Lect_note#2_Plant_Nutrient_MRQ_2011 _Long-Distance Transport in the Xylem and Phloem

8/6/2019 Lect_note#2_Plant_Nutrient_MRQ_2011 _Long-Distance Transport in the Xylem and Phloem

1/51

Long-Distance Transport

in the Xylem and Phloem

Mochammad Roviq, 2011


2/51

Concept The long-distance transport of water and

solutes - mineral elements and low-molecularweight organic compounds - takesplace in the vascular system of xylem and

phloem. Long-distance transport from the roots to the

shoots occurs predominantly in the nonlivingxylem vessels.

Coniferous trees lack the continuous systemof xylem vessels, and depend on tracheides

which are non-living conducting cells rangingin length from 2 to 6 mm.

In annual plant species too long-distancetransport in the xylem vessels may beinterrupted by tracheides, for example at the

root-shoot junction or in the nodes of thestem.

These structures pose an internal resistanceto xylem volume flow but simultaneouslypermit an intensive xylem phloem solutetransfer


3/51

Root pressure

Xylem transport is driven by thegradient in hydrostatic pressure(root pressure) and by the gradientin the water potential

The gradient in water potentialbetween roots and shoots is quitesteep particularly during the daywhen the stomata are open.

Values become less negative in the

following sequence: atmosphere leaf cells > xylem sap > root cells >external solution


4/51

Direction of long-distance

transport

Solute flow in the xylem from theroots to the shoots is thereforeunidirectional

Under certain conditions in the shoots

a counterflow of water in the xylemmay also occur, for example, fromlow-transpiring fruits back to theleaves

Long-distance transport in the phloem

takes place in the living sieve tubecells and is bidirectional.

The direction of transport isdetermined primarily by thenutritional requirements of the

various plant organs or tissues andoccurs, therefore, from source to sink


5/51

Transfer cells

During long-distancetransport, mineralelements and organicsolutes are transferred

between the xylem andphloem by extensiveexchange processes,referred to as loading andunloading.

The transfer is mediated byspecific cells called transfercells


6/51

Transfer cells are specialised

parenchymal cells. They are different from

the other cells of theplant structure in that,they are concerned withtransport of solutes.

They are present in highnumbers whereveractive absorption andsecretion are evident.

They are mostly found inthe close conjunction

with vascular cambialcells i.e., xylem andphloem


7/51

Composition of the Xylem Sap

Depend on factors such as plant species, mineralelement supply to the roots, assimilation ofmineral nutrients in the roots and nutrient

recycling. Composition and particularly concentration of

solutes are also strongly influenced by the degreeof dilution by water and are therefore dependent

on the transpiration rate and the time of day. Composition and concentration of xylem sap also

change typically during the ontogenesis of plants


8/51

Xylem Volume Flow (Pressurized Exudation at 100 kPa) and Mineral

Element Concentrations in the Xylem Sap of Soil-Grown Nodulated

Soybean During Reproductive Stage


9/51

Exchange Adsorption and Resorption

The interactions between cations are similar to those inthe AFS of the root cortex .

The degree of retardation (penghambatan) of cationtranslocation depends on the valency of the cation (Ca2+>

K

+

), its own activity, the activity of competing cations, thecharge density of the negative groups (dicots > monocots),the pH of the xylem sap which may vary between 5 and 7and the diameter of the xylem vessels.

The translocation rate in the xylem of heavy metal cationsis much enhanced when the ions are complexed, forexample, in the case of copper, zinc or cadmium


10/51

Resorption

Solutes can also be resorbed from the xylem(apoplasm) into the living cells (transport in thecytoplasm and vacuole) along the pathway of thexylem sap from the roots to the leaves

Resorption from the xylem can be the result either oftransient or permanent storage in the xylemparenchyma and other stem tissue, or of xylem-phloem transfer, mediated by specialized cells (xylemparenchyma transfer cells)

The resorption of certain mineral elements from thexylem sap is very pronounced and can have importantconsequences for the mineral nutrition of these plants


11/51

Natrophobic or natrophilic plant species

Na+ is retained mainly in the roots and lowerstem (Phaseolus vulgaris), whereas in natrophilicspecies (e.g. sugar beet) translocation into theleaves readily occurs


12/51

Resorspsi unsur mikro

Resorpsi dari cairan xilem di akar dan batang juga dapat

menjadi faktor penentu dalam distribusi unsur mikro pada

tanaman.

Sehingga ketika pasokan

molibdenum dalammedium hara tinggi, pada

tomat terjadi keracunan

jauh lebih awal daripada

kacang atau bungamatahari


13/51

Release or Secretion

Komposisi cairan xilem di sepanjangjalur transportasi juga dapat berubaholeh pelepasan atau sekresi zat terlarutdari sel-sel di sekitarnya.

