This is an "advanced" project worthy of college students, and as such very few directions are given. That does not mean that only college students should attempt this project. Creative high schoolers who can think logically should be able to make good headway in this problem. In fact, one grade schooler made a significant advance by being rather clever.* All methods will have to be worked out by the researchers. This has not been done before in its entirety.

Have you ever wondered how a tree can wave its topmost leaves in the sun for hour after hour and not get sunburned - burned to a crisp? Or to go to an even more extreme cases of desert plants'sitting atop white reflective sand and "catching the rays" over years with very little cloudy weather and showing few signs of damage? What does the plant have that is so effective against solar ultraviolet rays that rapidly damage DNA? UV light causes DNA damage so rapidly that it sterilizes bacterial cultures within minutes, and yet the chromosomes, chloroplasts and mitochondria of the leaves go unscathed for months on end. How can this be? What adaptations have been made by plants?

Two possible adaptations MAY have been made. One is that plants possess marvellously efficient DNA-repair systems, and the second is that the plant wears a superlative sunscreen. This second alternative seems the more probable once you notice that leaves have incredible powers of stopping UV from penetrating from top to bottom. You can demonstrate this quite easily by placing a fluorescent rock on a table, laying a leaf over the rock and then holding a bright short wave UV light overhead. If you get down and peek under the leaf, you will see that the rock is not glowing; take away the leaf and the rock glows. WEAR UV-OPAQUE EYE PROTECTION BEFORE TURNING ON THE UV LAMP! Don't even think of not wearing eye protection when the UV light is on. It's damaging effects on your eyes are very fast and will last a lifetime.

Now that you have convinced yourself that UV cannot pass through leaves, you are left again with two alternatives for why this could be: the leaves could be reflecting the UV light, or they they could be absorbing it - or they might be doing both! Showing which of these, or both, is true is the next logical step. Since you are a brilliant budding scientist, the methods for doing this will be left up to you - otherwise it would be this website's science project and not yours!

Your next move would be to start accumulating information about what is known about this phenomenon. You might talk with some botanists, who for the most part have never thought of this. That is good for YOU because it makes the project even more worthy of doing. Horticulturists, on the other hand can give you something to think about: if you wish to move a house plant outside, you must slowly acclimate the plant to the much brighter outdoors. First the plant should be set on the shady side of the house for a few days before moving into the sun. If you don't do this, the plant will be sunburned! Hmmm. What is the plant doing to acclimate? It might be revving up its DNA-repair systems, or it might be putting on more sunscreen - or both! But in any case, this acclimation stuff is interesting information and may eventually give your some "tools of comparison" - comparing the same species grown indoors with those grown outdoors.

What the Botanists Will Likely Say

Most of the time they will quickly divert away from what you are asking because they want to talk about what they know about. For too many scientists it is humiliating to admit to not knowing something! When you bring up the subject of the leaf's cuticle, they will probably start talking about how the cuticle prevents the evaporation of precious water. What you are asking is if the cuticle might be the leaf's sunscreen AS WELL AS the leaf's evaporation barrier. So make sure that the botanist with whom you are conversing with stays on the subject!

Let's suppose - hypothesize! - that sunscreen is the route the plant is taking. When you use sunscreen, do you drink it to put it inside your body? Where do you put it? Of course, between you and the sun - on your outside. Putting it on your inside would not protect the skin from UV damage. Perhaps - another hypothesis! - plants do it the same way. Perhaps they cover their leaves with some chemicals that are either highly reflective of UV or strongly absorb it so that, in either case, no UV penetrates into the cellular layers of the leaf.

When this author has brought this whole idea up to his botanist and plant physiologist friends, he was told that "of course" leaves absorb most of the UV so it won't damage their cells. That the cells were filled with UV absorbing compounds that would "of course" protect the DNA and other UV-sensitive structures. But, as you can see, that would be like your drinking sunscreen to protect against sunburn. And, by the way, the UV sensitive layers in your skin are (a) thicker than most leaves, and (b) also filled with lots of UV absorbing compounds. Remember that DNA itself is strongly UV absorbing - or else it wouldn't get damaged. So just because something is UV absorbing doesn't make it a sunscreen. It may be the target.

Again, getting back on track: re-read the last sentence above printed in black. Thus, if any UV penetrates into a cellular layer - any CELLULAR layer, that layer's DNA could be damaged and cause the cells to die - in minutes (unless the DNA-repair hypothesis is the true one). This should prompt you to get out your microscope and look at cross-sections of a number of leaves. Are ALL leaves from sun-loving plants coated with an "acellular" (cell-less) layer - a cuticle of sorts (see box above). Well, you will read that leaves generally have cuticles, but THE present question is - are these cuticles composed of cells or not? If cuticles are not cells, then there is not DNA to be damaged, and the sunscreen hypothesis still "flies;" otherwise the DNA-repair hypothesis gains strength.

Let's suppose - hypothesis again! - that the cuticles are not composed of cells. Our next question is: are cuticle layers UV-opaque because of either reflecting or absorbing UV - all the UV? (Close to "ALL" of the UV lest that which gets through cause DNA damage.)

At this point you have probably done some reading about leaf cuticles and you will have undoubtedly found that the books say cuticles are to prevent water loss (again, the box above!). No mention is made of UV protection. Hmmm! That is good for YOU, because it means that either someone has shown that your hypothesis about sunscreen is wrong, or that you are delving into a totally new realm of science. Be optimistic: it's probably the latter - you are poised to advance the cutting edge of science!

Now you will left on your own: a lot of ideas have just come your way, and now you must decide what you are going to do and how you are going to do them. Here are some essentially untried hints to start you off:

1. If you find that the cuticle is not composed of cells, can you in some way figure out either:
   
   1. how to strip off a layer of cuticle without any underlying layers. Inspect its cell-free status under the microscope. Is it UV opaque?
      
   2. how to dissolve off the cuticle layer without affecting the lower layers. Is the solution of cuticle UV opaque? (Make sure your solvent is not UV opaque!)
      
2. Delicately insert a known UV sensitive organism at various depths in a leaf (see figure below). If the leaf protects them from UV killing, you will know that there was something UV opaque between that sensitive organism and the UV light source. Imagine being able to insert this sensitive organism between the cuticle and the topmost cellular layer!

   Think about doing it this way. A culture of E. coli was tinted with blue food coloring and then inserted at various depths longitudinally into the broken edge of a thick leaf in the steps shown in the next series of figures. The blue flagged the location of the injections. The leaf is then exposed to an amount of UV that is necessary to kill all the bacteria on the surface of the leaf. Next the leaf is cracked open on the blue lines, which are swabbed and plated on MacConkey Agar, which allows E. coli to grow into red colonies. Few other leaf-bacteria can grow on this type of agar.