A new measurement technique reveals rapid post‐illumination changes in the carbon isotope composition of leaf‐respired CO2

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Abstract
We describ an open leaf gas exchange system coupled to a tunable diode laser (TDL) spectroscopy system enabling measurement of the leaf respiratory CO2 flux and its associated carbon isotope composition (δ13CRl) every 3 min. The precision of δ13CRl measurement is comparable to that of traditional mass spectrometry techniques. δ13CRl from castor bean (Ricinus communis L.) leaves tended to be positively related to the ratio of CO2 produced to O2 consumed [respiratory quotient (RQ)] after 24–48 h of prolonged darkness, in support of existing models. Further, the apparent fractionation between respiratory substrates and respired CO2 within 1–8 h after the start of the dark period was similar to previous observations. In subsequent experiments, R. communis plants were grown under variable water availability to provide a range in δ13C of recently fixed carbohydrate. In leaves exposed to high light levels prior to the start of the dark period, CO2 respired by leaves was up to 11‰ more enriched than phloem sap sugars within the first 10–15 min after plants had been moved from the light into the dark. The 13C enrichment in respired CO2 then decreased rapidly to within 3–7‰ of phloem sap after 30–60 min in the dark. This strong enrichment was not observed if light levels were low prior to the start of the dark period. Measurements of RQ confirmed that carbohydrates were the likely respiratory substrate for plants (RQ > 0.8) within the first 60 min after illumination. The strong 13C enrichment that followed a high light‐to‐dark transition coincided with high respiration rates, suggesting that so‐called light‐enhanced dark respiration (LEDR) is fed by 13C‐enriched metabolites.