Plant Physiology Notes Brief Study of Mechanism of Ion Uptake And

Plant Physiology Notes

Brief study of mechanism of ion uptake and transport in Plants

1. Passive Uptake of Ion Absorption 2. Active Uptake of Ion Absorption (Metabolic).

Plants absorb minerals as ions

Plants absorb minerals from the soil in the form of inorganic ions. Earlier it was thought that the absorption of minerals takes place in plants along with the absorption of water. In fact, the process of mineral absorption and water absorption are two separate processes. In order to absorb any minerals from the soil, it should be dissolved in the water.

Plants absorb most of the minerals through the roots. The large surface area of roots and its ability to absorb minerals from the soil even in minor concentration makes the roots more efficient in mineral absorption than any other organs. In roots, the mineral absorption usually takes place through the meristematic region of the root tip.

Minerals can be absorbed by both Passive and Active Methods

There are two types of mineral absorptions based on the involvement of metabolic energy. They are (1) Passive minerals absorption (2) Active minerals absorption.

(1). Passive Mineral Absorption Passive mineral absorption is a passive process and it does not require the expenditure of metabolic energy. This type of mineral absorption occurs along the concentration gradient by simple diffusion.

(2). Active Mineral Absorption Active mineral absorption is an active process and thus it requires the expenditure of metabolic energy (ATP). Active mineral absorption can occur both along and against the concentration gradient by osmosis or through special carrier proteins in the plasma membrane. The exact mechanism of mineral absorption varies:

The exact mechanisms by which the plants absorb mineral from the soil vary greatly for various types of minerals. There are three main mechanisms of mineral absorption processes in plants.

(1). Ion-Exchange (2).Carrier Concept (3). Donnan’s Equilibrium

(1). Ion Exchange In ion exchange process of mineral absorption, the ions adsorbed on the surface of the root can exchange with the ions of the same charge from the soil solution. For example, H ions adsorbed on the surface of root cells can be exchanged with K ions in the soil. Two theories have been proposed to explain the mechanism of ion exchange – (a) Contact exchange theory and (b) Carbonic acid exchange theory. 1

(a). Contact Exchange Theory: According to contact exchange theory, the ions adsorbed on the surface of root cells and clay particles are in continuous oscillation. When the root and clay particles are in close contact with each other, the oscillation radius of ions on the clay particles may overlap with the ions adsorbed on the root cells. When these oscillation radii overlap, there is a possibility of spontaneous exchange of ions between clay particles and root surface.

(b). Carbonic Acid Exchange Theory: According to carbonic acid exchange theory, the CO produced in the root cells due to respiration combine with water to form carbonic acid (H CO ). The carbonic acid is immediately dissociated into H and HCO ions. The H ions thus formed are exchanged with cations on the clay particles.

(2). The Carrier Concept According to this theory, the plasma membrane is completely impermeable to some ions. The absorption of these ions is facilitated by some special proteins on the plasma membrane called ‘Carrier Proteins’. First, the carrier proteins are combined with the ions to form the carrier‐ion‐complex. The carrier‐ioncomplex can move across the plasma membrane. When the carrier‐ion‐complex reaches the inner surface of the membrane, they release the ions into the lumen of the cell. After this, the carrier protein will go back to the outer surface to accept new ions.

(3). Donnan Equilibrium The Donnan equilibrium explains the accumulation of some mineral ions inside the cell against the concentration gradient without the expenditure of metabolic energy. According to this theory, inside the root cells, there are some ions called Indiffusable Ions or Fixed Ions, which do not diffuse outside through the plasma membrane. The plasma membrane is permeable to other type of cations and anions. If a cell has an accumulation of cations as fixed ions, such a cell can absorb anions from the soil in order to maintain the electrical potential balance. The absorption of anion in this case causes an equilibrium of both cations and anion inside the cell. This equilibrium is called Donnan equilibrium. By attaining the Donnan equilibrium, the root cells can absorb any minerals in their ionic form.

Photosynthesis

Green plants are also called producers. This is because they have the ability to produce their own food from the raw materials around them by a process called photosynthesis.

During photosynthesis radiant energy from the sun (sunlight) is absorbed by green plants. The energy is used to convert carbon dioxide, water and minerals the plants take in from their surroundings into sugar and gaseous oxygen.

Photosynthesis is critical to life on Earth. Without photosynthesis the food supply would finish and the Earth’s atmosphere would lose its oxygen.

