Chapter 9 Photosynthesis:
Physiological and ecological considerations
 Address the intact leaf under natural conditions
 The major environmental factors:
directly: light, temperature, and ambient [CO2],
indirectly: humidity, soil moisture
 Physiologists, agronomists, and ecologists
 Limiting factors for photosynthesis :
the slowest step determine the rate of photosynthesis
Rubisco activity, ribulose bisphosphate regeneration, triose phosphate
metabolism  for C3 plants
 A “supply” and “demand” function
Three light parameters:
spectral quality, amount, and direction
Full sunlight,
incandescent light,
fluorescent light
quanta
watt (W)= J/s
Systeme International unit, p.200
Under direct sunlight 2000 mole m-2s-1
 900 Wm-2
Lux or foot-candles:
lumen (lm):
the luminous flux on a unit surface, all points of which are at
unit distance from a uniform point source of one candle.
Intensity was expressed either as foot candles (lm ft-2) or
lux (lm m-2)
based on the perception of light by the human eye, which is
maximally sensitive to light within the green region of the
spectrum, at 555 nm.
Instruments
calibration
Leaf anatomy maximizes light absorption
fluorescence
About 85 to 90% of PAR is
absorbed by leaf
Transparent to visible light and
convex (focus light) at epidermal
cells
Palisade cells: light to pass through
sieve effect: chlorophyll is confined to the chloroplasts
light channeling: through the central of vacuole and air space between the cells
Spongy cells: light absorption
light scattering: increase the travel length
C3
Hairs, salt glands, and
epicuticular wax (reflect light)
Plant compete for sunlight
 A canopy configuration
 A rosette growth: leaves grow radially
 A branching structure: increase interception
 Sunflceks: one of the characteristics of shade plants
capture a short burst of sunlight
dandelion
Solar tracking:
 The leaf blade is perpendicular to the incident light
 alfalfa, cotton, soybean, bean, lupine, and mallow family
 Circadian rhythms: sun rise, sun set; intermittent cloud, night/dawn
 a blue-light response of leaf movement
 Photosensitive region: major leaf veins
 Pulvinus: is the organ to control the leaf orientation, at the junction between
the blade and petiole
 Heliotropism: paraheliotropic vs. diaheliotropic (short-lived plants)
Lupinus succulentus
向日葵?
Pulvinus: at the junction between the blade and petiole
The mechanism of leaf movement
– a rapid response to phytochrome involving ion fluxes across membranes causes
turgor changes
 pH change also involve
Plant acclimate and adapt to sun and shade
p. 672
If tolerance increases as a result of exposure to prior stress,
the plant is said to be acclimated (or hardened)
 environmental
A genetically determined level of resistance acquired by a
process of selection over many generations is adaptation
 genetic
p. 203
Glossary
Photosynthetic responses to light by the intact leaf
Light compensation point (LCP): the photon flux when photosynthetic
CO2 assimilation equal to CO2 release by mitochondria respiration
LCP varied with species and developmental conditions
A C3 plant
The carboxylation capacity of
rubisco or the metabolism of
triose phosphates
Light compensation point
saturation
Sun plants:
10~20 μmole m-2 s-1
Shade plants:
1~5 μmole m-2 s-1
Maximal photosynthetic rate
sun  shade plants
Maximum quantum yield
the slope (p. 131)
Quantum yield:
No. of photochemical products / Total No. of quanta absorbed
Gas solubility/ temp
C4 plants: CO2 concentration
C3 plants: photorespiration
lower temp, higher
quantum yield
Growth conditions
affect the photosynthetic properties of a leaf