Plant Growth and Development

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COURSE CODE: COURSE TITLE: NUMBER OF UNITS: COURSE DURATION:

PCP 504 PLANT GROWTH AND DEVELOPMENT 3 UNITS 3 HOURS PER WEEK

COURSE DETAILS: COURSE DETAILS:
Course Coordinator: Email: Office Location: Other Lecturers:
DR A.O.OLAIYA [e-mail] Room 131, COLPLANT Prof V.I.O OLOWE, Prof P.O.ADETILOYE, DR M.O.ATAYESE, DR O.S SAKARIYAWO

COURSE CONTENT:
Seed germination and dormancy. Juvenility and senescence. Translocation and respiration in crops, role of environmental resources. Water and water stress in plants. Light and solar radiation, role of plant nutrients. Photosynthesis, plant growth and partitioning of assimilates. Yield limiting factors and yield components. Growth regulators – auxin, gibberellins, cytokinins etc, Plant growth and mearsurements, Growth analysis: RGR,NAR,LAI etc, Plant development: roles of plant organs like leaf, stem, root,flower,fruit and seed.

Practical : Students will be taken through various instruments and methodologies in plant growth measurement and analysis tools.

COURSE REQUIREMENTS:
This course is compulsory for all students in the college of plant science and crop production. In view of this, students are expected to participate in all the course activities and have minimum of 75% attendance to be able to write the final examination.

READING LIST:
1. The physiology of tropical Crop production by G.R.Squire , C.A.B International, 1990. 2. Laboratory methods of soil and plant analysis: A working manual by J.R.Okalebo ; K.W. Gathua and P.L. Woomer, Tropical Soil and Fertility Programme, 1993

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E LECTURE NOTES
SEED VIABILITY AND VIABILITY TESTING Viability of a seed refers to the ability of a seed to germinate and produce a “normal” seedling. Seed viability is highest at the time of physiological maturity, though environmental conditions on the parent plant may not permit germination. After physiological maturity, the viability of seed gradually declines. The longevity of seed depends on crop species and on the environmental conditions to which they are exposed. SEED GERMINATIN STAGES: Irrespective of the criteria used for deciding on when seed germination has taken place, the following stages exists: 1. Germination This stage includes water imbibition and all the biochemical and physiological process that culminate in the emergence of the radical and the plumule. 2. Underground elongation At this stage of germination, elongation of both the radical and the plumule takes place at the expense of food reserve in the endosperm. The soil depth through which the plumule can emerge depends on the amount of food reserve in the endosperm. 3. Emergence It is at this stage that the aerial parts of the seedling emerge above the ground. Seedling emergence may be hypogeal if cotyledons remain below the soil surface, or epigeal if the cotyledons are forced above ground by elongation of the epicotyls. 4. Independent growth The period starts with the onset of photosynthetic activity by the seedling plant. Normal functioning of the seedling is established at this stage. With large-seeded species and sprouts from vegetative propagules, there is considerable overlap in time between parental independence and development of photosynthetic food source. GERMINATION TEST: Germination test is most commonly used method to determine seed viability. Seed germination is the consecutive number of steps that will cause a non-dormant or active seed to imbibe water and initiate changes that led to the development of the embryo until the emergence of those essential structures which are indicative of its ability to produce a normal plant. The seed takes up water firstly by imbibition of water is accompanied by swelling of the seed, and as the fine structure of the cells is restored, the metabolic activity beings, thus, signifying the commencement of germination. PROCEDURE 1. Select 30 seeds (beans) from the seed lot 2. Put filter paper, cotton wool or tissue paper in each of five Petri dishes. 3. Moisten the filter paper. 4. Place 10 seeds in each petri dish 5. Then moisten the seeds and cover with another petri dish 6. Place the petri dish in dark (laboratory cupboard would be adequate) Note: by dividing each seed into five sets of 10 seeds each, you are replicating the study. Replication is necessary so as to be sure that results obtained are not due to chance.

