PHOTOPROTECTIVE MECHANISMS IN PLANTS-PART II

#plantphysiology #mscbotany #botany #plants #photosynthetic #photosynthesis #photoprotection #photoprotective_mechanisms
#chlorophyll as the main light-absorbing pigment in the light-harvesting complex, the inner antenna, and also in the reaction centres, is very efficient in absorbing.
If the energy is not efficiently used, the spins of the electrons in the excited state can rephase and give rise to a lower energy excited state: the chlorophyll triplet state.
Chl triplet states formed
either directly by intersystem crossing from a singlet excited chlorophyll, or
by charge recombination reactions in the reaction centres.
1O2 is formed from Chl triplets and can react with proteins, pigments, and lipids and is thought to be the most important species responsible for light-induced loss of PSII activity, the degradation of the D1 protein (protein of the reaction centre of PSII) and for pigment bleaching.
In the reaction centre of PSII, the first detectable radical pair formed after excitation by light is P680+Pheo−, with P680 being the primary electron donor and pheophytin the primary electron.
The next step of electron transfer after the formation of the primary radical pair (P680+Pheo−)leads to the reduction of the primary quinone acceptor QA.
Subsequently, P680+ is reduced by electron donation from the redox active tyrosine TyrZ, which itself obtains an electron from the water oxidizing complex.
Charge recombination reactions can occur when the forward electron transport cannot proceed.
If the primary quinone acceptor stays reduced because of a block of the forward electron transport the yield of the primary charge separation is lowered...