Fresh green leaves contain good quantity of flavonoids contents which show high anti-oxidant activity,  presence of oxygen and its binding to the molecules and its combustion depends on oxygenation mainly oxidation of products. Because oxidation chemistry concern with the vitality and action of certain enzymes, this activity in dry leaves reduced and vital enzymes destruct more than 40% of the flavonoids. Yes it happens that during heat and air long drying, the compounds can activated or totally destroyed by specific enzymes.Hence, brow dry leaves have only low amounts of flavonoids or their derivatives, mostly acids, no alcohols. Dry leaves contain tannins, starch, acids and amides of glucose, minerals moslty.

I think both types of leaves contains such kind flavonoids. Its depend on your handling process from bed to patients. Due to more sensitive to head, all flavonionds requires sophisticate handling process. 

Thanks for all your answers.

I am agreed with all above answers as they are supporting each other. In fresh leaves the water content is more and in drying process some of the  flavonoides gets destructed though in small quantity. In dried leaves if preservation is not proper then chances of fungus contamination are more, in more heat many flavonoides and aromatic compound gets evaporated. 

The flavonoid concentration is too low in some plants. Therefore is better to perform the extraction in freeze dried plants. Other  drying methods can damage the flavonoids.

Plants subjected to biotic and abiotic stresses produce defensive chemicals in response to the stresses. Plants or their parts collected from the wild are further subjected to influences of plant age, stage of collection (vegetative, reproductive stages), season (wet rainy season, dry summer season, chilly winter season, leafless autumn season), time of the day etc.

Check more interesting answers in the following thread:

https://www.researchgate.net/post/Which_do_you_recommend_fresh_or_semi-dry_or_dry_biomass_tissue_for_an_aqueous_solvent_extract_or_for_extracting_isolating_separating_phytochemicals

Very good.

Dear Prof. Maitree

I am attaching a seminar paper on traditional Ayurvedic concepts on medicinal plants, the effect of different factors and quality control. 

Agreed with Sunisa. Fruits usually exhibited a reduction of phenolic compounds, but in some cases it can be observed an increment. My advice is trying different Antioxidant activity assays like DPPH, CUPRAC, ABTS, FRAP among others to get information from different points.

Food processing, of course, can alter polyphenolic compounds. However, some products from alteration still show powerful antioxidant properties. For instance in green tea leaves, although heating changes color from green to brown, the brown leaves somehow show more powerful antioxidant properties. The color change is one attribute for product characteristic, it does not always mean that the bioactitivity of compounds would degrade along the color.

On basis of my practical experience, flavones and flavonols glycosides and aglycones do not change their structure during drying and heat assisted extraction processes, while anthocyanidins and flavanols derivates are very sensitive to these operations leading to their condensed omologues; similarly, caffeoylquinic acid derivates can lead to different isomers during these operations. Given these, bioavalability and bioactivity of the plant derived products can be affected, thus explaining the occurence of similar/identical vegetal content and very different therapeutical/beneficial effects.

This, of course, depends on the plant and the chemical or what the plant is using the chemical for. For example, Artemisia annua's antimalarial artemisinin has been found to have higher abundance in the dead leaves even though it is produced in new tissues. It is my hypothesis that the plant is loading it into dying leaves to effectively treat the soil microbiome. So it is not unheard of that compounds can have a higher abundance in these tissues than would be expected or explained simply by loss of metabolic function or dry down. IMHO. However, in the case of flavonoids, it seems that plants easily secrete them from roots, and may not employ this round about method, but its something to think about if not entirely off topic...

As long as no heat exposure, there may not be a change in the flavonoid content in fresh and dry leaves. However fresh plant parts are more prefered for consistency of a phytoconstituent.

Marella: Not only heat affect the flavonoid content, but  certain active enzymes in plants such as polyphenol oxidase and glycosidases also could lower polyphenols and flavonoid content pattern as well.  

Heat can  destroy some enzyme activity resulting the color can be stable  or changed. For example, phenol oxidase  commonly found  in plants can change polyphenols to brown or  dark tannin in dry  leaves or fruits.  This reaction will not occur after fresh  plant is  heated. 

Yes, always there is difference in contents of fresh green and dried brown, particularly enzymes, aromatic compounds and flavonoids but always it is not possible to use green leaves.

Yes you will get some difference and it will depend on plant species to species.

Dear Maitree, you have raised a very interesting question. Your primary question is: “are there differences of flavonoids, contents and chemistry, in plant leaves between the fresh green one and dry brown one?”.

By “brown” leaves if you are referring to brown leaves that would have been subjected to oxidative browning (catalyzed by leaf enzymes), then the data obtained with such leaves would not be reflective of the flavonoid composition of fresh green leaves. You have specified: “During heat and air long drying, the compounds can be activated or totally destroyed by specific enzymes. “  These operations typically are made in the manufacturing of tea (from the processed leaf of Camellia sinensis).  Prolonged exposure to air can lead to leaf enzyme catalyzed oxidative transformations. Leaf polyphenol oxidases have two distinct activities (phenolase and catecholase activities): 1) hydroxylation of monophenols to ortho-diphenols, and 2) oxidation of ortho-diphenols to ortho-quinones.    Leaves contain peroxidases, which mediate oxidative changes in flavonoids. Endogenous ascorbate is known to prevent this activity. Ascorbate (in view of its redox potential) has the potential to protect several (hydroxylated) flavonoids and anthocyanidins from being oxidatively modified. 

Dear Maitree, you have also referred to tea flavonoid moieties. Tea leaf flavonoids are very stable. Green tea contains native catechins (flavanols). Green tea is manufactured by employing tender tea leaves; the processes involved are withering, rolling and steaming or pan firing (to minimize the oxidation of flavanols by tea leaf enzymes) and drying. Black tea production involves “fermentation”; tea leaves are exposed to air for prolonged periods in order to bring about  fermentation before they are subjected to steaming or smoke firing or heating and drying. The term “fermentation” refers to oxidative transformations undergone by tea flavonoids/phenolics involving natural browning reactions induced by oxidizing enzymes (polyphenol oxidases). Tea leaf peroxidases also are involved in these transformations.  While black tea is fully fermented oolong tea is a partially fermented one. Black tea contains catechin oligomers, oligomeric proanthocyanidins, theaflavins (theaflavin gallates) and other related constituents, which are generated upon natural oxidative transformations (catalyzed by ltea eaf enzymes).

Anthocyanidins, which contain the flavilium cation (example, cyanidin), are pretty stable but are affected by acid. Upon treatment with acid they are converted to (colorless) proanthocyanidins. Anthocyanidins from red grapes are quite stable after wine fermentation.