Plant Mineral Nutrition: Absorption and Circulation of Ions in Roots
Course: Plant Physiology
Code: 308 Bot
Students: Third year Botany
Lecture 2 (Plant Mineral Nutrition)
Dr. Taha Mohamed El-Katony
Absorption and translocation
The path of ions across the root
Absorption of salts by roots is both passive and active.
The movement of ions into the AFS (apoplasm) is passive
(by free diffusion).
Ions can move freely in the
wet cell walls (apoplast) of
the r
oot as far as the endodermis.
Further penetration is retarded by the
C
asparian strip.
The continuum of cytoplasm and the connecting
palsmodesmata is called the symplast.
The endodermal cells by virtue of the Casparian strip
present a barrier to the passive diffusion of ions.
The protoplast of endodermal cells is tightly attached to
the Casparian strip.
Ions can not pass through the walls of the endodermal
cells.
The only route available is through the protoplast.
How can ions be delivered into xylem?
It was supposed that there is a gradient of decreasing O
2
and
increasing CO
2
levels from the cortex to the stele.
The living cells adjacent to the xylem vessels therefore possess
low metabolic activity, are leaky and lose salts to the
lumen of xylem vessels.
Diffusion of salts back through the impervious Casparian strip
is not allowed and there is a unidirectional loss of salt
into the lumina of the xylem vessels.
Circulation of salts
Salts delivered to the xylem ducts of the root are translocated
upward to the shoot and, once there, are distributed
and redistributed throughout the plant.
Salts deposited in the leaves are withdrawn prior to
abscission and translocated to the reproductive organs or
younger leaves.
Circulation takes place in the vascular tissues.
By the use of radioactive tracers several pathways for the
translocation of salts were discovered:
1- in the xylem,
2- in the phloem,
3- laterally between the two tissues and
4- outward from the leaves.
Translocation of salts in xylem
Experimental evidence
1-
Ringing experiment:
The
upward translocation of
salts is unimpeded by removal of a ring of phloem
near the stem base.
2- L
arge amounts of salts occur in the xylem sap.
3-
32
P
travels upward to plant tip faster under high
transpiration (bright sunlight) than under low
transpiration.
4-
If transpiration by a leaf is diminished by covering
the leaf with a polyethylene bag, translocation of
minerals to that particular leaf is reduced
considerably.
5- Dissection experiment with radioactive tracers:
A: Procedure
i- The bark and xylem along a 9-inch length of a willow stem
were separated and a strip of impervious waxed paper
was inserted between them.
iii- The continuity of bark and xylem was undisturbed, and
the plant was left intact.
iv- The plant was allowed to absorb
42
K for 5 hours.
v- Sections of the treated and intact areas of the stem were
analyzed for
42
K.
B: Results and conclusion
1-
42
K
translocates upward in the xylem.
2-
Lateral interchange
of
42
K between phloem and
xylem (through cambium) occurs within the intact
region but
further translocation
either upward or
downward in the
phloem
is
slow
.
3- The small amounts of
42
K in the bark along the
stripped area suggests
that little
translocation
takes place in the
phloem
tissue.
Lateral translocation of salts
T
he above experiment revealed:
1- upward translocation of salts in xylem
and
2- lateral movement between xylem and phloem through
cambium.
Cambium regulates the amount of salts carried up in the
transpiration stream.
If the upward movement of salts were not regulated, certain
areas of the plant
(shoot and root tips) would
not be
accommodated.
The cambium position allows - both
metabolically
and
physically
-
re
gulation of the upward, lateral and
downward movement of salt.
The active accumulation of salt by the cambial cells acts
as a deterrent against an indiscriminate sweep of
salts upward in the transpiration stream.
The lateral movement from xylem to phloem correlates
inversely with element content in the phloem
high concentration of an element in the phloem, slows
down the lateral translocation into the phloem and
low concentration enhances lateral movement.
Translocation of salts in the phloem
The initial upward movement of salts occurs in the xylem.
But upward movement may occur also in the phloem.
Evidence on upward movement:
1- Ringing bark high up in the stem
retarded s
tem tip growth.
But ringing at stem base has no effect on salt nutrition of the whole
plant including stem tip.
Thus, the influence on stem tip growth was because of the blockage
of salts moving out of the lower leaves and transported upward in
the phloem and not because of the root-absorbed salts.
Evidence on downward movement:
2- Dissection experiment with Radioactive tracers.
Procedure
:
A- The bark was separated from wood by waxed paper
immediately below leaf petiole of a cotton plant.
B- Bark and wood were left intact.
C-
32
P was injected into leaf blade just above the
separated area.
