ROS generation in chloroplast is associated with PSI linked to photorespiration. the attached figure will explain which ROS form first and how, do refer it. ROS accumulations are very much dependent to each other and one ROS concentration is determined by how long and at what severity plant get exposed to stress. As superoxide anion are highly reactive, in aqueous solution it decayed to H2O2 and so got higher concentration in the organelle than others.

I'll try to give a very simple answer that basically explains the general situation. As for details, it will be better if you search for reviews that address this situation. There really are some excellent ones available.

ROS generation is associated with all electron transport processes in the living system and are considered as misfires of the system. In chloroplasts, this thought is extended as the process of photosynthesis is associated with energy transfer as well as electron transfer.

If all excitation and every electron got transferred perfectly, there would be no ROS, only completely successful transfer of energy. The thing to be remembered is that ROS can be generated at virtually every step that has a redox potential to reduce oxygen improperly. When an electron transport chain becomes saturated with electrons, electrons can be received by oxygen from electron donors that have a high reduced state lifetime. In chloroplasts, the same thing happens with excitation. Under conditions of intense illumination, excitation stays localised on light harvesting systems, leading to transfer of excitation to oxygen, generating singlet oxygen. Under low-intensity illumination, excitation transfer is quick and hence little generation of ROS.

As for the interplay between ROS, even these reactive species try to stabilise by reacting with other species. The more reactive the ROS, lesser will be its lifetime. Hydrogen peroxide is least reactive so it has the longest lifetime. Additionally, it also has the property of passing through membranes as its completely non-polar.

Please do remember that the lifetimes for all ROS are given in their biological environment where they have a number of biological molecules to react with. Hydrogen peroxide in a bottle lasts much longer than its 1 ms lifetime.

Hope this was useful to some extent at least.

These Ros are produce in normal conditions.. But under stress there are over production of them.soD enzyme scavenger for O2- to give H202 then different enzymes react with it to give molecular 02 and water instead of its transformation to the more dangerous radical OH-

Dear Duy,

It may not be easy to get superoxide anion radicals from singlet oxygen. Superoxide radicals are generated due to one electron reduction of molecular oxygen [generally due to unusual transport of electrons]. Singlet oxygen is indeed a ROS, but carotenoids can take care of them or the energy can be donated to chlorophyll. Superoxide anion radicals however can lead to generation of hydrogen peroxide, hydroxyl radicals, peroxy radicals, hydroperoxy radicals etc. To the best of my knowledge, superoxide radicals are generated whenever there is higher proportion of reduced state of NAD(P)+ [i.e. NAD(P)H] compared to oxidized state [i.e. NAD(P)+]. Although, Chloroplast and mitochondria are major sites for the production of ROS, they are produced in microbodies as well as in cytosol. Regarding your major query i.e. what is the equilibrium state?, I don't think anyone can address this query. It depends on the situation in which the living being is thriving in [type and extent of stress]. It do agree that they may be generated even under unstressed conditions, as some of them play crucial role in signal transduction and gene expression.

Being a plant physiologist (still in preliminary stages of understanding Nature invented processes), I always stress to my scholars, students and fellow colleagues that in a perfect/ideal plant system the rate of reduction of NADP+ to NADPH [through light harvesting photosynthetic machinery] must be equivalent to rate of oxidation of NADPH to NADP+ [through Calvin-Benson-Bessham cycle i.e dark reaction of photosynthesis]. Unfortunately, it is impossible to come across any such plant system. If NADP+ is not available for accepting the electrons driven by light harvesting photosynthetic machinery, the electrons are readily taken up by oxygen [which is abundant within the chloroplast compartment as it is produced as a by-product during photolysis]. For me equilibrium state can be linked to managing right status of reducing power.

Thank you very much everyone for taking the time to answer my question.