Tải bản đầy đủ - 0 (trang)
CHAPTER 2. FACTORS AFFECTING ROOT EXUDATION II: 1970–1978

CHAPTER 2. FACTORS AFFECTING ROOT EXUDATION II: 1970–1978

Tải bản đầy đủ - 0trang

94



M. G. HALE AND L. D . MOORE



solubilization of nutrients, in soil aggregation, and in effects on the pH of the soil

solution.

Soil microbiologists were the first to draw attention to the differences in

environment for microorganisms close to the root surface and at some distance

from the root. Hiltner (1904) is given credit for coining the term “rhizosphere,”

which is the sphere of influence of the root on its environment for a distance of at

least a millimeter or two from the root surface. Agronomists have since become

aware of “soil sickness” problems in the rotation of crops in which it is postulated that there is a chemical effect of residual compounds from previous crops

(Muller , 1966).

The intriguing idea that populations of rhizosphere organisms can be controlled by foliar applications of chemicals, which are either translocated to the

roots and exuded into the rhizosphere intact, or which change the pattern of

exudation and in this manner affect the resistant of the plant, has excited plant

pathologists and soil microbiologists.

In the last two decades a large body of literature has been developed on root

and other plant part exudates. Understandably, a large percentage of the experiments have been conducted under controlled environmental conditions, usually

with axenic (aseptic) seedlings in nutrient solutions. Those experiments conducted under natural environmental and soil conditions have usually involved the

assay of microorganism populations in rhizosphere and nonrhizosphere soils.

The use of 14C0, to label plant metabolites and to trace their movement from

leaves to roots and into the rhizosphere has begun to yield valuable information.

New quantitative estimates of carbon lost via the roots indicate that the quantities

are far greater than have been estimated previously. Such experiments have led to

refinements of the term “exudate” to denote the source of the organic compounds and whether they are water-soluble or insoluble; diffusible or nondiffusible; gaseous or volatile. One begins to hope that meaningful quantitative determinations of root exudates can be made in situ and that evaluation can be made

of the effect of exudation on the carbon balance of the plant as well as on the

ecology of the rhizosphere. The data accumulated have been substantive enough

that loss of organic compounds or organic matter into the soil from roots as they

grow cannot be ignored in interpretation or design of experiments.

Brief reviews of exudation include those of Tinker and Sanders (1975) and

Lespinat and Berlier (1 975) on factors affecting exudation. Bowen and Rovira

(1976) have reviewed exudation in relation to root diseases. Hale et al. (1978)

reviewed the principles and concepts of root exudation. The relationship of

exudates to soil microorganisms has been comprehensively covered in the two

volumes edited by Dommergues and Krupa (1978; Krupa and Dommergues,

1978). Since the review of Hale et al. (197 l), the factors affecting root exudation

have not been reviewed comprehensively.

The present review covers contributions to our knowledge and understanding

of the factors affecting root exudation that have been published during the period



ROOT EXUDATION



95



1970-1978. Since there has been increasing interest in seed exudation and

reevaluation of seedling root exudates in comparison with seed exudates, the

factors affecting seed exudation are included.



II. Plant Factors



A. GROWTH



I . Root Cups and Root Hair Secretion

Unevenly distributed layers of granular and fibrillar material (mucilage) cover

the outer surface of roots and root hairs. Mucilage was found on the roots of

sixteen plant species examined by Greaves and Darbyshire (1972); it seemed to

fill the space between the root and soil particles. Under nonaxenic conditions,

some of the mucilage may be of microbial origin. The nature of the mucilage or

slime is complex, but it seems to be composed primarily of polysaccharides.

Floyd and Ohlrogge (1971) collected droplets from the nodal roots of corn (Table

I) every 2 to 6 hours. Analysis of the hydrolyzate revealed the presence of

galactose, arabinose, xylose, fucose, and uronic acid in the ratio of 7:8:5: 11:3.

Some acid phosphatase and adenosine triphosphatase were also present. Slime

from the primary root tips of corn seedlings contained, in addition, the sugars

mannose, glucose, and rhamnose (Jones and Morre, 1973). In the unhydrolyzed

slime, galactose occurred in the largest quantities.