Misalnya, nonlegum yang diberi nitrat,konsentrasi nitrat di cairan xilemmenurun sejalan dengan semakinpanjang jalur, sedangkan konsentrasinitrogen organik, khususnya glutaminmeningkat

Dalam legum yang bernodulasi (di manaterjadi fiksasi N2), di sisi lain, rasioamida dengan asam amino bergesermendukung asam amino


14/51

Xylem Unloading in Leaves

Meskipun terjadi resorpsi sepanjang jalur di batang, sebagianbesar zat terlarut dan air diangkut pembuluh xilem ke daun.

Air diangkut dalam pembuluh utama ke bagian yang

mengalami penguapan cepat seperti tepi daun

Konsentrasi zat terlarutdapat naik dibagian

tertentu, misalnya, tepi

daun, tergantung pada

konsentrasi dan komposisi

larutan dalam cairan xilem

yang masuk daun, laju

kehilangan air melalui

transpirasi, dan panjang

jalur yang melalui daun


15/51

Precipitation..

Pencegahan akumulasi zat terlarut yang

berlebihan dalam apoplasm daun dapat

dicapai dengan pembentukan garam low

solubility di apoplasm, terutama untuk

menghilangkan kalsium larut dalamgymnosperms

This mechanism of precipitation

seems to be a safe way of coping with

a continuous xylem import of calcium

which is scarcely exported in the

phloem and where the ionic

concentrations in the symplasm have

to be kept very low.

Calcium oxalate crystals in the

apoplasm of needles. (Left) Micrograph

from the phloem of a needle from

Juniperus chinensis; (right) micrograph

of a stomatal pore of a needle from

Picea abies (L.) Karst.


16/51

Model for scavenging solutes from the xylem sap

(xylem unloading') in leaf cells.

The cells of the bundlesheath are sites of intensivenet proton excretion whichacidifies the apoplasm the

proton gradient across theplasma membrane acting asthe driving force forcotransport of amino acidsand ureides.

The activity of the protonpump in legumes is highbefore pod filling anddisappears during podformation

As- amino acids.


17/51

Effect of Transpiration

Rate on Uptake and

Translocation

Tingkat fluks air di akar(transportasi jarak pendek) dan

di dalam pembuluh xilem(transportasi jarak jauh)ditentukan oleh tekanan akardan laju transpirasi.Peningkatan tingkat transpirasimungkin, atau tidak mungkin,

meningkatkan serapan dantranslokasi unsur mineral dalamxilem tersebut. Peningkatandapat dicapai dengan berbagaicara

A. 'Passive' transport of mineral elements through

the apoplasm into the stele. B. More rapid removal

of mineral elements released into the xylem vessels

. C. Increase in the mass flow of the external

solution to the rhizoplane and eventually into the

apparent free space of the cortex, favoring active

uptake into the symplasm. E, Endodermis; X, xylem;

arrow, water flux (A to C see text).


18/51

Predominantly factors of uptake and

translocation rate:

1. Plant age. In seedlings and young plants witha low leaf surface area, enhancementeffectsof transpiration are usually absent; wateruptake and solute transport in the xylem tothe shoots are determined mainly by the root

pressure. As the age and size of the plantsincrease, the relative importance of thetranspiration rate, particularly for thetranslocation of mineral elements increases.


19/51

2. Time of day

In leaves up to 90% of the total transpiration isstomatal. During the light period, transpiration ratesand thus the potential enhancement of uptake andtranslocation of mineral elements are much higher

than during the dark period.


20/51


21/51

4. Type of mineral element. Under otherwisecomparable conditions (e.g. plant age andexternal concentration), the effect of

transpiration rates on the uptake and transportfollows a typically defined ranking order ofmineral elements.

transpirasi meningkatkan penyerapan dan

translokasi molekul tak bermuatan ke tingkatyang lebih besar daripada ion.

Ada hubungan erat antara tingkat transpirasi dantingkat penyerapan herbisida tertentu

translocation rate


22/51

Measured and Calculated Silicon Uptake inRelation to Transpiration (Water Consumption)

of Oat Plants


23/51

Effect of Transpiration Rate on

Distribution within the Shoot

The long-distance transport of a mineral elementexclusively in the xylem should be expected togive a distinct distribution pattern in the shootorgans that depends on both transpiration rates

(e.g., ml g-1 dry weight each day) and duration oftranspiration (e.g. age of the organ).

For example, manganese (where at the sameplant (maple tree) and similar leaf age the *sun

leaves' (high transpiration rates) have muchhigher manganese contents in their dry matterthan 'shade leaves' (low transpiration rates).


24/51

Increasing supply

The distribution of boron is alsorelated to the loss of water

from the shoot organ, as shownby the boron distribution inshoots of rape in response toan increasing boron supply

The typical gradient in thetranspiration rates in the shootorgans (leaves > pods seeds)corresponds to the gradient inboron content.

