Functional Equilibrium 

The shoot fixes C02 from the air, and the root extracts mineral nutrients and water from the soil. In this sense, there is a balance of shoot and root functions in a plant. After defoliation or root pruning, the plant acts so as to restore the balance of root and shoot functions. 

Source-Sink Relations 

Growth of non-photosynthesizing organs (sinks) is controlled by transport of sugars from photosynthesizing organs (sources) via the phloem. Sugars (mainly sucrose) are released from cells into the leaf apoplast (outside of the cell membranes) and move towards the conducting system (veins). Sucrose is actively loaded into phloem tissue and flows to sink sites under a pressure gradient. At the sink site, sucrose is unloaded from the phloem and used in growth processes or stored. (Marschner 5.1 p. 131, and 5.4 pp. 144-152) During vegetative growth photosynthesis is often sink limited, while in the reproductive stages source-limitation usually becomes more important. 

Phytohormones (Marschner 5.6.1, 5.6.2 pp 157-163 and 5.6.4 pp 166-172) 

Phytohormones play an important role in plant growth, source-sink relations and root-shoot relationships. The production and action of these substances is strongly influenced by environmental factors such as water stress and nitrogen deficiency (Marschner 14.7 pp. 535-536). 

There is a characteristic shoot:root ratio for each species at each growth stage. Shoot:root ratios tend to increase with plant size (decrease for root crops), reflecting increasingly preferential assimilate partitioning above ground (below ground for root crops). Thus, shoot:root ratio comparisons should be made at equal dry weight, or at equal plant developmental stage, not at equal time. 

Shoot:root ratios are influenced by changes in environmental conditions, such as light, nutrient availability, temperature and water supply. These changes usually reflect an adaptive advantage for the plant in acquiring the limiting resource. 

Q What factors affect root-shoot ratios ? 

  • The ratio of shoot to root growth varies widely between species, changes between phases in plant development, and can be modified by external conditions. 

  • At a gross level the ratio between shoot and root weight is a generally characteristic of the family or group. 
        Shoot and root growth of maize grown in hydroponics in the field (Miller et al, 1989. Canadian Journal of Soil Science, 69, 295-302. 
    • If either roots or shoots are pruned, the plant will tend to recover to the characteristic ratio. 
      Such observations help to explain benefits from rotational grazing, the practice of root pruning in tree nurseries, and the consequences of frequent lawn cutting. 
      Essentially the response can be seen as a source-sink phenomenon.  
        For example, if the ear is removed from corn, there is more carbon used for root growth. However, in this case, total carbon fixation may also decline, because there is a smaller total sink so that endpoint feedback occurs. 
    • If external conditions change, shoot-root ratio will be affected. 
        e.g.. There is a decline in the ratio in maize plants grown in the absence of P 
        Effect of P nutrition on shoot-root ratio (Anghinoni and Barber, 1980, Agronomy Journal, 72, 685-688). 
        The 12 day old plants were transferred to a P deficient solution. The shoot P concentration declined from 0.95% on day 1 to 0.27% on day 6. 
    • The root-shoot ratio increases if water is withheld from the rooting medium (Sharp and Davies, 1979) 

    This is not due to effects on carbon fixation as impacts on shoot growth take place at water potentials that are too high to restrict photosynthesis (Boyer, 1970). 

    The response of plants can be generalized as in the figure below. 




    Q What controls the rate of CO2 entry into leaves ? 
    • Radiation intensity 
    • Leaf water relations (stomatal aperture) 
    • The equilibrium concentration of CO2 in the sub-stomatal cavity 
    Q What are the essential differences between C3 and C4 plants for carbon fixation ? 
    • In C3 plants CO2 is fixed in mesophyll cells by carboxylation of a C5 compound, under Rubisco, leading to the formation of simple sugars. 
    • In C4 plants, CO2 is fixed in mesophyll cells by carboxlation of phosphoenolpyruvate. The product (oxaloacetate) may be reduced to malate, or undergo amination to aspartate. 

    These C4 compounds are transported to the cells of the sheath cells of vascular bundles in the leaf. There they are decarboylated and the C3 compound is recycled to the mesophyll cells. The CO2 released is refixed by the carboxylation of ribulosebiphosphate. 

    In the bundle sheath cells, the membranes are poorly permeable to CO2. Consequently the concentration of CO2 in the cells is high, but it is relatively low in the interstitial spaces. 



    Transport to the phloem elements in leaf veins is via the apoplast. Passage into the lumen of the phloem sieve tubes is an active process mediated by proton co-transport. 
    • Movement within the phloem depends on mass flow. The loading of sugars into the sieve cells decreases the water potential within the cells, so that water flows in from around the source. The inflow transports the assimilates towards the sinks. 
    • At the sinks there is evidence of both active transport of sugars into the apoplast and passive transport through the symplast. The establishment of concentration gradients will allow diffusion to contribute to movement
    Q What happens to the water at the sink end of the tubes ? 
    • It possibly is involved in the vacuolation process of newly formed cells.
    • Water may also be lost by evaporation by evaporation from new leaves that have little cuticular material. 
    • In roots water may move into the xylem vessels. 
    Q What are the factors that will most likely determine the supply of nutrients available to a given sink ? 
    • Demand of the tissue: metabolic activity.
    • Enzyme activity may be important in some cases eg at the sink, sucrose can be split by invertase thereby establishing a concentration gradient. Similarly if sucrose is converted into starch this will also set up a potential gradient. 
    • Mineral nutrient availability can affect carbohydrate supply. 

    • Plant growth controlling substances (phytohormones) play an important role. 
        Growth regulator Role Sites of biosynthesis Transport
        Cytokinins Cell division and expansion, RNA and protein synthesis, enzyme induction, apical dominance root meristems (shoot meristems, embryo in seed) In xylem from root to shoot
        Gibberellins Cell expansion, dormancy , induction of flowering, induction of enzyme synthesis shoot apex, expanding leaves, fruits (roots?) In roots to shoots in xylem, and shoots to roots in phloem. In symplast within shoots
        Auxins Cell expansion and division, 
        apical dominance, induction and activation of enzymes
        meristems of young tissue shoots to roots in phloem. Cell to cell by polar transport.
        Abscisic acid Abscission of leaves and fruits 
        Cell extension, stomatal closure. Inhibits DNA synthesis. Enzyme activation, membrane permeability
        fully differentiated tissues of roots and shoots mobile in both xylem and phloem
        Ethylene Aerenchyma formation, fruit ripening, tissue senescence. Enhances germination, epinasty, enhances flowering. All tissues, but ripening fruit and senescing tissue important Xylem transport from root to shoot
        Jasmonic acid Senescence, storage promotion, stomatal closure, inhibits cell growth. roots, shoots, fruits phloem mobile
      The genetic makeup of the plant and its environment affect both the phytohormone levels and the nature and response of the receptor cells.
          Possible relationships between the level and activity of phytohormones, receptors, and their action within the plant (After Marschner, 1995).
If you would like to download this file please click

Back to the Introduction