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Growth, carbon partitioning, water and ion uptake by the

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presented by '
Bihu Huang

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GROWTH, CARBON PARTITIONING, WATER AND ION
UPTAKE BY THE ROOT SYSTEMS OF PHASEOLUS
SEEDLINGS SUBJECTED TO SOIL
MECHANICAL IMPEDANCE AND

WATER DEFICIT STRESSES

By
BIHU HUANG

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

DEPARTMENT OF CROP AND SOIL SCIENCES
MICHIGAN STATE UNIVERSITY
1990

Q4222 -— 9‘4 7%:

ABSTRACT

GROWTH, CARBON PARTITIONING, WATER AND ION
UPTAKE BY THE ROOT SYSTEMS OF PHASEOLUS
SEEDLINGS SUBJECTED TO SOIL
MECHANICAL IMPEDANCE AND
WATER DEFICIT STRESSES

By

Bihu Huang

Soil mechanical impedance and water deficit are known to affect root
growth and decrease plant yield. Little is known about the interactive effects
of soil compaction and water deficit on the growth of roots and numerous
root activities associated with growth, such as the carbon requirements and
uptake of water and ions by roots subjected to mechanical impedance and
water deficit stresses. This study, using the soil core seedling test method
described by Asady et al. (1985), was designed to determine the separate
and combined effects of compaction and water deficit on the growth of dry
edible bean ( Phaseolus Elga_ris , L. variety Seafarer) roots and their associ-
ated uptake of H20, N, P and K, as well as the translocation of the photoas-
similates. Three bulk densities (1.1, 1.4 and 1.7 Mg m‘3) and two water
treatments (control and water deficit) were established in two greenhouse
experiments. Data collected 14 days after planting indicated that soil com-
paction and water deficit decreased root penetration ratios (RPR), root

Bihu Huang

lengths, and leaf areas. The efficiency of water absorption increased with
greater bulk density and was decreased by water deficits. Consequently, less
carbon was fixed and a greater proportion of transported carbon was lost by
stressed roots. The absorption of N, P and K decreased with the thickening
of roots in soils having greater bulk densities. As a result of these stresses,
total biomass accumulation decreased by almost 50% below the nonstressed
plants.

These data suggest that soil compaction and water stresses, which are

applied at the seedling stage, greatly reduce the competition of plants and
their growth may be retarded throughout the life of the plant.

ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to my major professor,
Dr. A.J.M. Smucker for his guidance, help and constructive criticisms. I am
special appreciate for his encouragement during all phases of this thesis.

Special appreciation is extended to my other committee members,
Dr D. Christensen from Crop and Soil Science, and Dr. R. Perry from Horti-
culture. Their contributions to the successful completion of this study is ack-

nowledged.

I would like to thank Dr. B. Ellis for providing me his lab for nutrient
analysis and for his guidance in analysis.

Acknowledgement is given to W. Berti for the assistance in the P and K
analysis. And soil test lab’s assistance in N analysis is appreciated.

ii

TABLE OF CONTENTS

P_ag_e

LIST OF TABLES ...................................................................................... v
_ LIST OF FIGURES ..................................................................................... vii
INTRODUCTION ....................................................................................... 1
CHAPTER 1 : LITERATURE REVIEW .................................................... 4
Soil Compaction ................................................................................ 4

Roots and Soil Mechanical Resistance ................................... 4

Roots and Soil Aeration .......................................................... 6

Roots and Temperature and Diseases ..................................... 8

Roots and Nutrients ................................................................ 9

Soil Compaction and Photosynthesis ...................................... 10

Water Stress ...................................................................................... 10

Root Growth and Water Availability ...................................... 10

Water Stress and Diseases ...................................................... 11

Plant Growth and Photosynthesis ........................................... 12

Literature cited .................................................................................. 14

CHAPTER 2 : CARBON PARTITIONING IN THE SHOOTS AND
ROOTS OF PHASEOLUS SEEDLINGS SUBJECTED
TO SOIL MECHANICAL AND WATER DEFICIT

STRESSES .......................................................................... 19
Abstract ............................................................................................. 19
Introduction ....................................................................................... 20
Materials and Methods ...................................................................... 22

iii

Results and Discussion ..................................................................... 26
Summary and Conclusions ................................................................ 34

Literature Cited ................................................................................. 35

CHAPTER 3 : WATER AND ION ABSORPTION EFFICIENCIES
BY THE ROOTS OF DRY BEAN SEEDLINGS
SUBJECTED TO SOIL MECHANICAL IMPEDANCE

AND WATER DEFICIT STRESSES ................................. 37

Abstract ............................................................................................. 37
Introduction ....................................................................................... 38
Materials and Methods ...................................................................... 39
Results and Discussion ..................................................................... 41
Summary and Conclusions ................................................................ 52
Literature Cited ................................................................................. 53
CHAPTER 4 : SUMMARY AND CONCLUSIONS ................................. 55

iv

LIST OF TABLES

CHAPTER 2
1. Dry bean root length responses to soil mechanical

impedance and water deficit stresses ........................

2. Dry bean shoot and root dry weight responses to
soil mechanical impedance and water deficit

stresses ............................................................................

3. Relative distribution in plant and soil of

1‘iC-label recovery in each experimental unit
3 hours after pulse labeling Phaseolus dry

beans 14 days after transplanting ....................................

CHAPTER 3

1. Plant tissue dry weight responses to soil
mechanical impedance and water deficit at

14 days after transplanting ..............................................

2. Leaf area and specific transpiration
responses to soil mechanical impedance
and water deficit at 14 days after

transplanting ....................................................................

3. Leaf density of 14 days-old dry bean seedlings
subjected to soil mechanical impedance and

water deficit .....................................................................

4. Root density of 14 days-old dry bean seedlings
subjected to soil mechanical impedance and

water deficit .....................................................................

5. Distribution of root length within three diameter
classes for dry edible beans subjected to soil

F3
m
(D

........... 27

........... 27

........... 31

........... 42

........... 42

........... 44

........... 44

mechanical impedance and water deficit for 14 days ................ 46

. Root length and shoot to root ratios of 14 days-old
dry bean seedlings subjected to soil mechanical
impedance and water deficit ....................................................... 46

. Root surface area sizes of 14 days-old dry bean
seedlings subjected to soil mechanical impedance
and water deficit ......................................................................... 48

. Total water, N, P and K in plant shoots and roots
after 14 days of exposure to soil mechanical
impedance and water deficit ....................................................... 48

. N, P and K ion absorption efficiencies by 14 days-old
dry bean seedling subjected to soil mechanical
impedance and water deficits ..................................................... 50

. Water absorption efficiency (W AE) for the 14th
day of dry bean seedlings subjected to soil mechanical
impedance and water deficit ....................................................... 50

. Water use efficiency (WUE) of dry bean plant
responses to soil mechanical impedance and
water deficit ................................................................................ 51

vi

LIST OF FIGURES

Page
CHAPTER 2
1. Three layer soil core assembly ................................................... 23

2. Whole plant biomass of 14 days-old dry bean
seedlings subjected to soil mechanical
impedance and water deficit ....................................................... 29

3. 14C-label fixation of 14 days-old dry bean
seedlings subjected to soil mechanical
impedance and water deficit ....................................................... 3O

vii

INTRODUCTION

Soil is the natural matrix for plant root growth and is the primary
natural resource of agriculture. The basic functions of the soil are designed
to produce green plants upon which human beings depend. Water and
nutrients which are absorbed from soil by the plant roots are among several
basic compounds for plant growth. Plant roots also provide mechanical sup-
port to the plant. Therefore, a good soil condition for plant growth should
include adequate aeration, optimum temperature, mechanical support and

enough water and nutrients.

