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Why Does Grass Become Greener after a Rain?

You have undoubtedly noticed how quickly a draught-striken lawn becomes green after the rains have returned - often in as little as 24 hours! How is this possible? Four alternatives come to mind:

1. The green chlorophyll is hidden away in the dormant plant - either in the roots, or opaque packages so the green doesn't show through, or it is chemically hidden by a subtle reaction that changes its color.
2. The amount of chlorophyll in the chloroplasts is rapidly increased making each chloroplast greener than it was before (see the figure to the left of the Title).
3. The number of chloroplasts in the cells are rapidly multiplied with the net effect of seeing that much more green (see the figure to the right of the Title).
4. Any combination of the above.

Now the problem is to test the plant "system" to see discover which reason or reasons are valid. Many of us are not unfortunate enough to have had a severe draught the will be broken tomorrow with plentiful rain. Those who are that fortunate should run outside right now and get some plant specimens and put the in the refrigerator so that they can be compared with the same plants after the rain comes tomorrow.

But what about the rest of us?

Here come one of the great scientific lessons: you set up your own controlled system. You might take a couple of patches of turf and set them in nursery flats. One you might water daily and the other is protected from the rain and is not watered for a week or ten days. While the first batch stays nice and green, the other will become pale, brown and not look happy at all. Right then and there you might microscopically compare the cells from the two flats for numbers of chloroplasts and green-ness of individual chloroplasts. Then you might wish to begin a normal watering regime with the draught striken batch, and periodically take microscopic views of the cells as they recover and become more and more green.

There is a parallel to this that you might find interesting, and may shed light on your botanical studies. That parallel deals with red blood cells (RBC's) and hemoglobin. Some people become anemic. Does this mean that the people have fewer RBC's (#1), or is it that each RBC has less hemoglobin in it than normal (#2)? And then the doctor gives the person iron supplements aong with a medicine that cures the person of anemia. What is happening to the RBC's during the cure period? Are the RBC's increasing in number, or is there just more hemoglobin in each pre-existing RBC?

Vocabulary: what do these words mean: chlorosis, chlorotic?

The very first thing you must do is find someone to help you see chloroplasts within cells under a microscope. You will need to make extremely thin slices of a leaf that has been embedded in paraffin wax. Even a simple microtome made from a nut-and-bolt works. All you need to be able to do is see the greenish dots within the cells. You must also be able to tell light green dots from dark green ones. And you must be able to count them in the cells. Where do you find a microscope if you are doing this project outside of school? Most medical doctors were required to own a microscope when they were in medical school. It is now probably gathering dust in some closet at home! Perhaps your family doctor will show you how to use hers or his, and let you use it. Do not begin doing the following unless you have access to a microscope and the skill to see chloroplasts.

There are two ways that leaves turn yellow (without the aid of manmade chemicals, of course). Thus this project would be very appropriate for two students - one working with one method and the other person on the other method. But in both cases the same plant species should be used. Selection of the right kind of plant is critical. Many plants when stressed beyond a certain point just die and will not recover except by seeds or resprouting of the roots. You need a plant that is draught tolerant and goes into some sort of dormancy during arid conditions. Most grasses do this, and that is why "grass" has been mentioned so much above.

1. The first way, obviously, is the one mentioned above - imposed draught. By 'imposed' is meant that you have experimentally prevented that plant from being watered. The person doing this aspect of the experiment will have two batches of grass that are grown under identical conditions except one is not watered. Here comes your first tough part: how do you ensure that both have identical conditions except for the watering? Suppose you supported a large window pane over one section of your lawn so that rain would not fall on it. In all other ways is it identical to another section of your lawn? Doesn't the glass block ultraviolet light? What about water runoff that flows under the glass? Lots of things you must contend with! When you think you know the answer to this problem - and one that YOU can afford and use, tell this website in an email by clicking .

   Once you have begun your plots of grass, you should daily take cell chloroplast counts. If the hypothesis about the changing number of chloroplasts is true, it should become markedly evident. Soon your "control" grass cells should have many times more chloroplasts per cell than in the arid grass cells. (If this is true, then you will want to find out how this came about in Number Phase Two. However, if the number per cell seemed to stay the same but the individual chloroplast dots seemed to get paler and paler, then you will want to check out how that might have occurred in Green-ness Phase Two.

   But before you proceed further, you should read about chloroplasts and be able to answer this question: How are chloroplasts like bacteria?

2. The second way is by blocking light from reaching the leaves. You may remember times when something was left on your lawn for several days and when removed the grass was pale or yellow underneath. Obviously daylight has been necessary to maintain the green-ness. The simple way to do this experiment is to take one flat of grass and place it in the back of your garage or shed where there is NO light coming in. The grass can "breath" and you can take it out at night and water it, let it drain for a few minutes, and then return it to the dark corner. Meanwhile an identical flat of grass is sitting happily outside in the sunshine. Just like your partner, above, who is looking at the effects of draught, you should daily take blades from both grass patches and microscopically check them out. (What is really nice is if you both use the same patch of "control" grass. By the way, what's a "control?") Just like your friend, once you have an answer, you should try to discover how the plant did what they did. If the cells in the dark plant had many fewer chloroplasts, click Number Phase Two, or if the number stayed the same but the chloroplasts became less green, then click Green-ness Phase Two.

The Reciprocal Experiment!

Now comes the "proof of the pudding:" can you run this experiment backwards? The "draught" person starts watering the brownish patch of grass, and the "darkness" person takes the yellowed grass out into the sunshine. Daily uncover the microscope and follow what is happening to the chloroplasts. Does the process go in reverse? Somehow, you know that the deprived grasses must recover and get back to the normalcy of the "control."

Chloroplast Numbers Per Cell Decreased

You have found that the number of chloroplasts decreased in the cells. This could happen in two ways:

1. The chloroplasts are consumed in the cell - either eaten up or they explode (apoptosis)
2. The cells continue to grow in number but the chloroplasts stop dividing, and so there is a constant number of chloroplasts that are divided among an ever growing number of cells.

In a careful review of your observations, which you have been recording all along!, can you discern which method was used by the plant?

And when you have a conclusion to your experiment, send an email to this website by clicking

Chloroplasts Became Pale

You have found that the number of chloroplasts remained the same in the cells but that the arid chloroplasts lost color. This could happen in two ways:

1. The chlorophyll molecules are consumed within the chloroplasts - either degraded or chemically changed to a colorless form.
2. The cells and the chloroplasts continue to grow in number but the chloroplasts stop making chlorophyll, and so there is a constant number of chlorophyll molecules to be divided among an ever growing number of chloroplasts.

When you have a conclusion to your experiment, send an email to this website by clicking

If you find yourself in this category, you have a problem worthy of a brilliant student's solution. You might try some sort of analytical procedure to quantify the amount of porphyrin in your samples. So hit the books! What is porphyrin? You will discover another connection between blood and chloroplasts!

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