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Biologi - Tingkatan 5 - Bantuan Kerjarumah
 Online Tutoring : Forum : Bantuan Kerjarumah (Tingkatan 5) : Biologi - Tingkatan 5 - Bantuan Kerjarumah
Topic Topic: jenis hormon dan fungsinya Post ReplyNew Question
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Riezz
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Posted: 31 March 2006 at 11:37am | IP Logged Quote Riezz

Tolong terangkan dgn lebih lanjut ttg jenis hormon dlm tumbuhan dan fungsinya....... Saya x berapa faham........

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Ruhil
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Posted: 31 March 2006 at 12:37pm | IP Logged Quote Ruhil

Auxins are a group of plant growth substances (often called phytohormones or plant hormones). Auxins play an essential role in coordination of many growth and behavioral processes in the plant life.

Overview

Auxins have been demonstrated to be the basic coordinative signal of plant development. Their transport throughout plants is complex, and often they also control action of other plant hormones. As a result, a plant can (as a whole) react on external conditions and adjust to them, without requiring a nervous system. They are sometimes referred to as cardinal plant hormones.

The most important member of the auxin family is indole-3-acetic acid (IAA), which is believed to be the most effective native auxin. It generates the majority of auxin effects in intact plants. However, molecules of IAA are chemically unstable, so they can't be used commercially.

Native auxins further include 4-chloro-indoleacetic acid, phenylacetic acid (PPA) and indole-3-butyric acid (IBA). Synthetic auxins include 1-naphthaleneacetic acid (NAA - with nearest effects to IAA), 2,4-dichlorophenoxyacetic acid (2,4D), and others.

Auxins are often used to promote root growth as a main compound of rooting stimulators (beneficial mainly in horticulture for treating of stem cuttings). They are also used to promote uniform flowering, and to set fruit and prevent premature fruit drop.

Used in high doses, it stimulates the production of ethylene, which stops growth and may cause leaves to fall and kill the plant. Some synthetic auxins such as 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) can be used as herbicides. Broad-leaf weeds like dandelions are much more susceptible to auxins than narrow-leaf plants like grass and cereal crops.

Hormonal activity

Auxins coordinate development at all levels of plants, from the cellular level to organs and ultimately the whole plant.

The plant cell wall is made up of cellulose and protein, and, in many cases, lignin. It is very firm and prevents any sudden expansion of cell volume, and, without contribution of auxins, any expansion at all.

On a cellular level

On the cellular level, auxins' presence is essential for both cell division and respective cell growth, resulting usually in its axial elongation. According to the "acid growth theory," auxins may stimulate cell elongation, for example, by causing responsive cells to actively tranport hydrogen ions out of the cell, thus lowering the pH around cells. This acidification of the cell wall region activates enzymes known as expansins, which break bonds in the cell wall structure, making the cell wall less rigid. When the cell wall is degraded (not entirely) by the action of auxins, this now-less-rigid wall is expanded by the pressure coming from within the cell, especially by growing vacuoles. However, this so-called 'acidification-theory' caused by auxin has yet to be proven.

Organ patterns

Growth and division of plant cells result in growth of tissue, and specific tissue growth contributes to the development of plant organs. Growth of cells contributes to the plant's size, but uneven localized growth produces bending, turning and directionalization of organs, for example, stems turning toward light sources (phototropism), growth of roots in response to gravity (gravitropism), and other tropisms.

Organization of the plant

As auxins contribute to organ shaping, they are also fundamentally required for proper development of the plant itself. Without hormonal regulation and organization, plants would be merely proliferating heaps of similar cells. Auxin employment begins in the embryo of the plant, where directional distribution of auxin ushers in subsequent growth and development of primary growth poles, then forms buds of future organs. Throughout the plant's life, auxin helps the plant maintain the polarity of growth and recognize where it has its branches (or any organ) connected.

An important principle of plant organization based upon auxin distribution is apical dominance, which means that the auxin produced by the apical bud (or growing tip) diffuses downwards and inhibits the development of ulterior lateral bud growth, which would otherwise compete with the apical tip for light and nutrients. Removing the apical tip and its suppressive hormone allows the lower dormant lateral buds to develop, and the buds between the leaf stalk and stem produce new shoots which compete to become the lead growth. This behavior is used in pruning by horticulturists.