Photosynthesis Reaction

The chemical equation for the process of photosynthesis is given as:

The photosynthesis equation is a simple representation for a very complex natural process. Within the photosynthesis process there are two distinctly separate stages, a photochemical stage followed by a biochemical stage.

The photochemical stage involves the radiant energy supplied by sunlight and involves reactions called light dependant reactions. Green plants contain a light absorbing pigment called chlorophyll. When a molecule of chlorophyll absorbs light it uses the energy to boost electrons to a higher energy level and the molecule is said to be excited. The electrons at the higher energy levels are transferred along chains of electron carrier molecules. The energy transfers of the electrons are responsible for the formation of key energy carrying molecules along with the splitting of the water molecule to oxygen and hydrogen.

These molecules then undergo the second stage of photosynthesis, the biochemical reaction. Here they react with hydrogen formed from the splitting of water in the photochemical stage and carbon dioxide from the atmosphere with the presence of enzymes to form the organic glucose molecules. The reactions in this stage are referred to as light‐independent reactions because they do not require light as they use the energy already provided by the light in the light‐dependent reactions. Thus, photosynthesis has transferred the energy from light to chemical energy in the sugar.

Mechanism of photosynthesis involves two distinct phases of reactions, viz. Primary photochemical reaction or light reaction and dark reaction or blackman's reaction or Calvin cycle.

Light reaction or primary photochemical reaction or hill's reaction:

The phase of reactions in photosynthesis which involves the direct sunlight is called light reaction. This reaction takes place inside the grana of chloroplast. In this reaction assimilatory power ATP and NADPH2 are generated with evolution of oxygen and photolysis of water. These assimilatory powers help in the fixation of carbon dioxide during dark phase of reaction.

Light reaction involves following steps:

(1) Absorption of light energy by chlorophyll pigment: a) Chief source of light energy for this reaction is the sun. b) All the incident light energy falling on green parts of plant is not absorbed only 1% a fraction of it is absorbed by pigments. c) Photosynthesis pigments absorb light energy only in the visible part of spectrum i.e. within 390 to 760 am. d) Chlorophyll pigment systems are the trapping centers of light. There are several form of chlorphyll a, and light. There are several form of chlorolyll and chlorophyll ‐ b constitute the photosystem along with other accessory pigment. e) Emerson and Arnold (1932) showed that 2500 chlorphyll molecules required to fix one molecule of CO2 in photosynthesis. f) Steinman (1952) observed that granular structure in chloroplast lamella may be the morphological expression at the physiological photosynthetic units called them as quantosomes.

(2) Transfer of light energy from accessory pigment to chlorophyll:

Light energy absorbed by the pigment other than chlorophyll a transferred to chlorophyll ‐a by resonance of photons. In the chlorophyll‐a primary photochemical reaction takes place.There are two pigment systems in the chlorophyll with p‐690 and p‐700.

(3) Activation of chlorophyll‐a molecule and photo excitation of the pigment: When pigment molecule in a photo system receives a photon of light, become excited and expels the extra energy level called excited second singlet state; then it comes to Meta stable state is called triplet state. From which it receive electron from outer source and return back to ground state.

(4) Photolysis of water: The electron being released by photo excitation makes the pigment unstable, for which supply of electron in bet by photolysis of water.

When pigment system II is active it receive light, the water molecule split into OH and H+ ions in presence of Mn++ and CL‐ ions. The OH ions unite form some water molecules again and O2 and electron are released. (5) Electron transport and the production of assimilatory

Excited electrons from the pigment system travels through a number of electron carriers reducing the NAND to NADPH2. The extra energy which carried along with electrons utilized in phoshorylation of ADP to form ATP. This above process is called electron transport or photophosphorylation.

There are two distinct paths of electron transport involves in light reaction. i. Noncyclic Photosphosphorylation: According to Arnon, the electron which ejected from pigment system ‐1 after photo excitation causes an electron hole in the p700 molecule. This ejected electron is trapped by FRS (Ferredoxin reducing substance which is an unknown oxidation reduction system with a redox notential E of 0.6 volts. The electron then

4 transferred to a non heme iron protein called ferrodoxin (FD) with E, 0 of 0.432 volt. From gerredoxin electron transferred to NADP, so that NAND is reduced to NANDPH.

