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Abstract

The effect of environmental factors on the post-illumination burst of CO2 (PIB) and O2 inhibition of apparent photosynthesis (APS) in wheat (Triticum aestivum L.) was studied in an open gas exchange system utilizing the mathematics of non-steady-state systems. Two components of inhibition by O2 are suggested: one is caused by photorespiration as measured from the maximum rate of the PIB, and the second is direct inhibition as taken as APS2%O2— (APSx%O2+ PIBx%O2) where X is the oxygen concentration.

A primary PIB which occurred from 16–28 s after the darkening of the foliage was attributed to photorespiration. No primary PIB was observed at 2% O2. At a CO2 concentration of 100 μ/1 in the atmosphere (about 2.5 μM based on leaf intercellular concentration) and at 30°C and 145 nE/cm2 nE/cm2·s, APS decreased curve-linearly with increasing O2 and reached an O2 compensation point of 560 μM (48% by volume), above which there was a net loss of CO2 in the light. The PIB increased with increasing O2 and became saturated at about 500 μM O2 but decreased above 900 μM O2. Direct inhibition of photosynthesis by O2 increased with increasing O2 concentration.

Decreasing CO2 concentration had an effect on the magnitude of the PIB similar to that of increasing O2. At 30°C and 21% O2, the PIB increased with decreasing CO2 down to the CO2 compensation point (I) of 1.4 μM (47 μM/l). Below Γ, both PIB and CO2 evolution into the air in the light (at 21% O2) increased and then decreased at CO2 below 0.8 μM.

The ratio of the PIB to APS2% o O2 increased linearly with increasing O2/CO2 ratio where O2 was held constant at 21% and CO2 was varied from 1.4 to 8.5 μM, while direct inhibition of photosynthesis expressed as a proportion of APS2%O2 remained constant over this range. At low CO2 concentration photorespiration as estimated by the PIB is the major part of O2 photosynthesis, while at atmospheric CO2 levels, direct inhibition is the major component.

The PIB and APS at 2% and 21% O2 increased hyperbolically with increasing irradiance and all became light-saturated at about 65 nE/cm2 s. The percentage total O2 inhibition of photosynthesis remained constant with increasing irradiance as did the relative contribution of direct O2 inhibition or photorespiration (PIB) to total O2 inhibition.

The PIB and APS at 21% O2 had similar temperature optima of 30°C when experimental conditions were adjusted to provide a constant internal O2/CO2 solubility ratio at varying temperatures. However, with a constant external CO2 concentration, the temperature optimum for the PIB shifted upward to 35°C while that for APS at 21% O2 remained at 30°C, which may be due to an increased O2/CO2 concentration in the leaf with increasing temperature.