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Net Photosynthesis, Carbon Dioxide Compensation Point, Dark Resperiration, and Ribulose-1,5-bisphosphate Carboxylase Activity in Wheat¹

Net photosynthesis, CO₂ compensation point, and ribulose-1,5-bisphosphate carboxylase (RuBPCase) activity were measured in the flag leaves of one hard wheat (Triticum durum Desf.) and one soft wheat (Triticum aestivum L.) from anthesis to an advanced state of senescence. Net photosynthesis was not significantly different in the two cultivars. A linear relation was found between net photosynthesis and RuBPCase attributable to the relatively low enzymatic activity. The slope of the line was found to be similar in the two cultivars, indicating similar carboxylation efficiencies. The CO₂ compensation points at 210 mL O₂ L^–1 air did not differ in the two cultivars while a slight difference was observed between the points measured at 22 mL O₂ L^–1 air only in mature leaves. The extrapolation of the CO₂ compensation points to zero O₂ concentration gave positive values indicating that mitochondrial respiration was not completely inhibited by light. Approximate values of this respiration were calculated for the two cultivars, based on a recent model of leaf photosynthesis. Dark respiration, measured as O₂ uptake by leaf slices, showed little change through maturation and senescence of the leaves. In mature leaves, this respiration was significantly higher in the soft wheat cultivar, while in senescent leaves, this difference disappeared.

Key Words: Triticum durum Desf. • Triticum aestivum L. • Hard wheat • Carboxylation efficiency

² Researchers, Institute of Plant Biochemistry and Ecophysiology, National Research Council, 00016 Monterotondo Scalo, Rome, Italy.

Received for publication April 25, 1985.

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