Department: Plant Physiology and Crop
Production
College of Plant Science

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A basic Biological Axiom

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Homeostasis
Control
Regulation
Growth
Nasty
Tropism
Photomorphogenesis
Thigmotropism
Osmoregulation,
Autopoiesis

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Understanding the concept of homeostasis, regulation and control
Life as organisational homeostasis and its biological implications

 biological stability
Basis for biological stability could be ascribed to circularity observed in living systems. For example interconnectedness and interrelatedness of biochemical pathways forming a coherent unit.

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Organisational invariance
Autonomy
Self-referentiality

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Physiological
Ecological

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Level of physiological activities is within certain limit for it to operate;
All physiological processes operate within certain concentration of solutes, temperature and pH.

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Presupposes that there is a certain correspondence; functional and structural between the biological system and its environment. This is evident in the cycle of certain elements in nature, such as water, nitrogen, carbon, phosphorus cycles and the formation of different adaptive mechanisms to various ecological conditions. One vivid example is the formation of different ecotypes of plant depending on their adaptability to available water.
Mesophytes
Hydrophytes
Xerophytes
Halophytes

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Biological stability = Coordination or control

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Perturbation: Any environmental factor, capable of disrupting system’s stability. These factors are Abiotic and biotic in nature
Sensor: element for detecting difference in status from the system goal. Within the context of a plant, there are different sensors; such as phytochrome, cryptochrome, phototropin and zeaxianthin.
Perceptor: plant organs
Model: The genetic composition of the plant
Goal: homeostasis
Information processing: signalling elements and signal transduction
Decision making: System survivability and senescence
Effector: Plant organ
Action: System’s response, in plant they could take the following forms; growth, nasty, morphogenesis, tropism and thigmotropism

Scope of balance
Water
Nitrate
Glucose
Temperature
Process nomenclature
Organ/regulatory mechanism
Animal Plant
Osmoregulation Kidney
Kidney
Thermoregulatio Skin
1. Active accumulation of osmolyte independent of cellular volume
2. Uptake of compatible ions
3. Ion extrusion and sequestration
Nitrogen cycle
Glycolysis and
Glycogenesis
Transpiration

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Basic concepts: Osmoregulation, transport, transporters, active and passive transport, primary and secondary transport, symport, antiport

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Water balance in plants and strategies for acclimation and adaptation
Osmoregulation as a mechanism for maintaining water balance in plant
Methods of eliminating waste product in plants
Transport mechanism in plant

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Synthesis and accumulation of osmolytes and osmoprotectants
◦ Organic nitrogen-containing
◦ Organic non-nitrogen containing
Uptake of compatible ions
Extrusion, sequestration and compartmentalisation of incompatible ions

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Amino acids e.g. proline, glycine betaine
Amino acids derivatives
Quaternary amino acids

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Sugars
Cyclic and acyclic polyols; mannitol, sorbitol
Fructans
Sulphonium compounds

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Accumulation of these substances in the cell
to the disruption of normal metabolic activities

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Water balance in cell
Osmoprotective functions such as the protection of the protein stability, scavenging reactive oxygen radical
Adjustment of cellular redox state and membrane stabilisation.

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Organs: Vacuole, Golgi bodies and
Endoplasmic reticulum, leaf

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Channels
◦ Selective (Potassium Inward Regulated Channel,
KIRC; Potassium Outward Regulated Channel,
KORC, Aquaporin)
◦ Non-Selective
Carriers; High and low affinity carriers
Pumps
◦ Electrogenic (H + / ATP-ase, H + /PP)
◦ Electroneutral

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Definition of growth:
◦ A process of irreversible increase by cell division and enlargement, including synthesis of new cellular material and organization of sub cellular organelles
◦ Process involving conversion of reserve materials into structural materials

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Increase in fresh weight
Increase in dry weight
Volume
Length
Height
Surface area

◦ Determinate – flower buds initiate terminally; shoot elongation stops; e.g. bush snap beans
◦ Indeterminate – flower buds born laterally; shoot terminals remain vegetative; e.g. pole beans

