Spring fever: Introducing this hormone-driven spectacular


For those of you who have lived all of your lives here in Hawaii, you probably have never experienced “spring fever.”

While a consensus on what causes spring fever will forever be debated, this much is known: Spring fever is the feeling of renewed energy, enthusiasm and restless behavior normally felt during March and April, but has also been referred to as a period of feeling weakness, fatigue and irritability.

For the gardener, it is a time to get the garden tools ready, finalize the annual planting list and get set for the coming regrowth of plants everywhere.

Spring fever is as much a feeling of anticipation of renewed life as it is a physical condition of enduring a long winter of cold days and short periods of light.

Today, human physiologists think hormones are the primary controllers of symptoms used to describe spring fever.

Cold, short periods of daylight and looking at leafless trees and shrubs are thought to be the hormone triggers.

The coming of spring brings a feeling of joy and renewed activity.

As for the fatigue and irritability, researchers indicated scurvy — a disease that results from a lack of vitamin C because of a the lack of fresh fruits and vegetables during the winter months — is a prime suspect.

In Hawaii, it is beautiful year-round, but hormones play an important role in plant growth and development.

Hormones work in concert with environmental clues to orchestrate the burst of growth associated with spring.

Day length is the primary trigger.

There are five general classes of plant hormones recognized: auxins, gibberellins, cytokinins, ethylene and inhibitors.

Auxin is the hormone associated with plant growth.

Its movement in the plant is only in one direction, away from the shoot and root tips, and is the primary reason for apical dominance.

Apical dominance is the ability of the shoot tip in certain plants to prevent the formation of lateral branches in plants such as ti-leaf, poinsettia and mums.

In these plants with strong apical dominance, the removal of the shoot tip results in a proliferation of side shoots to develop, resulting in a bushy plant.

Another important role of auxins is the development of adventurous roots in cuttings. Synthetic auxins are commonly used in plant rooting powders and liquids.

Gibberellins, or gibberellic acid, are known to control stem elongation by stimulating cell elongation and division. They are also involved in seed germination, flower and fruit maturation, and in breaking the dormancy of seeds, flower buds and leaf buds.

Hence, they are found in higher concentrations in shoot tips, germinating seeds and buds. More than 100 types of gibberellins have been identified, but most have not been characterized beyond this.

Gibberellins, or GAs, are used to influence growth of plants and fruits. In grapes, certain varieties are treated with GA sprays to promote the development of longer, larger fruit. The grape cluster is also elongated, leading to the creation of a looser bunch and easier disease control.

GA is sometimes used to elongate stems, especially those of flowers, to improve appearance or to free flowers from leaf cluster.

Cytokinins and kinetins are associated with cell division and are active in root and shoot tips, developing fruits and seeds, and young leaves.

Young developing coconuts are a good source of cytokinins. Cytokinins usually work with other hormones in controlling plant growth.

In association with auxins, high kinetin and low auxin concentrations favor shoot development, while low kinetin and high auxin favor root formation.

Branching is favored by kinetin, while higher auxin levels prevent branching. Higher levels of cytokinins can delay leaf senescence.

Ethylene is a different type of plant hormone, in that it is a gas, and can have a number of important effects on plants.

Ethylene can be produced in all plant parts because of stress, injury or just by growth. Roots, shoots, flowers and fruits all produce ethylene, and auxin promotes the production of ethylene.

Even ethylene promotes the production of more ethylene. Depending on your point of view, ethylene effects are helpful or harmful.

Bananas are forced to ripen with the use of ethylene, so you will always have ripe bananas to purchase. Green bananas can be ripened at home by using an ethylene source to initiate the process.

A good source is a banana that is changing color, as ripening banana fruit is a good source of ethylene.

Another source you can use is many types of flowers, especially vanda and dendrobium orchids. Place a few freshly picked flowers into a paper bag along with the green bananas and seal the bag closed. Leave the bag in a cool location for two to three days and check on the fruit.

Bananas will ripen sooner than if left on the counter.

Ethylene also can cause the premature fading of flowers and senescence of leaves.

The final hormone is a group referred to as the inhibitors; abscisic acid and phenolic-based inhibitors.

As the name implies, the function of these hormones is to inhibit a plant function, and in this case, it is cell growth and division.

Inhibitors are responsible for plant dormancy, as preparing plants for winter. Inhibitors prevent seeds from germinating at inappropriate times and assist in the survival of the species under natural conditions.

Under drought stress, abscisic acid increases, resulting in the closing of the stomata in the leaves and reducing water loss because of transpiration.

For more information on this and other gardening topics, please visit the CTAHR electronic publication website at http://www.ctahr.hawaii.edu/Site/Info.aspx or visit any of the local Cooperative Extension Service offices around the island. I can be reached at russelln@hawaii.edu.

This information is supplied by the University of Hawaii College of Tropical Agriculture and Human Resources.

 

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