Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways: IV. ANALYSIS OF REDUCED OXYGEN RESPONSE IN PANICUM MILIOIDES AND PANICUM SCHENCKII.

Reduced photorespiration has been reported in Panicum milioides on the basis of lower CO(2) compensation concentrations than in C(3) species, lower CO(2) evolution in the light, and less response of apparent photosynthesis to O(2) concentration. The lower response to O(2) in P. milioides could be due to reduced O(2) competition with CO(2) for reaction with ribulose 1,5-bisphosphate, to a reduced loss of CO(2), or to an initial fixation of CO(2) by phosphoenolpyruvate carboxylase. Experiments were carried out with Panicum maximum Jacq., a C(4) species having no apparent photorespiration; tall fescue (Festuca arundinacea Schreb.), a C(3) species; P. milioides Nees ex Trin.; and Panicum schenckii Hack. The latter two species are closely related and have low photorespiration rates. CO(2) exchange was measured at five CO(2) concentrations ranging from 0 to 260 microliters per liter at both 2 and 21% O(2). Mesophyll conductance or carboxylation efficiency was estimated by plotting substomatal CO(2) concentrations against apparent photosynthesis. In the C(4) species P. maximum, mesophyll conductance was 0.96 centimeters per second and was unaffected by O(2) concentration. At 21% O(2) mesophyll conductance of tall fescue was decreased 32% below the value at 2% O(2). Decreases in mesophyll conductance at 21% O(2) for P. milioides and P. schenckii were similar to that for tall fescue. On the other hand, loss of CO(2) in CO(2)-free air, estimated by extrapolating the CO(2) response curve to zero CO(2), was increased from 1.8 to 6.5 milligrams per square decimeter per hour in tall fescue as O(2) was raised from 2-21%. Loss of CO(2) was less than 1 milligram per square decimeter per hour for P. milioides and P. schenckii and was unaffected by O(2). The results suggest that the reduced O(2) response in P. milioides and P. schenckii is due to a lower loss of CO(2) in the light rather than less inhibition of carboxylation by O(2), since the decrease in carboxylation efficiency at 21% O(2) was similar for P. milioides, P. schenckii, and tall fescue. The inhibition of apparent photosynthesis by 21% O(2) in these three species at low light intensities was similar at 31 to 36% which also indicates similar O(2) effects on carboxylation. Apparent photosynthesis at high light intensity was inhibited less by 21% O(2) in P. milioides (16.8%) and P. schenckii (23.8%) than in tall fescue (28.4%). This lower inhibition in the Panicum species may have been due to a higher degree of recycling of photorespired CO(2) in these species than in tall fescue.