chlorophyll and the associated accessory 
 pigments allow plants to maximise use of the visible light 
 spectrum for photosynthesis .   by establishing internal 
 gradients in light capture ,
the green pigments enable plants to 
 respond rapidly to changes in the spectral environment , as well 
 as to exploit niche habitats .
equally , they provide some 
 protection against photoinhibition and photooxidation , the 
 damaging effects of excess quanta .
given the obvious benefits to their being green , why then do many 
 plant species produce red - pigmented leaves at one or more stages 
 in their life cycles ?
red - leafed flora are common throughout all 
 orders of the plant kingdom , from the basal liverworts to the 
 most advanced angiosperms .  
they occur in habitats as 
 diverse as the antarctic shoreline and the tropical rainforests , 
 are as abundant in arid deserts as in freshwater lakes , and seem 
 equally at home in the light - starved forest understorey as in the 
 sun - drenched canopy .   in many red - leafed species
the manufacture 
 of red pigments is transient , often associated with a discrete 
 developmental stage such as in the growth flushes of tropical 
 trees [ 3 , 4 , 
 5 ] , or in the senescing autumn foliage of 
 deciduous trees [ 6 , 7 , 
 8 , 9 ] .   in certain other species , 

however , red pigments can persist throughout the leaf 's entire 
 life span , or else they are induced and retained only 
 after the plant has experienced stress .
functional 
 implications of these red pigments in plants have been the focus 
 of a significant research output over the past decade . for most vascular plants , red colouration in leaves
is achieved 
 by anthocyanins , predominantly cyanidin-3-o - glucoside , 
 as a solution located in the vacuole of the plant cell .  
other 
 pigments  notably the betalains , certain carotenoids , 
 thiarubrine a , some terpenoids , and the 
 3-deoxyanthocyanins  also impart reddish colours in various 
 species ; these pigments have been less well studied than the 
 anthocyanins , but at least some of them have comparable functions 
 in leaves [ 12 , 13 , 
 14 , 15 , 
 16 ] .
the synthesis and vacuolar sequestration of anthocyanin molecules 
 represent a considerable metabolic investment 
 for plant cells .
first , there are metabolic costs associated with 
 enzyme production and activity ; at least seven enzymes are 
 involved in the biosynthesis of cyanidin from its precursors , 
 4-coumaroyl - coa and malonyl - coa . then there is a cost 
 associated with the conjugation of each cyanidin molecule to a 
 monosaccharide molecule .
finally , there are costs associated with 
 the transport of cyanidin-3-o - glucoside into the cell 
 vacuole via a tonoplast mg - atp - requiring glutathione carrier 
 .
this investment suggests that the accumulation of 
 anthocyanins in leaf cells is unlikely to be an 
extravagancy 
 without a vital function  . neither is it likely that 
 these red pigments are simply the default product of a saturated 
 flavonoid biosynthetic pathway , since the timing of anthocyanin 
 production is usually tightly controlled and often occurs in 
 tissues remote from those associated with other flavonoids 
 . on the contrary , a wealth of recent evidence , both 
 empirical and theoretical , ascribes a remarkable diversity of 
 functions to anthocyanins in leaves , many of them associated with 
 stress responses and some potentially critical to a plant 's 
 survival .
anthocyanins are arguably the most versatile of all 
 pigments , their multifarious roles in plant stress responses 
 stemming as much from the physicochemical property of light 
 absorption as from their unique combination of biochemical 
 reactivities .
recent advances in our understanding of these 
 various functions are the subject of this review .
anthocyanins in vivo absorb the green and yellow wavebands of 
 light , commonly between 500 and 600  nm 
 [ 20 , 21 , 
 22 , 23 , 
 24 , 25 ] . 

