EFFECTS OF EXOGENOUS APPLICATION OF HORMONES ON APICAL DOMINANCE IN THE MUNG BEAN (VIGNA RADIATA) PLANTS

Barnes, Manuel Miguel, Garcera, Acielle Angeli, and Manzanares, Rianna Institute of Biology, College of Science, University of the Philippines, Diliman, QC

ABSTRACT

This experiment, patterned from the classical experiments on auxin and cytokinin actions, aimed to determine the roles of the said hormones in apical dominace. Various set-ups of V.radiata seedlings were applied with IAA (auxin), kinetin (cytokinin) or a combination of both. The expected results manifested in some of the set-ups while in some, the opposite results were encountered. Instead of the seedlings with decapitated apical meristems that were applied with IAA exhibiting suppressed lateral bud development, high lateral bud growths were observed. The control also exhibited lateral bud growth. There were experimental results however that also aligned itself with the theoretical results. Intact plants whose lateral buds were applied with kinetin exhibited lateral bud growth as expected. Also seedlings whose decapitated apical meristems were applied with IAA and kinetin inhibited lateral bud growth as expected. The erroneous results were attributed to the length of time it took to make the final observations and to the factors such as greenhouse conditions which were beyond the experimenter’s control. It was suggested that measures be taken to ensure the reliability of the conditions the seedlings were exposed to and to utilize other plants apart from V. radiata.

I. INTRODUCTION

Apical dominance is one phenomenon that is studied regarding the coordination

of bud development in plants (Hopkins, 1999). As the shoot of a plant grows and the

apical meristem grows new leaf primordial, some cells on the angle between the stem and

the leaf primordium become separated from the apical meristem in its growth, and this

produces an axillary or lateral bud (Hopkins, 1999). But in most plants, the growth of

these lateral buds is arrested at an early stage, causing the bud to fail to grow (Hopkins,

1999). Removal of the apical meristem, however, stimulates the axillary buds to resume

their growth, and this is a common horticultural technique used for producing bushy

plants (Hopkins, 1999). This goes to show that the apical bud is able to exert a dominant

influence on the lateral buds, suppressing their cell division and growth (Hopkins, 1999).

Like other plant physiological processes, the phenomenon of apical dominance is

also controlled by plant hormones. In this study, two hormones were considered as

factors for apical dominance in the young plants of mung bean (Vigna radiata), namely,

indole acetic acid (IAA), which is an auxin, and kinetin, a cytokinin.

In 1934, K. V. Thimann and F. Skoog demonstrated that the hormone auxin can

substitute the presence of an apical meristem and, thus, still maintain apical dominance

despite the decapitation of the shoot tip (Hopkins, 1999). The most widely accepted

theory that explains the ability the shoot apex to suppress axillary bud development is

that the concentration of auxin for optimum axillary bud growth is much lower than that

for the elongation of stems (Hopkins, 1999). From the shoot apex flows out an abundant

stream of auxin towards the base of the plant, and this maintains an inhibitory

concentration of auxin at the region of the lateral bud (Hopkins, 1999). Removing this

supply of auxin would, of course, reduce the supply of auxin in the lateral bud area, and

relieve this bud of the inhibition to its growth (Hopkins, 1999).

Cytokinins were also shown to control the apical dominance phenomenon in a

plant, as they antagonize the effect of auxins (Hopkins, 1999). In several species,

application of cytokinins directly to the shoot apex or axillary bud will release the bud

from inhibition (Hopkins, 1999). In one study, tomato plants exhibiting strong apical

dominance were shown to have lower cytokinin concentrations than those with normal

dominance (Hopkins, 1999).

Other hormones such as abscisic acid (ABA) and ethylene also have effects on the

apical dominance of a plant. Experiments have proven that the concentration of ABA in a

lateral bud is under control of IAA moving down from the apex (Hopkins, 1999). The

application of ABA to the shoot apex releases the lateral buds from inhibition (Hopkins,

1999). Ethylene, on the other hand, is stimulated by auxin, but the studies regarding its

effects on apical dominance have obtained inconclusive results, and so the mechanism by

which it influences apical dominance remains undefined (Hopkins, 1999).

