Plant Hormones & Responses

* Plants have hormones (chemicals produced by organism that causes a physiological change) that regulate growth and development

    1.) Auxins: promote cell elongation, leads to cell growth, allows plants to respond to light


    2.) Gibberellins: promote cell elongation, causes plant to grow taller

        -Many dwarf plants are small because their cells cannot make gibberellins or their cells are not receptive to it

(see dwarf bonsai plant below)

Dwarf Jade Bonsai

    3.) Ethylene: simple gaseous plant hormone that speeds the ripening of fruits   


That red tomato is really green!

Do you remember the tomatoes that you ate when you were a kid? They were bright red, juicy, soft, and tasted great. 

Have you bought one in the supermarket lately? 

Not quite that tasty tomato of days past. 

In fact, you are likely to find that the tomatoes are hard, lack any juice, and are virtually tasteless. 

There are several things going on here: 

Yuk Reason #1 - The red tomatoes are really green tomatoes! (Yes, you read it correctly) It's no magic trick, but a feat of modern science. 

Ripe tomatoes are very soft, so they are picked green. Instead of letting them ripen, however, they are piled into a gas house and exposed to ethylene gas. 

Why ethylene gas? This is the same chemical that the tomato gives off internally when allowed to naturally ripen. The gassing tricks the tomato into producing that nice red color. It looks ripe to us, but it's not. 

Yuk Reason #2 - Home cooks and restaurants have told the growers that they wanted a firmer tomato. They don't want a mushy tomato - they want a firm one. A harder tomato means less juice - and a less appetizing salad veggie. Too bad they didn't ask the consumer. 

Yuk Reason #3 - Supermarkets want a tomato that looks good. No one wants to buy a damaged or bruised specimen, so the tomato producers have bred a tough skinned tomato so that it looks good on the grocer's shelves. 

So what do we get from modern science ? A tomato that has a great red color, is firm to the knife blade, and looks good. 

What we really get is a tomato that is actually green, not ripe, hard as a rock, tasteless, and downright awful. 

* Tropisms: plant's response to an external stimuli that comes from a certain direction

    A.) Phototropism: plants response to light

    B.) Geotropism or Gravitropism: plants response to gravity (causes stems and leaves to grow up, roots down)

    C.) Thigmotropism: plants response to touch

    * Nastic Movement: responsive movement of a plant that is not dependent on the direction of the stimulus

    (Ex: Mimosa pudica)

    (Ex: Venus Fly Trap)