D-
One hour
later, sections of stem were analyzed for
32
P.
Results and conclusion
1- Salts entering the main vascular stream from leaf move
primarily in a downward as well as upward direction in
the phloem.
2- Lateral transport between the vascular tissues takes place
where the phloem and xylem are not separated.
4- Both tissues may thus be involved with the upward
translocation of mineral salts moving out from leaves.
The movement of salts in the phloem is bidirectional.
This bidirectional movement might occur:
1- Simultaneously in the same sieve elements.
Or
2- In two different phloem channels, one toward the
tip and the other toward the base of the plant.
Outward movement of salts from leaves
In the
leaves of deciduous plants,
mineral salts move out of the
leaf
just prior to abscission.
The mineral nutrients moving out of leaves are N, K, P, S,
Cl and
(Mg and Fe).
Those remaining include Ca, B, Mn and Si.
Mineral nutrients move out of leaves primarily in the phloem.
When
32
P was introduced to leaves at different levels on the
plant: P from base leaves moves downward toward the root
while P from top leaves moves upward to tip.
Circulation and reutilization
Minerals are taken up in the transpiration stream.
Move primarily in the xylem vessels to the leaves.
Excess quantities are withdrawn from leaves
and
re-
translocated downward in the phloem.
Then they could be laterally transported into the xylem
w
here
upward translocation could take place again.
Elements such as N, K and P move readily in this circuit.
Calcium ascends the stem but does not relocate in the phloem.
Phosphorus
P is highly mobile in the plant and it is in continuous
circulation.
P mobility is an essential feature of plant growth.
P is needed in such metabolic schemes as photosynthesis,
starch synthesis, glycolysis, and the synthesis of fats and
proteins.
P is thus needed at various points in the plant.
A pool of P in a useable form is maintained throughout the
plant in a relatively uniform concentration.
Sulfur
Sulfur is mobile in plants but to a lower extent than P.
This is because of its rapid incorporation into metabolic
compounds.
Absorbed *S rapidly ascends in the xylem to the leaves.
Within 24 h most of the *S was found in the younger leaves.
The older leaves having lost their S to the young actively
growing leaves.
S is a constituent of protein and protein synthesis occurs
heavily in the younger leaves.
S, then, is freely mobile in the plant but is rendered
immobile rather quickly in metabolic reactions.
Iron
Mobility of iron in the plant depends on :
1- Fe concentration of the plant tissues.
Being highest at low Fe concentration in the plant and
decreases at high concentration.
2- P availability.
High P concentrations in the plant tissues render iron
immobile in the veins of the leaf.
3- pH of the rooting medium.
A pH of 4 gives high iron mobility and mobility decreases at
pH of 7.
Calcium
Absorbed *Ca ascends in the transpiration stream to the
different areas of the plant.
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In conclusion:
1-
T
here are four general directions of ion movement in the
plant:
upward, downward, lateral and outward.
3-
The upward translocation of salts takes place primarily in
the xylem, but also in the phloem.
4- Downward movement takes place in the phloem where
upward movement also occurs.
5- Movement of salts in the phloem is, thus, bidirectional.
6- Lateral movement occurs between xylem and phloem through
the cambium.
7- Movement of salts out of leaves is common, especially prior
to abscission, and occurs in the phloem.
Plant mineral nutrition involves the absorption and translocation of ions across roots. Salts are absorbed passively and actively, with ions moving into the root's apoplasm via free diffusion. The Casparian strip in endodermal cells acts as a barrier, allowing ions to pass only through the protoplast. Once absorbed, ions are delivered into the xylem for circulation throughout the plant, where they are essential for various metabolic processes.
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Course: Plant Physiology Course: Plant Physiology Code: Code: 308 308 Bot Bot Students: Third year Botany Students: Third year Botany Lecture Lecture 2 2 (Plant Mineral Nutrition) (Plant Mineral Nutrition) Dr. Dr. Taha Taha Mohamed El Mohamed El- -Katony Katony
Absorption and translocation Absorption and translocation The path of ions across the root The path of ions across the root Absorption of salts by roots is both passive and active. Absorption of salts by roots is both passive and active. The movement of ions into the AFS ( The movement of ions into the AFS (apoplasm (by free diffusion). (by free diffusion). apoplasm) is passive ) is passive Ions can move freely in the Ions can move freely in the wet cell walls ( the r the root as far as the endodermis. oot as far as the endodermis. wet cell walls (apoplast apoplast) of ) of Further penetration is retarded by the Further penetration is retarded by the C Casparian asparian strip. strip. The The continuum continuum palsmodesmata palsmodesmata is called the of of is called the symplast cytoplasm cytoplasm and and the the connecting connecting symplast. .
