Cut flowers begin to decline as soon as they are cut from the plant. For florists, as well as homeowners who like to have a fresh bouquet in the house, making them last as long as possible is important. There are entire industries that consist of manufacturing products to add to the water of cut flowers to prolong their life. The exact ingredients are a trade secret, but they are all designed to provide the right environment in the vase water to make cut flowers last as long as possible.
A biocide is simply an ingredient that kills bacteria that live in the vase water. The sugars that are in the stem sap and plant food are released into the water when the flowers are added to the vase. Bacteria thrives on these sugars and the bacteria populations increase immediately. When the flowers begin to pull water up the stem during the normal transpiration process, the bacteria clogs the stem and the vase life of the flowers is shortened. A biocide keeps the population of the bacteria under control or very low so the flowers can pull water up the stem without blockage.
Sugar or a sugar-type substance is what is considered the plant "food." This is what the flower uses for energy to stay alive. When a flower is closed or in the bud stage, it needs the energy created by the sugar to open and stay fresh. Sugar also increases bacteria populations so a biocide must be added to the floral preservative to counteract the explosion in bacteria that occurs when sugar is added to the water.
An acidifier is added to the water to lower the pH of the water to as low as 3.0. The reason that flowers last longer in a low pH water environment is that acidic water has an astringent effect that forces the flowers to take up water. Also, acidic water has a sterilizing and disinfecting effect on the vase water.