Violaxanthin pool

Physiologically, violaxanthin is an important component of the xanthophyU cycle a high light stress-induced de-epoxidation of the violaxanthin pool to the more photoprotective zeaxanthin is mediated by violaxanthin de-epoxidase (VDE). Violaxanthin and neoxanthin, an enzymatically (NXS)-produced structural isomer, are the precursors for the abscisic acid (ABA) biosynthetic pathway (Figure 5.3.1, Pathway 4 and Figure 5.3.2). In non-photosynthetic tissues, namely ripe bell peppers, antheraxanthin and violaxanthin are precursors to the red pigments, capsanthin and capsorubin, respectively (Figure 5.3.3B). 

Leaves were dark-adapted therefore, there is no detectable level of zeaxanthin. Concentrations are nmol pigment (mol chi a — b), the P value from one factor ANOVA is displayed below each column. V-A-Z = xanthophyll pool (violaxanthin, antheraxanthin, zeaxanthin) EPS = epoxidation state. Reprinted with permission from P. J. Ralph et al. [76]. 

While ascorbate is transported into isolated intact chloroplasts (Anderson et al., 1983b Beck et al., 1983 Foyer and Lelandais, 1996), the thylakoid membranes appear to have no carrier system to transport ascorbate into the thylakoid lumen (Foyer and Lelandais, 1996). This is surprising since the enzyme VDE, located inside the thylakoid, requires ascorbate to convert violaxanthin to zeaxanthin (Fig. 2). Upon illumination the pH ofthe lumen falls and VDE binds to the lumenal side of the thylakoid membrane and becomes active (Hager and Holocher, 1994 Rockholmand Yamamoto, 1996). The affinity of VDE for ascorbate is strongly dependent on pH, because ascorbic acid is the true substrate of the enzyme (Bratt et al., 1995). The addition ofHjOj to illuminated thylakoid membrane preparations induces a transient inhibition of zeaxanthin formation (Neubauer and Yamamoto, 1992), suggesting competition for ascorbate between the Mehler-peroxidase cycle and the VDE reaction. Since the ascorbate pool in the chloroplast stroma is substantial (10-50 rnM), sufficient amounts of ascorbic acid may be able to aoss the membrane by diffusion (Eoyer et al., 1983 Foyer, 1993 Foyer and Lelandais,... 

We calculated the total pool size of all the precursors to be some 30 times the size of the pool of free ABA and roughly equal to the amount of violaxanthin, lutein or -carotene. However, the long half-life of turnover for endogenous ABA, determined by 02 labelling, would reduce the size of the pool calculated on the... 

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