[00:00.00]W: Several of the processes related to growth and aging in plants involve the effects of hormones.[00:06.37]Today I’ll talk about two aging processes—the ripening of fruit and the falling of leaves in autumn.[00:13.43]These processes involve a hormone called ethylene.[00:18.07]What is ethylene? [00:20.50]It’s a chemical, a gas produced by plants as a hormone.[00:25.07]Ethylene is unique among plant hormones because it’s the only hormone that’s a gas.[00:31.41]The gaseous state is important because it enables ethylene to move through the plant.[00:37.69]Like all other hormones, ethylene triggers certain responses in the plant.[00:42.86]Ethylene is associated with a variety of aging processes.[00:47.44]One example of the aging process is the ripening of fruit.[00:52.26]The ripening of fruit involves several changes in structure and metabolism. [00:57.82]Some of these changes are aging processes.[01:00.91]One change is the weakening of cell walls, which softens the fruit.[01:06.29]Another is the decrease in chlorophyll content, which causes the fruit to lose its greenness.[01:12.38]Ethylene initiates or hastens both of these changes and also causes some ripened fruit to drop from the plant.[01:20.84]A chain reaction occurs during ripening, as ethylene triggers aging.[01:26.43]The aging cells then release more ethylene. [01:29.84]Because ethylene is a gas, the signal to ripen spreads from fruit to fruit.[01:35.24]You’ve heard the expression “One bad apple spoils the whole lot.”[01:39.24]Well, it really is true! Yes, Randy?
[01:42.97]M: So, when we buy fruit, like peaches that are still kind of green, and we store them in a plastic bag, and the peaches ripen faster—is this because of ethylene? [01:54.56] I mean, I think I get it. The bag traps the ethylene gas, and this speeds up the ripening.
[02:00.49]W: Right. That’s exactly what happens.[02:02.97]Ethylene accumulates inside the plastic bag, and this starts a chain reaction. [02:08.24]The signal to ripen spreads from one peach to another, and this causes all of the peaches to ripen faster.
[02:14.80]By the way, the role of ethylene was discovered in sort of an interesting way. [02:20.13]A century ago, citrus growers used to ripen fruit by “curing” it in sheds that were heated by kerosene stoves.[02:27.95] The growers thought it was the heat from the stoves that ripened the fruit. [02:31.93] However, they later discovered that the fruit didn’t ripen after they started using newer, cleaner-burning stoves.[02:39.13]Botanists figured out that the reason the fruit ripened in the sheds with kerosene stoves was because of ethylene—not heat—because, as it turns out, ethylene is a by-product of kerosene combustion.[02:52.85]Nowadays, a lot of fruit is ripened in huge storage containers that have ethylene gas piped in.[03:00.58]It’s a new variation on the old curing shed. [03:04.19]But in other cases, storage facilities have to slow down the ripening caused by natural ethylene. [03:10.54] For example, apples are stored in bins that are flushed with carbon dioxide. [03:15.94] Circulating the air with carbon dioxide prevents ethylene from accumulating, and carbon dioxide slows down the action of whatever ethylene isn’t flushed away.[03:26.85]This is how apples can be stored for several months before they’re shipped to supermarkets.
[03:32.23]Another aging process in plants where ethylene plays a role is the falling of leaves in autumn.[03:38.89]When a leaf falls from a tree, the breaking point is a layer near the base of the leaf’s stalk.[03:46.54]A change in the balance of two hormones—ethylene and auxin—controls the falling of leaves.[03:54.11] An aging leaf produces less and less auxin, a hormone that stimulates cell growth.[04:00.90] The drop in auxin makes the cells of the breaking layer more sensitive to ethylene.[04:06.54] As the influence of ethylene becomes stronger, the cells start producing enzymes that weaken the cell walls of the breaking layer.[04:15.54]Finally, with the help of wind or rain, the leaf breaks away at that layer and falls from the tree.
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