why chlorophyll a is always heigher than chlorophyll b in plant cultivated under salt stress ?
Chl a is the site of photochemical reaction. It absorbs light which is used in the reaction. Chlb supports Chla by supplying more light energy to Chla. Under shade when Chla fails to absorb more light, Chlb take leading part in supplying more light to Chla just to continue more photochemical reaction. Chlb is more adaptive to stress condition. Any situation plant possesses more Chla than Chlb. Maintenance of more Chla than Chlb is vital for survival .
Chla content is always higher than Chlb. They both participate in the formation of the light harvesting complexes, however only Chla participates in the formation of the reaction centers. The Chla/Chlb ratio is higher in shaded leaves because shade acclimated leaves tend to develop bigger light harvesting complexes. As a result, Chlb content increases at a greater extent relatively to the Chla content due to the absence of changes at the Chla-formed reaction centers.
Under stress condition the expression of enzyme responsible for degradation of chl b is reduced.
as chlorophyll b is greatly sensitive to salt stress and plant metabolism will then terns toward the metabolism of osmoregulants. Also Na ions affect the metabolism of chl b while chl a is tolerant to this
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Decreased chl b - could indicate dissociation of LHCII under salt stress and could have adaptive advantage by reducing linear electron flow and therefore reducing the need for alternative electron sinks at PSI while maintaining energy supply (ATPs) by a shift towards cyclic electron flow around PSI.
Even though chla is always higher then chlb generally, under salt, drought and heavy metal stresses some species have adapt or find avoidance strategies to these stressors, one such strategy is reduction in leaf area packing the chloroplasts which concentrates the chla and chlb.
Under different abiotic stress conditions, plants tend to incorporate more Chl b molecules into the outer-core of the antenna complexes (particularly LHCII that is associated with PSII) to counter limited photochemical efficiency and this may alter Chl a b-1 ratio.
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