When a photon of light absorbed by P690 form of chlorophyll a molecule in pigment system II, it gets excited and release electron. This ejected electron trapped by plastoquinone from where the electron follow down hill direction towards the pigment system 1, through a series of electron carriers or cytochrome systems and plastocyanin (PQ). The election while pass from Cyt‐b to Cyt‐f phosphorylation of ADP takes place as shown in above diagram.

ii. Cyclic photophosporylation:

a. Besides non‐cycles electron transport there may be transport of electrons takes place in light reaction in cyclic way. It involves only pigment system 1. b. This is rarely happen when the pigment system‐II is totally impaired or blocked by inhibitors like i‐ dimethy urea, or 3‐4 dichloro phynyl etc. c. There is no need of photolysis of water. d. When p700 molecule is excited in pigment system I by absorbing a photon of light the ejected electron is captured by ferredoxin. e. The electron then instead of reducing NADP fals back to p700 molecule involving electron pass from F.R.S to P.C. there is phosphpry lation takes place in previous manner; so that ATP molecule is generated. f. Thus light reaction is fully dependant on the radiant energy in terms of photon by which radian‐ energy from sun transformed into assimilatory powers NADPH2 and ATP which are utilised in the dark phase of reaction in order to fix CO2 forming sugar.

PHOTOSYNTHETIC PATHWAYS ‐ C3, C4 AND CAM

Dark reaction or Blackman’s reaction or Path of carbon in photosynthesis This is the second step in the mechanism of photosynthesis. The chemical processes of photosynthesis occurring independent of light is called dark reaction. It takes place in the stroma of chloroplast.

The dark reaction is purely enzymatic and it is slower than the light reaction. The dark reactions occur also in the presence of light. In dark reaction, the sugars are synthesized from CO2. The energy poor CO2 is fixed to energy rich carbohydrates using the energy rich compound, ATP and the assimilatory power, NADPH2 of light reaction. The process is called carbon fixation or carbon assimilation.

Since Blackman demonstrated the existence of dark reaction, the reaction is also called as Blackman’s reaction. In dark reaction two types of cyclic reactions occur 1. Calvin cycle or C3 cycle 2. Hatch and Slack pathway or C4 cycle

Calvin cycle or C3 cycle It is a cyclic reaction occurring in the dark phase of photosynthesis. In this reaction, CO2 is converted into sugars and hence it is a process of carbon fixation. The Calvin cycle was first observed by Melvin Calvin in chlorella, unicellular green algae. Calvin was awarded Nobel Prize for this work in 1961. Since the first stable compound in Calvin cycle is a 3 carbon compound (3 phosphoglyceric acid), the cycle is also called as C3 cycle. The reactions of Calvin’s cycle occur in three phases. 5

1. Carboxylative phase 2. Reductive phase 3. Regenerative phase Calvin cycle

Ribulose‐1,5‐bisphosphate acts as the carbon dioxide acceptor; following carbon dioxide addition, the resulting six‐carbon compound is cleaved into two three‐carbon 3‐phosphoglycerate molecules. The 3‐ phosphoglycerate is then converted to glyceraldehyde‐3‐phosphate using energy obtained from the light reactions of photosynthesis.

The glyceraldehyde‐3‐phosphate has several possible fates; however, the continuation of the carbon fixation process requires regenerating the ribulose‐1,5‐bisphosphate starting material. The cycle thus has three phases: carbon fixation, glyceraldehyde‐3‐phosphate formation and ribulose‐1,5‐ bisphosphate regeneration.

Rubisco The rate‐limiting enzyme in the Calvin cycle is ribulose 1,5‐bisphosphate carboxylase/oxygenase (better known as rubisco). In most plants, rubisco is a complex of 8 large (53 kDa) and 8 small (14 kDa) subunits (88). The large subunit is coded by a chloroplast gene, while the small subunit gene is located in the nucleus. In photosynthetic bacteria, rubisco is usually a dimer of proteins homologous to the plant large subunit.

Ribulose 1,5‐bisphosphate + CO2 + H2O 2x 3‐phosphoglycerate + 2 H+

The rubisco reaction thus yields two triose phosphate molecules from the single pentose bisphosphate. Note that ATP is not required for the rubisco reaction, a property that differs from most other carboxylase reacons; in this case, the reacon has a large negave G° because of the release of the two triose phosphates. 6

C4 cycle or Hatch and Slack pathway

It is the alternate pathway of C3 cycle to fix CO2. In this cycle, the first formed stable compound is a 4 carbon compound viz., oxaloacetic acid. Hence it is called C4 cycle. The path way is also called as Hatch and Slack as they worked out the pathway in 1966 and it is also called as C4 dicarboxylic acid pathway. This pathway is commonly seen in many grasses, sugar cane, maize, sorghum and amaranthus.

