Sometimes a pinch of sugar is added to water and fed to a plant that has wilted and hasn't been watered for a while. The sugar can help the plant quickly get back to normal. However, this doesn't always work and sometimes the plant might be too far gone to save.However, sugar is not usually added to the water that is fed to normal, healthy plants.Research photosynthesis, plants use sugar for energy.
The effect of loss of water in wilted plants as well as cut-flowers is an analogous process, that is, loss of turgor pressure (water pressure). Although, the effect on the cut-flowers is irreversible, the wilted plant may spring back to life. Plants have tiny holes in their leaves known as stomata that allow the exchange of O2 and CO2, but result in the loss of H20.
In principle, a continuous water column from the tip of a plants root to the high leaf on the plant is a continuous water column (like a chain of water molecules). As H20 evaporates from the top, that in turns pulls the chain of water molecules upward all the way from the root. As long as this turgor pressure is maintained the plant will remain upright (not wilt and slumped).
But, in conditions of low water and/or high temperatures greater than average evaporation from the leaves (which is actually called transpiration) occurs, and at some point the water column is no longer continuous.
However, when the stomata close, the plant can reverse some of this process by releasing stored water from nearby cells and thereby restore the continuous water column in the plant. Water also play an equally important role in photosynthesis, where it is broken down as an oxygen source, hydrogen ion and electron donor. Its role in photosynthesis is absolute.
No water no photosynthesis. So what the point? Well, the function of photosynthesis is to produce energy in the form of sugars (e.g. glucose, etc.) In the case of the cut flowers, you are temporarily breaking the water column in the plant, which is why you are supposed to cut the stems under water with something sharp.
The cut flowers are immediately put into a vase full of water or even cut in this container.
A sugar, antioxidant and anti-microbial agent (the little packets that come with cut-flowers) is poured into the vase. This solution replenishes the plants food supplies temporarily, since the water column was disrupted and food may have been lost. Flowers last much longer in the sugary solution, than in plain tap water or deionized water for that matter. Also, cutting the flowers after a day or to increases the water transport/transpiration potential of the plant. In the case of the wilted plant, sugar might temporarily help the plant, but in the absence of water any effect will be trivial and short-lived. The plant can make its own food when intact. It can't make its own water.
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