More than only nutrients, plant hormones determine growth and development of plants

General perception? is this that the plants need light, water, oxygen and nutrition as extrinsic factors to grow and develop. Is that all ? Plant growth depends on intrinsic factors? like intracellular genes or intercellular chemicals. These chemicals are called Plant Growth Regulators which are small and simple produced naturally by plants to regulate their growth and development. These PGRs or phytohormones are closely associated with elements that plant derive from mineralisation.?

It has been quite a customary for tea planters to apply Boron at seasonal dormancy of Tea to induce new leaf growth critically important for crop flow. Boron (B) is associated with several phytohormones, including?auxin, cytokinin, brassinosteroids, and Jasmonic acid. Deficiency of Boron is often felt in dry conditions after leaching out during? a heavy spell of monsoon or due to poor Organic content in soil. Point to be noted is this that application of B often induces new flush by rapid cell division at epical meristems and this is possible due to its ability for Auxin and Cytokinin promotion.

There are some nutrients that promote strong vegetative growth, and there are others that drive strong reproductive development. There are four nutrients that drive strong vegetative growth: nitrate nitrogen, potassium, chloride promoting Auxin formation while the 4th one in the league, Calcium promotes cytokinin formation.? Essentially every other nutrient, excluding those four, will trigger a slight reproductive response in plants. But there are three in particular that are prime movers of the reproductive category, helping drive bud initiation. These three are manganese, phosphorus and ammonium.

Of the four vegetative nutrients, three have a synergistic relationship with auxin. They drive rapid vegetative development, and they drive auxin formation. Those three nutrients are nitrate, potassium and chloride. When a plant has a generous supply of nitrate, potassium and/or chloride, this can trigger the response I just described. The plant has very rapid growth, but with auxin dominance. And because you have auxin dominance, you have very wide internode spacing — internodes that are six to eight inches apart or longer, depending on what plant and crop we’re talking about.

But the fourth of the vegetative nutrient drivers, calcium, does not have a synergistic relationship with auxin. Instead, it has a synergistic relationship with cytokinin. This, then, is the key. One can achieve the same amount of shoot extension when use calcium is used to drive vegetative growth as use of nitrate or potassium or chloride.

There is both vegetative growth and reproductive growth happening all the time in every single cell within the plant. There is never a complete dominance of one function over the other —there is a relatively sensitive fulcrum balance between the two and hence a slight change? can yield spectacular shift between vegetative or reproductive growth.Because there is both vegetative and reproductive growth happening all the time, very small nutrient applications can trigger a switch from one side of the balance to the other.?It is indeed possible to switch a plant from being vegetatively dominant to reproductively dominant with a single foliar application of manganese and phosphorus — the nutrients that have a very strong reproductive trigger.

Auxins drive vegetative growth, and they are produced in two locations inside the plant. First, they’re produced in growing shoot tips — the apical meristems. As the shoot tip is growing, it is producing auxin. The second place auxin is produced is in the developing seed. Auxin is a sugar magnet. Wherever auxin is in the highest concentrations, that is where sugar goes. Photosynthesis in the leaves of the plant produces sugars, and those sugars move out of the leaves in the afternoon and evening toward the sugar sinks — and those sugar sinks are determined by auxin concentration. So, when there is high auxin concentrations in developing seeds, or in the new shoots, that’s where most of the sugar is going to go.

Now, the interesting thing is that auxin, when it’s produced in the shoots or in the seeds — but particularly in the shoots — will move out of the shoot, down through the plant, and out through the root system to the growing root tips. And when it gets to the growing root tips, it shuts down cytokinin production.?

Cytokinins, unlike auxins, drive reproductive growth, and they are produced in growing root tips. This means that we need to have growing root tips every single day for new cytokinin production to occur — in order to maintain a balance between cytokinins and auxins. When cytokinins are produced in growing root tips, they then move to the upper part of the plant, and they slow down shoot growth. They slow down vegetative shoot growth, and they trigger reproduction.