Unfavorable soil physical conditions are known to be one of the major
limiting factors in agricultural production systems. Mechanization of agri-
culture, including utilization of heavy machinery and associated equipment,
coupled with poor management practices have resulted in compacted soils.
The compaction process causes soil particles to be in closer contact with
each other, decreasing pore diameters and reducing total soil porosity. These
adverse physical conditions reduce soil water permeability and aeration, and

increase soil evaporation.

The reduction of total porosity increases soil mechanical resistance
which affects seed gemination, seedling growth and emergence, and root
development. A compacted soil pan within the top 20 cm limits root
development and reduces the nutrient absorption areas which reduces shoot
growth and yields.

The absorption and utilization of water directly or indirectly affects
every aspect of plant growth. Since water used by crop plants is absorbed

through roots, the establishment and continued growth of an extensive, func-
tioning root system is of primary importance to plant establishment, sur-
vival, and growth (Kramer et a1. 1990). The movement of water into plants
largely depends on root distribution in the soil, both radially and longitudi-
nally. Hurd (1976) emphasized the importance of root depth, increased
branching and root diameter as beneficial factors for improving plant toler-
ance to soil water stresses Moreover, water uptake by the root system is the
primary factor for determining the water status of the shoot. Water deficits
in leaves will delay the transfer of assimilates from the photosynthetic tissue
into the conducting tissue. Along with cell elongation, photosynthesis,
translocation of sugars, and growth have been implicated to be among the
first processes to be affected by water stress (W ardlaw 1967).

Both soil compaction and soil water deficits are barriers to root growth.
Plants with deeper and diffuse root systems appear to have greater potentials
for absorbing water and nutrients because they increase the contact volume
with soil thereby increasing their absorption areas for water and nutrients.
In contrast, a small root system reduces the capacity for roots to achieve
their maximum function. As the consequence of this, plant growth is
reduced and yields will be lowered.

Many reports have summarized the effects of compaction and water
deficit on plant growth. Little is known about the interactions between soil

water and soil compaction. The main objectives of this study were:

1. To determine the effects of mechanical impedance and water stress on
root morphology and growth.

2. To determine carbon fixation and distribution responses to mechanical
and water deficit stresses.

3. To correlate the absorption efficiencies of water and ions to soil

mechanical and water deficit stresses.

CHAPTER 1

Literature Review

Of the two environments for the plant growth, the soil is more compli-
cated than the atmosphere (Russell 1977). Between the two parts of the
plant, the root and shoot, the root, which grows invisibly in the soil, seems
more complex than the shoot. Any of the chemical, physical and biological
stresses in the environment of the soil will inhibit root growth and plant pro-
ductivity. Soil compaction and water deficit are two major adverse environ-

mental conditions which reduce the growth and dynamics of root systems.

Soil Compaction

Soil compaction causes changes in the chemical, physical and biological
conditions of the soil. Combinations of changes in those properties cause
reductions in the yields of many crops. These reductions in yields were due
to losses of root function (Cannell and Jackson, 1981), increased root
disease (Burke et a1, 1980; Allmaras et a1, 1988) and excessive allocations of
photoassimilated plant carbon to the root system of stressed plants
(Smucker, 1984) as well as their interactive effects.

Roots and soil mechanical resistance

Soil mechanical impedance inhibits root elongation and penetration
altering root morphology and causing the roots to become thicker and more
branched. The processes involved in root elongation reflect cell division and

elongation and the response of cells to their environments. Cells of the root

4

are in mechanical equilibrium such that their internal and external mechani-
cal stresses balance. Dexter (1987) fit root growth data into a model and
found that root elongation rates decreased linearly with increasing mechani-
cal stresses which were applied externally to the root. He also indicated that
external mechanical stress results from the resistance to root elongation by
the surrounding soil. Camp and Land (1964) reported the effect of soil com-
paction on cotton root anatomy. They found that cells of roots grown in
loose soil elongated longitudinally, whereas cells of roots grown in com-
pacted soil failed to elongate and expanded radially, causing the roots to
thicken. Prihar et a1 (1971) observed that, in the compacted soil, there is
greater production of sclerified cells in the cortical and vascular tissues.
They considered this to be a response to the extemal forces and this
response to the deformation of the interior cells. More regular layers of cort-
ical cells developed in the loose soil than in the uncultivated soil. This indi-
cates that compaction hinders the multiplication of cortical cells.

Baligar et al (1975) studied the soybean root anatomy as irrfiuenced by
soil bulk density. They increased bulk density from 1.65 Mg m'3 to 1.95 Mg
m‘3 and found that it altered root anatomy. In the high density soil layers a
larger percentage of root volume was Occupied by cell wall material, and the
transverse sections of the roots had a wavy outline with ruptured epidermal
cells. The vascular cylinder became more oval with a gradual increase in
both casparian thickening in the endoderrnis and wall thickening in the

xylem vessels.

Mechanical stress greatly limits root penetration. As soils are com-
pacted, the particles become closer and the average pore diameter is
reduced. When growing roots encounter soil pores which are smaller than

their diameters, continuing extension is possible only if the roots are able to

exert sufficient pressure to expand the soil pores or to reduce in diameter
thus, passing through existing pores ( Russell 1977). Wiersum (1957)
proved that roots were not only unable to decrease their diameter to enter
pores, but also usually increased their diameters when their elongation was
restricted by external pressure. Soil that had a resistance to a metal
penetrometer of 2-3 MPa would resist radial expansion of the root cavity
with a pressure of O.30-0.4 MPa. For bean roots, this condition was found to
cause a 75% reduction in the rate of root elongation (Taylor and Burnett
1964, Mazurak and Pohlman 1968, Fanell and Greacen 1966, and Greacen
et a1 1968). Tackett and Pearson (1964) studied the interaction between soil
compaction and soil oxygen content in cotton seedling root penetration into
a sandy clay loam subsoil. They found that a high soil bulk density restricted
root penetration regardless of the oxygen content. At low bulk densities root
penetration was hampered at oxygen concentrations of 5% or below, but
penetration was still considerably higher than in the highly compacted sam-
ples with high soil oxygen content. Root penetration apparently was more
limited by mechanical impedance than by poor aeration in the sand soil used
in that study. Hopkins and Patrick (1969) also showed the same result that
little or no penetration occurred at the highest compaction levels or at the

lowest oxygen content.