Uneven distribution of auxin: To cause growth in the required domains, it is necessary that auxins be active preferentially in them. Auxins are not synthesized everywhere, but each cell retains the potential ability to do so, and only under specific conditions will auxin synthesis be activated. For that purpose, not only do auxins have to be translocated toward those sites where they are needed but there has to be an established mechanism to detect those sites. Translocation is driven throughout the plant body primarily from peaks of shoots to peaks of roots. For long distances, relocation occurs via the stream of fluid in phloem vessels, but, for short-distance transport, a unique system of coordinated polar transport directly from cell to cell is exploited. This process of polar auxin transport is directional and very strictly regulated. It is based in uneven distribution of auxin efflux carriers on the plasma membrane, which send auxins in the proper direction.

Locations

  • Synthesized in shoot (and root) meristematic tissue
  • Synthesized in young leaves
  • Synthesized in mature leaves in very tiny amounts
  • Synthesized in mature root cells in even smaller amounts (speculative)
  • Transported throughout the plant more prominently downward from the shoot apices
  • Released by meristematic cells when they are in good growing conditions
  • Released by all cells when they are experiencing conditions that would normally cause a shoot meristematic cell to produce auxin (speculative)
  • Directly or indirectly induced by high levels of ethylene (speculative)
  • Peaks during the day

Effects

  • Stimulates cell elongation (if gibberellins are also present, the effect is stronger)
  • Stimulates cell division (if cytokinins are also present)
  • Induces formation and organization of phloem (and xylem)
  • Participates in phototropism, gravitropism, tropism toward moisture and other developmental changes
  • Induces new root formation by breaking root apical dominance induced by cytokinins
  • Induces shoot apical dominance
  • Directly stimulates ethylene synthesis (stimulation of ethylene in lateral buds causes inhibition of its growth and potentiation of apical dominance)
  • Inhibits (in low amounts) ethylene formation and transport of precursor
  • Inhibits abscission prior to formation of abscission layer (inhibits senescence of leaves)
  • Induces sugar and mineral accumulation at the site of application
  • Stimulates Flower initiation
  • Is sex determinator
  • Inhibits root hair growth and causes them to die back (speculative)
  • Stimulates the rate of metabolism of cells in the root, thus increasing their efficiency of water and mineral uptake(speculative)
  • Indicates when cells have more than enough sugar and gases available than are needed for existence at their present size. It is a shoot health indicator and growth signal, and one of its essential missions is to compliment the excess sugar and gases with an excess of root-derived water and minerals. It therefore induces new roots. If Cytokinin is present, this is an indication that the root is healthy and the plant is completely ready to grow. In this case, it simply cooperates with cytokinin to cause cell division and balanced plant growth. (speculative)
  • Appears in general to be induced at the site of high concentrations of sugar, but always moving in a direction away from this synthesis. Since Auxin attracts nutrients to the cell where it is, this transport of auxin away from sugar synthesis may partly explain the transport of sugar in the phloem to the roots. The sugar may just be following the auxin. (speculative)

Molecular mechanisms of auxin action

Although auxins and their effects have been known for a long time, mechanisms of action in plants have remained unknown for a long time. In 2005, it was demonstrated that the F-box protein TIR1, which is part of the ubiquitin ligase complex SCFTIR1, is an auxin receptor. This marking process leads to the degradation of the repressors by the proteasome, alleviating repression and leading to specific gene expression in response to auxins.

Another protein called ABP1 (Auxin Binding Protein 1) is a putative receptor, but its role is unclear.

Sorry tak dapat nak upload gambar



Edited by Ruhil on 31 March 2006 at 12:52pm
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Posted: 31 March 2006 at 12:50pm | IP Logged Quote Ruhil

Cytokinins are a class of plant growth substances (plant hormones) active in promoting cell division, and are also involved in cell growth, differentiation, and other physiological processes. Their effects were first discovered through the use of coconut milk in the 1940s.


There two kind of cytokinins:

Adenin cytokinins

Examples: kinetin, zeatin, benzyl adenine. The DNA base adenine is a structural analogue of cytokinins and have low cytokinin bioactivity.

