So what do plants smell? So let me describe to you some phenomena that we've known through history. In ancient China, for example, the Chinese peasants would burn incense in a room full of pears in order to get them to ripen. They were sure that they were, you know, that the incense had some magical property. Maybe through the Gods, that help the fruit ripen. Similarly, in the early 20th century, citrus farmers would ripen oranges in sheds that were heated with kerosene heaters. So what's common to these two scenarios? In China, they were picking the pears that weren't ripe, putting them in a room, lighting the incense. And in 2000 years later in Florida, they're picking unripe citrus fruit, keeping them in a shed, turning on a kerosene heater and then the fruit ripens. The lemons or the oranges ripen. What's similar in these two scenarios? Alright, well maybe you consider that it was heat, but then imagine the surprise that, you know, a few years later in Florida when electricity, or when electric heaters came into use, the citrus ripening sheds that put in electric heaters were useless in causing the oranges or lemons to ripen. What this showed that it wasn't the heat that was causing the fruit to ripen, it was actually the smoke itself. And in 1924, a USDA scientist discovered that within the smoke, there's one particular volatile chemical which is called ethylene. And that it's ethylene which is inducing the fruit ripening. So, here's the structure of ethylene. You could see it's a very small molecule, two carbons, four hydrogens. and it's a gas, of course. It diffuses in the air spaces within a plant and around the plant, and it can induce fruit ripening. Actually, plants are so sensitive that they can respond to one molecule within 100 million molecules of air. So in other words, somehow or another the fruit smells the ethylene gas. And then ripens, just sort of like when we smell smoke from a barbecue, we start to salivate. But this also leads to the question, why would fruits respond to this bi-product of kerosene smoke or of incense smoke? What do they care about what we're burning? Well, to answer that question, I think we need to go back to something that we all probably have experienced. If you have an avocado that you've bought in the market, but it's still very, very hard, how can you get this avocado to ripen? Well, I'm sure of you have said, well, you know? You would put it together in a bag with a very ripe banana. So if I would put this avocado together with this banana in this bag, and then come back in a while. We all know that the avocado will be ripe. Why is this? Well, the reason goes back to a finding from the 1930s by Richard Crane. Richard Cane, excuse me, from Cambridge University. And he discovered that ripening apples release ethylene. And actually, all fruits release ethylene as they ripen, naturally. Not from the kerosene smoke, they're making it themselves. And we now know that ethylene is a universal plant hormone found in all plants, necessary for fruit ripening. So, for example, we take a tomato. It gets to its normal size, it grows to its normal size, it's green. Then it starts releasing ethylene. And it's the ethylene gas that caused the green tomato to soften and to start turning red. It is the gas that induces fruit ripening. And that's also why, you know, you've, we've heard the expression, one rotten apple spoils the bunch. So if you have one piece of fruit that's overly ripe, it's giving off so much ethylene, it then induces all the other fruits around it to ripen. And so now if I go back, of course, to my avocado which has now been sitting in this bag with this overly ripened banana, we now have a ripe avocado which is ready to eat. So what we see happening here, for example, in an orange tree, is that the first orange as it matures, it starts releasing ethylene as it's ripening. Its neighboring oranges, we could even maybe call it its brother oranges, will then also be influenced by the ethylene coming from that first orange, which will induce them to ripen faster. They'll ripen and release more ethylene, so that the whole tree starts ripening together. Within the grove, they would also then be influenced by this wave of ethylene coming out of that tree, so that more or less, we have this whole tree and all of the other trees ripening at the same time. Now this begs a question that, what would be the ecological significance of this? Why through evolution, would there be an induced timing of fruit ripening? Can you think of an answer for this? post your answers to the forum and I'll discuss it in a second. Ecologically, we know that colored fruit is a great advertisement for animals, or humans, who want to come pick the fruit to eat it. From the plant's point of view, the plant wants to be eat, have its fruit eaten, that way it can disperse its seeds. So if all of the fruit ripen at the same time, there is an added increased display of color, which is more apt to call the herbivores that, the fruit-eating animals to come and eat the berries, eat the fruit, and then disperse its seeds. So from this point of view, can we actually say that the fruits are talking to each other? I'll deal with this a bit later in today's lecture. But before we go on, I want to give a little bit more information about ethylene. because, if you got the impression that it's only for fruit ripening, it's not. Ethylene is a major hormone that does regulate fruit ripening. But also regulates other plant processes, such as leaf senescence and petal senescence. Senescence is when the leaves age and fall off the off the tree or the plant. For example, what we see here, is cotton plants. That the one on this side has been kept in normal air. The one on the other side has been put in air where ethylene has been pumped into its box. As you can see, all of its leaves have fallen off, which shows how ethylene can induce senescence. this was actually also noticed in 19th century Victorian England, because in Victorian England, all of the lights were powered by gas. Kerosene lamps. And the kerosene lamps, as we learned, give off ethylene. Which had the awful side effect of killing most of the house plants. Because of this, the plant called Aspidistra, which is naturally resistant to ethylene, in other words, ethylene doesnât effect it, became one of the most popular houseplants in 19th century England. Because the ethylene, the by-product of the kerosene lamps, didn't kill it. Because of these qualities in inducing fruit ripening, or inducing leaf or petal senescence, ethylene is immensely important in agriculture. And farmers are looking for ways of actually removing ethylene from fruits that are in transit, or from flowers that are in shops, in order to prolong both fruit life and flower life on the shelf.