Roots and soil aeration

Poor productivity of the crop stand, which was accompanied by more or
less severely damaged root systems, in many cases, seems to be caused by
poor aeration. Gill and Miller (1957) reported that normal root growth was
adversely affected by reduction of the oxygen concentration of soil air to
10%. Where the roots were not subjected to a mechanical barrier, growth
continued at concentrations as low as 1% of oxygen. Wiersma and Mortland

(1963) showed that oxygen can be a limiting factor in the growth of sugar-
beets which may be corrected by the use of calcium peroxide. According to
Lemon and Erickson (1952) active root surfaces are covered with a water
film and since oxygen diffusion is only slightly soluble in water, the oxygen
diffusion rate in soil is the important factor and not the absolute percent of
oxygen. Stolzy et a1. (1961) reported that root growth was reduced at oxygen
diffusion rates below 23x10‘8 g cm'2 min‘l.

Soil compaction usually reduces the volume of air-filled pores and the
total porosity which reduces aeration and prohibits the diffusion cf oxygen.
Bertrand and Kohnke (1957) observed that oxygen diffusion was slower in
dense than in loose subsoils and a high moisture content intensified the res-
tricting effects. Restricted root growth by a dense subsoil was partly the
result of the subsoil acting as a mechanical barrier and partly due to the lack
of oxygen. Scott and Erickson (1964) conducted a greenhouse experiment
to determine the effects of soil oxygen on root development when plants
were grown with soil layers of bulk density 1.90 Mg m‘3 at 4 inches below
the surface. They found that fibrous roots branching from the tap root were
greater in amount in the dense layer where calcium peroxide was used as a
source of oxygen. Tomato and sugarbeet roots responded similarly to the
peroxide in the dense layer. Schumacher and Smucker (1979) showed that
oxygen uptake rate by dry bean roots were independent of solution flow
rates > 6.0 mL min"1 when inlet p02 of the nutrient solution was 0.21 aim.

They also suggested that a greater oxygen supply may be required at the root
surface to prevent anoxia when roots are mechanically impeded. Asady and
Smucker (1989) showed that oxygen diffusion rates of the soil were below
the critical level at a bulk density of 1.7 Mg if3 during the vegetative
growth stages of dry edible beans

Roots, temperature and disease

Soil temperature is one of the limiting factors for root growth. Stafford
and McMichael (1989) reported that primary root length, number of laterals
and lateral root length of guar were greatest at 30-35C. Branching intensity
of was greatest at 35C. Primary root length was significantly correlated with
number of laterals and lateral root length in all three seed sources which
were used in their experiment. Since soil compaction causes changes in soil
thermal conditions, the interaction effects of compaction and temperature is
an important study objective. Burke (1965) and Burke and Nelson (1965)
have shown that loose soil and temperatures favorable for rapid plant growth
tend to counteract root rot diseases. Soil temperature lower than the
optimum for plant growth aggravate root rot (Burke 1964, Burke 1965).
Burke et a1 (1980) showed that bean roots penetrated compact (1.5 Mg m'3)
Fusarium- fumigated soil more readily when soil temperatures during the
night and day were 21 and 27 C, respectively, than when they were 16 and
21 C. Root penetration of compacted soils was greater at a constant soil
temperature of 21 than at 16 C, with plant tops exposed to either 21 or 27 C.
Soil temperature affected root growth much less in loose (1.2 Mg m‘3) soil
than in compact (1.5 Mg m'3) soil.

Soil compaction has also been reported to reduce yield by increasing the
incidence of root diseases (Gray and Pope 1986). Field experiments were
conducted for two years on a Drummer silty clay loam soil to detemrine if
soybean ( film % (L.) Merr.) stand establishment and Phytophthora
root rot incidence were influenced by soil compaction. They found that the
number of plants killed by the fungus was significantly increased by soil
compaction for "Corsoy" (susceptible to Phytophthora megasperrna

 

Drechs.f.sp. glycinea, Kuan and Erwin) in both years. The yield of Corsoy
was lowered significantly in the compacted plots. The yields of Corsoy and
Corsoy 79 were also significantly different on the compacted plots. Fulton et
al. (1961) reported the incidence of Phytophthora root rot of soybean

 

increased in fine textured soils subjected to concentrated implement traffic.
They demonstrated that under greenhouse conditions a high incidence of
Phytophthora occurred when the infested soil was compacted. Burke, and
Nelson (1965) and Miller and Burke (1974) reported that soil compaction
increased the severity of Fusarium root rot of beans ( Phaseolus _vu_lgar;i§ L.),
caused by Fusarium m (Mart.) Appel and Wr. f. sp phaseoli (Burk)
Snyd. and Hans.

Roots and nutrients

Russell et a1. (1975) showed that when soil conditions were uniformly
favorable for plant growth, root growth was sufficient to supply the mineral
requirements of the plant, but when mechanical impedance interfered, root
growth and shoot yield were decreased. On soils of low fertility, even a
slight decrease in root growth adversely affected plant growth (Parish,
1971). Phillips and Kirkham (1962) found that increasing soil bulk density
resulted in coarser com (Zea mays, L.) roots and decreased P and K uptake.
If roots fail to supply adequate nutrients and moisture to the plants, growth
will be reduced, and eventually photosynthesis will decrease (Trouse, 1971).
Hallmark and Barber (1981) reported that increasing soil bulk density re-
duced root growth, shoot growth, and nutrient composition of shoots but in-
creased net nutrient influx. The detrimental effect of increasing soil bulk
density on shoot growth and nutrient status of soybean plants may have been
caused by decreased root growth, coarser roots , and lower root surface area

per unit of shoot. Wiersum (197 9) showed that shallow rooting can restrict

10

the uptake of a number of mineral nutrients. Barraclough and Weir (1987)
reported that early shoot growth and N content were substantially reduced
by subsoil pans because of the inaccessibility of mineral N in the subsoil to
the mechanically irnpedanced root system. Garcia et a1 (1988) reported that
nitrogen placement had a greater impact on N uptake when compaction

zones were present than when they were not present.

Soil compaction and photosynthesis

Compacted soil has been shown to reduce growth of many plant species
(Zimmerman and Kardos, 1961; Rosenberg, 1964; Taylor and Bruce 1968;
Trouse 1971). Akiyama and Kubo (1978) shoWed that photosynthesis of
first; nipponica and M l'amnica decreased when they were grown in

compacted soil and the decreases were greater in S. nipponica. Tu and Tan

 

(1987) investigated the effect of soil compaction on photosynthesis and
plant growth. They found that there was a highly significant reduction in
root biomass, shoot growth and total leaf area of plants grown in compacted
soil. Soil compaction also significantly reduced photosynthetic rate and in-

creased root rot severity resulting in reduced growth.