Effect of increasing boron

application to the soil on the

distribution of boron in the

shoots of rape


25/51

II. Phloem Transport

Principles of Transport and Phloem Anatomy

Composition of the Phloem Sap

Mobility in the Phloem Transfer between the Xylem and Phloem


26/51

Principles of Transport and Phloem

Anatomy

Long-distance transport inthe phloem takes place inliving cells, the sieve

tubes The principles of the

transport mechanism inthe phloem wereproposed as early as 1930by Munch inpressure

flow hypothesis(Druckstromtheorie)based on the principle ofthe osmometer.

Cross-sectional area of a vascular bundle from

the stem of maize. Inset: sieve tube with sieve

plate pores and T-protein'. (From Eschrich,

1976.)


27/51

Release or unloading at the sink

Munch suggested that solutessuch as sucrose areconcentrated in the phloem ofleaves (i.e.,phloem loading) andthe water is sucked into thephloem, creating a positiveinternal pressure.

This pressure induces a massflow in the phloem to the sitesof lower positive pressurecaused by removal of solutes

from the phloem. Flow rate and direction of flow

are therefore closely related tothe release or unloading at thesink.


28/51

Munchpressure flow hypothesis


29/51

This type ofpressure-driven mass flow in thephloem differs from thatin the xylem in three

important ways: (a)Organic compounds arethe dominant solutes inthe phloem sap; (b)transport takes place in

living cells; and (c) theunloading of solutes atthe sink plays animportant role.


30/51

Source-sink regulated transport

An example for a primarilysource-sink regulated transportof a mineral nutrient is shownin for phosphorus.

After application to one of the

two mature primary leaves,the labelled phosphorus istransported to the shoot apexand the roots whereastransport to the other primary

leaf is negligible. In contrast, sodium is not

transported to the shoot apexbut exclusively movesdownwards (basipetally) to the

Retranslocation of labeled phosphorus

(P) and sodium (Na) after application to

the tip of a primary leaf of bean.

Autoradiogram, 24 h after application.


31/51

Composition of the Phloem Sap

Phloem sap has a high pH (7-8) and contains highconcentrations of solutes, on average 15-25% dry matter.

The main component is usually sucrose, which maycomprise up to 90% of the solids. The proportion of sucroseto other solutes depends on the site of phloem sapcollection, it is very high, for example near the ear ofcereals

In addition to sucrose, among the other organic solutesamino acids are usually present in high concentrations theamides glutamine and asparagine may represent up to 90%of this fraction, whereas the concentrations of nitrate andammonium are usually very low


32/51

Comparison of the Levels of Organic and Inorganic Solutes in the

Phloem and Xylem Exudates ofNicotiana glauca


33/51

Mobility in the Phloem

The classification is, ofcourse, only a firstapproximation as certain

factors are ignored, forexample, genotypicaldifferences or the nutritionalstatus of plants.

For the macronutrients,

except calcium (phloemmobility is generally high,and for the micronutrients itis at least intermediate withthe exception of manganese.


34/51

Transfer between the Xylem and

Phloem

In the vascularbundles, phloem andxylem are separated

by only a few cells(Lihat gambarsebelumnya).

In the regulation oflong-distancetransport, exchange

of solutes betweenthe two conductingsystems is veryimportant


35/51

Water and solute flow


36/51

Transfer from xylem to phloem

Dari perbedaan konsentrasi yang ditampilkan didalamnya adalah jelas bahwa transfer dari floemke xilem dapat terjadi downhill, melalui membranplasma dari tabung saringan, jika sebuah gradien

konsentrasi yang cukup ada. Sebaliknya, bagi sebagian besar zat terlarut

organik dan anorganik ditransfer dari xilem kefloem biasanya merupakan uphill transport

melawan gradien konsentrasi antara apoplasm(xilem) dan symplasm sel xilem sekitar parenkimdan sel-sel floem


37/51

Higher demand

The xylem-to-phloem transfer is ofparticular importance for themineral nutrition of plants,because xylem transport isdirected mainly to the sites(organs) of highest transpiration,which are usually not the sites ofhighest demand for mineralnutrients.

This transfer of organic and

inorganic solutes can take place allalong the pathway from roots toshoot, and the stem plays animportant role in this respect mostlikely via transfer cells

Long-distance transport in xylem (X)

and phloem (P) in a stem with a

connected leaf, and xylem-to-phloem

transfer mediated by a transfer cell (T).


38/51

Mineral Nutrients with High Phloem

Mobility

For mineral nutrients withhigh phloem mobility suchas potassium, phosphorus

or nitrogen as amino-N therelative importance ofphloem and xylemtransport into an organmainly depends on thestage of development of

the organ as shown in foramino-N during the life ofan individual leaf.


