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Advances in agronomy volume 35


ADVANCES IN

AGRONOMY
VOLUME 35


CONTRIBUTORS TO THIS VOLUME
C. R. ADAIR
PHILLIP
BARAK

T. T. CHANG
YONACHEN

S. K . DEDATTA

D. L. FR~ESNER
C. HAGEDORN
R. J . HANKS


G. HUCK
MORRIS

T. H . JOHNSTON
M. B. KIRKHAM
W. E. KNIGHT

F? MIEDEMA
V. P. RASMUSSEN
N . K. SAVANT

HOWARD
M. TAYLOR

V. H. WATSON


ADVANCES IN

AGRONOMY
Prepared in cooperation with the
AMERICAN
SOCIETY
OF AGRONOMY

VOLUME 35

Edited by N. C. BRADY
Science and Technology Bureau
Agency for International Development
Department of State
Washington, D . C .

ADVISORY BOARD
H. J. GORZ,CHAIRMAN

E. J. KAMPRATHT. M. STARLING

J. B. POWELL J. W.BIGGAR
M. A. TABATABAI


M. STELLY,
EX OFFICIO,
ASA Headquarters
1982

ACADEMIC PRESS
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ISBN 0-12-000735-5
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82 83 84 85

98 76 5 4 3 2 1


CONTENTS
CONTRIBUTORS
.....................................................
PREFACE
...........................................................

ix
xi

THE RHIZOTRON AS A TOOL FOR ROOT RESEARCH

Morns G . Huck and Howard M . Taylor
I . Introduction ................................................
I1. Physical Designs: General Types ..............................
111. Construction Details and Design Features ......................
IV. Some Techniques for Observing and Recording Root
System Parameters ..........................................
V. Experimental Design: Data Acquisition and Analysis ............
VI . Summary: Advantages and Disadvantages of Rhizotrons for Use
in Root Investigations ........................................
References .................................................

1
2
10
20
27

32
33

THE CONSERVATION AND USE OF RICE GENETIC RESOURCES

T. T. Chang. C . R . Adair. and T. H . Johnston
I.
I1 .
Ill .
IV.
V.
VI .
VII .

38
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
Diversity in Rice Genetic Resources ...........................
45
Recent Efforts in Genetic Conservation ........................
Dissemination and Evaluation of Germ Plasm . . . . . . . . . . . . . . . . . . 58
68
Preservation of Germ Plasm ..................................
71
Use of Germ Plasm ..........................................
80
Endeavors for the Future .....................................
85
References .................................................

THE EFFECTS OF LOW TEMPERATURE ON Zea mays

I? Miedema
1. Introduction ................................................
11. Freezing Injury .............................................
V

93
94


vi

CONTENTS

Ill .
IV.
V.
VI .

Damage by Low Nonfreezing Temperatures ....................
Growth and Development at Suboptimal Temperatures . . . . . . . . . .
Breeding for Low-Temperature Adaptation .....................
Summary ...................................................
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

95
103
119

124
124

AGRICULTURAL USE OF PHOSPHORUS IN SEWAGE SLUDGE

M . B . Kirkham
I.
I1 .
I11 .
IV.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Concentration of Phosphorus in Sludges .......................
Agricultural Use of Phosphorus in Sludges .....................
Summary and Conclusions ...................................
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

129
131
144
154
156

SUBTERRANEAN CLOVER IN THE UNITED STATES

W. E . Knight. C . Hagedorn. V. H . Watson. and D . L . Friesner
I . Introduction ................................................
11. Potential Use of Subclover

...................................
Ill . Breeding Subclover . . . . . . . . . . . . . .
1v. Seed Characteristics . . . . . . . . . . . . .
......................................
V. Nitrogen Fixation

166

167

171

v1. Fertilization and N

VII .
VIII .
IX .
X.

Climatic Variations
.......................
Establishment and Management . . .
Morphological Char
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References .......................
....

