ABSTRACT
In vascular plants, three elements, carbon (C), hydrogen, and oxygen, comprise 96% of the plant’s dry weight. Source leaves are the primary sites of C reduction and the main photosynthetic organs exporting reduced C to growing sinks. It is well known that in almost all species, sugars, starch, and amino acids accumulate in leaves during the daytime and export of assimilates derived from these reserves occurs both concurrently with photosynthesis and subsequently during night periods (Fondy and Geiger, 1982; Gordon, 1986; Kalt-Torres et al., 1987; Wardlaw, 1990; Geiger and Servaites, 1994; Weise et al., 2011). Our overall knowledge of translocation processes has been derived from diverse experimental approaches (Canny, 1973; Geiger, 1980; Milburn and Kallarackal, 1989; Olrich and Komor, 1989; Farrar, 1993a,b; Heldt, 1997; Gamalei, 2002). For example, imaging techniques that include light, electron, and uorescent microscopy using dyes or proteins provide valuable qualitative data on intercellular connections and export (Peterson
13.1 Introduction .......................................................................................................................... 279 13.2 Methodologies Used to Estimate Immediate Carbon Export ............................................... 281
13.2.1 Gas Exchange and Differential Dry Weight Analysis .............................................. 281 13.2.2 Isotopes of C ............................................................................................................. 281
13.2.2.1 Mass Isotope 13C ........................................................................................ 282 13.2.2.2 Radioisotope 11C ........................................................................................ 282 13.2.2.3 Radioisotope 14C ........................................................................................ 283
13.3 Steady-State 14CO2 Labeling and Measurement of Immediate C Export Rates during Photosynthesis ......................................................................................................................284 13.3.1 Open-Flow Gas Analysis System .............................................................................284 13.3.2 14CO2 Feeding and Monitoring of 14C Retention ......................................................285 13.3.3 GM Detector Counting Efciency ............................................................................286 13.3.4 Calculation of Concurrent Export during Steady-State 14CO2 Feeding ...................286 13.3.5 Estimating Export of 14C Assimilates during a Light or Dark Chase Period.....289 13.3.6 Partitioning of 14C in the Fed Leaf and Plant ........................................................... 289
13.4 Case Studies ..........................................................................................................................290 13.4.1 Photosynthesis and Export under Stress ...................................................................290 13.4.2 Immediate Export in Natural Photosynthetic Variants ............................................ 291
13.5 Summary .............................................................................................................................. 293 References ......................................................................................................................................294
