Effects of plant hormones

Plant hormones play a vital role in regulating growth and development. There are many different types of hormones and their effects depend on several factors like plant type, stage of the plant’s development, amount of hormones and combination of hormones. Most hormones have more than one function eg. the following are responses that auxin is known to cause:

• Stimulates cell elongation 

• Stimulates cell division in the cambium and, in combination with cytokinins in tissue culture 

• Stimulates differentiation of phloem and xylem 

• Stimulates root initiation on stem cuttings and lateral root development in tissue culture 

• Mediates the tropistic response of bending in response to gravity and light 

• The auxin supply from the apical bud suppresses growth of lateral buds 

• Delays leaf senescence 

• Can inhibit or promote (via ethylene stimulation) leaf and fruit abscission 

• Can induce fruit setting and growth in some plants 

• Involved in assimilate movement toward auxin possibly by an effect on phloem transport 

• Delays fruit ripening 

• Promotes flowering in Bromeliads 

• Stimulates growth of flower parts 

• Promotes (via ethylene production) femaleness in dioecious flowers 

• Stimulates the production of ethylene at high concentrations

(Refference: http://www.plant-hormones.info/auxins.htm)

In the observed article (Micropropagation of Alstroemeria x hybrida `juanita‘ ) the effects of different kind of cytokinines and auxines alone or in combination with growth retardant were studied in relation to rhizome branching, arial shoot production and rooting of rhizome.

Alstroemeria x hybrida is a plant which can be reproduced by rhizome. According to that, large numbers of roots per rhizome and many strong shoots is the aim of in vitro grown Alstroemeria x hybrid cultures.

The results of this experiment show that one hormone has more than one effect and in combination with one or several other hormones the results are again different. BAP, a cytokinin, has an effect on shoot length and number of roots per rhizome. A smaller amount of BAP causes greater shoot length and greater numbers of roots per rhizome. Combined with paclobutrazol and flurprimidol, two growth retardants, the shoot length is shorter than with just BAP. When also added NAA, an auxin, to the media the number of roots per rhizome increases rapidly.

According to these results, the best effects are achieved with little BAP (1.5mg/l) combined with NAA and flurprimidol.

Plant hormones are also used as herbicide, which means you have to know what you are doing by adding hormones to a culture. The right amount at the right time is vital for a plant. In nature the plants find the right amount of hormones at the right time themselves. In an in vitro culture the hormone application has to be well studied to success.  Hormones are just needed in a very small amount, in in vitro cultures that can be hard to achieve. In practice there is often a big amount of the whole matrix mixture made so that the amount of hormones in every little plant box is accurate.

Overall, plant hormones can have many different effects and for in vitro cultures the challenge is to get the time, the amount, the type of hormone and the combination of hormones right.