Transport in Plants

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Elements most readily mobilised are phosphorus, sulphur, nitrogen and

potassium. Some elements that are structural components like calcium

are not remobilised.

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Some of the nitrogen travels as inorganic ions, while most of it is carried

in the organic form such as amino acids and related compounds.

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Small amounts of P and S are also carried as organic compounds. There

is also exchange of materials between xylem and phloem. Hence, we

cannot clearly say that xylem transports only inorganic nutrients while

phloem transports only organic materials.

PHLØËM TRÅÑSPØRT: FLØW FRØM SØÜRÇË TØ SÏÑK

It is the long distance movement of organic substances (food,

primarily sucrose) from a source (region of synthesis the food) i.e.,

the leaf to a sink (region of storage or utilisation of food) through

the phloem.

The source and sink may be reversed depending on the season,

or the plant’s needs. E.g. In early spring, the sugar stored in

roots is moved to the tree buds for growth and development of

photosynthetic apparatus. Thus, root becomes the source and

buds the sink.

The direction of movement in the phloem can be upwards or

downwards, (bi-directional). In xylem, the movement is always

upwards (unidirectional). Hence, food in phloem sap can be

transported in any direction.

Phloem sap is mainly water and sucrose, but other sugars,

hormones and amino acids are also translocated.

THË PRËSSÜRË FLØW (MÅSS FLØW) HÝPØTHËSÏS

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It is the hypothesis that explains the mechanism of translocation of

sugar (phloem transport).

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The glucose prepared at the source (by photosynthesis) is converted to

sucrose (a disaccharide).