Lecture 0+1, Intro+Growth and Development

Question 1

Introduction to plant growth

Answer 1

-Ability to photosynthesise: readily obtain energy and nutrients (AUTOTROPHS)
-Never evolved the complexity that enables mobility in animals
-Rigid anatomy (cell wall): tough and rigid
-Cells are added progressively to the body in a continuous mode (whereas in animals they grow, stop, and old are replaced)
-Presence of meristematic tissues (cells constantly able to divide)
-Adaptive growth (plasticity, only in plants) is crucial to cope with the environment: grow in line with the environment; more competition for light=taller.

Question 2

General introduction

Answer 2

Agroecosystem is 40% of terrestrial ecosystems. Human health, well being and life quality impacted by:
-Soil infertility, desertification, land degradation
-Agronomic losses due to water and temperature extremes
-Decline in quantity and quality of agricultural produce

-Crop pests, disease outbreaks, newer weed infestation
-Loss of agrobiodiversity (flora and fauna) and species habitat loss
-Huge water scarcity, fallow land and fluctuation in market food prices

Question 3

Growth and Development: 1. External and Internal Factors

Answer 3

(All influence each other)
-Developmental genes/transcription factors
-Phytohormones
-Environmental factors

Question 4

Growth and Development: 2. Developmental Features

Answer 4

1. Plant architectural traits
   -Plant height
   -Number of tillers/branches
   -Leaf/branch angle
   -Prostrate habit
2. Leaf Features
   -Morphology (leaf length, width, area, shape)
   -Anatomy (organization of mesophyll, bundle sheath and vasculature)
   -Leaf growth rate and stay green features
3. Vasculature features
   -Vein density
   -Vein diameter
   -Interveinal distance
   -Proximity of veins to mesophyll/bundle sheath

Question 5

Growth and Development: 3. Physiological effects

Answer 5

1.Plant Architectural traits affect
-Amount of light interception
-Sink strength

2. Leaf features affect
   -Photosynthetic capacity
   -Source strength
3. Vasculature features affect
   -Transport of photosynthates
   -Source-sink relationship.

All three then determine the BIOMASS and YIELD of the plants.

Question 6

3 major stages of plant growth

Answer 6

1. Embryogenesis
2. Vegetative development
3. Reproductive Development

Question 7

3 major stages of plant growth: Embryogenesis

Answer 7

not covered in this course:
1. zygotic stage
2. globular stage
3. heart stage
4. torpedo stage
5. mature stage

Question 8

3 major stages of plant growth: Vegetative development- ROOT DEVELOPMENT

Answer 8

1. Develops from a differentiated tissue
2. Undifferentiation stimulus
3. Cell cycle with asymmetrical cell division lateral root primordium
4. Expansion of these cells, new differentiation and emergence:
   -Initiation and growth: stimuli such as
   a.) Hormones (mainly auxin)
   b.) gene expression (some are NOT auxin-dependent: ABERRANT LATERAL ROOT FORMATION)
   c.) Nutrients and stress (salinity)
   -Nutrient availability regulate the structure and function of the root system
   -Case study: Nitrogen (N); High [NO3-] Stimulates growth of long and sparsely-branched primary roots
   High [NH4+] represses root growth, short, thick and highly branched roots

Question 9

3 major stages of plant growth: Vegetative Development- SHOOT ARCHITECTURE

Answer 9

-Phytomers: structural modules of shoots
-Branching greatly varies among the species:
a.) genetically determined
b.) extremely responsive to the environment
c.) plasticity

-Source of phenotypic plasticity:
a.) auxin is an essential regulator of bud opening
b.) axillary buds= dormant buds
c.) apical dominance:
-inhibitory influence of the primary shoot apex on the growth of the axillary bud(s)
-when primary shoot tip removed, apical dominance is suppressed and axillary buds grow out into branches.

Question 10

3 major stages of plant growth: Reproductive development- OVERALL

Answer 10

The ‘green revolution’: shift in agriculture from small, family operated farms to large, industrial-scale agribusiness; increased use of mechanisation, GMOs, irrigation, fertilisers and pesticides.

Pros: Greatly increased efficiency of land use (higher yield per ha), short-term profitability and food supply

Cons: Soil erosion, biodiversity loss, surface water contamination, Farmer’s dependency on big corporations.

-Until mid-20th century, domesticated cereals like wheat+rice were tall (like the wild ancestors they derived from)

Question 11

3 major stages of plant growth: Reproductive Development- PHYSIOLOGY

Answer 11

Green Revolution: reduced height+improved yield
-Reduced height/ Rice: GA20-oxidase and sd1 gene
-Reduced height / Wheat: Rht alleles

Lodging: collapsing stems, with loss of harvestable grain
-The most important genes controlling plant height can manage plant’s ability to synthesise/respond to Gibberellins
a.) In rice, semi-dwarf varieties result from a defect in gibberellin biosynthesis
b.) In wheat, gibberellin insensitivity is the basis for the dwarfing trait

Semi-dwarf varieties of wheat:
1.Norin 10 dwarfing genes from a Japanese variety Daruma crossed with high-yielding American wheat

2. Most semi-dwarf bread wheat genotypes have Rht (reduced height) alleles from the variety Norin 10
   Rht proteins negatively regulate GA signalling