It is well known that enzyme activity differs not only between
plant species, but also between distinct tissues as well as developmental
stages of the same plant. Asparagus spears are developing,
immature shoots. Thus, different sections of the shoots are in different
stages of plant development. In this study, PAL activity of control
spears differed significantly between both, spear segments exhibited
33.3 pkat mg−1 protein in the fully developed base section and
9.4 pkat mg−1 protein in the immature apical part (Table 1). This
supports findings by Goldstein, Jennings, and Marsh (1971, 1972)
and Hennion, Little, and Hartmann (2006), who reported a high PAL
activity in the basal portion of freshly harvested asparagus spears,
with PAL activity declining toward the tips. In studies on monocotyledonous,
basipetally growing bamboo stem segments, which are also
consumed as a vegetable, PAL activity was high in the meristematic
tissue. In contrast to the acropetally asparagus shoot, meristematic
tissue of grass bamboo is located in the basal zone of the shoots (Lu
& Xu, 2004). Thus, asparagus, as a developmental active shoot, is
more differentiated in the basal section and revealed a higher PAL
activity. Basal spear tissues show a high content of lignin, which is
essential for cell wall rigidity and stem stiffness (Waldron &
Selvendran, 1992). Due to the fact that phenolic compounds are
precursors for lignin, higher PAL activity may facilitate lignification
in basal segment of asparagus.
Compared to PAL, POD activity was more pronounced in both,
apical and base segments. However, within the different parts, activity
of POD was the opposite of that found for PAL. POD activity of control
spears differed significantly in the apical section (13.2 nkat mg−1
protein) compared to the base segment (8.8 nkat mg−1 protein)
(Table 1). Generally, peroxidase and its isoenzymes play an important
role in the plant development of many species (Brownleader et al.,
2000), including asparagus (Takeda, Kotake, Nakagawa, Sakurai, &
Nevins, 2003). Numerous peroxidase isoenzymes are present in
asparagus, e.g. guaiacol peroxidase, syringaldazine peroxidase, coniferyl
peroxidase being responsible for lignification and, thus, cellwall rigidity
(Whetten, MacKay, & Sederoff, 1998). Besides this important role in
lignification, peroxidases also evoke cross-linking of polysaccharides
or extensions of cell walls, and, hence, have an essential impact on cell
wall construction (Brownleader et al., 2000). This was also found for
bamboo stems, where the activity of water-soluble peroxidase was
enhanced in tissues revealing a high physiological activity (Yoshizawa,
Satoh, Yokota, & Idei, 1989). Thus, higher POD activity in meristematic
tissue of the apical asparagus section was expected as also reported by
Haard, Sharma, Wolfe, and Frenkel (1974).
Inwhite asparagus shoots, the most important phenolic compounds
are the flavonols quercetin, rutin (Guillen et al., 2007; Hou, Luo, & Kong,
2009) and kaempferol (Hou et al., 2009). In the present study, rutin and
kaempferol were detected only in trace amounts in the spear tips
(Fig. 1) and even below the analytical threshold level in the basal
segments. This is consistent with investigations of Wang et al. (2003)
as well as Zhu et al. (2007), who reported a pronounced gradient in
rutin content, declining from the apical of the asparagus spears to the
base. In the present study, quercetin was the predominant flavonol in
asparagus spears. This might be due to the fact that the phenol profile
is strongly dependent on the cultivar and/or growing conditions (soil,
temperature, humidity etc.). The contents of quercetinwere also higher
in apical than in spear sections (Table 1). In contrast, for its glucosidic
derivatives (QMG and QDG) no difference in the two asparagus sections
of the control shoots were determined. Hence, glycosylation to its
water-soluble derivatives QMG and QDG may facilitate the translocation
of antioxidant compounds to sink areas such as meristematic
tissues. However, due to their glucosidic moieties, QDG and QMG are
only minor antioxidants (Bors, Heller,Michel, & Saran, 1990). Although
being also a quercetin glycoside, rutin is both, highly water soluble and
an effective antioxidant (Rice-Evans, Miller, & Paganga, 1996). In order
to protect the meristematic tissue, rutin might be translocated faster
than the other quercetin derivatives to areas where antioxidants are
required immediately.