Phenylurea cytokinins

Example: N, N'-diphenylurea

Although their chemical compositions differ, there is a structural correlation between adenine cytokinins and urea cytokinin, and both show similar biological activities.

Location, Characteristics and Occasions for Synthesis Induction

•Synthesized in root and shoot meristematic tissue

•Synthesized in meristematic regions of roots

•Synthesized in mature roots – small amount

•Rapidly transported in xylem stream at the plant level

•At the cell level, transported by purine transporters

•Reduced in plants suffering drought

•Peaks during the day

•Synthesized in mature shoot cells

*Released by meristematic cells when they have enough minerals and water to support   both

•themselves and any dependent cells

Effects

Cytokinins are generally promoting shoot development and inhibiting root development, although they are necessary for cell division in both shoot and root apical meristems.          Released by all cells when they are experiencing conditions that would normally cause a shoot meristematic cell to produce CK

           Directly or indirectly induced by high levels of GA/BA

           Promotes Chlorophyll production and leaf unrolling

           Promotes photosynthesis

           Stimulates cell broadening

           Promotes shoot formation

           Promotes the unloading of sugar from phloem

           Causes the outgrowth of secondary shoot buds – breaks shoot apical dominance/lateral bud development

           Delays leaf senescence

           Stimulates cell division with Auxin

           Participates in morphogenesis

           Promotes stomatal opening (theoretical)

           Induces xylem and phloem

           Directly induces GA/BA at high levels (theoretical)

           Inhibits C4 Photosynthesis

Stimulates the rate of metabolism of cells in the shoot (that are not at their peak metabolism rates) in response to an

           increase in the levels minerals and water (theoretical)

           Inhibits root growth (theoretical)



Edited by Ruhil on 31 March 2006 at 12:53pm
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Ruhil
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Posted: 31 March 2006 at 1:03pm | IP Logged Quote Ruhil

Ethylene

 

General

Systematic name

Ethene

Molecular formula

C2H4

SMILES

C=C

Molar mass

28.05 g/mol

Appearance

colourless gas

CAS number

[74-85-1]

Properties

Density and phase

1.178 g/l at 15C, gas

Solubility in water

Insoluble

Melting point

−169.1 °C

Boiling point

−103.7 °C

Structure

Molecular shape

planar

Dipole moment

zero

Thermodynamic data

Std enthalpy of
formation
ÄfH°gas

+52.47 kJ/mol

Standard molar
entropy
S°gas

219.32 J·K−1·mol−1

Hazards

MSDS

External MSDS

EU classification

Very flammable (F+)

 

R-phrases

R12, R67

S-phrases

S2, S9, S16,
S33, S46

Flash point

Flammable gas

Explosive limits

2.7–36.0%

Autoignition temperature

490 °C

Supplementary data page

Structure and
properties

n, år, etc.

Thermodynamic
data

Phase behaviour
Solid, liquid, gas

Spectral data

UV, IR, NMR, MS

Related compounds

Other alkenes

Propene
Butene

Related compounds

Ethane
Acetylene

Except where noted otherwise, data are given for
materials in their
standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references

Ethylene (or IUPAC name ethene) is the simplest alkene hydrocarbon, consisting of four hydrogen atoms and two carbon atoms connected by a double bond. Because it contains a double bond, ethylene is called an unsaturated hydrocarbon or an olefin.

The molecule cannot twist around the double bond, and all six atoms lie in the same plane. The angle made by two carbon-hydrogen bonds in the molecule is 117°, very close to the 120° that would be predicted from ideal sp2 hybridization.

Nomenclature

From 1795 on, ethylene was referred to as the olefiant gas (oil-making gas), because it combined with chlorine to produce the oil of the Dutch chemists (ethylene dichloride), first synthesized in 1795 by a collaboration of four Dutch chemists.

In the mid-19th century, the suffix -ene (a Greek root added to the end of female names meaning "daughter of") was widely used to refer to a molecule or part thereof that contained one fewer hydrogen atoms than the word being modified. Thus, ethylene (C2H4) was the "daughter of ethyl" (C2H5). The name ethylene was used in this sense as early as 1852.