Water Stress

Root growth and water availability

A good root system is important for drought resistance. Elkins et a1.
(1977) reported that mating depth improved drought-tolerance for the sandy
soils of Alabama. They found that if the roots of crops, such as cotton, com
or soybeans, are confined to the top 15 cm of the soil'profile, then the crop
will become water-stressed after only 3 days without rain. However, if the

ll

roots can penetrate to 100 cm, then the crop will not become water stressed
until after 20 days without rain. Roots which grow down through a seed-bed
and encounter a strong, untilled subsoil beneath, may be unable to penetrate
the subsoil and may be deflected horizontally. They will continue to grow
horizontally along the top of the subsoil until either the seed-bed dries out
‘ and roots wilt and cease elongation, or until they find some path of low
resistance down through the subsoil via a fissure or biopore. Kramer and
Spomer (1990) reported that roots experiencing a single water deficit of
short duration (0-20 min.) exhibited a nearly linear proportional response of
growth rate (0 to 1.6 mm hr'l) to tissue water potentials over the range from
-1 to -10 MPa. Expansion ceased at -10MPa, defining this as the root’s wilt-

ing point.
Water stress and disease

Ayres (1982) grew barley in soil columns which were inoculated with
powdery mildew ( _E_rysiphe gm f.sp. hordei Marchal) when two leaves
had fully expanded. Half the plants were not watered after inoculation so
that they suffered water deficits at gradients of bulk soil water potential of
between 0.6 and 1.8 MPa developed between the uppermost (O to 12.5 cm
deep) and lowest (37.5 to 50.0 cm deep) regions of the profile. He found that
infection reduced the amount of 14C fixed in watered and drought stressed
plants, but only in drought stressed plants did infection reduce the percen-
tage of total assimilates that were translocated to roots. Water deficits
stimulated root growth, particularly in healthy plants, making the differences
in root growth greater between healthy and infected plants subjected to
drought. Reductions in root growth caused by infection were most marked in
the upper drier regions of the soil profile, while reductions in translocation
were most marked in the middle regions.

12

The reduction in shoot growth caused by infection was similar in
watered and drought stressed plants. The percentage of total assimilates that
were translocated out of healthy third leaves was not affected by infection of
the 2 lower leaves, but was reduced by water deficit. Infection increased
translocation into first and second leaves at the expense of the remainder of
the shoot, especially in droughted stressed plants.

Plant growth and photosynthesis

Low soil moisture increases intemal plant water deficits and detrimen-
tally influences growth and yield through a number of physiological
processes, including carbon exchange rate and translocation of assimilates.
(Mederski et al., 1973; Wardlaw, 1969). Silvius et a1. (1977) reported that
drought decreased carbon dioxide exchange rate and altered assimilate dis-
tribution, producing growth modifications that favor efficient use of the lim-
ited supply of fixed carbon. They found that at leaf water potentials of -l.5
to -2.0 MPa, soybean plants displayed alterations in 14C distribution among
plant parts corresponding to alterations in dry weight distribution. Compared
to well-watered plants, relatively more 14C was found in the root of water
stressed plants before the pod-filling stage. Finn and Brun (1979) reported
that 14C assimilation and specific nodule activity decreased and stomatal
resistance increased with increasing water deficit. Sung and Krieg (1979) re-
ported that photosynthetic rates were reduced as midday leaf water poten-
tials declined from -1.4 to -2.7 MPa in cotton and sorghum. They suggested
that photosynthetic rates were reduced with increasing water deficits prior to
any significant changes in translocation rates suggesting that photosynthesis
is the more sensitive of the two processes. Wardlaw (1967) indicated that
the effect of water deficit on the translocation process is related to the avai-
lability of photosynthate more so than by a direct effect on the mechanisms

13

of the translocation processes. In soybean, it has been found that net pho-
tosynthesis is decreased by about 60% when the water leaf water potential
falls between -300 to -2000 KPa and is reduced by 80% at -4000 KPa
(Boyer 1970).

In conclusion, a great amount of work has already been completed
which reveals that soil compaction and water deficits alter soil temperature,
water, nutrient availability and other soil processes resulting in reduced up-
take of water and ions by stressed roots. Reduced availability of photosyn-
thates and reduced biomass caused by the root disease responses to compac-
tion and water deficit add to these problems. This study was focused on the
combination effects of these two stresses on the root growth and the conse-

quents of those stresses.

Literature Cited

.Akiyama, T. and S. Kubo. 1978. Treading on the grassland. Photosyn-
thesis and dry matter production of native grasses. Bull. Nat. Grassland Res.
Inst. 12:23-31. Nat. Grassland Res. Inst., Nishinasumo, Tochigi, Japan.

Allmaras, R.R., Kraft, J.M., and Smucker, A.J.M. 1988. Soil compac-
tion and crop residue management effects on root diseases of annual food
legumes. pp. 627-647. In: RJ. Summerfield (ed.). World Crops: Cool Sea-
son Food Legumes. Kluwer Academic Pub., Dordrecht.

Asady, G.H., A.J.M. Smucker and M.W. Adams. 1985. Seedling test for
the quantitative measurement of root tolerances to compacted soil. Crop Sci-
ence 25 :802-806.

Asady, G.H. and A.J.M. Smucker. 1989. Compaction and root
modifications of soil aeration. Soil Sci. Soc. Am. J. 53:251-254.

Ayres, PG. 1982. Water stress modifies the influence of powdery mil-
dew on root growth and assimilate import in barley. Physiol. Plant Path.
21:238-293.

Baligar, V.C., V.E. Nash, M.L. Hare and J .A. PriceJr. 1975. Soybean
root anatomy as influenced by soil bulk density. Agron. J. 67:842-844.

Barraclough, PB, and AH. Weir. 1987. Effects of a compacted subsoil
layer on root and shoot growth, water use and nutrient uptake of winter
wheat. J. Agric. Sci., Camb. 110:207-216.

Bertrand, A.R., amd H. Kohnke. 1957. Subsoil conditions and their ef-
fects on oxygen supply and the growth of com roots. Soil Sci. Soc. Am.
Proc.21:135-140.

Boyer, J .S., 1970. Leaf enlargement and metabolic rates in corn, soybe-
an and sunflower at various leaf water potentials. Plant Physiol. 46:233-235.

Burke, D.W. 1964. Time of planting in relation to disease incidence and
yields of beans in central Washington. Plant Dis. Rep. 48:789-793.

Burke, D.W. 1965. Plant spacing and Fusarium root rot of beans. Phyto-
path. 55:757-759.

14

15

Burke, D.W. and CE. Nelson. 1965. Effects of row and plant spacings
on yields of dry beans in Fusarium-infested and noninfested fields. Wash.
Agric. Exp. Sta. Bull. 664:1-60.

Burke, D.W., D.E. Miller, and A.W. Barker, 1980 Effects of soil tem-
perature on growth of beans in relation to soil compaction and Fusarium root
rot. Phytopath. 70:1047-1049.

Camp, CR, and Z.F. Lund. 1964. Effect of soil compaction on cotton
roots. Crops Soils 17:13-14.

Cannell, R.Q. and Jackson, MB. 1981 Alleviating aeration stresses. pp.
141-192. In: G.F. Arkin and HM. Taylor (eds.). Modifying the Root En-
vironment to Reduce Crop Stress. ASAE Monograph No. 4. Amer. Soc. of
Agric. Eng, St. Joseph, MI.