Paul ef al. ( 1 975) estimated the slime production rate of excised root tips of

corn 1-5 cm long to be 0.79 pg hr-' per root tip in a medium containing 40 mM

sucrose, 0.5 Hoagland's nutrient, and boric acid. Fucose made up 39%, galactose 30%, and arabinose plus xylose 22% of slime sugars. Fucose occurred in the

slime of all cultivars studied. Since fucose occurred only in the slime polysaccharide, its presence could be used as a marker for slime production (Bowles and

Northcote, 1972). Fucose apparently occurred only in the membrane fraction of

maize seedling root tips (Bowles and Northcote, 1972).

The hypothesis of Jones and Morre (1967) that the Golgi apparatus in corn is

involved in the secretion of root cap slime was confirmed in detailed biochemical

studies by Paul and Jones (1975 a,b, 1976). The slime originated in a layer one to

three cells deep at the root cap surface. It accumulated between the plasmalemma

and the cell wall and was eventually lost to the exterior of the cell wall.

Leppard (1974) used the scanning electron microscope to discover microfibrils

in the rhizoplane of the wheat root tip between the root cap and the root hair

zone. The microfibrils were determined to be polygalacturonic acids and because

of their physical structure could form microhabitats for microorganisms. Leppard

and Ramamorthy (1975) have suggested a role for the microfibrils in ion uptake.



TABLE I

Common Name, Scientific Name, and Reference for Plants Mentioned by Common Name

in the Text

Common name



Scientific binomial



Alfalfa



Medicago sativu L.



Barley



Hordeum vulgare L.



Bean



Phaseolus vulgaris L.



Bermuda grass

Broom rape



Cynodon dactylon (L.) Pers.

Orobanche rumosa L.



Cabbage

Corn



Brassica oleracea L .

Zen muys L.



Cotton



Cossypium herbeum L.

Gossypium hirsutum L.

Cucumis sativa L.

Linum usitatissiumum L.



Cucumber

Flax



Hyacinth bean

Lupine

Margosa

Marigold



Eucalyptus calaphylla R. Br.

Eucalyptus marginata Donn

ex. Sm.

Dolichos lablab L.

Lupinus angustifolia L.

Azudirachra indico Juss,

Tagetes erecta L.



Pea



Pisum sativum L.



Peanut



Arachis hypoguea L.



Gum



Citations

Hamlen et al. (1972, 1973)

Rao (1976)

Barber and Gunn (1974)

Barber and Lee (1974)

Barber and Martin (1976)

Manning e r a / . (1971)

Vancura and Stanek (1975)

Vancura and Stotzky (1976)

Wyse et al. (1976)

Singh and Singh (1971)

Ballard et al. (1978)

Hameed et al. (1973)

Vancura and Stotzky (1976)

Barber and Gunn (1974)

Barlow (1974)

Bowles and Northcote (1972)

Clowes and Woolston (1978)

Floyd and Ohlrogge (1971)

Hussain and Vancura (1970)

Jones and Morre (1973)

Kohl and Matthaei (1971)

Paul and Jones (1975a.b. 1976)

Paul er a / . (1975)

Vancura et al. (1977)

Vancura and Stotzky (1976)

Vancura and Stotzky (1976)

Booth (1 974)

Vancura and Stotzky (1976)

Ballard er al. (1978)

Hameed ei at. (1973)

Malajczak and McComb (1977)

Malajczak and McComb (1977)

Bhat et al. (1971)

Young e f at. (1977)

Alam et a / . (1975)

Alam et a / . (1975)

Hameed ( 197 1)

Hameed el a/. (1973)

Beute and Lockwood (1968)

Brannstrom (1977)

Christenson and Hadwigh (1973)

Short and Lacy (1976)

Tietz (1975)

Van Egaraat (1975a,b)

Vancura and Stotzky (1976)

Griffin er al. (1976)

Hale and Griffin (1974)



TABLE I-Continued

Common name



Pine, lodgepole

Ponderosa



scots



Scientific binomial



Pinus contortu Dougl. ex Loud

Pinus ponderosa Laws



Pinus sylvestris L.



