Oven drying of plant tissues means that plant cut or harvested, i think no more reaction takes place after harvest . Accumulation of malate in cells is due to harvest time, when harvested in the early morning, the leaves have ten times the malic acid content as when harvested in the late afternoon. Especially, when grown under water stress and hot environment.

Nobody should dry plant material below 80 °C. There are two exceptions (1) assement chemical constituents that are known to be damaged ... very few!! ... and if this is your task freeze dry! and (2) leaf litter, the decomposability changes by heat drying. In all other cases the standard set by the International Biological Program in the 1960 is 80°C. I do not know why and when that stupid habit of drying at 60 or even lower °C came into use. If you bring in fresh material evaporative cooling during the first half hour can cause T to be non-lethal which causes exorbitant rates of respiration. So, Gamal, your concern is correct. Glad you put that on the market. Microwave shock killing before drying is always a good idea. 

Dear Dr. Gamal,

Oven drying at 50ºC would be fine. Higher temperatures would be an error. Most of the secondary metabolites degradate at temperatures more than 60 degrees.

You must stop metabolism inmediatly when you harvest. The best way is to introduce the samples in liquid N until you dry.

Best regards.

Well the question started unclear: If you are interested in dry matter (what most people are who dry) than 50 °C would be really bad and departs from the internationally accepted IBP definition of dry matter. If your interest is delicate chemistry don't heat-dry at ALL , but apply freeze drying. For things like carbohydrates like sugars and starch, there is no measurable effect up to 100 °C or even beyond. We had explored that in detail years ago when we had to dry plant material in remote places with primitive devices...

This sounds really interesting.. Does anyone have experience in microwave drying for fatty acid profile analysis?

Dear Colleagues

Thank you all for the helpful answers to my question. My concern about the possiblility of malic acid formation during oven drying of Portulaca shoots was due to the further use of the oven dried material to prepare an aqueous extract to test its allelopathic effect on seed germination and seedling growth. I assume that the occurrence of actual allelopathic effect of the extract could possibly be attributed to its low pH due to the formed malic acid during drying. Is it possible?

Dear Dr. Gamal.

I assume that you have prepared the aquous extract with plant power or small leaf pieces. This process release weak acids from the plant, and lowers the pH of the solution. This process occurr even in allelopathic C3 plants. Of course, one of the elements that induce allelopathy in this extract is probably the pH and probably other organic molecules of low molecular weight released. After abscision I can not expect an increase in malic acid.

Best regards. It is a pleasure discuss about Science

Dear Dr Gonzalez

Thank you for yoyr helpful discussion and comments.

Actually I was very pleased when you and other colleagues have cobtacts to help me explian what is going on.

I wish you a happy properous day.