The C4 plants show a different type of leaf anatomy. The chloroplasts are dimorphic in nature. In the leaves of these plants, the vascular bundles are surrounded by bundle sheath of larger parenchymatous cells. These bundle sheath cells have chloroplasts. These chloroplasts of bundle sheath are larger, lack grana and contain starch grains. The chloroplasts in mesophyll cells are smaller and always contain grana. This peculiar anatomy of leaves of C4 plants is called Kranz anatomy. The bundle sheath cells are bigger and look like a ring or wreath. Kranz in German means wreath and hence it is called Kranz anatomy.

The C4 cycle involves two carboxylation reactions, one taking place in chloroplasts of mesophyll cells and another in chloroplasts of bundle sheath cells. There are four steps in Hatch and Slack cycle:

1. Carboxylation 2. Breakdown

Crassulacean Acid Metabolism (CAM) cycle or the dark fixation of CO2 in succulents

CAM is a cyclic reaction occurring in the dark phase of photosynthesis in the plants of Crassulaceae. It is a CO2 fixation process wherein, the first product is malic acid. It is the third alternate pathway of Calvin cycle, occurring in mesophyll cells. The plants exhibiting CAM cycle are called CAM plants. Most of the CAM plants are succulents e.g., Bryophyllum, Kalanchoe, Crassula, Sedium, Kleinia etc. It is also seen in certain plants of Cactus e.g. Opuntia, Orchid and Pine apple families.

CAM plants are usually succulents and they grow under extremely xeric conditions. In these plants, the leaves are succulent or fleshy. The mesophyll cells have larger number of chloroplasts and the vascular bundles are not surrounded by well defined bundle sheath cells. In these plants, the stomata remain open during night and closed during day time. The CAM plants are adapted to photosynthesis and survival under adverse xeric conditions. CAM plants are not as efficient as C4 plants in photosynthesis. But they are better suited to conditions of extreme desiccation. CAM involves two steps: 1. Acidification 2. Deacidification

Comparison of the plants of C3 and C4 cycle

Biological Nitrogen Fixation

The conversion of atmospheric nitrogen into the nitrogenous compounds through the agency of living organisms is called biological nitrogen fixation. The process is carried out by two main types of microorganism: those which live in close symbiotic association with other plants and those which are “free living” or non‐symbiotic.

Biological nitrogen fixation (BNF) is the process whereby atmospheric nitrogen is reduced to ammonia in the presence of nitrogenize. Nitrogenize is a biological catalyst found naturally only in certain microorganisms such as the symbiotic Rhizobium and Frankia, or the free‐living Azospirillum and Azotobacter and BGA.

Nearly 80% of Earths atmosphere contains nitrogen in the form of a highly inert di‐nitrogen (N = N) which most plants cannot utilize as such. The atmospheric di‐nitrogen (N2) consists of two nitrogen atoms linked by a triple‐covalent bond. About 225 kcal of energy is required to break this triple bond which is difficult to achieve.

The phenomenon of reduction of inert gaseous di‐nitrogen (N2) into ammonia (NH3) through the agency of some microorganisms so that it can be made available to the plants is called as biological nitrogen fixation or diazotrophy.

Nitrogen Fixers:

Among the earth’s organisms, only some prokaryotes like bacteria and cyanobacteria can fix atmosphere nitrogen. They are called nitrogen fixers or diazotrophs. They fix about 95% of the total global nitrogen fixed annually (‐200 million matric tones) by natural process.Diazotrophs may be asymbiotic (free living) or symbiotic such as given below:

(i) Free Living Nitrogen Fixing Bacteria: Azotobacter, Beijerinckia (bothaerobic) and Clostridium (anaerobic) are saprophytic bacteria that perform nitrogen fixation. Desulphovibrio is chemotrophic nitrogen fixing bacterium. Rhodopseudomonas, Rhodospirillum and Chromatium are nitrogen fixing photoautotrophic bacteria. These bacteria add up to 10‐25 kg, of nitrogen/ha/annum.

(ii) Free living Nitrogen Fixing Cyanobacteria: Many free living blue‐green algae (now called cyanobacteria) perform nitrogen fixation, e.g., Anabaena, Nustoc, Aulosira, Cylmdrospermum, Trichodesmium. These are also important ecologically as they live in water‐logged sods where denitrifing bacteria can be active. Aulosira fertilissima is the most active nitrogen fixer in Rice fields, while Cylindrospermum is active in sugarcane and maize fields. They add 20‐30 kg Nitrogen/ha/annum.

