The first question: This principle is the same as why pickles are easy to be preserved for a long time. Bacteria do not grow easily in salt water. I think you are a graduate student and easy to figure out. I am an undergraduate, so I don't have to waste too much time. You write papers, you might as well search online.
Second: This involves an osmotic pressure problem. First of all, it needs an element for plants not to wither. What is it? You search online. I am a high school student. I don't remember exactly what it is. Can't talk nonsense. You search, talk about osmotic pressure, flower stalk: that is, the cut fracture, the cells are still alive and well, just as a semi-permeable membrane (this is what the laboratory calls it). When flowers are inserted into ordinary water, the osmotic pressure of the upper layer of the semi-permeable membrane, that is, the cells of the flowers, is relatively high, and the osmotic pressure of the lower layer is relatively low, so the pressure tends to be balanced, and a large amount of water will penetrate through the semi-permeable membrane to the upper layer, that is, the cells, so that the cells can absorb it, but at the same time, the solutes (various elements) in the cells will partially, I mean partially, or relatively easily, penetrate through the semi-permeable membrane into the clear water below. Why do I say partially? Because the semipermeable membrane is a layer of cells here, it is alive, so it will actively absorb useful things in the water, but because of osmotic pressure, it is still constantly discharged, after all, not so much. Flowers thus maintain the blooming element (it seems to be the element "boron", it seems! ! ) will continue to die and wither.
After adding salt, the osmotic pressure on both sides of the semipermeable membrane is not as large as before, but there is still osmotic pressure, which can continuously absorb water and replenish the water lost by the surface evaporation and self-respiration of the flowers, and the chance of the flowers' own organic matter passing away will also be reduced, because the osmotic pressure difference is smaller than the original.
note:
Some people may mention that salt is added to supplement sodium ions for flowers to absorb, thus maintaining the osmotic pressure balance within and between cells of flowers themselves. This statement makes sense. That's because of living things, I mean plants that grow normally. In that case, because the plant itself will wither and die, new leaves and flowers will be born, so sodium will be lost and new sodium will be needed to form new cells. But this is a flower in a vase (of course, some plants only grow in water, such as daffodils), and only evaporate and breathe by themselves, so sodium will not be lost, let alone replenished.
My hands are tired. I don't know if it's useful to you ~ ~ hehe.