183

188
1x9

PREDICTING CROP PRODUCTION AS RELATED TO PLANT WATER STRESS

R . J . Hanks a n d V. I? Rasmussen
I.
I1 .
Ill .
IV.
V.
VI .

Introduction ................................................
Review of the Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Measuring ET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Estimating ET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Estimating Yield ............................................
Growth Stage Effects ........................................

193
194
199

201
203
205


CONTENTS

VII . Rasmussen and Hanks Spring Wheat Model . . . . . . . . . . . . . . . . . . . .
VIII . Rasmussen and Kanemasu Winter Wheat Model . . . . . . . . . . . . . . . .
IX . Hill. Johnson. and Ryan Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. Morgan. Biere. and Kanemasu Model for Corn . . . . . . . . . . . . . . . . .
XI . Other Models with Moisture Stress Included . . . . . . . . . . . . . . . . . . .
XI1 . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

x

vii
205
207
209
211
212
213
214

IRON NUTRITION OF PLANTS IN CALCAREOUS SOILS

Yona Chen a n d Phillip Barak
I . Introduction

................................................

217

XI . Soil Iron Compounds and Methods for Their Extraction . . . . . . . . . 218
111 . Iron Nutrition of Plants ......................................
Iv. Correction of Iron Deficiency .................................
References .................................................

222
230
238

NITROGEN TRANSFORMATIONS IN WETLAND RICE SOILS

N . K . Savant a n d S . K . De Datta
I . Introduction ................................................
I1 . Chemical Nature of Soil Nitrogen .............................
111. Physical and Physicochemical Processes Relevant to
Nitrogen Transformations ....................................
I v. Biochemical Nitrogen Transformations .........................
V. Fate of Fertilizer Nitrogen ...................................
VI . Regulating Nitrogen Transformation Processes . . . . . . . . . . . . . . . . . .
VII . Unresolved Challenges .......................................
References .................................................

INDEX .............................................................

241
244
249
261
286
291
293
294
303


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CONTRIBUTORS

Numbers

in

parentheses indicate the pages on which the authors’ contnbutions begin

C. R. ADAIR* (37), Agricultural Research Service, U . S . Department of
Agriculture, Beltsville, Maryland 20705
PHILLIP BARAK (217), The Seagram Centre f o r Soil and Water Sciences, The Hebrew University of Jerusalem, Rehovot, Israel
T. T. CHANG (37), Department of Plant Breeding, International Rice
Research Institute, Manila, Philippines
YONA CHEN (217), The Seagram Centre f o r Soil and Water Sciences,
The Hebrew University of Jerusalem, Rehovot, Israel
S. K. D E DATTA (241), Department of Agronomy, International Rice
Research Institute, Manila, Philippines
D. L. FRIESNER (165), Department of Agronomy, Mississippi State University, and Mississippi Agricultiml and Forest Experiment Station,
Mississippi State, Mississippi 39762
C. HAGEDORN (163, Department of Agronomy, Mississippi State University, and Mississippi Agricultural and Forest Experiment Station,
Mississippi State, MissisJippi 39762
R. J . HANKS (193), Department of Soil Science and Biometeorology.
Utah State University, Logan, Utah 84322
MORRIS G. HUCK (l), Agricultural Research Service, U . S . Department of Agriculture, Auburn University, Auburn, Alabama 36849
T. H . JOHNSTONt (37), Agricultural Research Service, U . S . Department of Agriculture, University of Arkansas Rice Research and Extension Center, Stuttgart, Arkansas 72160
M. B . KIRKHAM (1291, Evapotranspiration Laboratory, Kansas State
University Waters Annex, Manhattan, Kansas 66506
W. E. KNIGHT (163, Crop Science Research Laboratory, USDA-ARS,
MissiJAippi State, Mississippi 39762
I? MIEDEMA (93), Foundation f o r Agricultural Plant Breeding, 6700 A C
Wageningen, The Netherlands
V. I? RASMUSSEN (193), Department of Soil Science and Biometeorology, Utah State University, Logan, Utah 84322