In 1866, the German chemist Augustus von Hofmann proposed a system of hydrocarbon nomenclature in which the suffixes -ane, -ene, -ine, -one, and -une were used to denote the hydrocarbons with 0, 2, 4, 6, and 8 fewer hydrogens than their parent alkane[1]. In this system, ethylene became ethene. Hofmann's system eventually became the basis for the Geneva nomenclature approved by the International Congress of Chemists in 1892, which remains at the core of the IUPAC nomenclature. However, by that time, the name ethylene was deeply entrenched, and it remains in wide use today, especially in the chemical industry.

Chemistry

The double bond is a region of slightly higher electron density, and most of ethylene's chemistry involves other molecules reacting with and adding across its double bond. Ethylene can react with bromine, chlorine, and other halogens, to produce halogenated hydrocarbons. It can also react with water to produce ethanol, but the rate at which this happens is very slow unless a suitable catalyst, such as phosphoric or sulfuric acid, is used. Under high pressure, and, in the presence of a catalytic metal (platinum, rhodium, nickel), hydrogen will react with ethylene, saturating it.

Production

Ethylene is produced in the petrochemical industry via steam cracking. In this process, gaseous or light liquid hydrocarbons are briefly heated to 750–950 °C, causing numerous free radical reactions to take place. Generally, in the course of these reactions, large hydrocarbons break down in to smaller ones and saturated hydrocarbons become unsaturated.

The result of this process is a complex mixture of hydrocarbons in which ethylene is one of the principal components. The mixture is separated by repeated compression and distillation.

Another process is catalytic cracking where it is used in oil refineries to crack large hydrocarbon molecules into smaller ones. Use of zeolite as a catalyst allows the cracking to be achieved at a lower temperature. It is an important way of separating alkenes from alkanes using a fractionating column.

Theoretical considerations

Although ethylene is a relatively simple molecule, its spectra is considered to be one of the most difficult to explain adequately from both a theoretical and practical perspective. For this reason, it is often used as a test case in computational chemistry. Of particular note is the difficulty in characterizing the ultraviolet absorption spectrum of the molecule. Interest in the subtleties and details of the ethylene spectrum can be dated back to at least the 1950s.

Uses

Chemistry

Ethylene is used primarily as an intermediate in the manufacture of other chemicals, especially plastics. Ethylene may be polymerized directly to produce polyethylene (also called polyethene or polythene), the world's most widely-used plastic. Ethylene can be chlorinated to produce ethylene dichloride (1,2-Dichloroethane), a precursor to the plastic polyvinyl chloride, or combined with benzene to produce ethylbenzene, which is used in the manufacture of polystyrene, another important plastic.

Smaller amounts of ethylene are oxidized to produce chemicals including ethylene oxide, ethanol, and polyvinyl acetate.

Ethylene is also a widely-used refrigerant in commercial low temperature systems due to the low boiling point.

Ethylene was once used as an inhaled anesthetic, but it has long since been replaced in this role by nonflammable gases.

It has also been hypothesized that ethylene was the catalyst for utterances of the oracle at Delphi in ancient Greece.

Ethylene is used in greenhouses and is sprayed on crops to speed ripening.

Ethylene as a plant hormone

Ethylene functions as a hormone in plants. It stimulates the ripening of fruit, the opening of flowers, and the abscission (or shedding) of leaves. Its biosynthesis starts from methionine with 1-aminocyclopropane-1-carboxylic acid (ACC) as a key intermediate.

"Ethylene has been used in practice since the ancient Egyptians, who would gas figs in order to stimulate ripening. The ancient Chinese would burn incense in closed rooms to enhance the ripening of pears. It was in 1864, that leaks of gas from street lights showed stunting of growth, twisting of plants, and abnormal thickening of stems (the triple response)[see plant senescence](Arteca, 1996; Salisbury and Ross, 1992). In 1901, a russian scientist named Dimitry Neljubow showed that the active component was ethylene (Neljubow, 1901). Doubt discovered that ethylene stimulated abscission in 1917 (Doubt, 1917). It wasn't until 1934 that Gane reported that plants synthesize ethylene (Gane, 1934). In 1935, Crocker proposed that ethylene was the plant hormone responsible for fruit ripening as well as inhibition of vegetative tissues (Crocker, 1935). Ethylene is now known to have many other functions as well." - from (plant-hormones.info)