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EXERCISE 1 Observe dates of germination in each petrel dish no more of the seed lots will germinate. Note the total number of seeds that germinated, find the average and determine germination percentage of each seed lot. Observations Replicates 1st count 2nd count 3rd count 4th count 5th count Total no. % Germ. (Days) (Days) (Days) (Days) (Days) 1 2 3 4 5

Percentage germination = Total no. of seeds that germinated X Total no. of seeds planted

100

DORMANCY: A dormant but viable seed is one, which fails to germinate at a favourable temperature when supplied with water and air. Dormant but viable seed eventually germinate but only when some special condition has been satisfied, such as a particular treatment with light or low temperature. Dormancy is therefore biologically important, providing a mechanism for dispersal of the plant. Dormancy and the breaking of dormancy ensure that the seed germinates only at a certain time of the year. Dormancy therefore preserve the seed against temporarily unsuitable conditions such as may occur during periods between seed collection and storage. A physiologically sound seed may remain quiescent and therefore may germinate due to a lot of prevailing factor. CAUSES OF DORMANCY: 1. Dormancy due to seed coat Due to impermeability of seed coat to water and oxygen some seeds having hard and tough seed coat fail to absorb them and ultimately cannot germinate. Sometimes the hardness of the seed coat is the factor as found in seeds of clover and sweet pea and some other tree and non-tree legumes. In such cases germination is delayed till the seed coat is decayed in the soil as a result of bacterial action. The same is true with the seeds of some plants belonging to families Leguminosae, and Malvaceae, where there is also the mechanical barrier afforded by the seed covering to embryo emergence. 2. Dormancy due to immature embryos: Some seeds are shed before the embryo is mature and thus are not fit to grow. Such seeds require an ‘after ripening period’ during which certain changes occur within the seed. It is ripening period’ during which period certain changes occur within the seed. It is believed that there is some change in acidity, enzyme activity and respiratory rate or production of some growth promoting hormones or may be inactivation of some germination-inhibitor substances within seeds. Once embryo development is complete the seed then germinate without and special treatment. 3 Dormancy due to chemical inhibitors:

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The seeds present inside the juicy fruits such as Azadiracta indica, Mileci exelca, Gmelina arborea, oranges and tomatoes do not germinate while in the fruit where plenty of liquid is present. It is due to the presence of germination-inhibitory substances. Coumaruins and parascorbic acids are known to cause such inhibitions. 4. Dormancy requiring for after-ripening in dry storages: At the time of harvesting, seeds of many plants are dormant but they do not require any special treatment to overcome dormancy. Simply keeping them under dry storage conditions at normal temperature over a period ranging from a few weeks to several months, overcome dormancy. Many of common cereals such as barley, rice, oats\, wheat etc show this type of dormancy. 5. Dormancy requiring chilling treatment Many seeds of temperate species show dormancy, which is overcome by chilling. Simple examples of this category are the freshly harvested seeds of apple, rose and peach which will not germinate if planted under moist conditions at 200C but will germinate, if planted under moist conditions at 0-50C for several weeks and then transferred to warmer conditions.

6. Dormancy due to light sensitivity: The germination of many seed is affected by light. Such seeds are said to be photoblastic. The seeds in which germination is stimulated by light are called positively photoblastic seeds e. g Alilicia excels, Amarathus reflexus,Digitalis purpurea, lycopersicum esculentum, Nicotiana tabacum etc. whereas those in which germination is inhibited by light are negatively photoblastic e. g Silene armeria, Nemophala insignis etc.

Practical 2b: Seed dormancy Dormancy is the state in which growth is temporarily suspended. A dormant but viable seed is one which fail to germinate even when conditions are optimum for germination. Dormant but viable seed eventually germinate but only when some special condition has been satisfied, such as a particular treatment with light or low temperature. This is an important survival mechanism in plants. The causes of dormancy are varied and include: i) Rudimentary embryos ii) Physiologically immature embryo (inactive enzymes) iii) The presence of germination inhibitor iv) Mechanically resistant seed coat v) Impermeable seed coat i) and ii) may require a period of after-ripening. The third category includes seed coats that can be leached with water to remove the chemical inhibitor and thus allows germination. This feature adapts a specie not to germinate until considerable moisture is present in its environment. Fourth and fifth categories resist the ready diffusion of oxygen, carbon dioxide and water. In legumes and some grasses, this may be referred to as hard seed. This type of dormancy can be broken by SCARIFICATION before planting to permit better water uptake, germination and stand establishment. Mechanical scarification involves rubbing the seeds against an abrasive surface. Excessive or careless scarification may damage the seed. Methods of scarification include the following seed treatments. a) Shaking with sand; b) Cutting with knife;

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c) Rasping with a file; d) Rubbing with emery cloth or sand paper; e) Soaking in concentrated sulphric acid or alcohol. All of these treatments are design to weaken the hard seed coat so that water may be taken up more readily.

