4. Role of the plant hormones for the development and maturation of the fruit
Like animals, plants have DNA and hormones. Plant hormones (phytohormones) are crucial for the development of the plant during all its life, including during fruit growth.
Plant hormones are tiny chemical messengers that help the plant grow, develop and protect itself against a stressful environment and attacks from bacteria, fungi, animals. We speak of abiotic and biotic stresses.
The hormones are essential for all aspects of plant life. Here we will focus only on fruit development. There are five crucial hormones responsible for fruit and seed development: Abscisic acid (ABA), Auxin, Cytokinin, Ethylene, Gibberellins.
Abscisic acid (ABA) is also known as the stress hormone. It helps fight against drought stress and high temperatures by closing the stomata, those tiny pores by which the plants are breathing. So the water is not evaporating. For the same reasons, ABA is essential for seed dormancy and desiccation, where the seed loses its water to support long storage.
Auxin (IAA, Indole-3-acetic acid) is known as a growth hormone. It affects every aspect of plant development. It is used in the rooting powder in gardening because it promotes root growth. Auxin promotes cell elongation. So it helps the plant to respond to gravity and light. We speak of tropism. Have you ever notice that the leaves and shoots of the plant at home are turning toward the windows? This is because of auxin! At high concentrations, it is also used as an herbicide.
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Cytokinins work together with auxin during plant development. Cytokinin is important for senescence (plant aging). It promotes shoot growth. It is an essential hormone for the clonal propagation of plants in breeding processes.
Ethylene is a gaseous hormone. It controls fruit ripening, flower senescence, and sex determination in some species. It also helps the plant to respond to some stresses. Ethylene promotes elongation of the roots and the shoots. Did you know that the powder you receive with a flower bouquet in the flower shop containing an inhibitor of ethylene? It helps to preserve your flowers fresh for a longer time!
Gibberellins (GA) are essential for growth, seed germination, fruit growth, and sex determination in monoecious species (see Chapter 1). The semi-dwarf species developed by Norman Borlaug (see Chapter 3) are producing less GA. So their stems are shorter, and the plants can produce more seeds.
Two phases during fruit development, two sets of hormones
I guess you noticed that, in general, the fruit is much bigger than the flower. This is the case for tomatoes, cucumbers, apples,…
First, the fruit will expand and grow. For this, the fruit needs some auxin and GA. But some fruits are different. For example, strawberries will only use auxin, while grapes will only need GA.
Fruit growth depends on the fertilization of the ovules. The seeds produce auxin. It moves to the fruit and stimulates GA production in the fruit. Together they promote fruit growth. This is why not all the flowers are producing fruits, especially if they were not pollinated!!
However, nowadays, we can find fruits without any seeds. We say the fruits are parthenocarpic. For example, bananas are sterile. The fruit is developing, but not the seeds. They are those tiny black dots in the middle of the banana. The plant is propagating vegetatively with its roots.
How to produce banana seeds
Then, fruits are ripening. This is the maturation process. The fruit is becoming softer. Flavour and colors are changing. It is mainly the work of ethylene. At the same time, the seeds accumulate ABA to accumulate reserves of nutrients. ABA also helps the seed to survive the desiccation process. Then the seeds become dormant, waiting for good growth conditions to germinate.
Climacteric versus non-climacteric fruits
Some fruits ripen after being harvested. The fruit will become softer and sweeter. These fruits are called climacteric fruits. Apple, apricot, avocado, banana, tomato, kiwi, pear, peach are climacteric.
Others won’t mature after harvest. They are non-climacteric fruits. Cucumber, eggplant, grape, orange, raspberry, strawberry are non-climacteric.
Climacteric fruits are harvested when they are not yet fully ripened. They are transported in controlled-atmosphere storage conditions. It means low oxygen (O2) and high carbon dioxide (CO2). These conditions suppress the production of ethylene by the fruits. So the fruits are not overripe when reaching the shop shelves.
„Cover the petiole of your bananas with plastic wrap! It will slow down the ripening, and your bananas will stay green longer.“
„Help climacteric fruits to ripen faster by putting them close to a mature banana or apple!“
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