The isotopic composition of Light-Enhanced Dark Respiration (LEDR) in C3 and C4 plants

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Mirabal, Susan
thesis / dissertation description
Light-enhanced dark respiration (LEDR) has been studied in C3 plants as a method to elucidate the careful balance between photosynthesis and respiration yet little is known about the metabolic properties by which it occurs in C4 plants. Using a tunable diode laser gas exchange system, we analyzed transient isotopic shifts in leaf-respired CO2 associated with LEDR at 10Hz frequency from Populus deltoides (C3), Amaranthus hypochondriacus (C4 NAD-ME), and Zea mays (C4 NADP-ME). The results suggest that: vii (1) Post-photosynthetic fractionations are different between C3 and C4 (and within C4) plants, where A. hypochondriacus exhibited an unexpected LEDR response (Fig. 3, Table 3), (2) In Z. mays, post-illumination respiration was not dependent on light but its isotopic composition (δ13CRL) reflected changes in the previous illumination period. Depleted values of δ13CRL found here may be due to NADP-malic enzyme fractionation, and (3) The decarboxylation of various metabolic substrates is likely responsible for the differing post-illumination respiration responses in all species. Especially important are malate decarboxylation (in P. deltoides) and pyruvate decarboxylation (in A. hypochondriacus and Z. mays). We attributed the differences in the isotopic composition of LEDR in the above-mentioned plant species to variations in their cellular metabolism, compartmental fractionation and mesophyll and bundle sheath leakiness