A

ACID TREATMENT i) Place the concentrated sulphuric acid (95%) in a beaker. ii) Place 10 seeds of Leucaena leucophala iii) Immerse the seeds until covered. iv) Allow the seeds to soak for 5 minutes. v) Remove the seeds from the acid and wash in cool running water for 5 to 10 minutes to remove all acid. vi) Stir the seed carefully during rinsing. vii) Dry seeds viii) Plant in germinating bags provided (other seeds are Centrosema spp, Stylosanthes gracilis and cotton- Gossypium spp) HOT WATER TREATMENT Select 10 seeds from Leucaena leucocephala and small quantity of Corchorus olitorus (ewedu) and put them in polyethylene bags. Heat water to near boiling and submerge the seed lots. Remove from water and allow to cool. Then air dry the seeds. Plant the treated seeds in soil-filled polythene bags. Also plant untreated seeds in germinating bags as control. Compare the results obtained in A and B.

B

EMERY CLOTH TREATMENT Select 50 seeds of the following species Calopogonium, green grain, centrosema, lima bean seed, soybean and maize. Rub 25 seeds of each specie between two pieces of emery cloth. Germinate seeds, both treated and untreated of each specie in Petri dishes with cotton or filter papers, and observe the effect of scarification. Conduct this experiment in four replicates.

Exercise Items 1 2

Species Soybean Green grain

% germination Control

Scarified

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3 4 5 6

Calopogonium Centrosema Lima bean seed Maize

A. TRANSLOCATION Translocation is the movement of dissolved materials throughout the plant. The rate at which photosynthetic (Ps) products such as sucrose move from the leaves to the sink organs controls the rate of photosynthesis. Plant species that have high Ps rates also have relatively high translocation rates. This is consistent with the idea that effective removal of Ps products maintains rapid CO2 fixation. Severe infection of leaves by pathogens often so severely inhibits Ps rates that these leaves become sugar importers instead of sugar exporters. The adjacent healthy leaves then gradually attain marked increases in Ps rates, suggesting that enhanced translocation has reduced CO2 fixation. There are four principal pathways for translocation of materials after uptake by the roots of leaves of a plant. a. Movement in the xylem along the transpiration stream. It allows the upward movement of organic materials in the xylem from the soil solution into foliage. b. Through the phloem or other cells such as ray parenchyma. This is the major pathway of movement of materials applied to the leaves. Subsequently, phloem flow may take solutes up to the stem apex as easily as down to the lower parts of the plant. c. Through the cell walls. Aqueous network through the cell walls is described as apoplast (outside protoplast). It is the principal region of the apparent free space. d. Through the intercellular spaces. The rapid systemic permeation of gases and volatiles through the plants indicates a ready movement through the intercellular spaces. Most of the metabolic sinks in plants are connected with the source by phloem elements in vascular strands. Sugars move from source to sink down the concentration gradients. Translocation occurs in the sieve tubes of the phloem and although other sugars and derivatives and also nitrogenous compounds may be found in the phloem exudates, the most important and general constituent is the disaccharide sucrose. The movement of photosynthate or metabolites from the surrounding mesophyll cells of the leaf into the conducting tissues of the phloem is known as “phloem lading”. The process of loading is selective, as shown by the failure of certain sugars and organic acids to be transferred while others moved in readily. Entrance into the sieve tubes is apparently independent of concentration differences between the mesophyll and sieve tubes in the case of sucrose, amino acids and organic acids. Both the process of transport to the sieve tubes and the terminal step of passage into the

