Experimental tests of the Münch–Horwitz theory of phloem transport: effects of loading rates*

Magnuson, C. E.; Goeschl, J. D.; Fares, Y.

doi:10.1111/j.1365-3040.1986.tb01572.x

Cited by 28 publications

(23 citation statements)

References 15 publications

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“…Second, changes in the rate of phloem loading resulting from changes in photosynthetic production may have significant effects on phloem sucrose concentrations independent of sink demand. Magnuson, Goeschl & Fares (1986), working with 11 C tracers, obtained results that suggest that phloem sugar concentrations change in response to diurnal changes in phloem loading and changes in CO 2 concentration. Third, changes in water potential gradients between sinks and sources may require large changes in phloem osmotic concentration gradients to maintain flux even when there is no change in sink demand (Boersma, Lindstrom & Childs 1991).…”

Section: Discussionmentioning

confidence: 99%

Importance of leaf versus whole plant CO₂ environment for photosynthetic acclimation

Sims

Luo

Seemann

1998

Plant Cell & Environment

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, unloading of the phloem at the sinks, or utilization for growth of sinks. To determine the relative importance of leaf versus whole plant level limitations on carbohydrate utilization at elevated CO 2 , and the possible effects on the regulation of photosynthetic capacity, we constructed a treatment system in which we could expose single, attached, soybean leaflets to CO 2 concentrations different from those experienced by the rest of the plant. The single leaflet treatments had dramatic effects on the carbohydrate contents of the treated leaflets. However, photosynthetic capacity and rubisco content were unaffected by the individual leaflet treatment and instead were related to the whole plant CO 2 environment, despite the fact that the CO 2 environment around the rest of the plant had no significant affect on the total non-structural carbohydrate (TNC) contents of the treated leaflets. These results necessitate a re-evaluation of the response mechanisms to CO 2 as well as some of the methods used to test these responses. We propose mechanisms by which sink strength could influence leaf physiology independently of changes in carbohydrate accumulation.Key-words: Glycine max; Fabaceae; acclimation; carbohydrates; carbon dioxide; photosynthesis; rubisco; soybean. INTRODUCTIONElevated CO 2 increases photosynthetic rates in the short term (minutes to hours) but over the long term (days to weeks) photosynthetic capacities often decrease in response to elevated CO 2 [for a review, see Griffin & Seemann (1996)]. Photosynthetic acclimation to CO 2 may result from feedback inhibition of photosynthetic enzyme synthesis when carbohydrate utilization does not match demand (Stitt 1991). A carbohydrate feedback signal could be generated in response to a limitation in carbohydrate utilization at many different points; export of triose phosphates from the chloroplast, sucrose synthesis and phloem loading, transport in the phloem, unloading of the phloem at the sinks, or carbohydrate utilization for growth of sinks. Studies of carbohydrate allocation in plants tend to assume that one or another of these factors constitutes the major limitation (Farrar 1996). However, control theory suggests that the most efficient utilization of resources results when a plant adjusts the capacities of each limiting step so that the overall process is equally limited by all factors (Farrar 1996). In this case, we would expect that both whole plant and leaf level limitations to carbohydrate utilization would contribute to carbohydrate feedback in plants grown at elevated CO 2 .Most tests of the carbohydrate feedback hypothesis have applied rather drastic and unnatural treatments to increase the carbohydrate content of leaves, for example feeding sugars to leaves or intact plants (Krapp, Quick & Stitt 1991;Krapp et al. 1993;van Oosten, Wilkins & Besford 1994;Winters et al. 1994;Jones, Lloyd & Raines 1996), cold girdling of petioles (Krapp et al. 1993;Krapp & Stitt 1995), heat girdling of petioles (Goldschmidt & Huber 1992), and transforming plant...

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Section: Discussionmentioning

confidence: 99%

Importance of leaf versus whole plant CO₂ environment for photosynthetic acclimation

Sims

Luo

Seemann

1998

Plant Cell & Environment

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“…Indeed, modeling (Tyree et al, 1974; and empirical studies (Passioura & Ashford, 1974;Magnuson et al, 1986) have shown the OGPF hypothesisFin its simplest formFto be a virtual certainty for small plants. Additionally, the relatively recent recognition that sieve plates are nominally clear of P-proteins further justifies this certainty (Knoblauch & van Bel, 1998;Eckardt, 2001;Knoblauch et al, 2001), and has only bolstered the study of assimilate loading in source tissues (van Bel & Gamalei, 1992;van Bel, 1993;Komor et al, 1996;Turgeon & Medville, 1998;Patrick et al, 2001) and unloading in sink tissues (Oparka, 1990;Farrar, 1993;Minchin & Thorpe, 1996;Patrick, 1997;Oparka & Turgeon, 1999;Oparka & Cruz, 2000), as well as the molecular biology of the transfer of water, sucrose, and aminenitrogen between the phloem and xylem (Patrick et al, 2001).…”