foliage appears red because of the 
 subtraction of yellow - green light from the spectrum of light 
 reflected from the leaf 's surface .
interestingly , the amount of 
 red light that is reflected from red leaves often only poorly 
 correlates to anthocyanin content ; leaf morphology and 
 the amount and distribution of chlorophyll are apparently the 
 stronger determinants of red reflectance .
the property of 
 anthocyanins to absorb light provides a mechanism for several 
 important functions in leaves .
the red colours of anthocyanic leaves have been proposed both to 
 attract and to repel various animal species .
burns and dalen 
 
 postulated that red - orange autumn foliage of canadian 
 shrub species would accentuate the conspicuousness of 
 black - coloured fruits to birds .  
experimental manipulations of 
 fruit and background foliage colours confirmed that the black - red 
 contrast was indeed an effective enhancer of fruit - removal rates 
 by   avian dispersers .
certain insects , on the other hand , seem to 
 preferentially avoid eating red - pigmented leaves .
california 
 maple aphids , for example , readily colonise yellow - orange leaves 
 of japanese maples , yet they largely ignore red - leafed 
 individuals . similarly , leaf - cutting ants from the 
 tropical forests of panama browse significantly less on red 
 leaves than on green leaves . to these and other insects 

leaves that 
 are rich in chlorophyll as well as anthocyanin tend to be brown 
 or even black , mimicking dead foliage or else serving to 
 camouflage leaves against the exposed soil and litter of forest 
 floors [ 30 , 31 , 
 32 , 33 ] . 

even brilliant red or scarlet leaves 
 can appear dark to nonmammalian folivores , which lack red light 
 receptors [ 5 , 34 ] . 

the gains to be had from herbivore deterrence would offset metabolic costs to the plant associated 
 with anthocyanin biosynthesis . by intercepting the high - energy quanta
, anthocyanic cell vacuoles 
 can prevent important photolabile molecules from degradation by 
 green light .  
an elegant example of this was described recently 
 for the silver beachweed ( ambrosia chamissonis ) , a composite that grows at exposed , sunny 
 locations along the california coast .
the plant holds large 
 amounts of thiarubrine a , a potent defence compound that is toxic 
 to insects , bacteria , and fungi .
thiarubrine a is 
 photolabile ; even short exposures to visible light and/or 
 ultraviolet radiation render it inactive [ 36 , 
 37 ] .
however , 
 the tissues in a chamissonis that contain thiarubrine a 
 are shielded from light by a sheath of cells containing a mix of 
 two anthocyanins , cyanidin-3-o - glucoside and 
 cyanidin-3-o-(6'-o - malonylglucoside ) .
the 
 anthocyanins absorb quanta that would otherwise lead to the 
 destruction of thiarubrine a , and thereby contribute 
 significantly to the defensive armoury of the plant .
when leaves receive more light energy than can be used in 
 photochemistry , they show a characteristic decline in the quantum 
 efficiency of photosynthesis , termed photoinhibition . 
 under severe conditions the chloroplasts generate reactive oxygen 
 species , which have the potential to destroy thylakoid membranes , 
 damage dna , and denature proteins associated with photosynthetic 
 electron transport .  
anthocyanins have been shown in many plant 
 species to reduce both the frequency and severity of 
 photoinhibition , as well as to expedite photosynthetic recovery 
 [ 23 , 39 , 
 40 , 41 , 
 42 , 43 , 
 44 ] . in red - osier dogwood
( cornus 
 stolonifera ) , for example , a 30-minute exposure to strong white 
 light   reduced the quantum efficiency of photosynthesis by 60% 
 in red leaves , but by almost 100% in acyanic leaves . 

when the plants were returned to darkness , the red leaves 
 recovered to their maximum potential after only 80 minutes , yet 
 their acyanic counterparts had not achieved the pretreatment 
 state even after six hours .
anthocyanins protect leaves from the stress of photoinhibitory 
 light fluxes by absorbing the excess photons that would otherwise 
 be intercepted by chlorophyll b.   although red leaves 
 absorb more green light in total , their photosynthetic tissues 
 actually receive fewer quanta than do those of acyanic leaves 
 because the energy absorbed by the cell vacuole can not be 
 transferred to the chloroplasts . as a result , under 
 light - limiting environments the photosynthetic efficiencies of 

red leaves are often slightly lower than those for acyanic leaves 
 [ 4 , 22 , 
 45 , 46 , 
 47 , 48 , 
 49 ] . under strong light
, however , the 
 anthocyanins serve as a useful optical filter , diverting excess 
 high - energy quanta away from an already saturated photosynthetic 
 electron transport chain .  
chloroplasts irradiated with light 
 that has first passed through a red filter have been shown to 
 generate fewer superoxide radicals , thereby reducing the 
 propensity for structural damage to the photosystems . 