Rattlebox

Red clover

Red pepper



Crotularia mediccgineu Lank

Trifolium prutense L.

Cupsicum unnum L.



Rice



Oryza sutivu L.



Sorghum



Sorghum vulgare Pers.



Soybean



Glvcine mux L.



Squash



Cucurbita pep0 L.



Sugar maple

Sunnhemp



Acer saccharum Marsh.

Crotuluriu junceu L.



Tobacco



Nicotianu tabucum L.



Tomato



Lycopersicon esculentum Mill.



Urid

Western wheat grass

Wheat



Phaseolis mungo L

Agropyron smithii Rydb.

Trificum uestivum L. & Thell.



Citations

Hale et a / . (1977)

Thompson ( 1978)

Reid and Mexal (1977)

Reid (1974)

Harley ( 1969)

Vancura and Stotzky (1976)

Krupa and Nylund (1972)

Krupa and Fries (1971)

Sullia (1973)

Bonish (1973)

Alagianagalingen and

Rarnakrishnan (1972)

Asanuma et al. (1978)

Yoshida and Takashi (1974)

Balasubramanian and

Rangaswami (1973)

Ballard et ul. (1978)

Hameed et (I/. (1973)

Werker and Kislev (1978)

Lee and Lockwood (1977)

Shapovalov (1972)

Tingey and Blum ( 1973)

Magyarosy and Hancock (1974)

Vancura and Stotzky (1976)

Smith (1970, 1972)

Balasubramanian and

Rangaswami (1973)

Ballard et ul. (1978)

Joyner (1975)

Ballard et ul. (1978)

Hameed et a / . (1973)

Vancura and Stotzky (1976)

Wang and Bergeson (1974)

Rao (1976)

Bokhari and Singh (1974)

Ayers and Thornton (1968)

Barber and Martin (1976)

Bowen and Rovira (1973)

Holden ( 1975)

Jalali (1976)

Jalali and Suryanarayana (1971,

1972, 1974)

Leppard ( 1974)

McDougall (1970)

McDougall and Rovira (1970)

Martin (1977a,b)

Rovira and Ridge (1973)

Srivastava and Mishra (1971)

Vancura et a / . (1977)

Vagnerova and Macura ( 1974)



98



M. G . HALE AND L. D.MOORE



In sorghum, a fibrillar, mucilaginous layer was found on the surface of the root

hairs (Werker and Kislev, 1978). A pectic material seemed to arise from the

endoplasmic reticulum, but the fibrillar material outside the cell walls arose from

the Golgi bodies and mitochondria.



2. Sloughage

As plant roots grow both in length and in diameter, some of the outer tissues

are sloughed and decompose either by autolysis or through the activities of microorganisms. Sauerbeck and Johnson (1976) suggested that the total rhizosphere

deposition amounts to three to four times as much organic substance as is found

in the roots at harvest and postulated that such deposition leads to intensive

turnover of organic matter in the rhizosphere. Martin (1977a) stated that there

was a significant formation of soil organic matter during active growth of wheat

roots and concluded that the contribution directly from roots without the intervention of microorganisms has been underestimated. He estimated that as much as

8% of the shoot carbon ‘ended up in rhizosphere organic matter and that most of

the carbon lost from the plant was the result of autolysis of cortical tissue. Three

stages were outlined (Martin 1977b) for root decomposition of wheat: ( a ) continuing release of low-molecular-weight constituents from degenerate epidermal

and cortical tissue and sloughed root cap material starting before tillering and

continuing to the flowering stage; ( b ) invasion of epidermal and cortical tissue

by soil microflora, causing breakdown of cell walls; and ( c ) decomposition of

epidermal tissue following death. The term “root exudate” is misleading, as it

has been used and should be reserved for those water-soluble and diffusible

compounds lost from roots (Martin, 1977b). Additional terms should be used

specifically to designate sources of organic compounds such as root lysate,

mucigel, cell wall residues, and intact plant cells. Similar thoughts led Warembourg and Morrall (1978) to conclude that use of I4CO2to label metabolites and

measure carbon loss from roots may be a more reliable way of estimating exudation than collecting exudates and sloughed material over a long period of time.