*Present address: 3 Bedwell Lane, Concordia, Bella Vista, Arkansas 72712
?Present address: 13 C & H Circle, Stuttgart. Arkansas 72160.

ix


X

CONTRIBUTORS

N. K . SAVANT* (241), Department of Agronomy, International Rice Research Institute, Manila, Philippines
HOWARD M. TAYLOR (I), Department of Agronomy, Iowa State University, Arnes, Iowa 50011
V. H. WATSON (165), Department of Agronomy, Mississippi State University, and Mississippi Agricultural and Forest Experiment Station,
Mississippi State, Mississippi 39762

*Present address: International Fertilizer Development Center. PO. Box 2040. Muscle
Shoals, Alabama 35660.


PREFACE
During the past 25 years, the developing countries of the world have
doubled their food production. Increased use of food-producing inputs
such as irrigation, fertilizers, and monetary credit is responsible for much
of this remarkable achievement. But in most countries increased food
production has been based on the development of new and improved
technologies and on policies which encourage farmers to use these technologies. Agricultural scientists in both the developing and developed
countries have produced these new technologies, and soil and crop scientists certainly have done their share.
This volume provides evidence of agronomists’ contributions to the
world’s ability to produce food. Rice, the food crop for most of the
world’s poor, is the subject of two articles. One deals with the genetic
resources of this crop, and the other with transformations of nitrogen in
paddy soils.
The effects of stress on crop production are addressed in three articles:
one concerned with low temperatures, one with plant water stress, and
one with calcareous soils. They illustrate continuing attempts to address
the problems of large and important food-producing areas.
The remaining three articles likewise focus on practical problems facing
food producers. One concerns an important forage crop, subterranean clover, and the role it plays in modern agriculture. A second reviews the use
of rhizotrons in root research and reminds us of the significance of roots,
especially in relation to water utilization. The third summarizes work on
sewage sludge as a source of phosphorus for agriculture.
We express appreciation to the scientists from different countries who
have made these important contributions.

N. C . BRADY

xi


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ADVANCES IN AGRONOMY, VOL 35

THE RHIZOTRON AS A TOOL FOR
ROOT RESEARCH
Morris G. Huck* and Howard M. Taylor?
*Agricultural Research Service, U S . Department of Agriculture, Auburn University,
Auburn, Alabama and ?Department of Agronomy, Iowa State University, Ames, Iowa

I. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.............
11. Physical Designs: General Types. . . . . . . . . . . . .
A. Simple Pits or Boxes with Glass Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B , Multicompartment BelobGround Observation Facilities
C. Special Adaptations to Investigate Specific Problems .....................
111. Construction Details and Design Features. . . . . . . . . . .
............
A. Window Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Control of Soil Physical Properties in Reco
IV. Some Techniques for Observing and Recording
A. Measurement of Root System Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B. Population and Spatial Distribution over Time: The Sum of Growth and Death
.................
Rates in Each Localized Area . . . .
C. Validation of Root Density Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. Experimental Design: Data Acquisition and Analysis
.....
A. Controlled Environments versus Characterization of Natural Environments. . . .
B. Measurement of Root Functions: Water Removal, Mineral Uptake, and
Biological Oxygen Demand . . . . . . . . . . . . . . . . . . . . . . . . .
C. Selection of Data for Analysis and Storage. . . . . . . . . . . . .
D. Questions Which Can Be Addressed in Rhizotron Experim
VI. Summary: Advantages and Disadvantages of Rhizotrons for Use in Root

......................
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1
2
2
3
S
10
I1
1s
20
21
26
21
21
29
29
30
30
32
33

1. INTRODUCTION
Roots of higher plants perform several important functions: they provide anchorage, supply water and minerals, and have a regulatory role as well. Because
they grow underground and are not easily accessible, roots have been much less
completely studied than plant shoots.
Many techniques have been used to increase the accessibility of plant roots.
Bohm (1979) reviewed the methods for studying root growth and distribution in
1

