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The [O.sub.2] and [CO.sub.2] compensation points ([O.sub.2] and [Co.sub.2]) of plants in a closed system depend on the ratio of [CO.sub.2] and [O.sub.2] concentrations in air and in the chloroplast and the specificities of ribulose bisphosphate carboxylase/oxygenase (Rubisco). The photosynthetic [O.sub.2] [ is defined as the atmospheric [O.sub.2] level, with a given [CO.sub.2] level and temperature, at which net [O.sub.2] exchange is zero. In experiments with [C.sub.3] plants, the [O.sub.2] [ with 220 ppm [CO.sub.2] is 23% [O.sub.2]; [O.sub.2] increases to 27% with 350 ppm [CO.sub.2] and to 35% [O.sub.2] with 700 ppm [CO.sub.2]. At [O.sub.2] levels below the [O.sub.2] [, [CO.sub.2] uptake and reduction are accompanied by net [O.sub.2] evolution. At [O.sub.2] levels above the [O.sub.2] net [O.sub.2] uptake occurs with a reduced rate of [CO.sub.2] fixation, more carbohydrates are oxidized by photorespiration to products of the [C.sub.2] oxidative photosynthetic carbon cycle, and plants senesce prematurely. The [CO.sub.2] [ from 50 ppm [CO.sub.2] with 21% [O.sub.2] to 220 ppm with 100% [O.sub.2]. At a low [CO.sub.2]/high [O.sub.2] 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 [CO.sub.2] fixation rate. Because of low global levels of [CO.sub.2] and a Rubisco specificity that favors the carboxylase activity, relatively rapid changes in the atmospheric [CO.sub.2] level should control the permissive [O.sub.2] [ that could lead to slow changes in the immense [O.sub.2] pool.

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.