This is a great question!

Let’s start with how water and plants interact and then we’ll talk about the color portion of your experiment.

When plants photosynthesis, they open the stomata on their leaves to bring in carbon dioxide (which they use with energy from the sun to grow). At the same time, when their stomata are open, they release oxygen and water molecules to the air. The process is called transpiration. When the water molecules leave the plant, a “chain” is created that pulls water molecules from the soil into the roots, from the roots into the stem, and from the stem into the leaves. I think about this interaction like a drinking straw with me being the atmosphere creating the “pull” that’s pulling the chain of water molecules from the soil (cup of water), through the plant (drinking straw), and into the atmosphere (my mouth).

Now what happens if I’m drinking water with a powdered drink mix or food dye? Well, my straw is pretty big and all the powdered drink mix or food dye pass easily through the straw into my mouth. But in a plant, the straws (xylem) are much smaller and, while water molecules easily move through the plant and out the stomata, the food dye molecules get “stuck” and are left behind in the plant, thus coloring your plant.

Ok, now on to your experiment with dye and radishes. The exciting thing is – I don’t know what would happen either! Using a plant with a light color (e.g., a plant with a white flower) is the easiest way to see if your dye stains the plant. But radishes have green leaves and the only white part of the plant is underground! Would the color stain the outside of the radish? Probably, because it’s growing in soil surrounded by colored water (similar to dying eggs). Would the color stain the inside of the radish? That’s an interesting question! I’d guess that the radish leaves would turn color too (because the food dye molecules will be present in the leaves) but I’m not sure if the color will be visible within a green leaf.

As for affecting the growth of radishes, I don’t think food dye would have an effect because I think of food dye as being benign – we eat it all the time without effects to our health. However, a powdered drink mix has a ton of other stuff – sugar, artificial flavors, added supplements, etc. and I don’t know how those additives would affect the growth of a plant. Plus not all food dyes are the same. I wonder if there’s a difference between sources of the food dye…?

A few comments on your experiment – 1) What do you think will happen to your radishes if you water them with colored water? Do you think they’ll grow bigger leaves? Will the leaves change color? Will the radish bulb change color? Will it grow a smaller bulb?
Creating an educated guess (hypothesis) on what will happen during your experiment is important to identify so you can collect the appropriate data. (e.g., if you think the leaves will change color – make a color scale to quantify leaf “greenness”. If you think the leaves will grow bigger – measure the length and width of the leaves. If you think the bulb will be smaller – measure the height and width, or the radius of each bulb.

2) Make sure you have a control so you can verify the impact of your food dye treatments. For example, have a treatment where you water your radishes with only one difference – no food dye. This way you can be confident that the differences you observe in your radishes are due to the addition of food dye and not to different sources of water, more sunlight, type of soil, etc.

Good luck! I’m excited for you and your project.

Have you ever wondered how plants pull water up against gravity? After all, they don’t have a heart to pump it up.

Plants take in water using a process called evapotranspiration. It’s kind of a mouthful, but you can see that the first part of the word comes from “evaporation.” Now, take a close look at that word, and you can see “vapor.” As you may know, when liquid water turns into water vapor, we call it evaporation. The liquid water molecules get energized by heat and move a lot more, becoming vapor. The molecules themselves don’t change, they’re just spread out a lot more.

In a plant, the water mostly evaporates off the leaves. There are microscopic leaf holes called stomata (one is a stomate). When these open, the heat of the sun causes the water inside the plant to evaporate and leave through the stomata. The transpiration happens when water exits the leaves.

But here’s an interesting thing about water molecules, they stick together (cohesion) and to other things, like the tubes inside a plant (adhesion). Imagine that the water molecules are like a stack of magnets that stick together. So when one water molecule leaves through a stomate, it pulls another water molecule up behind it. This pull goes all the way down to the roots, where water is pulled out of the soil. Think of the journey of a water molecule being pulled from the soil to the top of a redwood tree. It’s powered by the sun.

Here's a good site on evapotranspiration: evapotranspiration

If things are dissolved in the water, like nutrients or dye, they can get pulled along for the ride.

To make your research even more interesting, you might look at some variables that could increase the rate of evapotranspiration. Think about things like temperature, wind (could be from a fan), or the amount of water.

One challenge in your study is whether you will be able to see the dye once it’s in the plant. The green pigment chlorophyll may be so dark that you can’t see the dye. It might take a long time for the dye to move up. I wonder whether it will reach the inside of the radishes.

There are two processes at work here. The first is capillary action by which the surface tension of the water inside of the plant draws water up out of the soil below, and the second is evaporation of water out of the leaves of the plant, which continues to draw more up through the aforementioned surface tension.

However, I'm not sure why you need this for your proposal. The goal of your experiment is to find out whether the dye gets drawn up into the plant, and whether it harms the plant in the process, right? In that case, what matters is that you have a control (a bunch of radish plants that you are giving ordinary water) along with your treatment (the radishes that you are giving the colored water). That you understand the mechanism by which plants draw water up out of the soil is nice, and something that would fit well into the discussion of the report that you will write after the experiment is concluded, but is only semi-relevant to your proposal to do the experiment in the first place.

Watering your radishes with colored water won’t change how they grow, but it may allow you to see how the water moves through the plant. People do this with white flowers a lot as an experiment because it is harder to see in other, darker plants. Plants usually take up water through their roots, leaves, or stems if the stalk has been cut. As the plant sucks up the water, the coloring dissolved in the water will move upwards too. Since the inside of a radish is white, I’m guessing you will be able to see the colored water move through the plant if you use enough dye. It also helps that the root of a radish is so close to the vegetable part. Good luck with your experiment!