(iii) Symbiotic Nitrogen Fixing Cyanobacteria: Anabaena and Nostoc species are common symbionts in lichens, Anthoceros, Azolla and cycad roots. Azolla pinnata (a water fern) has Anabaena azollae in its fronds. It is often inoculated to Rice fields for nitrogen fixation.

(iv) Symbiotic Nitrogen Fixing Bacteria: Rhizobium is aerobic, gram negative nitrogen fixing bacterial symbionts of Papilionaceous roots. Sesbania rostrata has Rhizobium in root nodules and Aerorhizobium in stem nodules. Frankia is symbiont in root nodules of many non‐leguminous plants like Casuarina and Alnus. 9

Xanthomonas and Mycobacterium occur as symbiont in the leaves of some members of the families Rubiaceae and Myrsinaceae (e.g., Ardisia). Several species of Rhizobium live in the soil but are unable to fix nitrogen by themselves. They do so only as symbionts in the association of roots of legumes.

Symbiotic Nitrogen Fixation: Both Rhizobium sp. and Frankia are free living in soil, but only as symbionts, can fix atmospheric di‐ nitrogen. The symbiotic nitrogen fixation can be discussed under following steps:

(i) Nodule formation

It involves multiple interactions between free‐living soil Rizobium and roots of the host plant. The important stages involved in nodule formation are as follows‐Host Specificity. The roots of young leguminous plants secrete a group of chemical attractants like flavonoids and betaines. In response to these chemical attractants specific rhizobial Tells migrate towards the root hairs and produce nod (nodulation) factors. The nod factors found on bacterial surface bind to the lectin proteins present on the surface of root hairs.

At these regions wall degrades in response to node‐factors and Rhizobia enter the root hair invagination of plasma membrane called infection thread. The infection thread filled with dividing Rhizobia elongate through the root hair and later branched to reach different cortical cells.

The Rhizobia are released into the cortical cells either single or in groups enclosed by a membrane. The Rhizobia stop dividing, loose cell wall and become nitrogen fixing cells as led bacteroids .The membrane surrounding the bacteroids is called peribacteroid membrane. The infected cortical cells divide to form nodule.

(ii) Mechanism of nitrogen fixation

The nodule serves as site for N2 fixation. It contains all the necessary bio‐chemicals such as the enzyme complex called nitrogenase and leghaemoglobin (leguminous haemoglobin). The nitrogenase has 2 components i.e. Mo‐Fe protein (molybdoferredoxin) and Fe‐protein (azoferredoxin).The nitrogenase catalyzes the conversion of atmosphere di‐nitrogen (N2) to 2NH3. The ammonia is the first stable product of nitrogen fixation.

During nitrogen fixation, the free di-nitrogen first bound to MoFe protein and is not released until completely reduced to ammonia. The reduction of di-nitrogen is a stepwise reaction in which many intermediates are formed to form ammonia (NH3) which is protonated at physiological pH to form NH4+.

Assimilation of Ammonia:

The ammonia produced by nitrogenase is immediately protonated to form ammonium ion (NH4+). As NH4+ is toxic to plants, it is rapidly used near the site of generation to synthesize amino acids. Amino acids synthesis takes place by three methods: reductive animation,

10 catalytic amination and transamination.

(i) Reductive amination: In this process, glumate dehydrogenase (GDH) catalyzes the synthesis of glutamic acid.

(ii) Catalytic amidation: It is a two step process catalyzed by glutamine synthetase (GS) and glutamate synthetase (glutamine – 2‐ oxyglutarate aminotransferase, or GOGAT).

(iii) Transamination: Glutamate or glutamic acid is the main amino acid from which other amino acids are derived through transamination. The enzyme aminotransferases (= transaminases) catalyze all such reactions. Transamination involves transfer of amino group from one amino acid to the keto group of keto acid.

Glutamate (amino donor) + Oxaloacetate (amino acceptor) → Aspartate (amino acid) + 2 oxyglutarate

Nitrate Assimilation:

Nitrate cannot be utilized by plants as such. It is first reduced to ammonia before being incorporated into organic compounds. Reduction of nitrate occurs in two steps:

1. Reduction of nitrate to nitrite:

It is carried out by an inducible enzyme, nitrate reductase. The enzyme is a molybdoflavoprotein. It requires a reduced coenzyme NADH or NADPH for its activity which is brought in contact with nitrate by FAD or FMN.