This study aims to investigate the effects of exogenously applied IAA and kinetin

on the apical dominance of the mung bean plant (Vigna radiata).

II. MATERIALS AND METHODS

Mung bean (Vigna radiata) seeds were germinated from seeds into young plants,

of which thirteen were transplanted into plastic cups with loose soil, one plant in each

cup. The treatments used in this study, using two plant set-ups per treatment, were the

following:

Labels Treatments A

(A1 & A2) Intact plants drops of kinetin + ethanol + Tween20 applied to lateral bud

B (B1 & B2)

intact plants two drops of ethanol + Tween20 applied to lateral bud

C (C1 & C2)

decapitated at apical meristem lanolin paste + two drops of kinetin applied to decapitated surface

D (D1 & D2)

decapitated at apical meristem lanolin paste + IAA (Indole Acetic Acid) applied to decapitated surface

E (E1 & E2)

decapitated at apical meristem lanolin paste + IAA + two drops of kinetin applied to decapitated surface

Control No treatments were applied Table 1: Treatments and Set-ups

Three plant set-ups were left untreated to be used as a control set-up. The plants

were treated for a second time after three days in the greenhouse and were set aside again

for another four days. Observations and measurements were done a week after the first

treatment, according to the following parameters: 1) length of the lateral bud of the first

trifoliate of each seedling, and 2) fresh weight of lateral buds.

III. RESULTS AND DISCUSSION

Below is a summary of the initial morphological observations of the mung bean plants.

Treatment & Replicate Observation A1 4 fully expanded trifoliate; 1 trifoliate w/ 1 leaf abscissed A2 2 fully expanded trifoliate B1 6 fully expanded trifoliate; 2 trifoliate w/ 1 leaf abscissed B2 3 fully expanded trifoliate; 1 trifoliate w/ 1 leaf abscissed C1 not very tall; 3 expanded trifoliate; 1 juvenile but

expanded (apical)-cut C2 3 expanded trifoliate-1 cut (apical); violet spots on stem

of youngest trifoliate D1 2 older leaves at base; 2 expanded trifoliate D2 4 expanded trifoliate-1 cut (apical) E1 3 expanded trifoliate; 1 leaf destroyed in 1 trifoliate; 1

pair of old leaves at base; looks broken E2 5 expanded trifoliate; 1 pair of old leaves at base

Table 2: Initial morphological observations of plant seedlings

From the information above, it is clearly shown that the plants are of somehow

normal conditions before they were subjected to the previously mentioned treatments.

Although it cannot be avoided that there were already some aberrations as to the state of

the plant, this were still insignificant to the effects of the applied hormones.

The following table summarizes the average values of the parameters observed

after the plants were exposed for one week to the treatments.

Treatments Length* (cm) fresh weight** (g) Leaf emergence (number of trifoliates)

A 4.85 0.1401 1 B 2.4 0.0472 1 C 0.65 0.0239 1 D 2.6 0.0443 0 E 0.6 0.0342 0

Control 7 0.1350 0.5 Table 2: Final observations (*length of lateral buds of first trifoliate;**fresh weight of lateral buds)

From the table above, it can be observed that the intact plants (A &B) have longer lateral

bud lengths when kinetin was applied (A). Amongst the plants whose apical meristems were

decapitated (C, D & E), lateral buds were longest in those whose apical meristems were applied

with IAA (D) and shortest in those whose apical meristems were applied with both IAA and

kinetin. It can also be observed that the control had the longest lateral bud length. The lateral bud

of the intact seedlings, which was that applied with kinetin also exhibited the heaviest fresh

weight while the seedlings whose apical meristems were removed and applied with kinetin were

the lightest. There were no new trifoliates in seedlings exposed to treatments D & E while those

applied with treatments A, B and C had one each. Only half of a trifoliate leaf developed in the

control seedlings.

The following graph summarizes the length of the lateral buds exposed to the different

treatments. It shows the relationship between the different treatments (x-axis) and the lateral bud

lengths, their fresh weights and the leaf emergence in the seedlings. The graph clearly shows the

observations mentioned previously.