However, Griffin er al. (1976) were able to measure quantitatively the amounts

of sloughed cortical tissue and root caps from axenic peanut roots growing in

nutrient solution. Microscopic observations of roots indicated that many

sloughed cells and tissue fragments remained loosely attached to the roots even

after a sonication treatment, so that collection of the sloughed material resulted in

an underestimation of the amount. Carbon, hydrogen, and nitrogen content of the

sloughed material was calculated and found to be 15.2%, 3.0%, and 1.3%,

respectively. For 1.5 mg of sloughed material per gram of root dry weight per

week, the loss amounted to 510 pg of carbon, 86 pg of hydrogen, and 66 pg of

nitrogen. Bowen and Rovira (1973) have indicated that the insoluble mucilaginous exudate of wheat roots, including sloughed root cap cells, accounted for

0.8-1.6% of the root carbon and 80% of the total carbon released into soil.



ROOT EXUDATION



99



Regeneration of sloughed corn root caps appears to result from the increased

activity of the quiescent center in the apex of the root (Barlow, 1974). Barlow

raised questions concerning the mechanism by which regeneration was controlled

and the sort of messenger involved in initiating activity of the quiescent center

after root cap sloughing and attributed it to stresses imposed by the root cap cells.

Clowes and Woolston (1978) estimated the sloughage into water of root cap cells

of primary seedling roots of corn at different densities of roots in the medium.

When root density ranged from 50 to 250 roots per liter, sloughed root cap cells

ranged from 7000 to 3000 cells per root per day. Lower root densities increased

the sloughage, as did more frequent changes of water surrounding the roots. One

wonders if a dense population of roots and less frequent water changes resulted in

less agitation and dislodgement of sloughing cells rather than any effect of root

density on the sloughing process per se.

3 . Type of Root System



Monocotylendenous plants, dicotyledenous plants, and gymnosperms have

been used in exudation studies. Most work has been done on primary seedling

roots or adventitious roots (Hale et a / ., 1978). Smith (1970) invented techniques

by which he could study exudation from lateral root tips of sugar maple. Reid

(1974) examined exudation from roots of 9- and 12-month-old Ponderosa pine

and 7-year-old lodgepole pine (Reid and Mexal, 1977). The mother roots of

lodgepole pine exuded four times as much I4Ccompounds as did the mycorrhizal

roots, even though the mycorrhizal roots accumulated more 14C from the photosynthesizing shoots. In terms of older suberized roots of other species contributing to the rhizosphere organic matter, no information is available.

In a complex natural situation it is conceivable that the type of root system

could be a factor dependent on the variations of environment in which the parts of

the system were growing. For example, different parts of the root system could

be subjected to different moisture stresses and thus different kinds of exudation.

It is conceivable also that the sloughing pattern is different in those roots in which

an active cambium gives rise to secondary tissues than it is in those roots or parts

of roots that do not have secondary tissues.



B. INJURY



Mechanical injury occurs during the normal process of lateral root growth as

the laterals force or digest their way from the site of initiation through the cortical

cells to the root surface (Esau, 1953). McDougall and Rovira (1970) reported

that the lateral root zone of wheat roots was a region of exudation, but the

exudation came primarily from the emerging lateral root tips and not from the

rupture of cells and tissues of the older root. Furthermore, microscopic observa-



100



M. G. HALE AND L. D. MOORE



TABLE I1

Sources and Possible Causes of Injury Resulting in Increased Exudation"



