Page 286 - e-Book

pathway are occur in the cytosol, then the oxidation of L-galactono-1,4-lactone via

Cytochrome-c takes place in the mitochondria, this step suggests the formation of ascorbic
acid biosynthesis requires energy metabolism and the cellular redox state.

FUNCTIONS OF ASCORBIC ACID IN PLANTS:
Ascorbic acid play different functions in plants such as, it act as a major redox buffer

(Pignocchi and Foyer, 2003), serve as a co-factor for several enzymes and as a major
antioxidant (Smirnoff and Wheeler, 2000), it regulates cell division and growth in plants

(Kerk and Feldman, 1995), it is also involved in signal transduction (Pignocchi and Foyer,

2003; Noctor et al., 2000), etc.
Role of L-Ascorbic acid in Apoplast:

Due to the environmental stresses, changes in the concentration of ascorbic acid in the

apoplast are observed by various workers (Davey et al., 2000). This finding indicates that, the
apoplastic ascorbic acid is involved in the function of protection of cell membranes against

oxidative damage, with special reference to ozone and atmospheric pollutants (Horemans et
al., 1994 and Asard and Cauberg, 1995). In Phaseolus, the activity of monodehydroascorbate

reductase (MDHAR) has been accounted in the plasma membrane (Horemans and Caubergs,
1994; Asard et al., 1995) and ascorbate peroxidase (APX) activity was observed in

extracellular matrix of several species (Ranieri et al., 1996) rather in the apoplastic region.

Moreover, the endogenous level of ascorbic acid in recent times has been recommended as
important in the regulation of developmental senescence and plant defense against pests

(Pastori et al., 2003; Barth et al., 2004 and Pavet et al., 2005).
Role of L-Ascorbic acid as an enzymatic co-factor:

L- ascorbic acid act as a co-factor in many enzymatic reactions in plants as well as in the
animals. According to different researchers these are mono or dioxygenases, they contain iron

or copper at active site of the enzyme, which require L- ascorbic acid for maximum yield

(Padh, 1990; Levine and Hartzell, 1987; Ginter, 1989; Ghosh et al., 1997 and Jung and
Wells, 1997). L- ascorbic acid, retain the transition metal ion centres of the enzymes in a

reduced form and thus act as a enhancer of the reaction (Padh, 1990).

Role of L-Ascorbic acid as an anti-oxidant:
L- ascorbic acid is known well for its one of the important function as anti-oxidant i.e.,

reactive oxygen species (ROS) (Smirnoff and Wheeler, 2000; Conklin, 2001; Conklin and
Barth, 2004). Being the reactive oxygen species it fights enzymatically and non-

enzymatically against free radicals. The redox reaction of ascorbate gives one electron
oxidation product of ascorbate i.e., mono-dehydroascorbate (MDHA) radical and it is perhaps

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