ISBN 0-12-000735-5


2

MORRIS G . HUCK AND HOWARD M . TAYLOR

field soils. One method he reviewed was the observation of roots growing in soil
behind transparent walls.
Sachs (1873), one of the pioneers in the study of plant roots, used a simple
soil-filled box with one glass wall. The facilities for studying root growth gradually became more complex. Recently, various kinds of sophisticated underground chambers have been constructed which permit plant roots to be studied
under replicated conditions while shoots are exposed to field environments.
These larger root observation laboratories have been called “rhizotrons,” the
Greek “rhizos,” meaning root and “tron,” meaning device for studying. (The
word was coined to parallel the words “phytotron,” “edaphotron,” “cyclotron,’’ and other specialized facilities used in modern science.)
This article will confine its coverage to facilities where plant roots can be
visually observed growing in soil while their tops are growing either under
outdoor conditions or in transparent enclosures exposed to sunlight. Although we
shall mention other facilities as well, we shall concentrate more heavily on
design features and operating characteristics of the Auburn, Alabama, and
Ames, Iowa, rhizotrons where the authors have conducted root research.

I I . PHYSICAL DESIGNS: GENERAL TYPES

A. SIMPLE
PITSOR BOXESWITH GLASSWALLS

The simplest rhizotron design is a transparent panel covering a vertical face of
soil that contains growing roots. The roots are observed from an adjoining pit
that usually is covered to exclude light and rainfall and to partially control
temperature fluctuations. Kolesnikov (197 l ) , Schuurman and Goedewaagan
(197 I ) , and Bohm (1979) have reviewed the extensive observational literature
that appeared in the early part of this century dealing with root observations from
transparent wall pits. These pits are inexpensive to construct, easy to operate,
and yield data suitable for demonstrations or for obtaining qualitative ideas about
behavior of root systems. Pearson and Lund (1968), for example, used a soil pit
with a transparent face, of which the bottom was recessed more than top, to show
that root growth of cotton generally preceded extensive shoot development.
In another simple design, plants are grown in soil-filled boxes with a transparent side which is usually covered to exclude sunlight. During the experiment,
the box can be either outside under field environmental conditions or inside a
greenhouse or growth chamber under controlled conditions. Even with boxes
large enough to permit plants to grow to maturity, construction costs of these


THE RHIZOTRON AS A TOOL FOR ROOT RESEARCH

3

transparent-sided boxes are relatively low; a major disadvantage is that root
temperatures are much closer to above-ground air temperatures than to soil
temperatures encountered under field environments.
B . MULTICOMPARTMENT
BELOW-GROUND
OBSERVATION
FACILITIES

Because of the variability inherent in all biological research, controlled and
replicated experiments are required. Under different soil conditions, root growth
and function can vary widely. A logical approach to the study of interactions
between soil factors and root function was construction of larger, more elaborate
facilities where many replicated (or replicable) treatments could be installed in
the same location under identical climatic conditions. In the usual arrangement,
viewing surfaces of adjacent compartments line the sides of a long tunnel,
permitting more economical construction than if the same number of compartments were constructed individually and buried separately in the soil. The central
walkway is generally covered with some type of roof to exclude sunlight and to
moderate below-ground temperature fluctuations. Instrument shelters and soil
preparation areas are easily accessible from all soil compartments.