Dexter, AR. 1987. Mechanics of root growth. Plant Soil 98:303-312.

Elkins, C.B., R.L. Haaland and CS. Hoveland. 1977. Grass roots as a
tool for penetrating soil hardpans and increasing crop yields. In Proc. 34th

Southern Pasture and Forage Crop Improvement Conf. Auburn, Alabama,
pp. 21 -26.

Farrell, D.A., and EL. Greacen. 1966. Resistance to penetration of fine
probes in compressible soil. Aust. J. Soil Res.4:1-17.

Finn, G.A., and WA. Brun. 1979. Water stress effects on C02 assimila-
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Fulton, J .M., C.G. Mortimore and A.A. Hilderbrand. 1961. Note on the
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Garcia, R, RM. Cruse and AM. Blackrner. 1988. Compaction and ni-
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take. Soil Sci. Soc.Am.J. 52:792-798.

 

Gill, W.R., and RE. Miller. 1957. A method for study of the influence
of mechanical impedance and aeration on the growth of seedling roots. Soil
Sci. Soc. Am. Proc. 20:154 -157.

Gray, LE. and R.A. Pope. 1986. Influence of soil compaction on soybe-

16

an stand, yield, and Phytophthora root rot incidence. Agron.J. 78: 189-191.

Greacen, E.L., D.A. Farrell, and B. Cockroft. 1968. Soil resistance to
metal probes and plant roots. Trans. 9th Int. Congr. of Soil Sci., Adelaide,
1:769-679.

 

Hallmark, W.B., and SA. Barber. 1981. Root growth and morphology,
nutrient uptake, and nutrient status of soybeans as affected by soil K and
bulk density. Agron. 1.73:779-782.

Hopkins, RM. and W.H. Patrick, Jr. 1969. Combined effect of oxygen
content and soil compaction on root penetration. Soil Sci. 108:408-413.

Hurd, EA. 1976. Plant breeding for drought resistance. In: Water
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Kramer, J .D. and L. A. Spomer. 1990. Water requirement for corn root
expansion growth. Soil Sci. Plant Anal. 21(1&2), 91-106.

Lemon, E.R., and AB. Erickson. 1952. The measurement of oxygen
diffusion in the soil with a Pt. electrode. Soil Sci. Soc. Am. Proc. 16:160-
163.

Mazurak, A.P., and K. Pohlman. 1968. Growth of com and soybean
seedlings as related to soil compaction and matric suction. Trans. 9th Int.
Congr. Soil Sci. 1:813-822.

Mederski, HJ., D.L. Jeffers, and DB. Peters. 1973. Water and water
relations. P. 239-266. In B.E. Caldwell (ed.) Soybeans: Improvement, Pro-
duction, and Uses. Agronomy 16. Am Soc. Agron., Madison, Wis.

Miller, DE. and D.W. Burke. 1974. Influence of soil bulk density and
water potential on Fusarium root rot of beans. Phytopath. 64:526-529.

Parish, D. H. 1971. Soil conditions as they affect plant establishment,
root development, and yield. p.277 -291. In K.K. Barnes, WM. Carleton,
H.M. Taylor, R.I. Throckmorton, and GE. Vanden Berg (ed.) Compaction
of Agricultural Soils. Am. Assoc. Agric. Eng, St. Joseph, Michigan.

Phillips, RF... and D. Kirkham. 1962. Soil compaction in the field and
corn growth. Agron. J. 54:29-34.

Prihar, S.S., M.R. Chowdhary, and T.M. Varghese. 1971. Effect of

l7

post-planting loosening of unstable soil on the anatomy of com root. Plant
Soil 35:57-63.

Rosenberg, NJ. 1964. Response of plants to the physical effects of soil
compaction. Adv. Agron. 16:181-196.

Russell, R.S., R.Q. Cannell, and MJ. Goss. 1975. Effect of direct dril-
ling on soil conditions and root development. Outlook Agric. 8:227 -232.

Russell, RS. 1977. Plant Root System: Their Function and Interaction
with the Soil. pp. 1 and 170-171.

Schumacher,T.E. and A.J.M. Smucker. 1979. Mechanical impedance
effects on oxygen uptake and porosity of drybean roots. Agron.J. 7 3:51-55 .

Scott T.W. and A.E. Erickson. 1964. Effect of aeration and mechanical
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Agron. J. 56:575-579.

Silvius, J .E., R.R. Johnson, and DB. Peters. 1977 . Effect of water stress
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of development. Crop Sci. 17:713-716.

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growth and diffusion rates as functions of oxygen concentration. Soil Sci.
Soc. Am. Proc. 25:463-467.

Sung, EL, and DR. Krieg. 1979. Relative sensitivity of photosynthetic
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64:852-856.

Tackett, J .L. and R.W. Pearson. 1964. Oxygen requirements for cotton
seedling root penetration of compacted soil cores. Soil Sci. Soc. Am. Proc.
28:600-604. -

18

Taylor, HM. and RR. Bruce. 1968. Effect of soil strength on root

growth and crop yield in southem United States. Int. Congr. Soil Sci. Trans.
9th 1:803-811.

Taylor, H.M., and E. Burnett. 1964. Influence of soil strength on the
root growth habit of plants. Soil Sci. 98:174-181.

Trouse, AC 1971. Soil conditions as they affect plant establishment,
root development, and yield. pp. 253-278. In: K.K. Barnes, W.M. Carleton,
H.M.Taylor, R.I. Throckmorton, and GE. Vanden Berg (eds.) Compaction
of Agricultural Soils. Am. Assoc. Agric. Eng., St. Joseph, Michigan.

Tu, J .C. and SC. Tan. 1988. Soil compaction effect on photosynthesis,
root rot severity, and growth of white beans. Can. J. Soil Sci. 68:455-459.

Wardlaw, LP. 1967. The effect of water stress in relation to photosyn-
thesis and growth. Aust. J. Biol. Sci. 20:25-39.

Wardlaw, LP. 1969. The effect of water stress on translocation in rela-
tion to photosynthesis and growth: II. Effect during leaf development in
Lolium temulentum L. Aust. J. Biol. Sci. 22:1-6.

Wiersma, D. and MM. Motland. 1963. Response of sugar beets to
peroxide fertilizations and its relationship to oxygen diffusion. Soil Sci.
75:355-360.

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of pores to their penetration by roots. Plant Soil 9:75-85.

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Zimmerman, RP. and LT. Kardos. 1961. Effect of bulk density on root
growth. Soil Sci. 91:280-288. .

