There are many fluorescence probes which can be used but detection is done by fluorescence devices, So I will not go into them, for simple photographic imaging (you just need a simple camera; your phone may work) of ROS ,  nitroblue tetrazolium (NBT) (for superoxide anion radical) and 3,3'-Diaminobenzidine (DAB) (for hydrogen peroxide) can be used. For detailed protocol I will recommend " Imaging of Reactive Oxygen Species In vivo. by Steven M. Driever, Michael J. Fryer, Philip M. Mullineaux, Neil R. Baker. Plant Signal Transduction Methods in Molecular Biology Volume 479, 2009, pp 109-116." and "Imaging of photo-oxidative stress responses in leaves. Fryer MJ, Oxborough K, Mullineaux PM, Baker NR. J Exp Bot. 2002 May;53(372):1249-54". 

Hello. You could try my method, published in Tree Physiology, "Development-specific responses to drought stress in Aleppo

pine (Pinus halepensis Mill.) seedlings" (Alexou 2013). It works well on pine material. I dont know what kind of material you have but it should work well.

Level of malondialdehyde (a cytotoxic productof lipid peroxidations) gives an indirect indication regarding level of ROS. This can be measured easily using spectrophotometeric protocol [you just require TCA for extraction and TBA (i.e. thiobarbituric acid)].

The level of malondialdehyde is as Pardha-Saradhi (with all due respect) said, an indirect indication. I would not advise it though, because it does not take into account the antioxidative capacity of the plant cell. Measuring the damage of ROS has little to do with measuring actual ROS levels. The same levels of ROS may have a completely different impact on a plant depending on the levels of antioxidants and the capacity to increase those in a period of time that remains unknown until we investigate experimentally. Measuring levels of malondialdehyde is a nice complement for assessing the damage of a certain level of ROS. Generally, don't try to measure the impact of something without measuring this "something" too. Measuring "damage" does not say much about how antioxidants were used (enzymatic or not) ie without investigating "what kind of fight" the plant conducted. Perhaps severe damage took place with low levels of ROS.

Dear Dr. Maria,

I certainly agree that there is no perfect simple method to quantitatively determine ROS.

The basic question asked is "How could I quantitatively measure reactive oxygen species in plants?"

When we need to quantify/measure ROS in plants at any given time, is it necessary for us to simultaneously measure antioxidant activity? Yes, I certainly agree that measuring antioxidant activity will certainly give us idea regarding the capacity of plant or its part to detoxify ROS.

I also suggested the protocol based on the available facilities with Sergey Bombin.

Please do not hesitate in criticizing/correcting me. I believe that it is essential for us to correct each other for the health of scientific research and that’s what seems to be the goal of ResearchGate.This is a good mode to educate each other.

Thank you

Thank you all very much, especially Dr. Maria and Dr. Pardha-Saradhi

Dear Dr. P. Pardha-Saradhi

Thank you for your kind words. In such experiments there are 3 things we need to measure: ROS, antioxidants, and damage as malondialdehyde levels. Unfortunately in most papers only antioxidants are measured. Damage is deducted by their levels, and so is the oxidative stress paradoxically. ROS levels are probably the most interesting of the 3. It all starts with what threat a plant deals with. I was just trying to make that particular point. I hope I did not overdid it. PS: Maria is my first name. :)

Dear Dr. Maria Alexou,

I completely agree and fully appreciate your answer.

Thanks

Most of the best modern age sensors for ROS are small-molecule based fluorescent probes. A good number of such probes are available at Invitrogen / Molecular Probes. However, many of these were developed almost a decade ago. Despite popular use, it is known in chemical literature that they have loads of problems and limitations in selectivity and sensitivity. You can check out (search in the search engine) best probes from chemistry papers (most will claim excellent biological applications) in JACS, Organic Letters, Chemical Science, even PNAS. ID a lab making such probes and request from the PIs accordingly. Although requests may not be always reciprocated, good probes mean a good start, at least.