Chlorophylls and their Degradation in Nature - Photobiogeochemistry of Organic Matter - page 729

during processing is caused by conversion of Chls to pheophytins, which is also
influenced by pH (Blair and Ayres 1943 ; Gupte et al. 1964 ; Minguez-Mosquera
et al. 1989 ; Koca et al. 2007 ). Chl degradation reactions can be caused by several
chemical, photoinduced or enzymatic processes, including simultaneous actions of
enzymes, weak acids or pH changes, oxygen, light and heat. Such processes can
lead to the formation of a large number of degradation products (Hayakawa and
Timbers 1977 ; Koca et al. 2007 ). Major chemical degradation processes are pheo-
phytinization, epimerization, pyrolysis, as well as hydroxylation, oxidation or pho-
toinduced oxidation (Mangos and Berger 1997 ).
The green colour of vegetables can be altered to an olive green under mild
acidic conditions, whereas hydrogen ions can transform Chls to their corre-
sponding pheophytins by substitution of the magnesium ion in the porphyrin ring
(Minguez-Mosquera et al. 1989 ; Gold and Weckel 1958 ; Gunawan and Barringer
2000 ). Preferential degradation of Chl b in the degreening of 'Satsuma' mandarin
( Citrus unshiu Marc.) is found in ethylene-treated fruits and in fruits ripening on
the tree. In contrast, Chl a is predominantly degraded in non-treated fruits (Keishi
1979 ). Methyl jasmonate and ethylene can markedly enhance the mRNA levels
and chlorophyllase activity, which presumably accelerates leaf senescence and
fruit ripening (Drazkiewicz 1994 ; Smart 1994 ; Creelman and Mullet 1997 ; Jacob-
Wilk et al. 1999 ; Tsuchiya et al. 1999 ). Stimulatory effects by methyl jasmonate
and ethylene also indicate that chlorophyllases are key enzymes for senescence or
ripening.

5.4 Mechanism for Degradation of Chl

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The key PSII reactions of Chls are photooxidation, involving attack of 1 O 2 , HO

or H 2 O 2 , and enzymatic degradation (see also chapter “ Photosynthesis in Nature:
A New Look ” ) (Takamiya et al. 2000 ; Brown et al. 1991 ; Gossauer and Engel
1996 ; Hörtensteiner 2006 ; Kräutler and Hörtensteiner 2006 ; Moser et al. 2009 ;
Hörtensteiner and Kräutler 2011 ). The processes occurring under high irradiance
or UV light and high temperature have been documented in the photosynthesis
chapter (see chapter “ Photosynthesis in Nature: A New Look ”). Three Chl cata-
bolic enzymes, such as chlorophyllase, pheophorbide a oxygenase, and red Chl
catabolite reductase (RCCR) are susceptible to play key roles into Chl degrada-
tion, either during leaf senescence and fruit ripening or in response to pathogens
and wounding (Hörtensteiner 2006 ; Hörtensteiner and Kräutler 2011 ; Kariola
et al. 2005 ; Azoulay Shemer et al. 2008 ).
The mechanism responsible for the degreening of plants and the degradation of
Chl involves enzymatic reactions in two phases, through several chain reactions
(Fig. 4 ) (Takamiya et al. 2000 ; Hörtensteiner 2006 ; Kräutler and Hörtensteiner
2006 ; Moser et al. 2009 ; Hörtensteiner and Kräutler 2011 ). In the first phase,
Chl degradation is caused by the removal of the phytol tail (dephytylation) and
of the central Mg atom (magnesium dechelatase). Dephytylation occurs first by