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Annuals
◦ Herbaceous (nonwoody) plants
◦ Complete life cycle in one growing season
◦ See life cycle of angiosperm annual

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Biennials
◦ Herbaceous plants
◦ Require two growing seasons to complete their life cycle (not necessarily two full years)
◦ Stem growth limited during first growing season;
Note vegetative growth vs. flowering e.g. celery, beets, cabbage, Brussels sprouts

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Perennials
◦ Either herbaceous or woody
◦ Herbaceous roots live indefinitely (shoots can)
 Shoot growth resumes in spring from adventitious buds in crown
 Many grown as annuals
◦ Woody roots and shoots live indefinitely
 Growth varies with annual environment and zone
 Pronounced diurnal variation in shoot growth; night greater

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Variation in pattern with species and season
Growth peaks in spring, late summer/early fall
◦ Spring growth from previous year’s foods
◦ Fall growth from summer’s accumulated foods
Some species roots grow during winter
Some species have some roots ‘resting’ while, in the same plant, others are growing

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Definition:
◦ Process of qualitative change in a living system over time

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Development is phasic in nature, i.e. progression from one physiological system state of the meristerm to another
Identified are two phases; vegetative and reproductive phases
Plant system possesses the capability of development to progress autonomously
The identifies phases of development are irreversible
Development process is controlled by various environmental and genetic factors, mainly; temperature and photoperiod (G X PX T)
Photoperiod gene and vernalisation genes possesses delaying impact on the process of development
Temperature effect is through Q
10 effect on the activities of the enzymes and ultimately on the biochemical reaction

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Phasic development
◦ embryonic growth
◦ juvenility
◦ transition stage
◦ maturity
◦ senescence
◦ death
During maturation, seedlings of many woody perennials differ strikingly in appearance at various stages of development

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Juvenility
◦ terminated by flowering and fruiting
◦ may be extensive in certain forest species
Maturity
◦ loss or reduction in ability of cuttings to form adventitious roots
Physiologically related
◦ lower part of plant may be oldest chronologically, yet be youngest physiologically (e.g. some woody plants)
◦ top part of plant may be youngest in days, yet develop into the part that matures and bears flowers and fruit

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Life spans among plants differ greatly
◦ range from few months to thousands of years
◦ clones should be able to exist indefinately
Senescence
◦ a physiological aging process in which tissues in an organism deteriorate and finally die
◦ considered to be terminal, irreversible
◦ can be postponed by removing flowers before seeds start to form

Parameters for comparison
Energy dimensio n
Scope of changes
Implicati on of changes
Induction factor
Scope of induction
Cumulative effect
Aging
Passive
Senescen ce
Active
Accumulative Increase in entropy
Deteriorative/ degradative
Loss of homeostasis
(dynamic equilibrium)
Time
Time
Hormone
Environmental factors nutrient
Unprogrammed
Uncontrolled
Loss of system identity
Programmed controlled
System death
(Loss of system functionality)

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Phases
◦ Flower induction and initiation
◦ Flower differentiation and development
◦ Pollination
◦ Fertilization
◦ Fruit set and seed formation
◦ Growth and maturation of fruit and seed
◦ Fruit senescence

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DNA directs growth and differentiation
◦ Enzymes catalyze biochemical reactions
Structural genes
◦ Genes involved in protein synthesis
Operator genes
◦ Regulate structural genes
Regulatory genes
◦ Regulate operator genes

◦ Believed to include:
 Growth regulators
 Inorganic ions
 Coenzymes
 Environmental factors; e.g. temperature, light
 Therefore . . .
 Genetics directs the final form and size of the plant as altered by the environment

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Flower induction and initiation
◦ What causes a plant to flower?
 Daylength (photoperiod)
 Low temperatures (vernalization)
 Neither

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Photoperiodism: Phenomenon of plant response to relative length of day to night
◦ Short-day plants (long-night; need darkness)
◦ Long-day plants (need sufficient light)
◦ Day-neutral plants (flowering unaffected by period)
Change from vegetative to reproductive