Source

Microflora and

microfauna

Cultural practices

Growth



Environmental stress



Cause

Permeability changes, dissolution by enzymes, lysis, puncture, releases

of toxins and growth substances

Pesticides, cultivation, fertilizers, water stress, mineral nutrient

deficiency or toxicity

Lateral root eruption, abrasion by soil particles, sloughing off tissues

and root caps, pressure from cambial activities, permeability changes

with age and stage of development, regrowth of injured roots

Water stress, 0 concentration, COPconcentration, temperature

extremes, pH, salt concentration



"Reprinted with permission from M. G.Hale et al. (1978).



tion of the emerging root tips and careful handling of roots during the experiments suggested that exudation by the root apices was not because of injury. On

the other hand, Van Egeraat (1975a) reported that exudation of ninhydrinpositive compounds from pea roots was greater at sites of lateral root emergence,

probably because of injury. Van Egeraat showed that homoserine was abundantly

present in roots but not in root exudates except where injury occurred either by

emerging lateral roots or by damaging roots with a needle. Previously Ayers and

Thornton (1968) had demonstrated that wheat roots growing in sand consistently

exuded greater amounts of ninhydrin-reacting compounds than did roots growing

in nutrient solutions. They attributed the difference to abrasive injury caused by

the sand particles. Hale and Griffin (1974) used peanut fruits in various stages of

development to demonstrate increased exudation as a result of mechanical injury.

Peanut fruits were scarified over a quarter of the surface of each fruit, and the

amount of sugars released into an ambient solution was measured. In a 24-hour

period over 100 times as much sucrose was exuded from injured immature fruits

and 24 times as much from injured mature fruits as from the comparable uninjured fruits. Some sources and causes of injury that might lead to increases in

exudation are listed in Table 11.

C. ONTOGENETIC DEVELOPMENT



The age and stage of development of plants are factors that affect the amount

and kind of exudates, as noted in previous reviews (Rovira, 1969; Hale et al.,

1971, 1978). Some of the more recent contributions to our knowledge center

around distinguishing between exudates emanating from seeds and seedlings.

As dry seeds imbibe water, they release gaseous and volatile compounds

identified as ethanol, methanol, formaldehyde, acetaldehyde, formic acid,



101



ROOT EXUDATION



ethylene, and propylene (Vancura and Stotzky, 1976). Dried or autoclaved seeds

did not evolve the compounds, but pulverized wetted seed material did. Imbibing

seeds of bean, corn, and cotton released larger quantities than did imbibing seeds

'of cucumber, squash, and cabbage (Table 111). Most of the exudation occurred in

the first 2 days during germination and preceded the appearance of the radicle.

Evolution of volatiles was independent of alternating light and darkness but

appeared to be inversely related to seed size (Stotzky and Schenk, 1976). It had

been shown in earlier work that amounts of nonvolatile exudates from seeds are

directly correlated with seed size (Vancura and Hanzlikova, 1972).

The age of stored seeds can significantly affect exudation as they imbibe

water. Short and Lacy (1976) examined miragreen pea seeds and found 10 times

as much exudate from 8-year-old seeds as from 1-year-old seeds. Highest rates of

exudation occurred from the micropyle, but significant amounts exuded through

the seed coat from the cotyledons.

The contribution of cotyledons to root exudation from seedlings has been

studied by Vancura and Stanek (1975). The effects of cotyledon and primary

leaf removal from bean plants led them to conclude that compounds stored in

the cotyledons moved to the roots and that the exudates reflected this movement.

The amount of exudation of cucumber plants up to the twenty-fourth day was

always lower when cotyledons were removed and was always higher when

TABLE In

Summary of Volatile and Gaseous Metabolites Released by Germinating Seeds"-"



Seed

Bean

Cabbage

Corn

Cotton

Cucumber

Pea

Pinus raribea

Pinus palusrris

Pinus ponderosa

Pinus taeda

Radish

Red alder

Squash

Tomato



Methanol



Ethanol



Formaldehyde



+

0

+

+



+



+



0



+

i

+

+

+

0



0

0



+



+

+



+

+



+

+

t



+

+

-t



+

+

+



Formic

acid



Ethylene



Pcopylene



0



+



+



+



+



+

+



+



+



+

+

0



0



0



+



+



0



0



+



+

+



+



+

0



+



+



~



"Reprinted with permission from Vancura and Stotzky (1976).

b + indicates compound present, 0 not present.