Figure 1: Graph summarizing the relationships of the parameters measured

The growth of lateral buds of plants is suppressed with the presence of auxin in its apical

meristem – a phenomenon termed apical dominance. This was an adaptation developed by plants

to ensure their survival in case some grazing animals manage to decapitate their apical meristem.

The full action of the hormone auxin, demonstrated in the experiment by IAA, can be fully

appreciated by decapitating the apical meristems and replacing it with lanolin infused with IAA.

The expected result would be to promote apical dominance and suppress lateral bud development.

It is therefore expected that treatment D would still have minimal lateral bud growth. However

the reverse happened in the experiment. Amongst the seedlings whose apical meristems were

decapitated, treatment D exhibited the longest lateral buds. This result should be expected if

cytokinins, modeled by kinetin in this experiment, are applied to the lateral buds of the same plant

thus creating an imbalance in the auxin-cytokinin concentration in favor of cytokinin. It has been

known that a supply of cytokinin in lateral buds promotes its growth even if the terminal bud is

present. This is proven by the results of treatment A where the lateral bud was applied with

kinetin and it exhibited the longest lateral bud length even though the plant has intact apical

meristems. The cytokinin concentrations outweigh the auxin concentration and this led the lateral

bud to develop. According to Cline (1997), increases in the length of the lateral bud can be

detected in some species within hours of apex removal. This may have been the case for the

plants exposed to treatment D. Auxin may have easily been depleted from the apex that even if

IAA was applied to it, the presence of cytokinins in the lateral buds was far greater. This would

lead the lateral buds to develop and along the way they may acquire their own auxins and their

development as the new apical meristem may commence. Treatment C would be expected to also

exhibit a large lateral bud development due to the suppression of auxin in the apical meristem.

The apical meristem, having been cut has also been supplied with kinetin. This would further

increase the cytokinin concentrations and throw the auxin-cytokinin equilibrium off balance. This

was not the observed case however, as treatment C exhibited one of the lowest lateral bud

lengths. This may again be due to the fact that the time frame for observing the effects of the

treatments was rather long thus affecting the actions of the hormones. The application of

combined IAA and kinetin in the seedling’s apex should have also prevented lateral bud

development. There is sufficient balance in their concentrations that would lead each hormone to

perform its function. This was observed in the actual results since treatment E did exhibited one

of the lowest lateral bud lengths. The control is also supposed to exhibit a low lateral bud

development but the reverse was true for the experiment.

V. CONCLUSIONS AND RECOMMENDATIONS

There could have been other intervening factors beyond the researches control that might

have given rise to such deviant results. Examples would have been greenhouse conditions and the

quality of the germinated seedlings. It is suggested that the next experiments employing the same

concepts should have measures that would sufficiently ensure the quality and reliability of the set-

ups. If the time and resources would also permit the utilization of a different plant, then that

possibility should also be explored to acquire a variety of data unlimited by the inherent

characteristics of V. radiata seedlings.

Apical dominance is regulated by the combined actions of auxin and cytokinins. Their

balance is important in preventing lateral buds from developing and ensuring the apical growth of

plants. Modifications in this arrangement can either induce or suppress lateral bud development,

and this kind of experimentation has been helpful in several industries such as the production of

bushy Christmas trees and production of plant sculptures that has been profitable for many artistic

people. Plant hormones work either synergistically or antagonistically with each other and the

auxin-cytokinin interaction is one of the antagonistic hormone interactions that produce profitable

and aesthetically important plants in communities and establishments worldwide.

V. LITERATURE CITED

Hopkins, WG. 1999. Introduction to Plant Physiology. 2nd edition. John Sons, Inc. Cline, G. M. 1997. Concepts and Terminology of Apical Dominance. American Journal of Botany 84(9): 1064-1069 http://plantphys.info/apical/apical.html (accessed October 1, 2007) http://www.cnr.vt.edu/dendro/forestbiology/htmltext/chapter5.htm (accessed October 2, 2007)