BLEEdING
It is the exudation of sap or watery solution from the cut or injured parts of the plant, e.g., Agave, Acer, Vitis, etc. It
occurs due to root pressure, phloem pressure, local pressure in xylem (stem pressure) and latex or resin.
upTAKE ANd TRANSpORT OF mINERAL NuTRIENTS
uptake of mineral ions
Unlike water, all minerals cannot be passively absorbed by the roots. Two factors account for this: (i) minerals are present
in the soil as charged particles (ions) which cannot move across cell membranes and, (ii) the concentration of minerals
in the soil is usually lower than the concentration of minerals in the root. Therefore, most minerals must enter the root
by active absorption into the cytoplasm of epidermal cells. This needs energy in the form of ATP.
The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake
of water by osmosis. Some ions also move into the epidermal cells passively. Ions are absorbed from the soil by both
passive and active transport. Specific proteins in the membranes of root hair cells actively pump ions from the soil into
the cytoplasm of the epidermal cells. Like all cells, the endodermal cells have many transport proteins embedded
in their plasma membrane; they let some solutes cross the membrane, but not others.
Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that
reach the xylem. Note that the root endodermis because of the layer of suberin has the ability to actively
transport ions in one direction only.
Translocation of mineral ions
After the ions have reached xylem of the roots through active or passive uptake, or a combination of the two, their
further transport up the stem to all parts of the plants is through the transpiration stream.
Though it is generally considered that xylem transports inorganic nutrients while phloem transports organic nutrients, the
same is not exactly true. In xylem sap, nitrogen travels as inorganic ions, as well as organic form of amino acids
and related compounds. Small amounts of P and S are passed in xylem as organic compounds. There is also exchange of
materials between xylem and phloem. Therefore, mineral elements pass up xylem in both inorganic and organic form.
They reach the area of their sink, namely young leaves, developing flowers, fruits and seeds, apical and lateral meristems
and individual cells for storage. Minerals are unloaded at fine vein endings through diffusion. They are picked up by cells
through active uptake.
There is remobilization of minerals from older senescing parts. Nickel has a prominent role in this activity. The senescing
leaves send out many minerals like nitrogen, sulphur, phosphorous and potassium. Elements incorporated in structural
components are, however, not remobilized, e.g., calcium. The remobilized minerals become available to young growing
leaves and other sinks.
phLOEm TRANSpORT: TRANSLOCATION OF ORGANIC SOLuTES
Formation of organic food or carbohydrate occurs by green parts of the plant (leaves) as a result of photosynthesis.
Non-green parts depend upon green parts for organic food and thus food is transported from green parts to non-green
parts of the plant. This movement of organic food or solute in soluble form, from one organ to another organ is called
translocation of solutes. For example: from leaves to stem and roots for consumption.
Translocation of solutes takes place always from higher concentration (source or supply end) of its soluble form to lower
concentration of its soluble form (sink or consumption end). Translocation of solutes mostly occurs in form of sucrose.