Plant growth and development

1. PLANT DEVELOPMENT

2. Stages in plant’s developmental
cycle
 Seed germination
 Juvenility
 Maturity
 Flowering
 Fruiting
 Seed development
 Senescence

3. GERMINATION AND
DORMANCY
 Dormancy – periods of suspended growth
usually in seeds and buds
 During this period:
 Seeds fail to germinate
 Buds show depressed axis elongation and
development of a compact array of nodes
enclosed in bud scales
 Seeds and buds partly dehydrated
 Respiration at very low level – with high
respiratory quotient (high CO2/O2 ratio)
 Closing down of synthetic capabilities of organ

4. SEED GERMINATION
 Processes involved:
 Water imbibition
 Formation of enzyme systems
 Commencement of growth and radicle
emergence
 Seedling growth

5. 0
10
20
30
40
50
60
70
80
90
dormant
Latent phase Second increase
jn fresh weight
nondormant
Initial
increase in
fresh
weight

6.  Initial increase in fresh weight – hydration of seed
tissues
 Latent phase – seed develops metabolic systems
necessary for growth and the enzyme systems
involved
 Enzymes: may be released or activated from
existing proteins or from de novo synthesis ie.
amylase, amylopectin glucosidase, ammonia
lyase, protease, nitrate reductase, etc.
 RNA synthesis - repressed
 Protein synthesis
 2nd increase in fresh weight
 Radicle emergence
 DNA synthesis

7. Seedling Growth
 From the emergence of the radicle from the
seed to the emergence of the seedling apex
from the soil, seedling growth is subterranean
 In the absence of light, subterranean growth is
a process of exaggerated elongation, with the
stem modified in several ways to facilitate
progress through the soil
 In grasses, growing point is enclosed in a
cylinder, the coleoptile and stem elongation
occurs primarily in the mesocotyl

8.  In dicots, modifications include:
 Compaction of the stem apex into a hook and lack
of leaf expansion
 Cotyledons of some species arise from the soil
and become photosynthetic like leaves

9. Seed dormancy
 Causes:
 Seed impermeability due to mechanical restriction
by the seed coat
 Presence of a hygroscopically activated valve in
the hilum
 Presence of growth inhibitors,ABA, phenolics
especially flavonoids in the embryo or in the seed
coat

10. Breaking Dormancy
 Seed coat scarification
 Mechanical rupturing of the seed coat by
cutting, abrasion, puncturing, freezing,
boiling, etc
 By soaking in concentrated sulfuric acid
 Light treatment – photoperiodic exposures
 Temperature treatments – low or high
temperature treatments
 Chemical treatments
 Cytokinin, gibberellin, potassium nitrate, thiourea

11. Juvenility
 Period when the plant is capable of
exponential increases in size, when flowering
cannot be readily induced and when the plant
develops characteristic morphological forms
(leaves, stems, thorns, tendrils, etc.)

12. Juvenile morphology
 Leaf form
 In some species, juvenile leaves are simple which
become increasingly lobed towards maturity
 In pine, juvenile leaves are short and in spirals,
mature ones in fascicles
 In pea, juvenile leaves are reduced to scales
 Stem
 in citrus, juvenile stems have thorns
 In ivy, juveniles are creeping vines that mature
into shrubs

13. Juvenile Physiology
 Juvenile wood roots more readily than mature
wood – may be due to higher endogenous
auxin levels in juveniles
 In apples, apple plants are repeatedly cut back
to induce juvenile basal shoots which will then
become the source of cuttings

14. Modifying juvenility
 Done to:
 intensify growth and pass through the juvenile
phase more rapidly e.g. fertilizer application
 Retard growth to obtain early flowering e.g.
Grafting mature scions onto juvenile stocks or
retard growth to delay flowering or to obtain a
desired bushy appearance e.g. pruning

15. MATURATION
 State of development which is capable of
flowering
 Achieved through gradual transition of
morphology, growth rate and flowering
capacity
 Development of mature leaf and stem forms
 Reduction in growth rate
 Production of flowers, fruits and seeds

16.  In the apical meristem, there is gradual
increase in the height of the meristematic
dome, gradual increase in RNA, and increased
activity of lateral meristems close to the apical
dome

17.  Flowering provides the plant with the
mechanism for genetic outcrossing, and a
means of securing a great variety of genetic
recombinations by which new adaptations are
made possible
 Fruiting or fruit development results of
pollination followed by fertilization
 Parthenocarpy – fruit development without
fertilization, fruits are seedless
 Apomixis – Seed development without fertilization
Flowering and Fruiting

18. SENESCENCE
 Deteriorative process which naturally
terminate the functional life of an organ or an
organism
 In some plants, gradual encroachment of
deteriorative processes while in others this
may be abrupt deterioration that lead to death
 Natural deteriorative changes in contrast to
aging which refer to changes without reference
to the natural development of death

19.  Central function of senescence is to facilitate
turnover
 Turnover of organisms - while most animals
experience turnover through predation or
parasitism, plants are only rarely removed by
physical removal and in many instances
achieve turnover through programmed
senescence
 Turnover of substrates, enzymes, nucleic
acids, organelles, etc

20. Plant Senescence
 In annual and biennial species, death commonly
occurs upon completion of fruiting
 Causes:
 mobilization of nutrients out of the vegetative parts
into the reproductive parts causes senescence
through a starvation process
 Deterioration of photosynthetic efficiency due to
decrease in chlorophyll and protein content of leaves
 Suppression of root development during fruiting.
Since roots are a major source of cytokinins, there is
drastic decrease in their levels leading to senescence
 Meristem sencescence

21. Leaf Senescence
 Decline in chloroplasts resulting to loss of
chlorophyll
 Increase in chromoplasts leading to increase
in carotenoid pigments
 Decline in ribosomes and rough ER
 Development of lesions in the nuclear
membrane
 Disruption of the cell membrane resulting to
increased permeability