'Ethanol present for all seeds.

dEthylene and propylene found in all seeds examined (four species).



I02



M. G. HALE AND L. D. MOORE



TABLE IV

Scientific Names of Bacteria, Fungi, and Nematodes Mentioned in the Text

Scientific binomial



Citations



Bacteria

Pseudomas purida (Trevisan) Migula

Rhizobium spp



Rhizobium leguminosarum Frank em.

Baldwin & Fred

Xanthomonas phaseoli var. fuscans

(Burkh) Start & Burkh



Vancura er a / . (1977)

Allen (1973)

Currier and Strobel (1976)

Rao (1 976)

Van Egaraat (1975)

Vancura and Stanek (1975)



Fungi

~



Alrernaria hutnicola Oudemans

Aspergillus niger van Tiegham

Beijerinckiu sp.

Boletus variegatus Fr.

Endogone sp.

Fomes annosus (Fr.) Cke.(Fomatopsis

annosus)

Fusarium solani f . sp. cucurbitae Snyd.

& Hans.

Fusarium solani f. sp. pisi (F. R. Jones)

Snyd & Hans.

Helminfhosporium sativum Pam., King. &

Bakke

Penicillium cirrinum Thom

Penicillium herquei Bainier & Sartory

Penicillium simplicissimum (Oud.) Thom

Phyrophfhora cinnamotni Rands

Puccinia graminis f. sp. tritici Eriks. and

E. Henn.

Thielaviopsis basicola (Berk. & Be.) Ferr.

Trichoderma harzianum Rifai

Trichoderma lignorum Rifai

Trichoderma viride Pers. ex. Fr.

Verricillium albo-utrum Reinke & Berth.



Nematodes

Helicorylenchus indicus Siddiqi

Hoploluimus indicus Sher.

Meloidogyne incognita (Kofoid & White)

Chitwood

Rorylenchulus reniformis Linford &

Oleviera

Tylenchorhynchus brassicae Siddiqi

Tylenchus filiformis Butschi



Sullia ( 1 973)

Sullia (1973)

Bhat er al. (1970)

Krupa and Fries (1971)

Krupa and Nyland (1972)

Jalali and Dornsch (1975)

Krupa and Nyland (1972)

Magyarosy and Hancock (1974)

Beute and Lockwood (1968)

Jalali and Suryanarayana ( 1971 )

Hameed (1971)

Sullia (1973)

Hameed (1971)

Malajczuk and McComb (1977)

Srivastava and Mishra (1971)

Lee and Lockwmd (1977)

Joyner ( 1975)

Sullia (1973)

Brannstrom (1977)

Booth ( I 974)

Alam et al. (1975)

Alarn et (11. (1975)

Alarn e f a/. (1975)

Hamlen et a/. (1973)

Wang and Bergeson (1974)

Alam et al. (1975)

Alam er al. (1975)

Alam er ul. (1975)



ROOT EXUDATION



103



TABLE V

Influence of Age on Effectiveness of Root Exudeates

on Percentage of Germination of Ombanche Ramosa



Seeds"

~



Plant



5 weeks



7 weeks



Flax

Sorghum

Tomato

Marigold



5.0

5.2

I .8

0.0



7.5

8.2

2.2

0.0



"Reprinted with permission from Hameed e/ a / .

(1973).



primary leaves were removed. As reserves in the cotyledons are depleted,

changes in exudation over time are important with respect to pathogen attack

(discussed in Section V,B,2,b). For example, increases in exudation of isoleucine and glutamic acid occurred beginning on the twenty-fourth day. Disappearance of the pathogen Xunrhomonas phuseoli (Table IV) from the bean root

rhizosphere was attributed to the disappearance of glutamic acid in the root

exudates.

