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C3 Photosynthesis and the Evolution of Leaves
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C3 Photosynthesis and Evolution of LeavesMechanism of C3 Photosynthesis
Photosynthesis: trapping (fixation) of CO2 and its
subsequent reduction to carbohydrate, using hydrogen
from water and light energy
- Conversion of CO2 and water to oxygen and sugars
using light energy
Adaptations for maximum sunlight absorption and
minimise water loss (at the same time allowing efficient
gaseous exchange)
1. Trapping light energy
- chlorophylls (a – primary pigment and b), accesoory pigments (cartotenoidsL β-
carotene, xanthophyll)
- chlorophyll a peaks at 430 nm, 660 nm, chlorophyll b peaks at 450nm, 63-640 nm
(blue, red light absorbed)
2. Light absorbed by photopigments excites electrons in the pigment molecules
- Pigments arranged in light-harvesting clusters – Photosystems
- Several hundred accessory pigments pass the energy absorbed to the primary at
the reaction centre
Light reactions – chlorophyll a
molecule absorbs light reaction
(tetrapyrrole ring structure
with phytol chain)
- Conjugated double bonds
can abosorb light – excited
by photon, emitting an
election
- Resonance energy transfer allows energy to be
passed from one chlorophyll molecule to another
PSI and II embedded into thylakoid membrane
Most abundant protein on earth: Rubisco
, Evolution of
megaphylly leaves and success of
land plants
Early vascular
plants (eg. Rhynia) in the Devonian did not
have leaves
Bryophytes (eg. mosses) had no xylem or
phloem
Mosses don’t have true leaves (no veins),
clubmosses have microphylls (leaves of
small size with one vein)
Megaphylls: no clear definition/single
evolutionary origin – leaves of generally larger
size with complex venation
Leaves have evolved independently multiple times (evolved
at least 2 independent occasions)
Microphylls
Distinctive vasculature, and usually unbranched venation, thought to have
evolved from spine-like enation and predate megaphylls in terrestrial fossil
record
, Megaphyll evolution
Megaphylls altered evolutionary trajectory of
terrestrial plant and animal life, biogeochemical
cycling of nutrients
Earliest ancestral vascular plants (dating back to late
Silurian 410 Myr ago – Edwards and Wellman, 2001) –
composed of simple or unbranched axial stems with
sporangia but no leaves
- Plants remained leafless for next 40-50 Myr, with
megaphylls finally becoming widespread at close
of the Devonian period (360 Myr ago)
Based on fossil record – Rhynia initial equal branching
Overtopping – Certain stems grew longer than others
Planation: all stems grow in the same plane
Webbing: Leaf blade forms around stems
Earliest land plants ~423 million years ago, leaves formed in the Devonian period
(360-400 mya), Flowers formed in the Cretaceous period (~130 mya)
Surprise in the delayed appearance of leaves of a seemingly simple developmental
modification
- Palaeontological evidence shows the structural framework necessary for
assembling a simple laminated leaf blade (meristem, vasculature, cuticle,
epidermis) was in place long before advent of large megaphylls
- Same interval marks unparalleled burst in evolutionary innovation in the history
of platn life – witnessed from rise of trees from herbaceous ancestors,
evolution of complex life cycles, including the seed habit
- Tiny deeply incised megaphylls of rare early-Devonian plant Eophyllophytum
bellon from Chinese rocks shows that plants had the capacity to produce a
simple megaphylly, but plants were not able to develop the leaf morphology till
much later…why?
Understanding of megaphyll evolution stemmed from Zimmerman’s telome theory
1. Ancestral form of a dichotomising axis branching out in 3 dimensions and
typified by rhyniophytes represents basal sate
2. Evolutionary “overtopping” followed producing a main axis bearing reduced,
lateral, determinate photsynthetic stems, branching out in three dimensions (eg.
trimerophytes)
3. 3D lateral branch systems of terete stem segments then become flattened into
a single plane (eg. cladoxyleans)
4. Webbing of photosynthetic mesophyll tissue joined flattened segments of the
lateral branches to form a laminate leaf blade (eg. some prgymnosperms)
- Transformation of branch into leaf achieved by simple modification of existing
organs rather than a major change in body plan
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