MadSci Network: Botany

Re: Do microwaves have any harmful effects on plant growth?

Date: Thu Jan 27 13:34:08 2000
Posted By: Joseph E. Armstrong,
Area of science: Botany
ID: 947562869.Bt

1. How microwaves work.
Heat is a measure of how active or excited molecules are. Water molecules in cold water move slower than water molecules in hot water. Water molecules in ice move so slowly that they arrange themselves into crystals.

Water molecules in steam move so fast the water is a vapor. Microwave ovens cook food by beaming microwaves (very short waves) into the food which vibrates the water molecules. This increases their energy causing them to move faster. This generates heat and the heat released actually cooks the food.

Cooking any living thing kills the cells. Seeds are living organisms, embryonic plants, and they can be killed by cooking them. However, dormant seeds have a very low water content and as a result are difficult to cook. Look at any bean soup recipe; dry beans take a long time to cook. So yes, microwaves can cook any living thing if they are subjected to enough microwave energy.

2. Design of the experiment
Why didn't your experiment work the way you thought it should? You predicted that the longer exposure to microwave energy would reduce the germination. You got unexpected results because you did not have enough information to design the experiment correctly. Seeds do die during dormancy especially the longer they are stored. You don't know what percent will fail to germinate under any circumstances. With just two seeds both could easily be dead just by chance, bad luck in choosing seeds from the pack. How much microwave energy is needed to "cook" and kill dry seeds? I don't know. You would have to find out by experimentation. First I would germinate 10 sets of 100 seeds to find out how many on average will grow. Then I would microwave 10 lots of 100 seeds for increasing amounts of time until none germinate (15 sec, 30 sec, 60 sec, 120 sec, 240 sec, etc.). So good experiments need lots of seeds and replications. Different types of seeds may show different results both in terms of viability and in terms of microwave damage. If you soaked the seeds in water for 24 hours prior to microwaving, you might find that less energy did more damage because the seeds will contain more water. Dry dormant seeds may have as little as 4% water. Not much water, so it will generate very little heat. Seeds also germinate at different speeds. You can not tell if moderate amounts of microwave energy stimulated faster germination unless you use many more seeds. Any good gardener knows that seeds soaked over night will germinate faster.

By the way this is why popcorn pops. The grains are 1-seeded fruit with a very hard fruit wall. The soft center contains water. Microwaves heat the water until it turns to steam which expands rapidly exploding the grain inside out, increasing its size and decreasing its density.

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