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Low temperature induction
Vernalization
◦ “making ready for spring”
◦ Any temperature treatment that induces or promotes flowering
◦ First observed in winter wheat; many biennials
◦ Temperature and exposure varies among species
◦ Note difference/relationship to dormancy
Many plants do not respond to changed daylength or low temperature; agricultural

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Flower development
◦ Stimulus from leaves to apical meristem changes vegetative to flowering
◦ Some SDPs require only limited stimulus to induce flowering; e.g. cocklebur – one day (night)
◦ Once changed the process is not reversible
◦ Environmental conditions must be favorable for full flower development

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Pollination
◦ Transfer of pollen from anther to stigma
◦ May be:
 Same flower (self-pollination)
 Different flowers, but same plant (self-pollination)
 Different flowers/plants, same cultivar (selfpollination)
 Different flowers, different cultivars (cross-pollination

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Self-fertile plant produces fruit and seed with its own pollen
Self-sterile plant requires pollen from another cultivar to set fruit and seed
◦ Often due to incompatibility; pollen will not grow through style to embryo sac
◦ Sometimes cross-pollination incompatibility

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Pollen transferred by:
◦ Insects; chiefly honeybees
 Bright flowers
 Attractive nectar
◦ Wind
 Important for plants with inconspicuous flowers
 e.g. grasses, cereal grain crops, forest tree species, some fruit and nut crops
◦ Other minor agents – water, snails, slugs, birds, bats

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What if pollination and fertilization fail to occur?
Fruit and seed don’t develop
Exception: Parthenocarpy
◦ Formation of fruit without pollination/fertilization
◦ Parthenocarpic fruit are seedless

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Fertilization
◦ Angiosperms (flowering plants)
 Termed double fertilization
◦ Gymnosperms (cone-bearing plants)
 Staminate, pollen-producing cones
 Ovulate cones produce “naked” seed on cone scales

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Fruit setting
◦ Accessory tissues often involved
 e.g. enlarged, fleshy receptacle of apple and pear
 True fruit is enlarged ovary
◦ Not all flowers develop into fruit
◦ Certain plant hormones involved
◦ Optimum level of fruit setting
 Remove excess by hand, machine, or chemical
 Some species self-thinning; Washington Navel Orange
◦ Temperature strongly influences fruit set

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Fruit growth and development
◦ After set, true fruit and associated tissues begin to grow
◦ Food moves from other plant parts into fruit tissue
◦ Hormones from seeds and fruit affect growth
◦ Auxin relation in strawberry fruits
◦ Gibberellins in grape
◦ Patterns of growth vary with fruits

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Change of Appearance
Scope:
Pigmentation
Green→ yellow or other characteristic colours
Dimensions:
Increase in the activity of chlorophyllase
Sequestration of pigment
Development of carotenoid and anthocyanin in the presence of light and phytochrome
Unmasking of certain pigments

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Changes in Texture
Scope:
Softening
Hard→ Soft
Dimensions:
Hydrolysis of
Cell wall (solubilisation of pectic substances in middle lamellae via methylation of galaturonic acid, reduction in size of polygalacturonide or both
Cell content

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Changes in Flavour
Scope:
Development of characteristic
Aroma
Taste
Polymers→ monomers
Loss of astringency
Dimensions:
Production of the secondary metabolites
Hydrolytic changes of biopolymers

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Changes in condition
Scope:
Increasing degree of perishability
Climacteric respiratory pattern
Non-climacteric respiratory pattern
Dimension:
Catabolic process>Anabolic process
Increasing activity of growth inhibitors e.g.
C
2
H
2 and ABA

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Light
Temperature
Water
Gases

 Quality- Photosynthetic Active Radiation (400nm-
700nm), photomorphogenesis, phytochrome absorbs red (660nm) and far-red (730nm) but not at same time
◦ Quantity- Phototropism
◦ Duration- Photoperiodism

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Temperature
◦ correlates with seasonal variation of light intensity
◦ tropical-region growth between 25 ° C and 35 ° C
◦ high light intensity creates heat; sunburned, heat stress
◦ low temp injury associated with frosts; not common in the tropics