CHAPTER 2

Carbon Partitioning in the Shoots and Roots of Phaseolus Seedlings Subject-
ed to Soil Mechanical and Water Deficit Stresses

ABSTRACT

The carbon requirements by roots subjected to soil compaction and wa-
ter deficit are essentially unknown. This study was conducted in the green-
house and consisted of experimentally compacted soils having bulk densi-
ties of 1.1 and 1.7 Mg m‘“3 for each water treatment (water control and water
deficit). Data collected 14 days after planting indicated that the dry biomass
of plants decreased with increased bulk density and greater water deficit.
The amount of 14C in plants decreased with increased bulk density and with
greater water deficit. The translocation of 14C was restricted by water deficit
but increased by compaction. Plants on the noncompacted and non-drought
stressed treatment, 97% of fixed 1“C was found in plants, and only 3%
leaked to soil. In contrast up to 9% of 1“C was lost to the soil by plants sub-
jected to compaction and/or drought stresses.

l9

20

INTRODUCTION

It is generally believed that soil environmental stresses influence the
loss of plant carbon resulting in reduced yields by stressed plants (McCree,
1982, Smucker, 1984). Studies of the physical factors which influence root
growth often include the characterization of those physical and biological
soil processes which are associated with bulk density. Soil compaction usu-
ally changes the existing balance between the liquid, gas, and solid phases
of the soil volume resulting in greater bulk densities and reductions in soil
porosity (Hillel 1982). Root growth is greatly reduced by these changes
(Russell, 1977). Soil drought is another stress affecting plant yield. Many
plant physiological processes would be delayed or stopped if plants were
restricted by water deficiency (Salisbury, et a1, 1978). Low soil moisture
content increases internal plant water deficits and detrimentally influences
growth and yield through a number of lowered physiological processes, in-
cluding carbon exchange rate and translocation of assimilates (Mederski et
al., 1973, Wardlaw, 1969). Sung and Krieg (1979) reported that photosyn-
thetic rates were reduced by greater water deficits prior to any significant
changes in the translocation rates, suggesting that photosynthesis is the more
sensitive of the two processes. Whereas the effect of water stress on the
translocation process was related to the availability of photosynthates, more
so than by a direct effect on the mechanisms of the translocation processes
(W ardlaw, 1967). Silvius et a1. (1977) reported that compared to well-
watered plants, relatively more 14C was found in the root of water stressed
plants before pod-filling stage.

Asady et al. (1985) reported that container experiments can be used to
measure the influence of soil bulk densities on the growth of roots. This

21

method has been used to demonstrate that root penetration ratios and root
lengths decline linearly with decreasing air filled porosity (Asady et al.,
1985). His study focused on the impedance effects of soil mechanical pro-
perties on root growth. Reduced root growth lowered the yields of plants,
primarily because more carbon appeared to be lost, thereby reducing the ac-
cumulation of photosynthates by plants subjected to soil compaction
stresses. The main objective of this study was to quantify the allocation of

photoassirnilates within plants and soils which were subjected to compaction
and water deficit.

22

Materials and Methods

This study was conducted in the greenhouse using the soil core see-
dling test described by Asady et. a1. (1985). Three polyvinyl chloride (PVC)
cores with a diameter of 7.6 cm and wall thickness of 0.64 cm were used for
the layered soil assembly (Fig. 1). Heights of the top, middle and bottom
layers were 2.5, 2.5 and 7.6 cm, respectively. Each core was filled with a
clay soil, (illitic, calcareous, mesic, Aeric Haplaquept) containing 53% clay
and having a consistency ranging from very plastic, when wet, to very hard
when dry. Soils were sieved to a range of 1.0 to 2.0 mm aggregates, and
compacted into a middle layer at a gravirnetric soil moisture content of
18%. Compression of soil to a desired bulk density of either 1.1 or 1.7 Mg
m"3 was achieved by pressing a specific quantity of soil into each layer by a
hydraulic press as described by Asady et a1. (1985). The bulk density of top
and bottom layers was 1.1 Mg m‘3. The three layers were wrapped with 5cm
wide plastic-impregnated duct tape, saturated for 24 hours, and drained. Dry
edible bean seeds (var. Seafarer) were surface sterilized with 0.1% hyper-
chlorite solution and germinated in the dark on wet paper towels at 25C for
three days. Two seedlings were transplanted to each core, and covered with
soil. The core assembly was placed on 48x48 cm tension tables (Hillel,
1980) which were established on the greenhouse bench. Each consisted of a
fine mesh screen and blotting paper saturated for 24 hours and deleted of all
the air bubbles. The tension tables were equipped with a Marionette burette
reservoir which supplied water to the soil from 40 cm below the tension
table by matric sorption. The tension tables were covered with a white plas-
tic fihn having water tight ports for the individual soil cores which prevented
water loss by surface evaporation. Plants were irrigated with tap water. Plant
nutrition was maintained by the natural soil fertility.

 

 

 

 

 

 

 

Fig. 1. Three layer soil core assembly where
chambers A and C contain control soils and
B contains the soils compacted to bulk
densities of 1.4 and 1.7 Mg rn"3 at a
gravimetric soil moisture content of 18%.

24

This study included two water treatments: 1) control, which was continuous-
ly irrigated by the tension table, and 2) short term water deficits, developed
by removing the cores from the tension table on the fourth and seventh days
after planting and re-establishing contact with the tension table after 60
hours. Cores of water deficit treatments were temporarily resaturated before
returning them to the tension tables and equilibrated to a matric potential 0f
40 cm. Each treatment was replicated four times.

Plants were pulse-labeled with 14C02 on the fourteenth day. The soil
surface was enclosed by a layer of parafilm sealed with transparent cellulose
tape around the plant stems. Each soil core was enclosed by a gas-tight po-
lyethylene plastic bag chamber. A small vial was attached to the middle of
the inside of the labeling chamber. The radioactive label was introduced by
injecting 0.5 ml HCl into the reaction vial followed by injecting 30111 of
NaH 14C03. The injection holes in the plastic bag were sealed with tran-
sparent tape immediately following injection. Ten minutes after injecting
the 14C-label, 0.2 ml 1N NaHC03 was injected into the reaction vial to re-
move any trapped 14C02. Concentrations of C02 surrounding the plant
during the three hours labeling period were monitored by injecting 0.5 ml
gas samples into an infra red gas spectrophotometer (Beckrnan model 865,
Beckman Industries, Fullerton, CA). Remaining l4C02 was removed from
the fixation tent by vacuum extraction and trapped by scrubbing through 8
ml of 1N KOH. Plants were harvested at the surface of the soil and leaf area
was measured by leaf area meter (Li-3000 Protable Area Meter, Licor, 4421
Superior Street P.O. Box 4425, Lincoln, Nebraska). The soil was cut into the
original three layers and the 14C02 contents of each layer was vacuumed to
remove respired 14C02 and trapped in 8 ml of 1N KOH. Root penetration

25

ratios (RPR) were determined by dividing the number of roots which exited
the middle core by the number which penetrated the top of the middle core
as outlined by Asady et a1 (1985). Subsamples of soil were frozen at -20 C
and stored for further evaluation. Roots were removed from the soil by
washing with the hydropneumatic elutriation system (Smucker, et a1, 1982 ).
Roots were frozen and stored at -20C. The plant shoots were also frozen at
-20C and held until further processing. All the frozen plant materials were
freeze-dried. Leaf, stem, root and moisture soil samples were combusted
using an oxygen saturated oxidizer (Biologocal Oxidizer-OX400, R.J. Har-
vey Instrument Corporation, Nillsdale.N.J.) at a temperature of 680 C (Ca-
talyst) and 900 C (Combustion zone) for 2 to 3 minutes. The l4C02 pro-
duced by this oxidation process was trapped by a 2:1 solution of Carb-Sorb
II (Packard Instrument Company, INC. 2200 Warrenville RD, Downers
Grove .Illinois 60515) and Safety-Solve (Research Products International
Crop. 410 N. Business Center Dr., Mount Prospect, IL 60056) and 14C-
isotope activities were determined by a scintillation counter ( TRI-CARB,
model 1500) for 5 minutes.

