It cannot be said, for sure, if there is any plant that does not have antioxidant activity but since all plants are exposed to oxidative agents so plants have elaborate antioxidant mechanisms to maintain steady state. Plants also have other protective mechanisms like secondary metabolites which are main actors in antioxidant activity of plants. Phytochemicals like phenolics are proved antioxidants. But this term "Antioxidants are beneficial" can be misleading in wake of latest research taht suggest that antioxidants may not act as antioxidants in body. The reasons are:

1. Antioxidants at certain concentrations act as pro oxidants hence may cause oxidative damage.

2. Anti oxidant activity results vary in vitro & in vivo because different factors are responsible for antioxidant effect in vivo.

3. Metabolic products of secondary metabolites or falvonoids may not act as antioxidants.

Since not all the plants have been assayed for antioxidant activities, it's difficult to say exception is not exist. However, all the plant are living towards oxidation stresses caused by oxygen in air, photosynthesis of them themselves, free radicals come from lipid autoxidation and so on. All of them have got antioxidant systems and substances due to evolution.

Highly agreed. Chen Shanqiao is right.

I think every plant have ability to give antioxidant action. It just depend on how much activity you expected and what reference standard you are using for the evaluation.

It cannot be said, for sure, if there is any plant that does not have antioxidant activity but since all plants are exposed to oxidative agents so plants have elaborate antioxidant mechanisms to maintain steady state. Plants also have other protective mechanisms like secondary metabolites which are main actors in antioxidant activity of plants. Phytochemicals like phenolics are proved antioxidants. But this term "Antioxidants are beneficial" can be misleading in wake of latest research taht suggest that antioxidants may not act as antioxidants in body. The reasons are:

1. Antioxidants at certain concentrations act as pro oxidants hence may cause oxidative damage.

2. Anti oxidant activity results vary in vitro & in vivo because different factors are responsible for antioxidant effect in vivo.

3. Metabolic products of secondary metabolites or falvonoids may not act as antioxidants.

Dear Shayaq please justify your following statement with any published report..

• Antioxidants at certain concentrations act as pro oxidants.

There is a lot of research published showing antioxidants act as pro oxidants in certain conditions and at higher concentrations. Check this review

http://www.sciencedirect.com/science/article/pii/S0003986108000489

Dear Shayaq please justify your following statement with any published report..

• Metabolic products of secondary metabolites or flavonoids may not act as antioxidants.

I also agreed plant in the universe possess rich antioxidant. In future characteristic antioxidant levels in different plant in different climatic environment.

every plants have antioxidant but different the levels

Maybe the question should be framed in terms of whether you are asking if it has antioxidant benefits if consumed by humans or animals or is limited to providing the plant with sufficient defense against free radicals to benefit it, but not mammals. Be most interested in your response. 

Majority of the medicinal plants have antioxidant activity.

I do agree with your skepticism that all plants are now assumed to contain anti-oxidants. I think we need to be careful as some plants may contain only trace amount of secondary metabolites which serve as anti-oxidant. We know that only some of the plant content could serve as anti-oxidant like flavonoids. Therefore, quantifying those useful metabolites will be valuable to  judge whether the plant is anti-oxidant or not. 

Thanks.

Not all plants have so far been examined for antioxidant activity. However almost every plant contain one or other secondary metabolites responsible of antioxidant property. The term 'Rich' is relative and can't be uniformly applied to all.

There is this controversy. Offlate some researchers have come up with an opinion that reporting for antioxidant property is not a big deal.  all plants have antioxidants. But the mode of research should be on a comparative ground. 

All living organisms have antioxidant mechanism and hence antioxidant properties - so plants are no exception.

I agree with Dipjyoti Chakraborty.

Dear Arvind

Check this thread.

https://www.researchgate.net/post/which_plant_is_commonly_used_as_a_source_of_natural_antioxidants_in_traditional_medicine

Not all plants have so far been examined for antioxidant activity. However almost every plant contain one or other secondary metabolites responsible of antioxidant property.

I think, every plant has some anti-oxidant property. It may only vary in degree. All the plants contain some amount of secondary metabolites.