Mass balance infers that the decrease in stem weight is related to the gain in kernel weight. Using 14CC>2 to radio-label assimilates to determine the inter-relatedness of organs within the plant can be quite useful. Pulse labeling at noon for 10 min. at late boot stage resulted in 5 to 14% of the assimilate later recovered in the grain at maturity (13). Most of the radioactivity assimilated after anthesis, whether labeled early or later in grain filling, measured 1-day, 7-days, or at maturity ended up in the grain (13). The speed of movement of a pulse of radioactivity in wheat is believed to be approximately 0.65 cm/min (52). Consequently the origin of radioactivity in the grain even 1 day after assimilation could be any organ in the plant, the stem, the root, the leaf blade, or sheath (21). Having 92% of the radio-label in the grain at maturity does suggest a high priority for movement to the kernel (21). How much of the radioactivity was first stored in the stem as fructan and later moved to the kernel cannot be determined. The distribution of radio-label was more accurately described theoretically when a continuous turnover of reserve biomass in the storage and maintenance compartments were included to map the time course of accumulation (Fig. 4 after 13). When assimilates were radio-labeled early during grain filling the accumulation of radioactivity was best described by drawing an additional arrow between the photosynthesis and grain biomass compartments of Fig. 4, (after 13). Labeling assimilates late indicates that radioactivity first went through the reserve biomass compartment before going to the grain compartment. The redistribution of ketose sugars during the later stages of grain fill (Fig. 1) is in agreement with these results. Mass balance and pulse-labeling allow us by inference to suggest that much of the stored reserves, mainly fructan, are mobilized to the grain.
Mass balance and pulse labeling are less informative when asking whether pre-anthesis stored reserves contribute to grain fill. The size of the source two weeks before anthesis was reported to have a stronger influence on yield than the source supply during grain fill (53). Gebbing et al. (28) realizing the limitations of mass balance and pulse-labeling to illustrate the importance of the pre-anthesis period on grain yield, employed steady state labeling using the stable isotope of carbon 13CC>2 (54, 55). The goal of steady state labeling is to make all of the carbohydrate in a particular pool have the same specific activity (13C/l2C) as that of the assimilated CO2. Wheat plants were grown at a different i3C/12C (5-27 %) than atmospheric 13C/12C (5 -8.3%) from booting till anthesis. Plants were sampled at anthesis and at maturity. The difference in abundance of 13C in water soluble carbohydrate + protein was taken to be the amount of carbon mobilized during grain fill (28).
The amount of mobilization ranged between 24-34 % of pre-anthesis assimilated carbon an amount 1.4-2.5 times as high as previously estimated (1) for above ground and root biomass remobilization. They attributed the difference to post anthesis incorporation of carbon into biomass (28). Obviously some of the reported loss in stem biomass during grain fill must be attributed to pre-anthesis stored carbon. It is unlikely that the amount contributed by stems will be much changed from that already estimated (1, 51). The amount of the water soluble carbohydrate attributed to fructan was not calculated but nearly all stored stem water soluble carbohydrate is fructan.
Sucrose is the substrate for fructan synthesis in stem storage tissues. Sucrose transported in the phloem may be unloaded from the phloem symplastically or apoplastically in the internode tissues (56). The difference between these pathways is in the free space hydrolysis of sucrose by invertase (57). It is possible that an invertase inhibitor could be active in the apoplast (57) so sucrose could travel via the free space to the storage cells without hydrolysis. Sink specific expression of the sucrose symporter genes SUT1/SUT2 has been reported for other plant species so sucrose could be the species arriving at the site of fructan synthesis in stem tissues (58). However in the axial pathway phloem unloading is probably symplastic controlled by diffusion gradients dependent on a low cytoplasmic sucrose concentration because of fructan synthesis in the vacuole (56). Because all the sugars in the pathway are osmotically active bulk flow in response to turgor pressure differences in tissues cannot be overlooked (56, 58). Sucrose may have a role in initiating the increase in fructan synthetic enzymes, which accompanies any increase in fructan storage (59). Fructan synthesis via SST (sucrose: sucrose fructosyltransferase) generates a trisaccharide and glucose (60). In the vacuole then glucose and sucrose are present, each of which has the ability to participate in regulation of partitioning as a signaling compound, through turgor or via metabolic regulation (58).
Recovering fructan from storage appears to be accompanied by an increase in fructan hydrolytic activity (39, 59). A reduction in the cellular content of sucrose might induce this change in hydrolase activity. Cellular hydrolysis may not be necessary since there is evidence that fructan oligomers are found in the apoplast along with hydrolytic enzymes under certain conditions (61). Fructans themselves may also be transported as has been reported for Agave deserti (62). However it is more likely that the increase in stem internode fructose initiates responses leading to the synthesis of sucrose from fructose (hexokinase, isomerase and SPS) and that sucrose is transported to the grain (63).
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