So, these two hormones, cytokinins and auxins have an antagonistic relationship. They’re constantly competing. Each is trying to be dominant inside the plant. Auxins are trying to be dominant — providing vegetative growth and suppressing cytokinin formation. Cytokinins are trying to be dominant — providing reproductive growth and slowing down shoot growth and auxin formation.?

As the seed and fruit begins developing, that fruit becomes the dominant auxin source, and it becomes the dominant sugar sink. So, most of the sugars that are being produced are moving into the fruit. This is appropriate ?as long as not so much sugars are moving into the fruit that they are sabotaging root growth and sabotaging shoot growth.?In other words, when fruit is developing, roots and shoots begin to receive less sugar. Sugar is always going to move into the fruit because the plant has a desire to reproduce.

For annual plants in particular when the root system no longer gets enough sugars in every 24-hour photoperiod, it no longer has the energy to keep growing. Root growth declines, or in many cases stops entirely. But the plant still needs cytokinin production, which takes place in growing root tips. So, when the growing root tips stop, there is halt the production of cytokinins. At this point, there are no cytokinins moving to the upper parts of the plant to trigger the initiation of reproductive buds.

This is the first indication that the root system has gone into decline. There is no longer any active root growth because sugars are all moving into the fruit and are no longer moving down to the root system. This also triggers a lot of root disease susceptibility. Challenges with?Phytophthora?or?Rhizoctonia?or?Anthracnose, or any number of various soilborne diseases, are exacerbated and begin developing quickly when you reach this stage of fruit development because the plant is not producing enough sugars to sustain both the fruit and the root system.?This doesn’t need to be the case, though. This is what is common — this is unfortunately normal — but it isn’t inevitable. Plants have the genetic capacity to photosynthesize to much higher levels and to be able to fill a fruit load and simultaneously sustain constant root growth and root development throughout the entire fruit-fill period.?

Plant Growth Regulators are either Plant Growth Promoters that promote cell division, cell enlargement, flowering, fruiting and seed formation (auxins, gibberellins and cytokinins) or Plant Growth Inhibitors that? inhibit growth and promote dormancy and abscission in plants. An example is an abscisic acid.

PGRs are required in tiny fragments in plants for critical and profound works what happens if they are applied in extra doses? Synthetic Auxin , a growth promoter kills plants in form of 2-4-D ;? ‘bakane’ disease of rice seedlings is caused by?the fungal pathogen?Gibberella fujikuroi,? the active substance causing the disease was identified as gibberellic acid, the growth promoter.

Gibberellins Increase the axis length in plants, delay senescence and help fruits to elongate and improve their shape.

Cytokinins help in the formation of new leaves and chloroplast., promote lateral shoot growth and adventitious shoot formation., help overcome apical dominance, promote nutrient mobilisation which in turn helps delay leaf senescence.

Abscisic Acid regulate abscission and dormancy, inhibit plant growth, metabolism and seed germination, Stimulates closure of stomata in the epidermis, increases the tolerance of plants to different kinds of stress and is, therefore, called ‘stress hormone’. Important for seed development and maturation. It induces dormancy in seeds and helps them withstand desiccation and other unfavourable growth factors.

Ethylene Affects horizontal growth of seedlings and swelling of the axis in dicot seedlings. Promotes abscission and senescence, especially of leaves and flowers. Enhances respiration rate during ripening of fruits. This phenomenon is ‘respiratory climactic’. Increases root growth and root hair formation, therefore helping plants to increase their absorption surface area.

To summarise, one or the other plant growth regulator influences every phase of growth or development in plants. These roles could be individualistic or synergistic; promoting or inhibiting. Additionally, more than one regulator can act on any given life event in a plant. Along with genes and extrinsic factors, plant growth regulators play critical roles in plant growth and development. Factors like temperature and light affect plant growth events (vernalisation) via plant growth regulators.

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