The endodermal cells by virtue of the The endodermal cells by virtue of the Casparian present a barrier to the passive diffusion of ions. present a barrier to the passive diffusion of ions. Casparian strip strip The protoplast of endodermal cells is tightly attached to The protoplast of endodermal cells is tightly attached to the the Casparian Casparian strip. strip. Ions Ions can cells cells. . can not not pass pass through through the the walls walls of of the the endodermal endodermal The The only only route route available available is is through through the the protoplast protoplast. .
How How can can ions ions be be delivered delivered into into xylem? xylem? It It was was supposed supposed that increasing increasing CO that there there is is a a gradient CO2 2 levels levels from from the gradient of of decreasing the cortex cortex to to the decreasing O O2 2 and the stele stele. . and The The living living cells low low metabolic metabolic activity, lumen lumen of of xylem cells adjacent adjacent to to the the xylem xylem vessels activity, are are leaky xylem vessels vessels. . vessels therefore therefore possess leaky and and lose lose salts possess salts to to the the Diffusion Diffusion of of salts is is not not allowed into into the the lumina salts back allowed and lumina of of the back through through the and there there is is a a unidirectional the xylem xylem vessels the impervious impervious Casparian unidirectional loss vessels. . Casparian strip loss of of salt strip salt
Circulation of salts Circulation of salts Salts delivered to the xylem ducts of the root are Salts delivered to the xylem ducts of the root are translocated upward to the shoot and, once there, are distributed upward to the shoot and, once there, are distributed and redistributed throughout the plant. and redistributed throughout the plant. translocated Salts deposited in the leaves are withdrawn prior to Salts deposited in the leaves are withdrawn prior to abscission and abscission and translocated translocated to the reproductive organs or to the reproductive organs or younger leaves. younger leaves. Circulation takes place in the vascular tissues. Circulation takes place in the vascular tissues.
By the use of radioactive tracers several pathways for the By the use of radioactive tracers several pathways for the translocation of salts were discovered: translocation of salts were discovered: 1 1- - in the xylem, in the xylem, 2 2- - in the phloem, in the phloem, 3 3- - laterally between the two tissues and laterally between the two tissues and 4 4- - outward from the leaves. outward from the leaves.
Translocation of salts in xylem Translocation of salts in xylem Experimental evidence Experimental evidence 1- Ringing experiment: The upward translocation of salts is unimpeded by removal of a ring of phloem near the stem base. 2- Large amounts of salts occur in the xylem sap. 3- 32P travels upward to plant tip faster under high transpiration (bright sunlight) than under low transpiration. 4- If transpiration by a leaf is diminished by covering the leaf with a polyethylene bag, translocation of minerals to that particular leaf is reduced considerably.
5 5- - Dissection Dissection experiment experiment with with radioactive radioactive tracers tracers: : A A: : Procedure Procedure i i- - The The bark were were separated was was inserted inserted between bark and separated and and xylem xylem along along a a 9 9- -inch and a a strip strip of of impervious between them them. . inch length length of of a a willow impervious waxed willow stem waxed paper stem paper iii iii- - The The continuity continuity of of bark the the plant plant was bark and left intact intact. . and xylem xylem was was undisturbed, undisturbed, and and was left absorb 42 42K K for iv iv- - The The plant plant was was allowed allowed to to absorb for 5 5 hours hours. . v v- - Sections Sections of of the analyzed analyzed for the treated treated and for 42 and intact intact areas areas of of the the stem stem were were 42K K. .
Stripped branch 42K in bark (ppm) Unstripped branch 42K in bark (ppm) 42K in wood (ppm) 42K in wood (ppm) Section SA 53.0 47 64 56 S6 11.6 119 S5 0.9 122 S4 0.7 112 87 69 S3 0.3 98 S2 0.3 108 S1 20.0 113 SB 84.0 58 74 67
B: Results and conclusion B: Results and conclusion 1- 42K translocates upward in the xylem. 2- Lateral interchange of 42K between phloem and xylem (through cambium) occurs within the intact region but further translocation either upward or downward in the phloem is slow. 3- The small amounts of 42K in the bark along the stripped area suggests that little translocation takes place in the phloem tissue.