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Water
◦ most growing plants contain about 90% water
◦ amount needed for growth varies with plant and light intensity
◦ transpiration drives water uptake from soil
 water pulled through xylem
 exits via stomates
◦ evapotranspiration - total loss of water from soil
 loss from soil evaporation and plant transpiration

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Gases
◦ Nitrogen is most abundant
◦ Oxygen and carbon dioxide are most important
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 plants use CO plants use O
2
2 for photosynthesis; give off O for respiration; give off CO
2
 stomatal opening and closing related to CO
2
2 levels?
 oxygen for respiration limited in waterlogged soils
 increased CO
2 levels in atmosphere associated with global warming
 additional pollutants harm plants

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Learning objectives:
Understanding the concept phytohormones and their roles in growth of plant
Classification of phytohormones and their roles in cell division, elongation and differentiation

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Phytohormones are physiologically active substances that affect plant growth and development in conjunction with other environmental factors.

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They are required in small quantity,
Transported from the site of synthesis to mediate physiological response in other parts of the plant.
The have organic origin
They are naturally occurring or synthetic
Non-nutrient chemicals:
◦ Brassinosteroids
◦ Jasmonic Acid
◦ Salicylic Acid
◦ Polyamines

Growth promoters:
1.
Auxins
2.
Gibberellins
3.
Cytokinins

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Growth Inhibitors
Ethylene
Abscisic acid

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See table 3 of the lecture note

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Learning Objectives:
Understanding of the basic principle of respiration
Understanding of the mechanism of respiration
Comparative analysis of aerobic and anaerobic (Fermentation) respiration
Factors affecting respiration
Importance of respiration in agricultural process

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 bio- oxidative process; involving loss of electron, proton and the addition of oxygen.
The process of converting sugars and starches into energy through a series of biochemical steps.
Biochemical process of degradation of biological polymers into monomers, with energy and other metabolites
Redox reaction

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Energy is released which is consumed in various metabolic processes essential for plant and activates cell division
It brings about the formation of other necessary compounds participating as important cell constituents
It converts insoluble food into soluble form
It liberates carbon dioxide and plays a part actively in maintaining the balance of carbon cycle in nature
It converts stored energy (potential energy) into usable form (Kinetic energy)

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Cytosol
Mitochodria

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Throughout the life of the plant

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Initial degradation (hydrolysis)
Partial degradation
(glycolysis/EMP/oxidative pentose phosphate pathway/Enter-Doudoroff pathway)
Total degradation (Krebs cycle and electronic transport system)

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It is common to all plants
It goes on throughout the life
Energy is liberated in larger quantity. In total,
38 ATP molecules are formed
The process is not toxic to plants
Oxygen is utilised during the process
The carbohydrates are oxidised completely and are broken down into CO
The end-products are CO
2
2 and H and H
2
2
O
O
The process takes place partly in cytosol
(glycolysis) and partly inside mitochondria
(Krebs cycle)

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It is a rare occurrence
It occurs for a temporary phase of life
Energy is liberated in lesser quantity. Only 2
ATP molecules are formed
It is toxic to plants
It occurs in the absence of oxygen
The carbohydrates are oxidised incompletely and ethyl alcohol and carbon dioxide are formed
The end-products are ethyl alcohol and carbon dioxide
The process occurs only in the cytosol

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Growth Respiration
Maintenance Respiration

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R = g r
G + m r
W
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Where:
R: Respiration g r: Coefficient of Growth Respiration
G: Growth Respiration
M r : Coefficient of Maintenance Respiration
W: Maintenance Respiration

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Structural maintenance of the cellular structures
Gradient of ions and metabolites across the membrane
Phenotypic plasticity
Turnover of macromolecules

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Active uptake of ions
Assimilation and reduction of NO
3 and SO
4
Synthesis of biological monomers
Polymerisation of biological monomers
Translocation of assimilates
Tools maintenance

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Relationship between photosynthesis, respiration and growth on crop performance