26

RESULTS AND DISCUSSION

The total root length was significantly decreased when bulk densities in-
creased from 1.1 to 1.7 Mg m'3. Water deficit further decreased the root
length. Therefore, the greatest total root lengths were found in nonstressed
plants whereas the smallest root length appeared in plants which were
stressed by both soil compaction and water deficit (Table 1). The data shows
that greater soil density decreased total root lengths by 56% while soil water
deficit decreased total root growth by 52%. Combinations of water deficit
and soil density decreased total root growth by 59%.

Root penetration ratios (RPR) were significantly reduced by compaction
and water deficit. The root penetration ratio in nonstressed control plants (of
the water control with a bulk density of 1.1 Mg m'3) was 7 to 12 times
greater than the stressed plants. More roots accumulated in the top and mid-
dle portions of the 3 layered containers when the middle layer was compact-
ed to 1.7 Mg m'3. Hence, the relative root lengths in the bottom layers of the
treatments having compacted layers were reduced because fewer roots were
able to penetrate through the more compacted middle layer (Table 1).

In addition to root length, root dry weight, leaf dry weight and stem dry
weight were also significantly decreased by compaction and water deficit
(Table 2). The greatest dry weights taken from each of these parameters
were always collected from nonstressed plants (Table 2). Both drought and
compaction reduced plant dry weights. Leaf area was reduced when bulk
density increased from 1.1 to 1.7 Mg m‘3 for both water treatments. The
greatest leaf area (220.2 cm2/plant) was obtained from nonstressed plants
(Table 2).

27

Table 1. Root length, root penetration ratio, and leaf area
responses to soil compaction and water deficit.

 

 

 

 

 

 

 

 

 

 

 

 

Water B,D, Total root Relative root length RPR
length Top Middle Bettom
Mg rn’3 cm %
Control 1.1 2793 14.56 22.50 62.93 64.75
1.7 1237 31.43 38.10 30.48 5.50
Stress 1.1 1345 20.53 32.68 46.78 8.75
1.7 1 140 34.75 36.35 28.80 6.00
13130.05 430 11.04 n.s. 20.14 13.12
Table 2. Dry bean shoot and root dry weight responses to
soil compaction and water deficit stresses.
Water B.D. Leaf area Leaf dry Stem dry Root dry
weight weight weight
Mg m’3 01112 3
Control 1,1 220.2 0.52 0.47 0.36
1.7 131.1 0.32 0.17 0.25
Stress 1.1 175.8 0.40 0.21 0.24
1.7 141.5 0.34 0.19 0.23
LSDMS 35.3 0.07 0.22 0.08

 

28

All the parameters discussed above could be used to indicate plant
growth responses to soil density and or drought stresses. The total dry matter
of the 14 days-old seedlings decreased with increased bulk density and wa-
ter deficit. There was no effect of compaction on the accumulation of dry
matter when plants were under water deficit. This indicated that water is a
more restricting stress for biomass accumulation than soil density. Even at
the low bulk density, the total dry matter remained very low when soil water
was deficient (Fig. 2).

The limitations of soil mechanical impedance and water deficits on
plant growth and reduced biomass were also demonstrated by the transport
of 14C-label through the plant and soil continuum. This approach, therefore,
appears to be another method for quantifying plant tolerance to drought and
mechanical impedance.

Labeled l4C02 was added to the dry beans to determine the distribution
of recently fixed plant carbon as well as its transport rates (Table 3). The
fixation of l4C-label decreased with water deficit. The greatest recovery rate
(92%) was found in the water control having low bulk density. Recovery of
l4COg-label was reduced to 40-45% by the compaction treatments during
the 3 hours labeling period (Fig. 3). The reduced recovery of 1“C02 by
compaction stressed plants resulted in lower fixation of the 1“C by these
plants.

This study demonstrated that the 14C distribution in the plants and soil
was influenced by both soil mechanical impedance and drought. Data in
Table 3 indicated that higher percentages of 14C remained in the leaves of
drought stressed plants than in the nonstressed plants, suggesting that water
deficits restricted carbon transport beyond the site of photosynthesis. Greater

g/core

Whole plant biomass

29

 

2.0
1.83
1.6;
1.4-3
1.2:
1.03
0.81
0.6:
0.4:

0.2-1

m Water control
CE Water deficit

 

 

 

 

 

0.0

 

Bulk density Mg m"3

Fig. 2. Whole plant biomass of 14 days—old

dry bean seedlings subjected to sail
mechanical impedance and water deficit.

30

 

n Water control
CZ: Water deficit

wow u M 666666» 6

COO)

 

     

  

V‘.‘.‘.‘.‘.‘.‘.‘.‘.‘.{.0A
heme» 66666666».

  

    
 

 

  

 

7

/
-:s

 
 

   

1
tyMgm

 

        

    

  

      

  

  

  

    

come».

P

/
.1

 

  

Bulk densi

 

 

100.0

80.0-

6666. 666666.
- 6.6?» 66666666666»!
q A ldr — d d
0. D O.
O 0 O
6 4 2

N x6609. 3302

 

0.0

Fig- 3- 14c-iabe1 fixation of 14 days old

dance and water defic

Impe

dry bean seedlings subjected to sail

mechanical

31

Table 3. Relative distribution in plant and soil af14C -label

in each experimental unit 3 hours after pulse labeling
Phaseolus dry beans 14 days after transplanting.