Location, Characteristics and Occasions for Synthesis Induction

  • Directly induced by high levels of auxin
  • Found in germinating seeds
  • Induced by root flooding
  • Induced by drought
  • Synthesized in nodes of stems
  • Synthesized in tissues of ripening fruits
  • Synthesized in response to shoot environmental, pest, or disease stress
  • Synthesized in senescent leaves and flowers
  • Rapidly diffuses
  • Inhibiting effects of ethylene on shoot growth (more specifically on stem elongation) reduced in the presence of light. Also ethylene levels are decreased by light
  • The above may be because light induces auxin synthesis and moderate auxin levels inhibit ethylene. (speculative)
  • Released in mature (and to a lesser extent immature cells) cells when they do not have enough minerals and water to support both themselves and any dependent cells. (speculative)

Effects

  • Stimulates leaf and flower senescence
  • Induces leaf abscission mainly in older leaves.
  • Induces seed germination
  • Induces root hair growth – this increases the efficiency of water and mineral absorption
  • Stimulates epinasty – leaf petiole grows out, leaf hangs down and curls into itself
  • Stimulates fruit ripening
  • Induces the growth of adventitious roots during flooding
  • Usually inhibits growth - although perhaps just shoot growth (speculative)
  • Affects neighboring individuals
  • Disease/wounding resistance
  • Triple response when applied to seedlings – root ? and shoot growth inhibition and pronounced hypocotyl hook bending
  • Inhibits stem swelling ? (Contradictory to the finding below – contradictory sources)
  • Stimulates cell broadening (and lateral root growth)
  • Interference with auxin transport (with high auxin concentrations)
  • Directly or indirectly induces auxin at high levels (speculative)
  • Inhibits the rate of metabolism of cells in the shoot so as to redirect resources to the root (speculative)
  • Is a general indicator of poor root health. Strategy of senescent leaves may to funnel more resources to the root. (speculative)
  • May be more active at night when root and mineral acquisition are, on average, lower (speculative)
  • Just as a role of auxin may be to increase minerals and water by shoot growth, ethylene may do this by shoot senescence. Cytokinin and auxin hormones are released when conditions are favorable for growth, for example during the day. Ethylene and gibberellin (or brassinosteroid) may be released when the plant must either cut back in size, or survive on stored resources, for example during the night. (speculative)
  • Induces flowering in pineapples
  • In food production, some plants are considered ethylene producers, while others are considered ethylene sensitive.


Edited by Ruhil on 31 March 2006 at 1:05pm
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Ruhil
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Posted: 31 March 2006 at 1:17pm | IP Logged Quote Ruhil

Gibberellins are a plant growth substance (phytohormone) involved in promotion of stem elongation, mobilisation of food reserves in seeds and other processes. Its absence results in the dwarfism of some plant varieties. Chemically all known gibberellins are gibberellic acids, a family of diterpene acids that are synthesized by the terpenoid pathway in plastids and then modified in the endoplasmic reticulum and cytosol until they reach their biologically-active form.

Much of our knowledge of the biosynthesis and molecular mechanisms of gibberellins comes from research on their role in triggering á-amylase release by the aleurone layer in seed germination.

Gibberellin was first isolated in 1926 by Japanese scientists. It was derived from the Gibberella fungus lamapitus.

Examples: Gibberellin 452D

Location, Characteristics and Occasions for Synthesis Induction

Synthesized in the embryo and germinating seeds

•Synthesized in the roots

•Increased in production in the dark when sugar cannot be manufactured, and decreased in production in the light

•Synthesized in apical meristems ? and young leaves ?

