Re: Why did a flower in SPRITE last longer than one in water?

Sprite contains sugar (sucrose). The sugar is used by cut flowers for 
energy as a substitute for the sugars normally produced by photosynthesis. 
Photosynthesis in cut flowers is minimal because of the low light levels in 
our homes and because many of the leaves are often removed. Not all cut 
flowers may benefit from sugar and may be injured if the sugar  
concentration is too high. Use of sugar is highly recommended for cut roses 
and snapdragons. 

Flower preservatives that dissolve in water consist of sugar and a 
compound(s) that acts as an acidifying agent and biocide, such as 
8-hydroxyquinoline citrate. A biocide kills microbes so they don't clog the 
xylem and prevent water uptake by the stem. The acidifying action increases 
water flow through the stem. Using a floral preservative can double or 
triple vase life of some cut flowers. 

In addition to containing sugar, I believe Sprite would also be acid. You 
might want to measure the pH of your three solutions and compare them. 

References 

Larson, R.A. 1992. Introduction to Floriculture. New York: Academic Press. 
 http://floralife.com/