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

LESSONS FROM CROP PLANT EVOLUTION . IMPROVING PHOTOSYNTHETIC RATE . CARBON PARTITIONING BETWEEN ORGANS . CHARACTERISTICS OF ESTABLISHED SOURCE·PATH·SINK SYSTEMS . Photosynthesis in Relation to Sink Demand . Phloem Loading . �r,Io"::; a fJnr::d"!:.�:.:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: Sink Properties .

Photosynthesis, carbon partitioning, and yield

Autoradiographic, plasmolysis, and 14 C-metabolite distribution studies indicate that the majority of exogenously supplied 14 C-sucrose enters the phloem directly from the apoplast in source leaf discs of Beta vulgaris . Phloem loading of sucrose is pH-dependent, being markedly inhibited at an apoplast pH of 8 compared to pH 5. Kinetic analyses indicate that the apparent K m of the loading process increases at the alkaline pH while the maximum velocity, V max , is pH-independent. The pH dependence of sucrose loading into source leaf discs translates to phloem loading in and translocation of sucrose from intact source leaves. Studies using asymmetrically labeled sucrose 14 C-fructosyl-sucrose, show that sucrose is accumulated intact from the apoplast and not hydrolyzed to its hexose moieties by invertase prior to uptake. The results are discussed in terms of sucrose loading being coupled to the co-transport of protons (and membrane potential) in a manner consistent with the chemiosmotic hypothesis of nonelectrolyte transport.

Phloem Loading of Sucrose

Photosynthetic Assimilation of Exogenous HCO3 by Aquatic Plants

Publisher Summary This chapter reviews three main aspects of K+ in crop production—namely, K availability in the soil, the function of K+ in the plant, and potash fertilizer application. The soil is considered as a source of K+ to plant roots. The use of K+ in practical crop production is also emphasized in the discussions on the physiological role of K+ in the plant and in fertilizer application. The potassium status of a soil may be assessed on its content of K+-bearing minerals because the amount of these minerals present in a soil gives some indication of the potential source of K+ to plants. However, in terms of the ability of the soil to supply K+ to plant roots, the quantity of K+-bearing minerals plays only an indirect role. More important in determining the K+ supply to plants are the soil K+ fractions. These fractions, which have been established experimentally using different extraction techniques, are soil solution K+, K+ adsorbed to clay minerals or humus, and K+ present in minerals.

Potassium in crop production.

Loading of 14C-labelled sugars from the hollow petiole of Ricinus communis L. was stimulated by potassium and by low pH in that both the 14C-activity and the sugar concentration of phloem sap collected from a nearby incision increased. A pH drop was observed in the solution perfusing a hollow petiole. This pH drop was greater in the presence of potassium and less in the presence of sugars, while the uncoupler CCCP induced a pH rise in the perfusing solution. Sugars were detected in the perfusing solution when it was buffered at pH>9. A model is proposed for a proton co-transport of sugars from the free space driven by a linked proton efflux/potassium influx pump.

F. Malek

Papers

Proton co-transport of sugars in phloem loading.