Lateral translocation of salts Lateral translocation of salts T The above experiment revealed: he above experiment revealed: 1 1- - upward translocation of salts in xylem upward translocation of salts in xylem and and 2 2- - lateral movement between xylem and phloem through lateral movement between xylem and phloem through cambium. cambium. Cambium regulates the amount of salts carried up in the Cambium regulates the amount of salts carried up in the transpiration stream. transpiration stream. If the upward movement of salts were not regulated, certain If the upward movement of salts were not regulated, certain areas of the plant areas of the plant (shoot and root tips) would (shoot and root tips) would not be accommodated. accommodated. not be The cambium position allows The cambium position allows - - both physically physically - - re regulation of the upward, lateral and gulation of the upward, lateral and downward movement of salt. downward movement of salt. both metabolically metabolically and and
The active accumulation of salt by the cambial cells acts The active accumulation of salt by the cambial cells acts as a deterrent against an indiscriminate sweep of as a deterrent against an indiscriminate sweep of salts upward in the transpiration stream. salts upward in the transpiration stream. The lateral movement from xylem to phloem correlates The lateral movement from xylem to phloem correlates inversely with element content in the phloem inversely with element content in the phloem high concentration of an element in the phloem, slows high concentration of an element in the phloem, slows down the lateral translocation into the phloem and down the lateral translocation into the phloem and low concentration enhances lateral movement. low concentration enhances lateral movement.
Translocation of salts in the phloem Translocation of salts in the phloem The initial upward movement of salts occurs in the xylem. The initial upward movement of salts occurs in the xylem. But upward movement may occur also in the phloem. But upward movement may occur also in the phloem. Evidence on upward movement: Evidence on upward movement: 1 1- - Ringing bark high up in the stem Ringing bark high up in the stem retarded s retarded stem tip growth. tem tip growth. But ringing at stem base has no effect on salt nutrition of the whole But ringing at stem base has no effect on salt nutrition of the whole plant including stem tip. plant including stem tip. Thus, the influence on stem tip growth was because of the blockage Thus, the influence on stem tip growth was because of the blockage of salts moving out of the lower leaves and transported upward in of salts moving out of the lower leaves and transported upward in the phloem and not because of the root the phloem and not because of the root- -absorbed salts. absorbed salts.
Evidence on downward movement: Evidence on downward movement: 2 2- - Dissection experiment with Radioactive tracers. Dissection experiment with Radioactive tracers. Procedure Procedure: : A A- - The bark was separated from wood by waxed paper The bark was separated from wood by waxed paper immediately below leaf petiole of a cotton plant. immediately below leaf petiole of a cotton plant. B B- - Bark and wood were left intact. Bark and wood were left intact. 32 32P was injected into leaf blade just above the P was injected into leaf blade just above the separated area. separated area. C C- - D D- - One hour One hour later, sections of stem were analyzed for later, sections of stem were analyzed for 32 32P. P.
Stripped plant 32P in bark (mg) Unstripped plant 32P in bark (mg) 32P in wood (mg) 32P in wood (mg) Section A 1.11 0.444 I 0.485 0.100 C 0.610 S1 0.554 0.064 0.160 0.055 S2 0.332 0.004 0.103 0.063 S3 0.592 000 0.055 0.018 S4 0.228 0.004 0.026 0.007 0.152 B 0.653
Results and conclusion Results and conclusion 1 1- - Salts Salts entering primarily primarily in in a a downward the the phloem phloem. . entering the the main main vascular downward as vascular stream as well well as stream from as upward upward direction from leaf direction in in leaf move move 2 2- - Lateral Lateral transport where where the transport between the phloem phloem and between the and xylem the vascular vascular tissues xylem are are not not separated tissues takes separated. . takes place place 4 4- - Both Both tissues translocation translocation of of mineral tissues may may thus mineral salts thus be be involved involved with salts moving moving out with the out from from leaves the upward upward leaves. .
The movement of salts in the phloem is bidirectional. The movement of salts in the phloem is bidirectional. This bidirectional movement might occur: This bidirectional movement might occur: 1 1- - Simultaneously in the same sieve elements. Simultaneously in the same sieve elements. Or Or 2 2- - In two different phloem channels, one toward the In two different phloem channels, one toward the tip and the other toward the base of the plant. tip and the other toward the base of the plant.
