The 02 and CO2 compensation points (02 F and C02 F) of plants in a closed system depend on the ratio of CO2 and 02 concentrations in air and in the chloroplast and the specificities of ribulose bisphosphate carboxylase/oxygenase (Rubisco). The photosynthetic 02 F is defined as the atmospheric 02 level, with a given CO2 level and temperature, at which net 02 exchange is zero. In experiments with C3 plants, the 02 F with 220 ppm CO2 is 23% 02; 02 F increases to 27% with 350 ppm CO2 and to 35% 02 with 700 ppm CO2.At 02 levels below the 02 F, C02 uptake and reduction are accompanied by net 02 evolution. At 02 levels above the 02 F, net 02 uptake occurs with a reduced rate of CO2 fixation, more carbohydrates are oxidized by photorespiration to products of the C2 oxidative photosynthetic carbon cycle, and plants senesce prematurely. The C02 F increases from 50 ppm CO2 with 21% 02 to 220 ppm with 100% 02. At a low C02/high 02 ratio that inhibits the carboxylase activity of Rubisco, much malate accumulates, which suggests that the oxygen-insensitive phosphoenolpyruvate carboxylase becomes a significant component of the lower CO2 fixation rate. Because of low global levels of CO2 and a Rubisco specificity that favors the carboxylase activity, relatively rapid changes in the atmospheric CO2 level should control the permissive 02 F that could lead to slow changes in the immense 02 pool.In contrast to the attention that regulation of atmospheric CO2 has attracted, a photosynthetic 02 compensation point (02 F) has not been described or considered as part of the global 02 cycle that has equilibrated the atmospheric 02 level at 21%. Although 02 inhibition of photosynthesis has been known for 75 yr (1) and its biochemical process has been recognized as photorespiration (2-8), the existence of an 02 F was not described because at high 02 levels, 1802 exchange and a lower rate of CO2 fixation continue, and plants senesce only slowly. High CO2 alleviates 02 inhibition and low CO2 intensifies it, as expected from the dual activities of ribulose bisphosphate carboxylase/oxygenase (Rubisco) (9). In the absence of 02 the Km (CO2) is 12 ,uM, which increases to 26-42 ,uM between 200 and 30°C with 21% 02. Reported Km (02) values for the oxygenase activity are between 250 and 400 ,vM 02 at 20°-30°C in the presence of low levels of CO2 (9).Photosynthesis contributes to the atmospheric 02 balance by oxygen production from water during CO2 assimilation in the C3 reductive photosynthetic carbon cycle. Net CO2 fixation by the carboxylase activity of Rubisco and subsequent reduction are illustrated on the left part of Fig. 1. The oxygenase activity of Rubisco initiates photorespiration via the C2 oxidative photosynthetic carbon cycle that composes both parts of Fig. 1. The C2 and C3 carbon cycles coexist and together constitute photosynthetic carbon metabolism (10, 11). In the complete C2 cycle the CO2 released is refixed to regenerate the ribulose bisphosphate to sustain the C2 cycle. Refixation of CO2 generates the same amount of ...

Mature fruit (kiwifruit) of Actinidia deliciosa var. deliciosa (A. Chev.), (C.F.) Liang and Ferguson cv. Haywood (Chinese gooseberry) were harvested and allowed to ripen in the dark at 20° C. Changes were recorded in metabolites, starch and sugars, adenine nucleotides, respiration, and sucrose and glycolytic enzymes during the initiation of starch degradation, net starch-to-sucrose conversion and the respiratory climacteric. The conversion of starch to sucrose was not accompanied by a consistent increase in hexose-phosphates, and UDP-glucose declined. The activity of sucrose phosphate synthase (SPS) measured with saturating substrate rose soon after harvesting and long before net sucrose synthesis commenced. The onset of sugar accumulation correlated with an increase in SPS activity measured with limiting substrates. Throughout ripening, until sucrose accumulation ceased, feeding [(14)C] glucose led to labelling of sucrose and fructose, providing evidence for a cycle of sucrose synthesis and degradation. It is suggested that activation of SPS, amplified by futile cycles, may regulate the conversion of starch to sugars. The respiratory climacteric was delayed, compared with net starchsugar interconversion, and was accompanied by a general decline of pyruvate and all the glycolytic intermediates except fructose-1,6-bisphosphate. The ATP/ ADP ratio was maintained or even increased. It is argued that the respiratory climacteric cannot be simply a consequence of increased availability of respiratory substrate during starch-sugar conversion, nor can it result from an increased demand for ATP during this process.

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