I . Observation Windows Covering Intuct, Native
Soil Profiles
The root observation laboratories built at East Malling, Kent, England, in
1961 and 1966 (Fig. 1 ) were prototypes for most of the larger root observation
facilities constructed since then. These facilities have been described by Rogers
(1969) and Rogers and Head (1963a,b, 1968), and were also mentioned in
reviews by Kolesnikov (1971) and Bohm (1979). The basic arrangement consists
of a long trench excavated with a mechanical scoop, leaving an undisturbed soil
profile on either side. A framework of interlocking precast concrete pillars and
lintels was erected in each trench, and a curved roof was cast in place, slightly
above ground level. Finally, observation windows were fitted between the concrete pillars, and soil, shaved from the trench wall at an appropriate depth, was
screened and replaced to ensure close contact between soil and the glass viewing
windows. An above-ground entrance hut and stairway were built at one end of
the tunnel.
It is also possible to construct a root observation facility containing a large
number of different, intact, native-soil profiles in which root growth could be
observed. Tackett et al. (1965) and Cannell et al. (1980) have published procedures for obtaining cylinders of relatively undisturbed soil 40-80 cm in diameter. These cylinders could be transported to a central installation for study (Cannell et al., 1980). A cross section of the cylinder plus soil would then be sliced


4

MORRIS G. HUCK AND HOWARD M. TAYLOR

FIG. 1. Observation tunnel in rhizotron at East Malling, Kent, England.

off the cylinder from top to bottom. The remainder of the cylinder would be
fitted with a viewing panel, which would be sealed to the remainder of the
cylinder wall. The cylinder bottom, with ceramic plates or small cylinders for
drainage, would permit maintenance of the desired water potential profile. Although this technique should be possible, we know of no instance where it has
been attempted on a large scale.
Facilities with a native-soil profile located behind the observation panels expose roots to the heterogeneous soil environment always found in the field. The
biological, chemical, and physical environments in this type facility are disturbed much less than in those facilities utilizing reconstructed profiles.
2. Observation Window Covering Artificially
Constructed Soils
Some experiments require precise information about the volume of soil occupied by root systems and the quantities of water, nutrients, or contaminants
located in that soil volume. Some of the newer rhizotrons have sacrificed the
ability to observe root growth in undisturbed soil to have better control over
experimental conditions. The rhizotron-lysimeter facilities at Guelph, Ontario


THE RHIZOTRON AS A TOOL FOR ROOT RESEARCH

5

FIG. 2. Entrance hut and stairway leading to observation tunnel in rhizotron at Guelph, Ontario,
Canada.

(Hilton et al., 1969, Fig. 2); Auburn, Alabama (Taylor, 1969, Fig. 3); Muscle
Shoals, Alabama (Soileau et al., 1974); Ames, Iowa (Taylor and Bohm, 1976);
and Columbus, Ohio (Karnok and Kucharski, 1979) use reconstructed soil profiles to attain the physical and chemical soil properties of interest for a particular
experiment.
C. SPECIAL
ADAITATIONSTO INVESTIGATE
SPECIFIC
PROBLEMS

1, Lysimeters with Root Observation Windows

Some rhizotrons can also be used as lysimeters. The Auburn rhizotron, for
example, was fitted with porous plates and suction tubes at the bottom of each
compartment to ensure adequate soil drainage. In many experiments conducted
there, 20-liter bottles have been installed in the drainage line to collect leachate
for analysis or water balance studies (Long and Huck, 1980b) (Fig. 4). Two
facilities have been constructed, however, to act specifically as combined


6

MORRIS G . HUCK AND HOWARD M. TAYLOR

FIG. 3. (a) Soybeans and corn growing in the rhizotron at Auburn, Alabama. Observation
compartments are surrounded by soil-filled borders at a slightly different height. The access stairway,
instrument trailer, and soil preparation building are visible in the background. Access tubes for soil
moisture measurement by neutron meter are visible in each root observation compartment. (b) Soil
has been removed from observation compartments in the foreground in preparation for another
experiment using a different soil profile. Sheet-metal dividers added during the filling process permit
separate observations to be made on each of four different species of turf grass in the next compartment, while remaining area is fallow in preparation for planting soybeans. Neutron probe access
tubes are capped to prevent entry of rainwater; vertical rods will support apparatus to measure stem
diameter (Huck and Klepper, 1977).

lysimeter-rhizotrons: Muscle Shoals, Alabama (Soileau et al., 1974), and Temple, Texas (Arkin et al., 1978).
The Muscle Shoals facility consists of 18 compartments, each 1 .0 m side-toside, I .2 m front-to-rear, and I .9 m high. Electrically controlled drainage and
water sampling equipment are located in a 0.7-m-high space below each compartment. Suction can be applied to a 30 X 30 m, porous ceramic plate located in
the bottom of each compartment. This facility allows researchers to (a) evaluate
fertilizer use-root growth-nutrient leaching, and nutrient balance relationships,
(b) measure mobility in soils and percolation losses of potentially toxic ions from
organic wastes, and (c) study effects of soil chemical and physical properties on
root development and crop yields.


