O₂-Insensitive Photosynthesis in C₃ Plants

Abstract

There have been a number of reports of no stimulation of photosynthesis and sometimes even an inhibition upon switching from air levels of 02 to low 02 pressure (29,31). This unusual behavior is sensitive to temperature (5, 14, 15), CO2 pressure, and light (18). In this paper, I show that as light is varied the inhibition of photosynthesis by 02 is nearly constant over a range of CO2 assimilation rates until the rate of CO2 assimilation approaches a maximum, whereupon the 02 inhibition becomes at first less severe, and then can even reverse. This 02-insensitive limitation to photosynthesis also occurs in water-stressed plants. The postillumination burst of CO2 was used to demonstrate that the lack of 02 sensitivity was not the result of reduced photorespiration. Since the 02-insensitive limitation of photosynthesis appears to set an upper limit on the rate of photosynthetic CO2 assimilation, it was studied in a weedy species with a high rate of assimilation (X. strumarium) and a crop species that has a maximum assimilation rate of less than half that of Xanthium (Phaseolus vulgaris). No differences in the nature of 02 insensitivity were noted, and results from both species are presented. Photosynthesis in plants having the C3 pathway of photosynthesis is known to be inhibited by air levels of 02 pressure when the partial pressure of CO2 is below 1000 ,bar. Present

CO2 Uptake and Electron Transport Rates in Wild-Type and a Starchless Mutant of Nicotiana sylvestris (The Role and Regulation of Starch Synthesis at Saturating CO2 Concentrations)

“…The present work also confirms the notion that the O2 enhancement of photosynthesis at CO, and light saturation (Viil et al, 1972(Viil et al, , 1977Sharkey, 1985) is related to Pi deficiency. The latter is greater in the mutant and so is the O2 enhancement.…”

“…Cell wall C02, pM (Viil et al, 1972(Viil et al, , 1977 or 02-insensitive photosynthesis (Sharkey, 1985) at CO, saturation. Data in Figure 1 suggest that this phenomenon is related to the decline of photosynthesis with increasing CO, concentrations in low O,.…”

Regeneration of Ribulose 1,5-bisphosphate and Ribulose 1,5-bisphosphate carboxylase/oxygenase Activity Associated with Lack of Oxygen Inhibition of Photosynthesis at Low Temperature

“…Our data confirm this for the lack of O 2 -inhibition at low temperature , and support an alternative explanation which is related to the concentration of inorganic phosphate (PJ in the chloroplast stroma (Sharkey, 1985). Photophosphorylation may be limited by Pj when organic phosphate levels increase due to an inbalance between CO 2 assimilation and assimilate export at high light intensity, low temperature, high p(CO 2 ) and low p(O 2 ).…”

“…However, Sharkey (1985) found that a lack of O 2 -inhibition of photosynthesis was always associated with a failure of increased CO 2 pressure to stimulate photosynthesis to the expected degree. Our data confirm this for the lack of O 2 -inhibition at low temperature , and support an alternative explanation which is related to the concentration of inorganic phosphate (PJ in the chloroplast stroma (Sharkey, 1985).…”

Photosynthetic Acclimation in Pea and Soybean to High Atmospheric CO2 Partial Pressure

“…Although such a mechanism might operate during short-tem transients of source-sink mismatch, it is unlikely to be a long-tenn mechanism of feedback regulation because it would not optimize the use of the leaf's metabolic resources. A test for this kind of feedback involving failure to recycle Pi back from the chloroplast is the 0, sensitivity of net photosynthesis (Sharkey, 1985): 0,-insensitive net photosynthesis is Pi limited. In the present study net photosynthesis of both pea and soybean, whether from elevated COn or not, was highly O2 sensitive when the measured leaves were at the ages when they showed photosynthetic down-regulation (Table 111) and contained high levels of starch (Table IV).…”