinations of these two sugars with that of
sucrose. Kromer and Kukulczanka (1985) found that
meristem tips of Canna indica survived better on a
mixture of 25 g/l glucose plus 5 g/l fructose, than on
30 g/l sucrose. Germination of Paphiopedilum orchid
seeds was best on a medium containing 5g/l fructose
plus 5 g/l glucose; a mixture of 7.5 g/l of each sugar
was optimal for further growth of the seedlings
(Pierik et al., 1988). In spite of its rapid hydrolysis to
glucose and fructose, sucrose appears to have a
specific stimulatory effect on embryo development in
Douglas fir, that was not observed when it was
replaced by the monosaccharides (Taber et al., 1998).
The general superiority of sucrose over glucose
for the culture of organised plant tissues such as
isolated roots may be on account of the more
effective translocation of sucrose to apical meristems
(Butcher and Street, 1964). In addition, there could
be an osmotic effect, because, from an equal weight
of compound, a solution of glucose has almost twice
the molarity of a sucrose solution, and will thus, in
the absence of inversion of the disaccharide, induce a
more negative water potential (see below).
Maltose. Plant species vary in their ability to
utilise unusual sugars. For instance, although
Gautheret (1945) could grow carrot callus on
maltose, Mathes et al. (1973) obtained only minimal
growth of Acer tissue on media supplemented with
this sugar. Similarly, growth of soybean tissue on
maltose is normally very slow, but variant strains of
cells have been selected which can utilise it (Limberg
et al., 1979), perhaps because the new genotypes
possessed an improved capacity for its active
transport. Later studies have given a more prominent
role to maltose as a component of tissue culture
media. Maltose serves as both a carbon source and as
an osmoticum. Compared to sucrose there is a slower
rate of extracellular hydrolysis, it is taken up more
slowly, and hydrolysed intracellularly more slowly.
Maltose led to a substantial increase in somatic
embryos from Petunia anthers (Raquin, 1983). It
also led to an increase in callus induction and plantlet
regeneration during in vitro androgenesis of
hexaploid winter triticale and wheat (Karsai et al.,
1994). Maltose also increased callus induction in rice
microspore culture, with an acceleration of initial cell
divisions (Xie et al., 1995). For barley microspore
culture, the inclusion of maltose led to a higher
frequency of green plants (Finnie et al., 1989).
Maltose has been reported to equal or surpass sucrose
in supporting embryogenesis in a number of species,
including carrot (Verma and Dougall, 1977;
Kinnersley and Henderson, 1988), alfalfa (Strickland
et al., 1987), wild cherry (Reidiboym-Talleux et al.,
1999), Malus (Daigny et al., 1996), Abies (Norgaard
1997) and loblolly pine (Li et al., 1998). The number
of plants regenerated from indica (Biswas and
Zapata, 1993), and japonica (Jain et al., 1997) rice
varieties was also greater when protoplasts were
cultured with maltose rather than sucrose. Transfer
from a medium containing sucrose or glucose to one
supplemented with maltose has been used by Stuart
et al. (1986) and Redenbaugh et al. (1987) to enhance
the conversion of alfalfa embryos. Similarly, maltose
led to a much higher germination rate from asparagus
somatic embryos than sucrose (Kunitake et al., 1997).
Lactose. The disaccharide lactose has been
detected in only a few plants. When added to tissue