the anthocyanins are therefore clearly a useful supplement to 
 other nonphotochemical quenching mechanisms such as the 
 xanthophyll cycle pigments .
recent studies involving mutants of 
 arabidopsis thaliana indicate that whereas xanthophylls 
 have a greater role in the protection of plants from short - term 
 light stress , the anthocyanins can be the more effective 
 photoprotectants over the long term .
the photoprotection hypothesis potentially explains why the 
 leaves of many deciduous trees turn red in the autumn . as leaves 
 senesce , nitrogen associated with their chloroplasts
anthocyanins would protect the degrading 
 chlorophyll from damaging light levels , thereby restricting the 
 formation of reactive oxygen that could jeopardize the resorptive 
 process [ 2 , 8 , 
 9 , 23 , 
 51 , 52 ] . 

consistent with this hypothesis , 
 nitrogen resorption has recently been shown to be more efficient 
 in wild - type than in anthocyanin - deficient mutants of three woody 
 species .
interest in the flavonoid family has increased in recent years 
 following the observation that these compounds act as sunscreens 
 against potentially damaging uv - b radiation .
foliar 
 anthocyanins have generally been included with other flavonoids 
 in this uv - b protective role .
consistent with this hypothesis , 
 the anthocyanins , particularly when acylated , absorb strongly in 
 the uv region [ 54 , 
 55 ] , are induced or upregulated in plant 
 tissues in response to uv irradiation [ 56 , 
 57 , 58 , 
 59 , 60 ] , and 
 mitigate dna damage in uv - b - irradiated cell cultures 
 [ 61 , 62 , 
 63 ] .
furthermore , certain anthocyanin - deficient 
 mutants of arabidopsis are hypersensitive to uv - b 
 , and red - leafed coleus varieties retain higher 
 photosynthetic efficiencies after uv irradiation than do 
 green - leafed varieties . notwithstanding
this body of evidence , there is now a growing 
 conviction that foliar anthocyanins can not be primarily concerned 
 with uv protection . unlike the colourless flavonoids , the 
 anthocyanins are usually located in the internal mesophyll tissue 
 rather than in the epidermis , the optimal site for uv 
 interception [ 33 , 
 65 ] .
for example , an 
 arabidopsis mutant with enhanced sensitivity to uv 
 radiation was found deficient in certain flavonoids , yet it held 
 normal amounts of anthocyanin .
similarly , the responses 
 of brassica rapa mutants to supplementary uv - b treatment 
 were for the most part independent of anthocyanin levels in the 
 leaves .
indeed , red - leafed plants of petunia 
 ( impatiens ) and rice have all been observed to perform 
 significantly worse than their green - leafed counterparts under 
 uv - enriched environments [ 68 , 69 , 
 70 ] .
noted that dna damage after prolonged uv treatment was 
 substantially greater in purple - leafed rice than in a 
 near - isogenic green line .   to repair uv - damaged dna , plants 
 employ photolyase , an enzyme that uses blue / uv - a light to 
 remonomerise the pyrimidine dimers .
the anthocyanins in purple 
 rice prevented the photoactivation of photolyase by absorbing 
 some of the blue / uv - a light incident on the leaves .
thus , any 
 short - term gain from the absorption of uv - b by anthocyanins would 
 be offset by their property to absorb visible light and thereby limit the rate of dna repair .
the red colours of anthocyanic leaves have been proposed both to 
 attract and to repel various animal species . burns and
dalen 
 