39/51

Mineral Nutrients of Low Phloem

Mobility; Example Calcium

Because of its low concentrations in the phloem sap (Section 3.3.2)the import of calcium into growth sinks such as shoot apices, youngleaves or fruits takes place nearly exclusively in the xylem, whereas

the import in the phloem is negligible as shown for castor bean inTable 3.11. This is in marked contrast to potassium of which most(terminal bud) and at least half (youngest leaves) of the total netimport takes place in the phloem. For magnesium phloem importcontributes to 25 and 40% of the total import, respectively.


40/51

Environmental Factors

To increase thecalcium contentof growing leavesor fruits,

increasing thetranspirationrates of the fruitsis more effective

than increasingthe calciumsupply in thesubstrate

Effect of Environmental Factors and Growth Rate

on Calcium and Potassium Content of Red Pepper

Fruits


41/51

Root pressure

High root pressure, as indicated by the intensity

of guttation, is closely correlated with an

increased concentration of calcium in expanding

leaves and either the absence of, or only mildsymptoms of, calcium deficiency (tip necrosis).

Magnesium, which is highly phloem mobile, is

only slightly affected by root pressure. Root pressure also strongly depends on root

respiration and oxygen supply to the roots.


42/51

Guttation


43/51

Retranslocation and Cycling of

Nutrients

With exception of calcium and presumably also

manganese, import of nutrients in the xylem and

export (retranslocation) in the phloem is a normal

feature throughout the life of an individual leaf. Several pieces of evidence indicate a rapid xylem-

to-phloem transfer in the leaf blades and

involvement of only a small fraction of the totalleaf content ('cycling' fraction;) in this process.


44/51

Between 82 and 100% of theexported mineral elementsare retranslocated in thephloem back to the roots, anda high proportion of thepotassium and magnesiumcycle, i.e., they are againloaded into the xylem andtransported to the shoots.

For calcium and sodium noprecise data can be given but

cycling is of minorimportance. For cycling ofpotassium, correspondingdata for other plant speciesare 20% in tomato and 30% inwheat and rye.


45/51

Remobilization of Mineral Nutrients

Import and export of mineral nutrients occursimultaneously during the life of plant organssuch as leaves (Table 3.16).

h l l d b h l


46/51

Physiological and biochemical

processes

Remobilization is based on a range of differentphysiological and biochemical processes:

utilization of mineral nutrients stored in vacuoles(potassium, phosphorus, magnesium, amino-N, etc.),

breakdown of storage proteins (e.g., in vacuoles of theparaveinal mesophyll cells of legumes;), or, finally,

breakdown of cell structures (e.g., chloroplasts) and

enzyme proteins thereby transforming structurallybound mineral nutrients (e.g., magnesium inchlorophyll, micronutrients in enzymes) into a mobileform.


47/51

Seed Germination

During the germination of seeds (or storage

organs such as tubers) mineral nutrients are

remobilized within the seed tissue and

translocated in the phloem or xylem, or both,to the developing roots or shoots.

As a rule, seedlings will grow for at least

several days without an external supply ofmineral nutrients


48/51

Vegetative Stage

During vegetative growth, nutrient supply to the roots isoften either permanently insufficient (as in the case of lowsoil nutrient content) or temporarily interrupted (when, forexample, there is a lack or excess of soil moisture).

Remobilization of mineral nutrients from mature leaves to

areas of new growth is thus of key importance for thecompletion of the life cycle of plants under theseexperimental conditions.

This behaviour (strategy) is typical for fast-growing cropspecies whereas for many wild species cessation of growth

occurs under adverse environmental conditions and, thus,redistribution of mineral nutrients plays a lesser importantrole


49/51

Reproductive Stage

Remobilization of mineral nutrients is particularlyimportant during reproductive growth whenseeds, fruits, and storage organs are formed.

At this growth stage root activity and nutrientuptake generally decrease, mainly as a result ofdecreasing carbohydrate supply to the roots ('sinkcompetition'.

Therefore, the mineral nutrient contents ofvegetative parts quite often decline sharplyduring the reproductive stage


50/51

Period Before Leaf Drop (Perennials)

As a rule, and similar to annual species, theextent of remobilization is high for nitrogen,potassium, phosphorus, and zinc, whereas theleaf contents of calcium, boron, iron and

manganese increase until leaf drop Remobilization of mineral nutrients (except

calcium and manganese) from the leaves towoody parts is a typical feature of perennial

species before leaf drop in temperate climates,and is closely related to the discoloration ofleaves in the autumn.


51/51

Terimakasih

Lect_note#2_Plant_Nutrient_MRQ_2011 _Long-Distance Transport in the Xylem and Phloem

Documents

Transcript of Lect_note#2_Plant_Nutrient_MRQ_2011 _Long-Distance Transport in the Xylem and Phloem