括号不翻译。。。。求
plant species, but also between distinct tissues as well as developmental
stages of the same plant. Asparagus spears are developing,
immature shoots. Thus, different sections of the shoots are in different
stages of plant development. In this study, PAL activity of control
spears differed significantly between both, spear segments exhibited
33.3 pkat mg−1 protein in the fully developed base section and
9.4 pkat mg−1 protein in the immature apical part (Table 1). This
supports findings by Goldstein, Jennings, and Marsh (1971, 1972)
and Hennion, Little, and Hartmann (2006), who reported a high PAL
activity in the basal portion of freshly harvested asparagus spears,
with PAL activity declining toward the tips. In studies on monocotyledonous,
basipetally growing bamboo stem segments, which are also
consumed as a vegetable, PAL activity was high in the meristematic
tissue. In contrast to the acropetally asparagus shoot, meristematic
tissue of grass bamboo is located in the basal zone of the shoots (Lu
& Xu, 2004). Thus, asparagus, as a developmental active shoot, is
more differentiated in the basal section and revealed a higher PAL
activity. Basal spear tissues show a high content of lignin, which is
essential for cell wall rigidity and stem stiffness (Waldron &
Selvendran, 1992). Due to the fact that phenolic compounds are
precursors for lignin, higher PAL activity may facilitate lignification
in basal segment of asparagus.
Compared to PAL, POD activity was more pronounced in both,
apical and base segments. However, within the different parts, activity
of POD was the opposite of that found for PAL. POD activity of control
spears differed significantly in the apical section (13.2 nkat mg−1
protein) compared to the base segment (8.8 nkat mg−1 protein)
(Table 1). Generally, peroxidase and its isoenzymes play an important
role in the plant development of many species (Brownleader et al.,
2000), including asparagus (Takeda, Kotake, Nakagawa, Sakurai, &
Nevins, 2003). Numerous peroxidase isoenzymes are present in
asparagus, e.g. guaiacol peroxidase, syringaldazine peroxidase, coniferyl
peroxidase being responsible for lignification and, thus, cellwall rigidity
(Whetten, MacKay, & Sederoff, 1998). Besides this important role in
lignification, peroxidases also evoke cross-linking of polysaccharides
or extensions of cell walls, and, hence, have an essential impact on cell
wall construction (Brownleader et al., 2000). This was also found for
bamboo stems, where the activity of water-soluble peroxidase was
enhanced in tissues revealing a high physiological activity (Yoshizawa,
Satoh, Yokota, & Idei, 1989). Thus, higher POD activity in meristematic
tissue of the apical asparagus section was expected as also reported by
Haard, Sharma, Wolfe, and Frenkel (1974).
Inwhite asparagus shoots, the most important phenolic compounds
are the flavonols quercetin, rutin (Guillen et al., 2007; Hou, Luo, & Kong,
2009) and kaempferol (Hou et al., 2009). In the present study, rutin and
kaempferol were detected only in trace amounts in the spear tips
(Fig. 1) and even below the analytical threshold level in the basal
segments. This is consistent with investigations of Wang et al. (2003)
as well as Zhu et al. (2007), who reported a pronounced gradient in
rutin content, declining from the apical of the asparagus spears to the
base. In the present study, quercetin was the predominant flavonol in
asparagus spears. This might be due to the fact that the phenol profile
is strongly dependent on the cultivar and/or growing conditions (soil,
temperature, humidity etc.). The contents of quercetinwere also higher
in apical than in spear sections (Table 1). In contrast, for its glucosidic
derivatives (QMG and QDG) no difference in the two asparagus sections
of the control shoots were determined. Hence, glycosylation to its
water-soluble derivatives QMG and QDG may facilitate the translocation
of antioxidant compounds to sink areas such as meristematic
tissues. However, due to their glucosidic moieties, QDG and QMG are
only minor antioxidants (Bors, Heller,Michel, & Saran, 1990). Although
being also a quercetin glycoside, rutin is both, highly water soluble and
an effective antioxidant (Rice-Evans, Miller, & Paganga, 1996). In order
to protect the meristematic tissue, rutin might be translocated faster
than the other quercetin derivatives to areas where antioxidants are
required immediately.
括号不翻译。。。。求