 postulated that red - orange autumn foliage of canadian 
 shrub species would accentuate the conspicuousness of 
 black - coloured fruits to birds .  
experimental manipulations of 
 fruit and background foliage colours confirmed that the black - red 
 contrast was indeed an effective enhancer of fruit - removal rates 
 by   avian dispersers .
certain insects , on the other hand , seem to 
 preferentially avoid eating red - pigmented leaves .
california 
 maple aphids , for example , readily colonise yellow - orange leaves 
 of japanese maples , yet they largely ignore red - leafed 
 individuals . similarly , leaf - cutting ants from the 
 tropical forests of panama browse significantly less on red 
 leaves than on green leaves . to these and other insects 

leaves that 
 are rich in chlorophyll as well as anthocyanin tend to be brown 
 or even black , mimicking dead foliage or else serving to 
 camouflage leaves against the exposed soil and litter of forest 
 floors [ 30 , 31 , 
 32 , 33 ] . 

even brilliant red or scarlet leaves 
 can appear dark to nonmammalian folivores , which lack red light 
 receptors [ 5 , 34 ] . 

the gains to be had from herbivore deterrence would offset metabolic costs to the plant associated 
 with anthocyanin biosynthesis .
by intercepting the high - energy quanta , anthocyanic cell vacuoles 
 can prevent important photolabile molecules from degradation by 
 green light .  
an elegant example of this was described recently 
 for the silver beachweed ( ambrosia chamissonis ) , a composite that grows at exposed , sunny 
 locations along the california coast .
the plant holds large 
 amounts of thiarubrine a , a potent defence compound that is toxic 
 to insects , bacteria , and fungi .
thiarubrine a is 
 photolabile ; even short exposures to visible light and/or 
 ultraviolet radiation render it inactive [ 36 , 
 37 ] .
however , 
 the tissues in a chamissonis that contain thiarubrine a 
 are shielded from light by a sheath of cells containing a mix of 
 two anthocyanins , cyanidin-3-o - glucoside and 
 cyanidin-3-o-(6'-o - malonylglucoside ) . the 
 anthocyanins absorb quanta that would otherwise lead to the 
 destruction of thiarubrine a , and thereby contribute 
 significantly to the defensive armoury of the plant .
when leaves receive more light energy than can be used in 
 photochemistry , they show a characteristic decline in the quantum 
 efficiency of photosynthesis , termed photoinhibition . 
 under severe conditions the chloroplasts generate reactive oxygen 
 species , which have the potential to destroy thylakoid membranes , 
 damage dna , and denature proteins associated with photosynthetic 
 electron transport .  
anthocyanins have been shown in many plant 
 species to reduce both the frequency and severity of 
 photoinhibition , as well as to expedite photosynthetic recovery 
 [ 23 , 39 , 
 40 , 41 , 
 42 , 43 , 
 44 ] . in red - osier dogwood ( cornus 
 stolonifera ) , for example , a 30-minute exposure to strong white 
 light   reduced the quantum efficiency of photosynthesis by 60% 
 in red leaves , but by almost 100% in acyanic leaves . 

when the plants were returned to darkness , the red leaves 
 recovered to their maximum potential after only 80 minutes , yet 
 their acyanic counterparts had not achieved the pretreatment 
 state even after six hours .
anthocyanins protect leaves from the stress of photoinhibitory 
 light fluxes by absorbing the excess photons that would otherwise 
 be intercepted by chlorophyll b.   although red leaves 
 absorb more green light in total , their photosynthetic tissues 
 actually receive fewer quanta than do those of acyanic leaves 
 because the energy absorbed by the cell vacuole can not be 
 transferred to the chloroplasts . as a result , under 
 light - limiting environments the photosynthetic efficiencies of 
 red leaves are often slightly lower than those for acyanic leaves 
 [ 4 , 22 , 
 45 , 46 , 
 47 , 48 , 
 49 ] . under strong light , however , the 
 anthocyanins serve as a useful optical filter , diverting excess 
 high - energy quanta away from an already saturated photosynthetic 
 electron transport chain .  
chloroplasts irradiated with light 
 that has first passed through a red filter have been shown to 
 generate fewer superoxide radicals , thereby reducing the 
 propensity for structural damage to the photosystems . 