THE RHIZOTRON AS A TOOL FOR ROOT RESEARCH

7

The Temple facility consists of 24 weighable steel chambers, each with one
vertical side of glass. These root observation chambers are inserted vertically
into concrete retaining liners buried in the soil. An A-frame and load-cell arrangement is used to weigh the chamber periodically and to lift the chamber from
the concrete liner for root observations. When compared to the Auburn or Ames


8

MORRIS G. HUCK AND HOWARD M. TAYLOR

FIG.4. (a) Corn (Zea mays L.) in a rhizotron experiment described by Long and Huck (1980b)
measuring vertical ion migration. Tensiometers and soil solution sampling tubes (Long, 1978) permit
detailed measurements as a function of depth and time. Vertical rods provide mechanical support to
help avoid injury to plants during sampling and measurement operations. (b) Observation tunnel in
the rhizotron at Auburn, Alabama. Drainage water from each compartment is collected in bottles
along the walls (see text).


THE RHIZOTRON AS A TOOL FOR ROOT RESEARCH

9

rhizotrons, where evapotranspiration rates must be determined from differences
in soil water content with time, the Temple facility provides opportunity for more
frequent, and relatively more precise, short-term measurements of water loss
from the soil profile.

2. Above-Ground Enclosures to Sample or Modify the
Aerial Environment
In yet another level of measurement sophistication, it is possible to collect
water vapor transpired from plants growing in a rhizotron compartment and
compare it with water loss as calculated from differences in water content profiles. This procedure requires that a transparent cover such as those described by
Leafe (1972), Lange et al. (1975), or Alberda (1977) be placed over the rhizotron compartment. Air inside these chambers is circulated through a nearby
air-conditioning unit to maintain temperature and humidity control. Water vapor
that condenses on the air-conditioning coils is trapped and measured (King,
1980; Wheeler, 1980) in the unit operating at Auburn. The Auburn unit follows a
design prototype described by Musgrave and Moss (1961) and modified by


10

MORRIS G . HUCK A N D HOWARD M. TAYLOR

Phene et al. ( 1 978). In the SPAR (Soil-Plant-Atmosphere-Research) chambers
of Phene et al. (1978) both above- and below-ground plant organs are monitored
in individually controlled environmental chambers.
Similar facilities are located in the Crop Root Research Laboratory, Department of Greenhouse Cultivation, Vegetable and Ornamental Crops Research
Station, Taketoyo, Japan. In these units, however, a mist or liquid nutrient
solution is used instead of soil in the root portion of the controlled-environment
chambers.
When the above-ground microenvironment is controlled and/or measured, as
in the enclosed-canopy facilities described above, it is possible to measure not
only transpiration but also net photosynthetic rates of plants growing in the
rhizotron bins. By monitoring the amount of CO, which must be added to
maintain a constant level in the circulating gas inside the above-ground chamber,
it is possible to estimate instantaneous net photosynthetic rates with much more
accuracy than in open systems (Samish and Pallas, 1973) or by inferences drawn
from changes in plant size.

