Photorespiration

Photosynthesis 6CO2 + 6H2O C6H12O6 + 6O2 Respiration (mitochondrial respiration) C6H12O6 + 6O2 6CO2 + 6H2O

Photorespiration C6H12O6 + 6O2 6CO2 + 6H2O

The relative rate of photosynthesis is a function of temperature.

Hew, Krotkov, Canvin, Plant Physiol 44:671, 1969

Sunflower leaves

The relative rate of photosynthesis is a function of temperature.

The relative rate of photosynthesis is a function of temperature.

Why is Rubisco called a “carboxylase-oxygenase?

- Alternative activity at high temperatures:

HO

The metabolismof phospho-glycolateinvolves enzymes In the peroxisomeand the mitochondrion, as well as thechloroplast

The metabolismof phospho-glycolateinvolves enzymes In the peroxisomeand the mitochondrion, as well as thechloroplast

The metabolism of phosphoglycolate involves enzymes In the chloroplast, peroxisome and the mitochondrion, as well as the chloroplast

2 RuBP + 2 O2 2 P-glycolate + 2 PGA O2 ATP 2 P PGA 3 PGA

CO2

2 RuBP + 3 O2 + ATP 3 PGA + CO2 + 2 Pi

Why does high temperature promote photorespiration?

CO2 vs O2 solubilities

An increase in temperature lowers the concentration of CO2 in the cytoplasm relative to the concentration of O2.

Is the formation of P-glycolate by rubisco an “evolutionarymistake,” which occurred because the primordial atmospherehad a low O2 concentration?

If high temperatures increase photorespiration, willglobal warming result in a decrease in plant productivity?

Or will an increase in atmospheric CO2 produce a general increase in plant productivity?

If high temperatures increase photorespiration, willglobal warming result in a decrease in plant productivity?

The high temperatures excluded gymnosperms, especially from low latitudes.

Some plants show better net photosynthesis at high temperaturesthan others.

Some plants show better net photosynthesis at high temperaturesthan others.

Apparent photosynthesis=

Gross photosynthesis- Photorespiration- Mitochondrial

Respiration

What accounts for thedifference in optimumtemperature betweenSpartina and Leucopoa?

Summary

•The apparent rate of photosynthesis is a function of temperature,with most plants showing a maximum at about 25oC.

•Rubisco can react with O2, to produce P-glycolate and P-glycerate.

•The metabolism of phosphoglycolate results in O2 uptake and CO2production: photorespiration.

•High temperature promotes photorespiration by increasing O2/CO2.

•Some plants show maximum photosynthesis at 35-45oC.