the anthocyanins are therefore clearly a useful supplement to 
 other nonphotochemical quenching mechanisms such as the 
 xanthophyll cycle pigments .
recent studies involving mutants of 
 arabidopsis thaliana indicate that whereas xanthophylls 
 have a greater role in the protection of plants from short - term 
 light stress , the anthocyanins can be the more effective 
 photoprotectants over the long term .
the photoprotection hypothesis potentially explains why the 
 leaves of many deciduous trees turn red in the autumn . as leaves 
 senesce , nitrogen associated with their chloroplasts
anthocyanins would protect the degrading 
 chlorophyll from damaging light levels , thereby restricting the 
 formation of reactive oxygen that could jeopardize the resorptive 
 process [ 2 , 8 , 
 9 , 23 , 
 51 , 52 ] . 

consistent with this hypothesis , 
 nitrogen resorption has recently been shown to be more efficient 
 in wild - type than in anthocyanin - deficient mutants of three woody 
 species .
interest in the flavonoid family has increased in recent years 
 following the observation that these compounds act as sunscreens 
 against potentially damaging uv - b radiation .
foliar 
 anthocyanins have generally been included with other flavonoids 
 in this uv - b protective role .
consistent with this hypothesis , 
 the anthocyanins , particularly when acylated , absorb strongly in 
 the uv region [ 54 , 
 55 ] , are induced or upregulated in plant 
 tissues in response to uv irradiation [ 56 , 
 57 , 58 , 
 59 , 60 ] , and 
 mitigate dna damage in uv - b - irradiated cell cultures 
 [ 61 , 62 , 
 63 ] .
furthermore , certain anthocyanin - deficient 
 mutants of arabidopsis are hypersensitive to uv - b 
 , and red - leafed coleus varieties retain higher 
 photosynthetic efficiencies after uv irradiation than do 
 green - leafed varieties . notwithstanding
this body of evidence , there is now a growing 
 conviction that foliar anthocyanins can not be primarily concerned 
 with uv protection . unlike the colourless flavonoids , the 
 anthocyanins are usually located in the internal mesophyll tissue 
 rather than in the epidermis , the optimal site for uv 
 interception [ 33 , 
 65 ] .
for example , an 
 arabidopsis mutant with enhanced sensitivity to uv 
 radiation was found deficient in certain flavonoids , yet it held 
 normal amounts of anthocyanin . similarly , the responses 
 of brassica rapa mutants to supplementary uv - b treatment 
 were for the most part independent of anthocyanin levels in the 
 leaves .
indeed , red - leafed plants of petunia 
 ( impatiens ) and rice have all been observed to perform 
 significantly worse than their green - leafed counterparts under 
 uv - enriched environments [ 68 , 69 , 
 70 ] .
. 
 noted that dna damage after prolonged uv treatment was 
 substantially greater in purple - leafed rice than in a 
 near - isogenic green line .   to repair uv - damaged dna , plants 
 employ photolyase , an enzyme that uses blue / uv - a light to 
 remonomerise the pyrimidine dimers .
the anthocyanins in purple 
 rice prevented the photoactivation of photolyase by absorbing 
 some of the blue / uv - a light incident on the leaves .
thus , any 
 short - term gain from the absorption of uv - b by anthocyanins would 
 be offset by their property to absorb visible light and thereby limit the rate of dna repair .
anthocyanins diminish the oxidative load in a leaf simply by 
 filtering out yellow - green light , since the majority of reactive 
 oxygen in plant cells is derived from the excitation of 
 chlorophyll .
purified solutions scavenge almost all species 
 of reactive oxygen and nitrogen with an efficiency up to four 
 times greater than those of ascorbate and -tocopherol 
 [ 72 , 73 , 
 74 ] .
recent experimental evidence indicates that this 
 antioxidant potential is indeed utilised by plant cells .  
in 
 arabidopsis , for example , strong light and low 
 temperatures caused more lipid peroxidation in 
 anthocyanin - deficient mutants than in wild - type plants . 

