Dr. Lew Feldman, Garden Director
In previous IGYAs, we discussed two major plant signaling molecules: auxin and gibberellin. In this third write-up on plant growth and development controls, we will consider the molecule ETHYLENE, agriculturally and commercially the most important plant signaling molecules. Unlike auxin and gibberellin, which are moved about the plant via internal cell pathways, ethylene is a gas. It affects plant development by moving through the atmosphere, from where it is made in a plant, to other parts of the same plant, or to other plants. Although ethylene’s effects have been known for hundreds, if not thousands of years, the modern discovery of ethylene can be traced to the use of gas for illuminating Victorian homes. It was found that most plants in rooms illuminated by gas lights died; flowers discolored, petals were lost, and leaves fell off. When the gas was analyzed it was found that ethylene was the component causing the damage to the plants. We now know that ethylene is a naturally occurring plant signaling molecule, having many effects on plant growth and development. Commercially, the most important process in plants affected by ethylene is fruit ripening. As fruits develop, they naturally produce a small amount of ethylene. For some fruits, ripening is triggered by or coincident with an increase in ethylene production in the fruit, including apples, bananas, kiwis, pears, persimmons, and tomatoes. Avocados, too, are caused to ripen by ethylene, but ripening will only occur after the avocado is picked, which is why avocados will not ripen if left on the tree. Picking an avocado induces damage where the fruit is severed from the branch, stimulating ethylene production and ripening of the avocado.
Unpicked avocados make little ethylene (Garden Betty).
Picking an avocado wounds (red arrow ) the fruit and stimulates ripening (Foodie Crush).
This observation that wounding induces ethylene underlies the saying that “one rotten apple spoils the barrel.” Meaning, if one apple is damaged in a barrel, it will produce ethylene, diffusing to all other apples in the barrel, causing them to ripen and spoil rapidly.
Knowing that ethylene can regulate fruit ripening has important economic consequences. Bananas, as most of us know, are picked green. When they arrive at the greengrocer, she/he can initiate ripening by treating the fruits with compounds that activate ethylene production.
Comparing ethylene-induced (chemically ripened) bananas with “naturally” ripened Bananas (Twitter).
Ethylene gas used for ripening fruits (Youtube).
It is also beneficial for produce managers to be able to delay ripening. For this to occur, the bananas (and other fruits) are stored in an environment in which the naturally produced ethylene is counteracted by introducing high carbon dioxide levels (CO2) into the storage atmosphere. At home, you can speed up the ripening of green bananas by placing them is a sealed environment (a bag) into which you also place a cut (damaged) apple, which releases ethylene. But manipulating the storage atmosphere for fruits takes considerable skill. If CO2 levels are too high or not correctly controlled, the fruits can be damaged, as is sometimes seen in apples.
Damage due to high levels of CO2 (WSU-tfrec – Washington State University)
Since ethylene signals the onset of ripening in many fruits, delayed ripening on some plants can be achieved by modifying the plant at the molecular level, thereby affecting its ability to “see” (detect) ethylene. Delayed Ripening Technology has been applied for use in tomatoes, melons, and papaya. Commercially, ethylene is especially important in the pineapple industry. Pineapples are difficult and expensive to harvest because of their knife-like leaves, making manual harvesting challenging. Machinery has been designed for this purpose, but this is only economically feasible if harvesting machinery can be used on fields with many pineapples. Ethylene treatments can promote and synchronize flowering in pineapples, thereby assuring that harvesting machinery can be deployed in pineapple fields having many ripe fruits.
Pineapple fields with ethylene-synchronized flowering (Soundcloud.com).
Ethylene also shortens many flowers’ shelf-life, including carnations, geraniums, petunias, roses, and many other plants. Harvesting flowers wounds the stem, resulting in ethylene production. Thus, often when you purchase cut flowers, you are also provided with a packet of powder, which will prolong the shelf life of the flowers by retarding ethylene production when dissolved in the vase water. Interestingly, aspirin also delays ethylene production, so if your florist neglects to give you a packet to add to the vase water, you can instead substitute an aspirin. An interesting application of Delayed Ripening technology is in floriculture where experiments are underway to apply the technology to delay flowers’ withering.