Section: Introductionmentioning

confidence: 99%

Application of a Single-solute Non-steady-state Phloem Model to the Study of Long-distance Assimilate Transport

Thompson

Holbrook

2003

Journal of Theoretical Biology

171

266

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“…The studies on phloem loading rate at the whole plant level were very few. They were based mostly on 11C experiments (Magnuson et al 1986, Thorpe and Minchin 1987, Farrar et al 1994, the carbon isotope with half live of several minutes, which allow the repetition of a few experiments on the same plant and to study the effect of che~rfical and physical stimuli. However, studies with lie cause many logistical problems and are possible only in laboratories located near the nuclear reactor where the I1C is synthesised (Magnuson et al 1982).…”

Section: Introductionmentioning

confidence: 99%

The effect of irradiance, p-chloromercuribenzenesulphonic acid and fusicoccin on the long distance transport in Zea mays L. seedlings

Sowiński

1998

Acta Physiol Plant

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The system consisting of a few proportional detectors with appropriate electronic components was earlier developed for in vivo studies of long distance transport in whole maize seedlings. [4CO2 assimilation rate (Pa), time of radioactivity appearing in the loading region (AT), transport speed in the leaf (TS1), transport speed between the leaf and the roots (TSr), the maximum radioactivity values detected in the leaf below the feeding area (RI) and in the mesocotyl (Rr) from leaves to roots in maize seedlings were calculated from the obtained temporal profiles of radioactivity. The study was undertaken to follow the changes in separate steps of long distance transport in maize seedlings as affected by two light irradiances and application of p-chloromercuribenzenesulphonic acid and fusicoccin, with the aim to investigate differen(steps of long distance transport, particularly phloem loading.The method used allows to study in vivo the different aspects of long distance transport in maize seedlings, both qualitatively and quantitatively. It was shown that the characteristics obtained from the radioactivity profiles corresponded to different steps of long distance transport, as assimilate synthesis, phloem loading, and phloem translocation. It was also demonstrated that although active phloem loading participate in assimi'late'export from the leaves, assimilate transport along the maize seedling might undergo accordingly to assimilate gradient, particularly under light irradiance higher than during the growth.List of abbreviations: F C -fusicoccin; P C M B S -pchloromercuribenzenesulphonic acid; G L -irradiance under the growth conditions, 200 mol-m-2-s -1; I l L -400 mol.m-2.s -1; Pa-14CO2 assimilation rate; A T -time of radioactivity appearance in the loading region; Ill -radioactivity of leaves near the feeding area, Rm -radioactivity of mesocotyl; TS1 -speed of translocation in the leaf blade; T S m -speed of translocation between leaf and mesocotyl.

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Experimental tests of the Münch–Horwitz theory of phloem transport: effects of loading rates*

Cited by 28 publications

References 15 publications

Importance of leaf versus whole plant CO₂ environment for photosynthetic acclimation

Importance of leaf versus whole plant CO₂ environment for photosynthetic acclimation

Application of a Single-solute Non-steady-state Phloem Model to the Study of Long-distance Assimilate Transport

The effect of irradiance, p-chloromercuribenzenesulphonic acid and fusicoccin on the long distance transport in Zea mays L. seedlings

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Experimental tests of the Münch–Horwitz theory of phloem transport: effects of loading rates*

Cited by 28 publications

References 15 publications

Importance of leaf versus whole plant CO2 environment for photosynthetic acclimation

Importance of leaf versus whole plant CO2 environment for photosynthetic acclimation

Application of a Single-solute Non-steady-state Phloem Model to the Study of Long-distance Assimilate Transport

The effect of irradiance, p-chloromercuribenzenesulphonic acid and fusicoccin on the long distance transport in Zea mays L. seedlings

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Importance of leaf versus whole plant CO₂ environment for photosynthetic acclimation

Importance of leaf versus whole plant CO₂ environment for photosynthetic acclimation