Plants need sucrose to survive. If your plants look in need of a quick boost, you might be tempted to supplement the sucrose they make by watering them with a sugar solution. Sugary drinks certainly give us an energy jolt; why not plants? But things aren't quite that simple. The effects of sugar water on plants are complicated and not altogether beneficial.
Plants receive water by a passive process called osmosis. This is the passage of water from across a semi-permeable membrane from the side where water is in higher concentration to the side where it's in lower concentration. Wherever the ratio of water to "stuff" is lower, that's where osmosis makes the water wants to go.
If you feed your plant a strong solution of sugar water, you create a situation in which the water outside its cells has a higher percentage of soluble material than does the water inside. The direction of osmosis reverses, causing water to exit plant cells or not be able to enter in the first place. The plant begins to die.
If dilute enough, sugar water may not cause this effect at first. In fact, California Science Fair participant Mary M. Karcher discovered that bean plants watered with 50 grams of sugar per liter of water grew stronger and larger than bean plants fed pure water over a period of 28 days. However, sugar molecules, being too large to pass through cell walls, remain behind in the soil. Continue watering with that same sugar solution and eventually the sugar will reach too high a concentration for water to continue entering plant cells.
Increased Microbe Activity
The increased sugar content in the soil serves as food for any number of microorganisms. Depending on which microbes are present, this can have beneficial or harmful effects on the plant. Beneficial effects include all the activity that goes on in hot compost: nitrogen fixation, toxin decomposition and nutrient production. These desired effects are the reason for adding sugar to a fertilizing agent, as described in U.S. Patent Application 20090266125. But some microbes excrete toxins that poison the plant, with whom they may also compete for necessary nutrients.
Though sugar can't cross plant cell walls, it can enter the plant's vascular system in other ways. In 1942, Dr. Herman Augustus Spoehr experimented with albino corn plants, which, lacking chlorophyll, cannot produce their own sucrose. They do not normally survive long. Spoehr cut the tips off the leaves in order to open up the veins, then submerged the leaf ends into small vials of a .3 molar sugar water solution. His plants survived for 3 to 4 months and even produced small ears of corn.
Cut Flower Preservation
Floral preservatives such as the product Floralife mainly consist of sugar. They work in a manner similar to Spoehr's sugar water vials, providing sucrose to the vascular system of cut flowers to keep them "alive" and fresh longer.