Outward movement of salts from leaves Outward movement of salts from leaves In In the the leaves leaf leaf just leaves of of deciduous deciduous plants, just prior prior to to abscission plants, mineral mineral salts abscission. . salts move move out out of of the the The The mineral mineral nutrients Cl Cl and and (Mg nutrients moving (Mg and and Fe) moving out Fe). . out of of leaves leaves are are N, N, K, K, P, P, S, S, Those Those remaining remaining include include Ca Ca, , B, B, Mn Mn and and Si Si. . Mineral Mineral nutrients nutrients move move out out of of leaves leaves primarily primarily in in the the phloem phloem. . When When 32 32P P was plant plant: : P P from while while P P from was introduced introduced to to leaves from base base leaves leaves moves from top top leaves leaves moves leaves at at different moves downward downward toward moves upward upward to to tip different levels toward the tip. . levels on on the the the root root
Circulation and reutilization Circulation and reutilization Minerals Minerals are are taken taken up up in in the the transpiration transpiration stream stream. . Move Move primarily primarily in in the the xylem xylem vessels vessels to to the the leaves leaves. . Excess Excess quantities quantities are translocated translocated downward are withdrawn withdrawn from downward in in the from leaves phloem. . leaves and and re re- - the phloem Then Then they upward upward translocation they could could be be laterally laterally transported transported into translocation could could take into the place again the xylem xylem w where again. . here take place Elements Elements such such as as N, N, K K and and P P move move readily readily in in this this circuit circuit. . Calcium Calcium ascends ascends the the stem stem but but does does not not relocate relocate in in the the phloem phloem. .
Phosphorus Phosphorus P is highly mobile in the plant and it is in continuous P is highly mobile in the plant and it is in continuous circulation. circulation. P mobility is an essential feature of plant growth. P mobility is an essential feature of plant growth. P is needed in such metabolic schemes as photosynthesis, P is needed in such metabolic schemes as photosynthesis, starch synthesis, glycolysis, and the synthesis of fats and starch synthesis, glycolysis, and the synthesis of fats and proteins. proteins. P is thus needed at various points in the plant. P is thus needed at various points in the plant. A pool of P in a useable form is maintained throughout the A pool of P in a useable form is maintained throughout the plant in a relatively uniform concentration. plant in a relatively uniform concentration.
Sulfur Sulfur Sulfur is mobile in plants but to a lower extent than P. Sulfur is mobile in plants but to a lower extent than P. This is because of its rapid incorporation into metabolic This is because of its rapid incorporation into metabolic compounds. compounds. Absorbed *S rapidly ascends in the xylem to the leaves. Absorbed *S rapidly ascends in the xylem to the leaves. Within 24 h most of the *S was found in the younger leaves. Within 24 h most of the *S was found in the younger leaves. The older leaves having lost their S to the young actively The older leaves having lost their S to the young actively growing leaves. growing leaves. S is a constituent of protein and protein synthesis occurs S is a constituent of protein and protein synthesis occurs heavily in the younger leaves. heavily in the younger leaves. S, then, is freely mobile in the plant but is rendered S, then, is freely mobile in the plant but is rendered immobile rather quickly in metabolic reactions. immobile rather quickly in metabolic reactions.
Iron Iron Mobility of iron in the plant depends on : Mobility of iron in the plant depends on : 1 1- - Fe concentration of the plant tissues. Fe concentration of the plant tissues. Being highest at low Fe concentration in the plant and Being highest at low Fe concentration in the plant and decreases at high concentration. decreases at high concentration. 2 2- - P availability. P availability. High P concentrations in the plant tissues render iron High P concentrations in the plant tissues render iron immobile in the veins of the leaf. immobile in the veins of the leaf. 3 3- - pH of the rooting medium. pH of the rooting medium. A pH of 4 gives high iron mobility and mobility decreases at A pH of 4 gives high iron mobility and mobility decreases at pH of 7. pH of 7.
Calcium Calcium Absorbed * Absorbed *Ca different areas of the plant. different areas of the plant. Ca ascends in the transpiration stream to the ascends in the transpiration stream to the Ca Ca is immobile in the phloem, and once delivered by the is immobile in the phloem, and once delivered by the transpiration stream, it remains stationary transpiration stream, it remains stationary.
In conclusion: In conclusion: 1 1- - T There are four general directions of ion movement in the here are four general directions of ion movement in the plant: plant: upward, downward, lateral and outward. upward, downward, lateral and outward. 3 3- - The upward translocation of salts takes place primarily in The upward translocation of salts takes place primarily in the xylem, but also in the phloem. the xylem, but also in the phloem. 4 4- - Downward movement takes place in the phloem where Downward movement takes place in the phloem where upward movement also occurs. upward movement also occurs. 5 5- - Movement of salts in the phloem is, thus, bidirectional. Movement of salts in the phloem is, thus, bidirectional. 6 6- - Lateral movement occurs between xylem and phloem through Lateral movement occurs between xylem and phloem through the cambium. the cambium. 7 7- - Movement of salts out of leaves is common, especially prior Movement of salts out of leaves is common, especially prior to abscission, and occurs in the phloem. to abscission, and occurs in the phloem.