•Produced in the stem rather than the growing tip ? (opposite finding to above – conflicting sources)

•Transported in non-polar, bidirectional manner, producing general responses

Released in all cells (more particularly)

root and mature cells) when they do not have enough sugar and oxygen to support both themselves and any dependent cell (speculative)

•Released in response to root, environmental, pest, and disease stress

•Directly induced by high levels of CK (speculative)

Effects

Stimulates shoot and cell elongation

Delays senescence of leaves

•Inhibits root growth

•Inhibits adventitious root growth

•Produces seed germination

•Antagonist promotes root growth and GA reverses this

•Promotes root initiation in low concentration in pea cuttings

•Stimulates bolting and flowering in biennials

•Regulates production of hydrolytic enzymes for digesting starches

•Inhibits CK bud growth on calluses

•Inhibits bud formation

•Inhibits leaf formation

•Breaking of dormancy

•Induces extra Chlorophyll production or more efficient methods of photosynthesis (C4 Photsynthesis). (speculative)

•Stimulates root senescence (speculative)

•Directly or indirectly induces CK at high levels (speculative)

*Inhibits the rate of metabolism of cells in the roots so as to funnel resources to the shoot for better acquisition of sugar

Is a general indication of poor shoot health. May be released when the plant or plant part does not have enough shoot derived nutrients (sugar and gases) to continue existing at the present size, let alone growing.

Action in general may be to increase sugar and gases, by either releasing stores, lowering root metabolism or inducing less

•needed root senescence.

May work with Brassinosteroid to

           produce its effects.

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Ruhil
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Posted: 31 March 2006 at 1:21pm | IP Logged Quote Ruhil

Abscisic acid (ABA), also known as abscissin, is a plant hormone.

Location, Characteristics and Occasions for Synthesis Induction

  • Released during desiccation (of vegetative tissues)
  • Found to peak at night
  • Synthesized in green fruit and seeds at the beginning of the wintering period
  • Moved within the leaf and can be transferred to the leaf from the roots by the transpiration stream
  • Rapidly-translocated
  • Produced in response to stress
  • Synthesized in leaves and stems (particularly when water-stressed)
  • Released by cells in danger of not having enough nutrients locally or good enough environmental conditions to survive
  • Capable of being synthesized by all cells.

Abscisic acid is defined as a plant growth regulator that acts mainly to inhibit growth, promote dormancy, and help the plant tolerate stressful conditions.

Abscisic acid is named so because it was believed that this plant growth regulator caused the abscission of leaves from deciduous trees in the fall. The plant's growth slows down, and then assumes a dormant state. This is the complete opposite of what auxin, gibberellins, and cytokinins, the other plant hormones, do to the plant. Inside the terminal bud, the hormone abscisic acid is produced. The slow growth and direction of leaf primordial develops scales to protect the dormant buds during the cold season.

This plant growth regulator inhibits the division of the cell in the vascular cambium, also preparing for the winter by suspending primary and secondary growth. The most impressive effect of abscisic acid is the inhibition of growth and the maintenance done on the dormancy of buds. Yet this is not enough to keep the dormancy of buds up for the long term.

 

Effects

  • Stimulates stomatal closure
  • Inhibits fruit-ripening
  • Encourages seed dormancy by inhibiting cell growth – inhibits seed germination
  • Inhibits the uptake of Kinetin
  • Activates the pathogen resistance response defense
  • Induces senescence in already-damaged cells and their proximate neighbors
  • Quickly puts a plant, organ, tissue or individual cell in a defensive posture (whatever this entails) in response to rapidly-developing nutrient or environmental stress that threatens their survival
  • Decreases metabolism in response to a newly-developing deficiency of nutrient or adverse environmental condition, such that the condition becomes survivable at the new lower level of metabolism 
  • Possibly induces cell dormancy or senescence by a climactic increase or sustained level, stimulating the synthesis of GA and/or Ethylene

A climactic rise or sustained level of ABA may be a prerequisite for the synthesis of any GA and/or Ethylene in that its presence indicates unusable or unsurvivable levels of Water, Sugar, Minerals and/or essential gases

 

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Ruhil
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Posted: 31 March 2006 at 1:26pm | IP Logged Quote Ruhil

hormon auxins (struktur IAA)

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Posted: 01 April 2006 at 10:12am | IP Logged Quote Riezz

TQ very much
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Amd_aft
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Posted: 04 April 2006 at 10:08pm | IP Logged Quote Amd_aft

pening

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Posted: 07 April 2006 at 9:35am | IP Logged Quote Iz_aan

pening betul......takde ke in BM??? mudah sket nak paham..... so, anyone can translate???

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