similarly , upon gamma irradiation , only those 
 arabidopsis plants that contained both anthocyanin and 
 ascorbic acid were able to grow and flower normally .
microscopic examinations of wounded leaf peels have shown that 
 red - pigmented cells eliminate h2o2 significantly faster 
 than do green cells .
it is not clear , however , whether 
 scavenging occurs predominantly by the red tautomers of 
 anthocyanin found inside the cell vacuole , or else by the 
 colourless tautomers in the cytosol .
both forms have impressive 
 antioxidant potentials [ 77 , 78 , 
 79 ] . in a model in vitro 
 system , the colourless tautomers of cyanidin 
 3-(6-malonyl)glucoside were found capable of scavenging up to 
 17% of the superoxide radicals generated by irradiated 
 chloroplasts .  
given their proximity to the organelle 
 sources of reactive oxygen , it may be that the cytosolic 
 anthocyanins , rather than those in the cell vacuole , provide the 
 greater contribution to antioxidant defence .
the degree to which anthocyanins contribute to the arsenal of 
 low - molecular - weight antioxidants ( lmwa ) varies among species . in 
 the young , red leaves of elatostema rugosum , an 
 understorey herb from new zealand ,
anthocyanins are the 
 predominant phenolic component of the lmwa pool . in 
 contrast , the red- and green - leafed morphs of the canopy tree 
 quintinia serrata both hold hydroxycinnamic acids as 
 their most concentrated lmwa .
thus it would seem that anthocyanin biosynthesis can 
 enhance but is not usually a prerequisite for protection from oxidative stress .
the induction of foliar anthocyanins has been implicated in the 
 acquisition of tolerance to many different kinds of environmental 
 stressors [ 11 , 80 , 
 81 ] .
anthocyanins , for example , are 
 associated with enhanced resistance to the effects of chilling 
 and freezing [ 82 , 83 , 
 84 , 85 , 
 86 ] , to heavy metal contamination 
 [ 87 , 88 , 
 89 , 90 ] , 
 to desiccation [ 91 , 92 , 
 93 ] , and to 
 wounding [ 76 , 94 , 
 95 ] .
it is not clear at this stage whether 
 the apparent ameliorative properties stem from one or more types 
 of mechanism .
chalker - scott [ 11 , 
 80 ] provided a compelling 
 case for a generalised role of anthocyanins as osmoregulators in 
 plant cells , since most types of suboptimal environments induce 
 water stress , either directly or indirectly .
others have argued 
 that the photoprotective   or the antioxidant 

 properties of anthocyanins are paramount .
regardless of their 
 mechanism , it is clear that anthocyanins offer multifaceted , 
 versatile , and effective protection to plants under stress .  
 anthocyanins , the pigments responsible for 
 spectacular displays of vermilion in the leaves of deciduous 
 trees , have long been considered an extravagant waste of a 
 plant 's resources . 
 contemporary research , in contrast , has begun 
 to show that the pigments can significantly influence the way a 
 leaf responds to environmental stress . 
 anthocyanins have been 
 implicated in tolerance to stressors as diverse as drought , 
 uv - b , and heavy metals , as well as resistance to 
 herbivores and pathogens . by absorbing high - energy quanta , 
 
 anthocyanic cell vacuoles both protect chloroplasts from the 
 photoinhibitory and photooxidative effects of strong light , and 
 prevent the catabolism of photolabile defence compounds . 
 
 anthocyanins also mitigate photooxidative injury in leaves by 
 efficiently scavenging free radicals and reactive oxygen species . 
 far from being a useless by - product of the flavonoid pathway , 
 
 these red pigments may in some instances be critical for plant survival . 
INTRODUCTION
CONSEQUENCES OF BEING RED
Herbivory
Protection of photolabile defence compounds
Protection of photosynthetic apparatus
Protection from ultraviolet radiation
FREE RADICAL SCAVENGING
AMELIORATION OF STRESS RESPONSES