Summary
This review reports on the processes associated with carbon transfer and metabolism in leaves and growing organs and the role of long-distance transport and vascular links in the regulation of carbon partitioning in plants. Partitioning is clearly influenced by both the supply and demand for photosynthate and is moderated by vascular connections and the storage capacity of the leaves and pathway tissues. However there appears to be little more than circumstantial evidence either that short distance transfer of carbon within either the source or the sink, or that long-distance transport in the phloem, are limiting photosynthesis or growth directly. Although individual biochemical and physiological processes relating to photosynthesis and growth may be well understood, the factors primarily responsible for the control of carbon partitioning in plants have not been clearly identified. There is a need for a greater understanding of organ initiation and development (source and sink formation and potential size), the clear identification of whether growth is sink or source limited (including possible sink-controlled photosynthesis) and a detailed assessment of the role of storage in buffering developmental and environmental changes in sink and source activity. Also more information is needed on the role of hormonal and nutritional factors in regulating source and sink activity (organ interactions not directly associated with carbon transfer).

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1990.tb00524.x

Tansley Review No. 27 The control of carbon partitioning in plants

https://www.cell.com/cell/pdf/0092-8674(90)90587-5.pdf

Rain-, Wind-, and Touch-Induced Expression of Calmodulin and Calmodulin-Related Genes in Arabidopsis

Macromolecular trafficking within the sieve element-companion cell complex, phloem unloading, and post-phloem transport were studied using the jellyfish green fluorescent protein (GFP). The GFP gene was expressed in Arabidopsis and tobacco under the control of the AtSUC2 promoter. In wild-type Arabidopsis plants, this promoter regulates expression of the companion cell-specific AtSUC2 sucrose-H+ symporter gene. Analyses of the AtSUC2 promoter-GFP plants demonstrated that the 27-kD GFP protein can traffic through plasmodesmata from companion cells into sieve elements and migrate within the phloem. With the stream of assimilates, the GFP is partitioned between different sinks, such as petals, root tips, anthers, funiculi, or young rosette leaves. Eventually, the GFP can be unloaded symplastically from the phloem into sink tissues, such as the seed coat, the anther connective tissue, cells of the root tip, and sink leaf mesophyll cells. In all of these tissues, the GFP can traffic cell to cell by symplastic post-phloem transport. The presented data show that plasmodesmata of the sieve element-companion cell complex, as well as plasmodesmata into and within the analyzed sinks, allow trafficking of the 27-kD nonphloem GFP protein. The data also show that the size exclusion limit of plasmodesmata can change during organ development. The results are also discussed in terms of the phloem mobility of assimilates and of small, low molecular weight companion cell proteins.

http://www.plantcell.org/content/plantcell/11/3/309.full.pdf

Cell-to-Cell and Long-Distance Trafficking of the Green Fluorescent Protein in the Phloem and Symplastic Unloading of the Protein into Sink Tissues

INTRODUCfION Techniques for Studying the Identity of Transported Solutes of Nitrogen . Nitrogen in Phloem and Xylem Fluids . Uptake. Assimilation. and Transport of Nitrogen by Roots . Transport of Nitrogen from Nodulated Roots of Nr/ixing Plants .. Initial Distribution of Xylem-Borne Nitrogen in Shoots .. Export of Nitrogen from Leaves in Phloem . Export of Nitrogen from Stem Tissue in the Phloem . RelationshipS Between Transport and Storage of Nitrogen .. Transport of Nitrogen to Deve/oping Fruits and Seeds Loading of Nitrogenous Solutes onto Xylem and Phloem .. Relationships Between Nitrogen Transport and the Production and Partitioning of Photosynthate .. CONCLUSIONS ..

Transport and Partitioning of Nitrogenous Solutes

Anatomy of Seed Plants.

The capacity of a growing leaf blade of Cucurbita pepo L. to import 14C-labelled photoassimilate is lost in a basipetal direction. Import into the lamina tip stops when the blade is 10% expanded. Development of the leaf progresses linearly with time and the lamina base stops importing when the blade is 45% expanded. Export capacity also develops basipetally and follows immediately the loss of import capacity, at least in the lamina base. The small amount of material initially exported from the leaf tip is redistributed to the still-importing leaf base, delaying export from the lamina until the blade is 35% expanded. Loss of import capacity by the petiole is both basipetal and dorsoventral. The proximal, adaxial portion of the petiole is the last region to cease importing 14C. Leaves of Beta vulgaris L. and Nicotiana tabacum L. also lose import capacity in a basipetal direction.

Leaf development and phloem transport in Cucurbita pepo: Transition from import to export

To determine the role of the apoplast in the loading of the minor veins of Cucurbita pepo L., leaves were examined either for the ability to release selectively the transport sugars, sucrose and st...

Leaf free space analysis and vein loading in Cucurbita pepo

Net photosynthesis, dark respiration and growth for leaf 5 of Cucurbita pepo L. plants grown under controlled conditions were measured and the data used for an assessment of the changes in carbon balance during growth of the leaf through expansion to maturity. The blade is first capable of net CO2 fixation when ca. 8% expanded but the initial rapid growth during this period is sustained almost entirely through imported nutrients. When the growth rate starts to decline rapidly the net photosynthetic capacity of the blade begins to increase. This increase is accompanied by an expansion of the intercellular spaces and by decreasing dark respiration measured at night and in dark periods during the day. The blade becomes completely independent of phloem imported nutrients and begins to export excess photosynthate when the phase of rapid decrease in relative growth rate is almost complete at about 45% expansion. Maximum net photosynthesis of ca. 11 mg CO2 h(-1) dm(-2) is achieved at 70% expansion. The first detectable synthesis of the transport sugars stachyose and raffinose in the blade coincides with the beginning of intralaminar phloem transport from the tip to the base of the leaf. The synthesis of sucrose, the other major transport sugar, is detectable at all stages of leaf development.

Leaf development and phloem transport in Cucurbita pepo: Carbon economy

Growth inhibition by mechanical stress.

Young leaves of Cucurbita pepo L. were examined by whole-leaf autoradiography and serial paradermal sections were examined by light microscopy to determine whether commencement of sugar export depends upon the minor vein phloem achieving structural maturity. Maturation of these veins develops progressively from the largest toward the smallest elements with the minor veins in the distal region of the leaf maturing before those in the proximal region. Commencement of sugar export is coincident with maturation of the abaxial phloem of the minor veins delimiting the areoles. The abaxial phloem elements of the larger minor veins, which are probably capable of vein loading too but border only relatively few areoles, mature before export starts. The adaxial phloem surrounding the areoles and the xylem elements, mature in advance of the abaxial phloem and well before the beginning of sugar export. It is therefore considered unlikely that structural development alone directly governs the initiation of export. The results suggest that some other rate controlling step is involved.

John A. Webb

Papers

Leaf development and phloem transport in Cucurbita pepo: Transition from import to export

Leaf free space analysis and vein loading in Cucurbita pepo

Leaf development and phloem transport in Cucurbita pepo: Carbon economy

Growth inhibition by mechanical stress.

Leaf development and phloem transport in Cucurbita pepo: Maturation of the minor veins.