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sieve tubes may require ATP. The concentration of sugars in leaves where they are produced is usually higher than that in the sinks. Only a fraction of the products of photosynthesis remain at the site of production in fully expanded leaves, most of them are translocated to other organs where they are either used as building blocks for various cell-constituents or deposited as storage products. A certain part is always lost through respiration during transport and at the final storage site. Photosynthathes (PS) are distributed within the plant at a definite distribution pattern. The pattern of movement changes continuously during the growth of the plant and exerts a profound effect on both the morphological form and the yielding properties of the plant. During the development of a single young leaf, it imports PS from other parts of the plant to build up its own structure; but very soon it becomes self-supporting and in a short time starts to export PS. As long as the plant is young, this export is mainly directed towards centres of active growth, such as developing leaves, root tips, or shoot apices; but later on much of the assimilate transport is diverted to storage organ such as fruits, grains, or tubers. Another expression of the distribution pattern is the fact that leaves in different positions on the plant may preferentially supply different growing organs with assimilates. The PS for the roots are thus mainly produced in the lower leaves and those for the apical parts in the upper leaves, whereas leaves in an intermediate position deliver assimilates in both directions. This general pattern is only relative, however, and may be modified both by differences in the internal organizations of the vascular system. Different centres of active growth may exhibit various strengths of “demand” for assimilates and therefore, reproductive organs often (but not always) dominate vegetative organs; shoot apices dominate root tips. The continues growth of higher green plants well supplied with water and inorganic nutrients depends primarily on i. ii. the accomplishment of photosynthesis in the leaves of the plants the transport of organic compounds from the leaves to heterotrophic cells which constitute metabolic sinks Underground parts – roots and a variety of storage organs are obvious examples of plant parts leading a heterotrophic existence and developing tubers are regarded as the classical sink for products of photosynthesis. Non-green aerial plant paerts – bilbs, flowers and fruits and most of the cells in stems and petioles also constitute a drain on photosynthetic products. Even within leaves there are many cells without chloroplasts and the autotrophic cells themselves consume PS in their own growth and respiration.. In a germinating seedling, the sink is the rapidly growing embryo and this is supplied by mobile organic materials produced during hydrolytic activity in the source storage organs – cotyledon or endosperm.

In a vegetative plant, the developing green leaves are a sink not only for their own PS but also for that produced in older leaves with appropriate vascular connections, which also support apical growth. The oldest leaves near the base of the plant, provided they receive adequate

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illumination export sugars to the roots. Developing buds and meristematic regions in the roots place demands on the available assimilates and compete successfully as sinks with developing leaves. The onset of flowering and subsequent fruit development have a marked effect on the redistribution of assimiliates. Fruits develop at then expense of vegetative growth and at this time the growth of roots may be restricted. The principal carbohydrate translocated in the sieve tubes appears to be sucrose, but in trees and some other plants this molecule may have condensed with it one or more molecules of galactose, to give rafinose, starchyose or verbascose in order of decreasing concentrations. Sucrose is also the principal carbohydrate translocated in herbaceous plants. Amino and organic acids, minerals, viruses, hormones and steroids are also known to be translocated. As sucrose moves through the sieve tubes it may be withdrawn into surrounding cells and hydrolyzed there so that whole-phloem analysis may give misleading results.

B. PARTITIONING OF ASSIMILATES Partitioning describes the distribution of new assimilates to growth of various plant parts and to respiration. Partitioning is subject to a control system whose flexibility is indicated by its capacity to respond appropriately to different environmental stimuli. Thus shading is likely to increase partition (relative to the total dry weight) to the laminae whereas water or nutrient stress improves partition to the roots. Removal of plant parts results in temporarily increased partition to other similar parts until partition relationship of intact control plants is re-established. All the vegetative sinks are potentially in competition during the vegetative phase, the dominant sink being sheath and stem. Yield depends on the translocation of assimilates in most species largely sucrose from the leaves or other photosynthetic tissues to the parts of economic importance – tubers, seeds, etc. Other parts – roots, stems, young leaves and fruits also rely on the assimilates produced by the mature leaves. The source and sinks are linked by a transport pathway of phloem sieve elements. Mature leaves are the primary sources of assimilates, from the current photosynthesis, but this can be supplemented by the mobilization of stored reserves/organs such as the stems, which may be described as secondary sources. In most dicots, cotyledons initially serve as source of reserves, but are subsequently raised above ground, when they assume a photosynthetic function which supports further growth until the first true leaves are photosynthetically competent. Storage function is taken over by endosperm from which mobilized reserves are absorbed by the cotyledons and then

translocated to the embryonic axis. Sucrose from the endosperm is accumulated in the phloem cells of the cotyledons largely sustain growth of the hypocotyls and roots. Elongation of the hypocotyls raises the cotyledon.

