Steady-State Carbon Flow in Photosynthesis and Photorespiration in Lemna minor L.: The Effect of Temperature and Ammonium Nitrogen

Fuhrer, Jürg; Erismann, Karl Hans (1984). Steady-State Carbon Flow in Photosynthesis and Photorespiration in Lemna minor L.: The Effect of Temperature and Ammonium Nitrogen. Photosynthetica, 18(1), pp. 74-83. Inst Experimental Botany, Prague

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Net photosynthetic rate, P N• of Lemna minor L., measured at 21% oxygen, 528 ng cm-3 CO2 and high photon fluence rate, was 7.9 ng C cm-2 s-1 at 25 °C. Carbon dioxide release from illuminated plants into CO2-free air was 1.10, and from the post-illumination burst a total release of photorespiratory CO2 of 1.52 ng C cm-2 s-1 was estimated. From these values, net C-fixation rate, FIN• was calculated after taking into account the rate of dark respiration in the light (0. 38 ng C cm-2 s-1). Based on the CO2 compensation concentration, r, and the intercellular CO2 concentration, the ratio between oxygenation and carboxylation of ribulose-1,5-bisphosphate, a, was derived. With the aid of a model for C3 photosynthesis, carbon flux rates between pools of intermediates were calculated from F1N and a, and the effect of temperature and ammonium nitrogen was investigated. Lowering the temperature from 25 to 15 °C resulted in lower values for FIN and a. Calculated rates of C-flow into the glycollate pathway were 5.92 and 3.55 ng C cm-2 s-1 at 25 and 15 °C, respectively. Calculated rates of CO2-release from glycine decarboxylation were in good agreement with the measurement (post-illumination burst) at both temperatures. When ammonium was used as nitrogen source instead of nitrate, a slightly stimulated rate of F1N was observed, but no change in a. Based on an enhanced flux of carbon into the glycollate pathway (7.26 ng C cm-2 s-1), a small increase in CO2 -release was calculated which did not agree with the measurement. It is suggested that ammonium stimulated glyoxylate decarboxylation. This reaction accounted for about 50% of the CO2 -release, whereas on nitrate nitrogen about 95% of the CO2 was derived from glycine.

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