2. Reduction of nitrate:

It is carried out by the enzyme nitrite reductase. The enzyme is a metalloflavoprotein which contains copper and iron. It occurs inside chloroplast in leaf cells and leucoplast of other cells. Nitrite reductase require reducing power. It is NADPH and NADH.

Reduction process also require ferredoxin which occurs in green tissues of higher plants. It is presumed that in higher plants either nitrite is trans‐located to leaf cells or some other electron donor (like FAD) operates in un‐illuminated cells. The product of nitrite reduction in ammonia.

PLANT HORMONES

Plant hormones or Phytohormones may be defined as an "an organic substance produced naturally in plants which control growth and other physiological functions at a site away from its place of synthesis". Thimann (1948) proposed the term phytohormones. As these hormones are synthesis by plants, they are also called phytohormones.

Phytohormones are active in small concentrations. They are capable of influencing physiological activities leading promotion, inhibition and modification of growth. These growth regulatory substances are generally grouped under five major classes, namely Auxins, gibbellins, cytokinins, ethylene and abscisic acid.

Types of Plant Hormones There are five general classes of hormones: auxins, cytokinins, gibberellins, ethylene, and abscisic acid.

Auxins

An auxin, indole‐3‐acetic acid (IAA), was the first plant hormone identified. It is manufactured primarily in the shoot tips (in leaf primordia and young leaves), in embryos, and in parts of developing flowers and seeds. Its transport from cell to cell through the parenchyma surrounding the vascular tissues requires the expenditure of ATP energy. IAA moves in one direction only—that is, the movement is polar and, in this case, downward. Such downward movement in shoots is said to be basipetal movement, and in roots it is acropetal.

Auxins alone or in combination with other hormones are responsible for many aspects of plant growth. IAA in particular:  Activates the differentiation of vascular tissue in the shoot apex and in calluses; initiates division of the vascular cambium in the spring; promotes growth of vascular tissue in healing of wounds.  Activates cellular elongation by increasing the plasticity of the cell wall.  Maintains apical dominance indirectly by stimulating the production of ethylene, which directly inhibits lateral bud growth.  Activates a gene required for making a protein necessary for growth and other genes for the synthesis of wall materials made and secreted by dictyosomes.  Promotes initiation and growth of adventitious roots in cuttings.  Promotes the growth of many fruits (from auxin produced by the developing seeds).  Suppresses the abscission (separation from the plant) of fruits and leaves (lowered production of auxin in the leaf is correlated with formation of the abscission layer).  Inhibits most flowering (but promotes flowering of pineapples).  Activates tropic responses.  Controls aging and senescence, dormancy of seeds. Synthetic auxins are extensively used as herbicides, the most widely known being 2,4‐D and the notorious 2,4,5‐T, which were used in a 1:1 combination as Agent Orange during the Vietnam War and sprayed over the Vietnam forests as a defoliant.

Cytokinins

Named because of their discovered role in cell division (cytokinesis), the cytokinins have a molecular structure similar to adenine. Naturally occurring zeatin, isolated first from corn ( Zea mays), is the most active of the cytokinins. Cytokinins are found in sites of active cell division in plants—for example, in root tips, seeds, fruits, and leaves.

They are transported in the xylem and work in the presence of auxin to promote cell division. Differing cytokinin:auxin ratios change the nature of organogenesis. If kinetin is high and auxin low, shoots are formed; if kinetin is low and auxin high, roots are formed. Lateral bud development, which is retarded by auxin, is promoted by cytokinins.

Cytokinins also delay the senescence of leaves and promote the expansion of cotyledons.

Gibberellins

The gibberellins are widespread throughout the plant kingdom, and more than 75 have been isolated, to date. Rather than giving each a specific name, the compounds are numbered—for example, GA1, GA2, and so on. Gibberellic acid three (GA3) is the most widespread and most thoroughly studied.

The gibberellins are especially abundant in seeds and young shoots where they control stem elongation by stimulating both cell division and elongation (auxin stimulates only cell elongation).

The gibberellins are carried by the xylem and phloem. Numerous effects have been cataloged that involve about 15 or fewer of the gibberellic acids. The greater number with no known effects apparently are precursors to the active ones.

Experimentation with GA3 sprayed on genetically dwarf plants stimulates elongation of the dwarf plants to normal heights. Normal‐height plants sprayed with GA3 become giants.