Plant hormones

I think I’m going to be moving into the plant phase of this blog. You see, a certain percentage of the GRE involves botany (they say it’s only 15-33%, but it seems like a lot more on one of the tests I am looking at). Also, in the next few week’s I’ll be running a week-long training for grade school teachers in botany, so I need to brush up on my skills. Here we go:

Today’s question:

Which of the following plant hormones hastens apple ripening?
A) Auxin
B) Gibberellin
C) Abscisic acid
D) Cytokinin
E) Ethylene

Let’s dissect the question. What exactly is a hormone? A hormone, in terms of this question, is any of a handful of plant compounds that control the growth and differentiation of plant tissues. Basically, hormones control lots of things having to do with plant growth and development.

This question entirely depends upon your knowledge of plant hormones. There’s not really a good way to guess your way through this one, which makes it a bit of a pain in the ass. Get your flash cards ready! Let’s go through the hormones.

Auxin: Auxin has to do with the growth of plant tissues. Anyone who has ever grown plants know they have a neat tendency to do things like grow towards the light, and the stems grow away from gravity while the roots grow down. (Oh! Try this: take a plant you have in a pot and place it on its side. After several days, the stem will bend so it is once again growing away from gravity. You can also put a plant in a dark room with one window, or in a box with a window cut in the end. After a few days, the stem will bend towards the light source, and the plant will start growing towards the light.) So what happens in these two situations? Well, plant cells have auxin in them. When light hits the tip (or growing center) of the plant from one side, then the auxin present on that side flees from the light (maybe it’s a vampire?) and concentrates on the other side of the cell. This causes the illuminated side of the plant to grow more slowly than the dark side. This over zealousness makes the plant tip grow towards the light, which allows the plant to get as much light energy as possible. The same basic thing happens with gravity. The auxin in root cells drop to the lowest point of the cell, which causes that tissue to grow faster than the higher points. Make sense? Of course it does. It all breaks down to this: auxin makes tissue goes faster. Wherever there’s a lot of it, that’s what grows.

Gibberellin: Giberellin is also in charge of growth, but in a different way than auxin. Gibberellin takes care of stem growth upwards, as opposed to which way the plant grows. This hormone is in charge of making stems grow tall really, really quickly, especially in those plants that are usually short. (Hey, when do you think this would happen? Perhaps in plants that are trying to compete for sunlight and need to outgrow their competition. Hmm.) It is also in charge of inhibiting new root formation, and stimulating new phloem cells. We’ll talk about the xylem and phloem in another post. They are rant worthy, that’s for sure. Finally, giberellin break the dormancy of buds and seeds and start the flowering in some plants during their first year of life.

Abscisic acid: Abscisic acid is in charge of stopping cell growth. This primarily happens when a plant needs to go dormant to avoid damage from excessive cold. This is what causes all the trees to stop growing during the winter and whatnot.

Cytokinin: All these hormones seem to govern plant growth, don’t they? Cytokinin is no different–this particular hormone causes cell division. It also has a neato interaction with auxin: in undifferentiated cells, the ratio of auxin and cytokinin becomes very important. If auxin is dominant, the cells turn into root cells. If cytokinin is dominant, then the cells turn into stem and eventually bud cells. Ah, the webs these hormones weave!

Ethylene: Ethylene seems to be the only hormone that doesn’t govern growth of tissues. This hormone travels through the air–noticed that all the other hormones stick to the tissues. This stuff goes everywhere. Ethylene is in charge of ripening fruit and the loss of leaves during the change of seasons. Have you ever heard of putting unripened fruit in a bag with half an apple? When fruit gets damaged or ripe, it gives off ethylene. Ethylene causes ripening, so putting a sliced apple with some unripened fruit causes the unripened fruit to ripen quickly. This is also why you don’t want bruised or over ripened fruit with fruit you don’t want to ripen too quickly–the ethylene will cause everything to ripen right up.

Now that we know everything there is to know about these five hormones, back to our question:

Which of the following plant hormones hastens apple ripening?

A) Auxin
B) Gibberellin
C) Abscisic acid
D) Cytokinin
E) Ethylene

Since we know that the first four are in charge of tissue growth, that leaves ethylene as our answer. Yay!

2 thoughts on “Plant hormones”

  1. Hello, as you can see this is my first post here.
    Hope to receive any help from you if I will have some quesitons.
    Thanks and good luck everyone! 😉

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