Ill. CONSTRUCTION DETAILS AND
DESIGN FEATURES
Most of the research groups constructing rhizotrons have modified the original
design of the East Malling root observation laboratory to fit specific research
requirements or to take advantage of more readily available construction materials. Examples are the concrete block retainer walls of the Mlanje, Malawi
(Fordham, 1972), Ames, Iowa (Taylor and Bohm, 1976), and Columbus, Ohio
(Karnok and Kucharski, 1979), rhizotrons or the brick and waterproof plaster
walls in Griffith, Australia (Freeman and Smart, 1976).
Some rhizotrons, such as those at Temple, Texas, Ames, Iowa, Columbus,
Ohio, and Griffith, New South Wales, are built flush with the soil surface to
minimize disturbance of airflow patterns and reduce reflectance of bare concrete
and metal surfaces. The rhizotrons at Woodward, Oklahoma (Shoop, 1978), and
at Mlanje, Malawi, have their passageways between compartments covered with
grass mulches to reduce the extra thermal load which would be created by a
concrete roof. The Ames, Columbus, and Auburn rhizotrons have soil-covered
roofs which allow plants growing in the experimental compartments to be surrounded by similar plants in guard rows. Figure 5 illustrates the manner in which
guard rows are planted on all sides of the test plants in the instrumented observation compartments at Auburn.


THE RHIZOTRON AS A TOOL FOR ROOT RESEARCH

11

A. WINDOWDETAILS

1. Orientation (Azimuthal, Vertical Angle, Height, and
Unsupported Width}
In rhizotrons designed to study the roots of row crops, the rows are customarily planted perpendicular to the glass windows so that the glass may be considered to represent a vertical slice across a typical row. Both depth and lateral
spread of the root system can be measured. Root density (as defined later) is
averaged in two spatial directions, and calculations of root growth, water movement, and other root activity parameters are expressed on a “per unit root
length” or “per unit soil volume” basis.
On the other hand, if the experiment is one in which rates of extension of a
particular root tip are desired (as opposed to mean population density of roots in
an average soil volume), then windows with a negative 10” slope (or more) can
be used (Pearson and Lund, 1968). In this orientation, geotropism will constrain
any root tips intercepting the window to grow along the glass and not turn back
into the soil. This configuration is useful for studying root elongation rates or
branching and branch initiation in anatomical studies of contiguous flow paths
from specific root tips to the base of a given plant’s stem. Autoradiographs are
readily prepared from the “bisected” root system resulting from this planting
and window configuration (see Fig. 9).
When all experimental plants grow adjacent to the window, and sloping glass
confines their root system, nearly half the entire root system can be seen at the
window. If the plants are grown in a row perpendicular to the window, on the
other hand, roots measured at vertical windows represent those intersecting a
typical plane through bulk soil.
Many root observation laboratories have reflective roofs made from such
materials as concrete or gravel in asphalt, which increase transpiration from
plants growing in adjacent rhizotron bins to levels far above that of plants
growing in comparable field plots. Transpiration rates of cotton plants grown in
the Auburn rhizotron before that facility was modified to eliminate a bare concrete roof above the walkway were about twice those of cotton plants growing in
an adjacent field soil (Taylor and Klepper, 1975).
One solution to this advective energy problem is to lower the passageway or
tunnel roof line so that its upper surface is flush with the surrounding soil surface
level and then to cover the roof with soil; however, this structural configuration
creates a root-viewing problem. If the roof line is dropped so that soil above the
roof is flush with the surrounding soil surface, those roots growing in the surface
soil (to a depth equal to the roof thickness) cannot be seen from the tunnel
windows (see side view, Fig. 6 ) .


12

MORRIS G . HUCK AND HOWARD M . TAYLOR

FIG.5. (a) Plastic tubing supplies irrigation water to guard rows of soybeans and maize which
have been planted on either side of observation compartments in the Auburn rhizotron. The central
observation tunnel is covered by only 15 cm of soil, so it must be resupplied with water at I - to 2-day
intervals to ensure normal growth of plants in the guard rows. (b) By midsummer, maize and
soybeans shown in Fig. 5a have grown into a closed canopy, shading exposed concrete boundaries.
Only the top of the constant environment chamber is visible above surrounding maize plants; airsupply ducts are completely shaded at this time.


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