Dry matter partitioning between roots, above ground vegetative growth, and reproductive growth are usually modified by water deficits. The response depends upon the species, when

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the stress occurs, its duration, and its severity. The increase in root-shoot ratio of crops under water deficits may reflect an increase in the proportion of assimilates allocated to the roots, or a change in the rate of death or turn over of roots relative to the shoot. An increase in root growth may indicate a greater density or a greater depth of roots. Soil and root resistances to water uptake are reduced when root length density increases, thereby permitting higher water flow rates through the plant and delaying the onset of severe plant water deficits. Whilst partitioning of dry matter to roots may enhance water uptake, it represents a loss to above ground dry matter production. An exception is root crops, such as cassava, where water deficit could be beneficial to economic yield, provided that the increased proportion of dry matter partitioned to the roots goes mainly to the tubers. Since photosynthesis is inhibited more than translocation during stress, dry matter produced before flowering may be transferred from the stem and roots to the grain during grain filling – “ Compensatory translocation” The amount of pre-anthesis assimilate partitioned to the grain is dependent on the timing and severity of water deficits as it affects the source-sink balance.

C. PHOTOSYNTHETIC POTENTIAL OF PLANTS There are basically two types of plants C-3 and C-4. In C-3 plants, the earliest labeled compound is 3-phosphoglycerate (PGA). The 5-Carbon substance, ribulose diphosphate (RuDP) combines with one molecule of CO2, presumably forming an unstable 6-carbon substance which immediately splits into two molecules of PGA, the 3-carbon substance which gives the C-3 cycle its name. Ribulose diphosphate + CO2 5C atoms 2 X PGA

1 C atom 2 X 3C atoms

The reaction is catalyzed by RuDP carboxylase C-4 plants, (most tropical grasses and cereals, except rice and a few dicots) have 4-carbon organic acid, malate or aspartate) as the immediately labeled product which is formed by the combination of 3-carbon phosphoenol pyruvate (PEP) with CO2 PEP + CO2 3C +1C PEP carboxylase malate or aspartate 4C

A feature of the C-3 is photorespiration which reduces net CO2 assimilation rates in all C-3 species especially at high leaf temperatures. C-4 plants actually contain the enzymes for C-3 photosynthesis including RuDP carboxylase but they are confined to certain cells in the leaf surrounding the smaller veins and called the bundle sheath cells. In the mesophyll cells, CO 2 is fixed to form the C-4 acid which are translocated to the bundle sheath cells where they dissociate to release CO2 which is re-fixed by RuDP carboxylase. The other dissociated product, probably pyruvate, is translocated back to the mesophyll.

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The net Ps rates of C-4 plants are higher than in C-3 plants, whereas the rate of water use is not greatly different from C-3 species, the WUE of C-4 plants may be 50-100% better than C-3 species.

SOME PHOTOSYNTHETIC CHARACTERISTICS OF C-3 AND C-4 PLANTS
Sn 1 CHARACTERISTIC Leaf anatomy C-3 No distinct bundle sheath of photosynthetic cells Ribulose diphosphate (RuDP) carboxylase C-4 Well organized bundle sheath , rich in organelles (starch storing chloroplasts) Phosphoenol pyruvate (PEP) carboxylase + RuDP carboxylase 1:5:2 250-350 3.9+0.6 Yes 0-10

2

Carboxylating enzyme

3 4 5 6. 7.

8.

Theoretical energy requirement (CO2:ATP:NADPH) Transpiration ratio g/H2O/g dry wt increase Leaf chlorophyll a to b ratio Requirement for Na+ as a micronutrient CO2 compensation point (ppm CO2) i.e CO2 conc. at which net Photosynthesis ceases Photosynthesis inhibited by 21% O2

1:3:2 450-950 2.8 +0.4 No 30-37

Yes

9. 10. 11.

Photorespiration detectable?. Optimum temperature for photosynthesis Dry matter production tons/ha/year

Yes 15-25oC 22+0.3

No.