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

watt (W)= J/s quanta Systeme International unit, p.200 Under direct sunlight 2000 mole m-2s-1  900 Wm-2 watt (W)= J/s

Lux or foot-candles: lumen (lm): Instruments calibration 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 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 The carboxylation capacity of rubisco or the metabolism of triose phosphates A C3 plant

Light compensation point 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) saturation

Quantum yield: No. of photochemical products / Total No 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