Fig. 6-4 Test plant and integrated image of 11C-photoassimilate flow A 11CO2 tracer was mixed with ambient room air and introduced into the gas cell (shaded area), into which a mature leaf has been inserted. The flow of radiolabeled compounds in the stem was captured as a movie image using PETIS. Subsequently, the total concentration (approx. 350 ppm) of CO2 in the gas feed was elevated to 1000 ppm, and then a second run of imaging was carried out with a fresh 11CO2 tracer.

Fig. 6-5 Estimated flow speeds and distribution ratios of photoassimilates The stem is not just a transport tube for photoassimilates. It also requires photoassimilates for its own growth. Photoassimilates leak from the transport flow in the stem and are distributed to the surrounding tissues. We have estimated the distribution ratios of photoassimilates at the first three internodes in a test stem, and have found that their total was lower under the enriched CO2 condition. This suggests that the amount of photoassimilates under transport was more than the capacity of the surrounding tissues under these conditions. We also estimated flow speeds in the internodes, and found that they increased up to two times under the enriched CO2 condition. This suggests that the driving force of the flow was elevated because of the increased photoassimilation which resulted from CO2 enrichment.

Carbon dioxide ( CO2 ) is converted into sugars or other organic compounds by photoassimilation in plant leaves. They are distributed appropriately to various organs, such as roots or fruits, by phloem transport. How do plants manage to regulate this transportation process when they have neither a heart nor a nervous system? We had previously developed the positron emitting tracer imaging system (PETIS) and successfully employed it to visualize transport of various radiolabeled compounds in intact plants. In particular, we had obtained movie images of photoassimilate transport in plant bodies by feeding a 11CO2 tracer into their leaves. In the present study, we have developed a new analytical method to analyze the movie data captured by PETIS, wherein we can estimate the transport of radiotracers numerically. This enables to understand photoassimilate transport not just as a movie image, but in terms of quantitative dynamics. Using this method, we have tested the response of dynamics of phloem transport when an elevated level of CO2 has been fed into a leaf of a broad bean plant (Fig. 6-4). More specifically, optimal equations were determined which reproduce the transport dynamics captured by PETIS, then the distribution and flow speeds of photoassimilates in the stem were calculated with the equations. The results suggest that the flow speeds and the relative allocation of photoassimilates for roots increase with enriched CO2 applied to a leaf (Fig. 6-5). Such quantitative analyses help to make clear the power balance between consuming organs (roots, fruits and so on) and supplying organs (mature leaves) for plant nutrition, and may make an important contribution to agricultural sciences.

Reference S. Matsuhashi et al., Quantitative Modeling of Photoassimilate Flow in an Intact Plant Using the Positron Emitting Tracer Imaging System (PETIS), Soil Sci. Plant Nutr., 51(3), 417 (2005).

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