As alfalfa plants aged, there were changes in the carbohydrate content of the

exudates (Hamlen et a l . , 1972). Initially, significant increases in the pentoses,

arabinose and xylose, were observed, but such increases were not observed in

exudates of plants 8 weeks old or older. Also, glucose, fructose, and mannose

increased in concentration from the fourth to the sixth week, but fructose and

mannose disappeared by the tenth week.

The age of the plant affects the exudation of substances that influence germination of Orabanche seeds (Hameed et a / . , 1973) (Table V). Further studies of

effects of exudates on Orubunche seed germination (Bailard et d.,1978)

showed that active exudates could be leached from the rooting medium of flax 3

days after transplanting, but only after 15 days from transplanting for tomato,

tobacco, and sorghum. The maximum effect on germination occurred after 20 to

25 days. This difference in time of appearance of active exudates from the

various species after transplanting could be a factor in resistance to parasitism by

Orabanche.



111. Effects of Environmental Factors



A. INDIRECT EFFECTS



Photosynthates may appear as root exudates in the rhizosphere in a variety of

organic forms, which indicates that the translocated material is usually acted



104



M. G. HALE AND L. D. MOORE

TABLE VI

The Influence of Defoliation on the Quantities of Compounds Exuded by Roots

of Sugar Maples"**

1969'

Compound



Control



1970d

Defoliated



1. Carbohydrates



Fructose

Glucose

Sucrose

2. Amino aciddamides

Alanine

Cystine

Glutamine

Glycine

Homoserine

Lysine

Methionine

Phenylalanine

Threonine

Tyrosine

3. Organic acids

Acetic

Malonic

~



~



~



2.6 t 0.2

0

7.3 ? 0.1

0



*



0.2

0.1

2.3 f 0.3

0.5

0.1

1.1 2 0.2

0.8 2 0.1

0

0

Trace

0



*



49.7 t 10.1

0

~~



4.3



* 0.1'



0

2.9 t 0.Zp

0

0.5 ? 0.1

3.4 f 0.5

0.3 f 0.1

0.3 2 O . l e

1.8 t 0.2'

0

0

Trace

0



24.3 t 9.4

0



Control



Defoliated



4.3 2 1.2

Trace

7.9 f 0.9



6.1 t 0.9

Trace

6.0 ? 0.3



1.8 t 0.4



1.7 2 0.1

0

4.3 2 1 . 1

1 . 1 t 0.3

1.8 2 0.3



0

3.6 2 0.7

1.9 t 0.3

2.4 2 0.6

0

1.5 f 0.1

2.7 t 0.6



3.4 2 0.3

0.9 2 0.7

63.2 2 11.1

Trace

~~



0.5 2 0.2'

1.4 f 0.4



3.1 2 0.5

O P



1 . 1 k 0.2



58.1 2 13.3



0



~



OReprinted with permission for Smith (1971).

bData are micrograms x lo-' of each material released during 14 days per milligram oven-dry

root.

Mean and standard error of three replicate determinations using one Composite exudate sample

from 19 and 20 roots of control and defoliated tree, respectively.

Mean and standard error of three replicate determinations using one composite exudate sample

from 17 and 23 roots of control and defoliated tree, respectively.

eControl and defoliated figures significantly different at 95% level.



upon metabolically before it appears in exudates. Those factors that affect rates

of photosynthesis and translocation will have an indirect effect on exudation. For

a more thorough discussion of sources and mechanisms of exudation, see Hale

et al. (1978).

A few investigations have appeared since the previous review (Hale et al.

1971) relating to the environmental effects of temperature and light on shoots

with consequent changes in root exudates. For example, Shapovalov (1972)

explained the effect of temperature on exudation of scopoletin from soybean and

oat roots by setting apart three stages based on the Qlo of the exudation rate. In

the stage 20-24"C, the process appeared to be one of diffusion from free space;

from 23 to 30°C, he claimed the process activated diffusion, probably across the

plasmalemma; and in the range of 4O-6O0C, exudation probably increased

sharply as a result of denaturation of protein.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

CHAPTER 2. FACTORS AFFECTING ROOT EXUDATION II: 1970–1978

Tải bản đầy đủ ngay(0 tr)

×