 

 

 

 

 

Water B.D. Leaf Stern Root Sail
Mg m % recovered

Control 11 47.29 32.37 17.76 2.59
1.7 29.14 34.95 26.63 9.28

Stress 1.1 77.82 11.07 7.65 3.47
1.7 46.17 25.21 21.02 7.61

LSDODS 38.21 as 13.15 6.92

 

32

bulk densities tended to reduced the accumulation of 14C - label in the
leaves for bath water treatments. Therefore, the highest percentage of 14C
in the leaf was found in the water deficit with low bulk density. While the
lowest percentage was found in the water control with high bulk density
(Table 3). No statistically significant deferences were observed for the per-
centages of 14C-label in the stems among the bulk densities and water treat-
ments, although there was a tendency for stems of drought stressed plants to
contain lower quantities of labeled carbon, relative to the quantities in the
leaves. It appeared that when the leaf contained a lower percentage of 14C,
then the stem had higher percentage of 14C. These data also show that
greater bulk densities enhanced the movement of photoassirnilates from the
leaves and throughout the plant and into the soil, as a lower percentage of
l4C-label remained in the leaves of plants with the greater bulk density.
Nevertheless, the 14C-labeled materials in the roots and soils were
significantly greater at the higher bulk density. The percentage of 14C lost
by roots appeared to be affected less by water deficit than by soil bulk densi-
ties. This may have occurred because roots grown in the compacted soils
needed more energy. which resulted from greater respiration rates to over-
come this resistance. The greater diameter and branching of roots subjected
to mechanically impeding soils may have contributed to the greater carbon
losses by bean plants (Schumacher and Smucker 1981). Consequently, a
greater percentage of 1“C accumulated in roots stressed by higher bulk den-
sities, and a greater percentage of 1“C leaked into the soil (Table 3). In this
study, relative percentages of 14C lost to the soil were observed to increase
nearly 3 times from B.D. 1.1 to 1.7 Mg m"3 under the water control.
Although the nearly 2 fold greater losses of the 14C-label by roots in greater
bulk density under draught stress were not statistically significant, 7.6%

33

losses would greatly modify both the plant and rhizosphere activities. In
summary, the reduced fixation of 14C02 and increased loss of 14C into the
soil were observed for the stressed plants. Therefore, less biomass accumu-
lated during the 14 days of growth.

SUMMARY AND CONCLUSIONS

Water deficit and soil compaction stresses reduced root growth. Conse-
quently, RPR, biomass, root length and leaf area decreased. These differ-
ences among the distribution patterns of labeled 14C within different plant
parts and the soil were the direct result of soil compaction and water deficit
environments. The reduced plant biomass probably resulted from the re-
duced C02 fixation and the increased loss of plant carbon compounds.
Further experiments are needed to correlate the losses of plant carbon cam-
paunds with the increases in the number of branches and greater diameters

of roots responding to the mechanical impedance and water deficit stresses

ACKNOWLEDGEMENTS

Financial assistance by the Institute of Intemational Agriculture, Michi-
gan Agricultural Experiment Station and CSRS 88-34114-3351 are grateful-
ly acknowledged.

34

LITERATURE CITED

Asady, G.H., and A.J.M. Smucker and M.W. Adams. 1985. Seedling
test for the quantitative measurement of root tolerances to compacted soil.
Crop Science 25:802-806.

Asady, G.H. and A.J.M. Smucker 1989. Compaction and modifications
of soil aeration. Soil Sci. Soc. Amer. J. 53:251-154.

Hillel, D 1982. Introduction to Soil Physics. Academic Press INC. pp.
176-181. New York.

Hillel, D. 1980. Fundamentals of Soil Physics. Academic Press Inc.
p122-165, 195-232. New York.

McCree, K. 1982. The role of respiration in crop production. Iowa State
J. Res. 56(3): 291-306.

Mederski, H.J., D.L. Jeffers, and DB. Peters. 1973. Water and water
relations. pp. 239-266. In: B.E. Caldwell (ed.) Soybeans: Improvement, Pro-
duction, and Uses. Agronomy 16. Am Soc. Agron., Madison, Wis.

Russell R. S. 1977. Plant Root Systems : Their Function and Interaction
with Soil. pp. 170-171. McGraw Hill, London.

Salisbury, F. and R. Clean. 1978. Plant Physiology. 2nd edition. Bel-
mont, Calif. : Wadsworth Pub. Co. PP. 363-365.

Schumacker, TE. and A.J.M. Smucker 1981. Mechanical impedance
effects on oxygen uptake and porosity of dry bean roots. Agron. J. 73:51-55.

Silvius, J.E., R.R. Johnson, and DB. Peters. 1977. Effect of water
stress on carbon assimilation and distribution in soybean plants at different
stages of development. Crop Sci. 17:713-716.

Smucker A. J. M., S.L. McBumcy and AK. Srivastava. 1982. Quanti-
tative separation of roots from compacted soil profile by the hydropneuma-
tric elutriation system. Agron. J. 74:500-503.

Smucker A. J. M. 1984. Carbon utilization and losses by plant root sys-
tems. In: S.A. Barber and DR. Bouldin (eds) Roots, Nutrient and Water
Influx, and Plant Growth Chap. 2. pp. 27-46.

35

36

Sung, F.J. and DR. Krieg. 1979. Relative sensitivity of photosynthetic
assimilation and translocation of carbon to water stress. Plant Physiol.
64:852-856.

Wardlaw, LR 1969. The effect of water stress on translocation in rela-
tion to photosynthesis and growth: II. Effect during leaf development in
Lolium temulentum L. Aust. J. Biol. Sci. 22:1-6.

 

Wardlaw, LP. 1967. The effect of water stress on translocation in rela-
tion to photosynthesis and growth: I Aust. J. Biol. Sci. 20:25-39.

CHAPTER 3

Water and Ian Absorption Efficiencies by the
Roots of Dry Bean Seedlings Subjected to Soil
Mechanical Impedance and Water Deficit Stresses.

ABSTRACT

Little is known about the interactive effects of soil compaction and wa-
ter deficit on the uptake of water and ions by plant root systems. This study
was designed to determine the separate and combined effects of compaction
and drought stress on the growth of dry bean roots and their associated up-
take of H20, N, P and K. Three bulk densities (1.1, 1.4 and 1.7 Mg m‘3). and
two water treatments (control and water deficit) were established during this
14 day experiment. Water deficit increased leaf thickness and sail compac-
tion increased root diameter. Both compaction and drought reduced plant
dry matter production. N, P and K absorption efficiencies decreased with the
thickening of roots in soils having greater bulk densities under water control
treatments. Only potassium absorption efficiency decreased by greater bulk
density under water deficit treatments. Water absorption efficiency increased
with greater bulk density and was decreased by water deficit. Lower tran-
spiration ratios were correlated to higher yields under water deficit condi-
tions, whereas lower transpiration occurred in the larger root systems which

developed in noncompacted or less compacted soils.

37

38

INTRODUCTION

A significant amount of work has been reported which reveals the ef-
fects of compaction on plant root anatomy and growth (Prihar, et al., 1971
and Baligar, et al., 1975). They reported that root elongation was inhibited
resulting in greater radial expansion and thicker roots when soils were in
high bulk density. Additional studies have shown that compaction reduces
the growth and yield of plants (Zimmerman and Kardos, 1961 , Trouse,
1971, Burke et al. 1972, Miller and Burke 1974). Wiersum (1979) reported
that shallow rooting restricts the uptake of mineral nutrients. Nutrient uptake
by roots has also been reported to be decreased when subjected to soil com-
paction (Russell et al. 1975, Trouse 1971, and Wiersum 1979). Increasing
soil bulk density resulted in decreased uptake of P and K by com ( Phillips
and Kirkham, 1962). The placement of nitrogen fertilizer had a greater im-
pact on N uptake when compaction zones were present than when they were
not present (Garcia et a1, 1988). This study was designed to evaluate the
growth of roots and their ion and water absorption efficiencies by stressed

roots subjected to soil compaction and water deficit.

39

MATERIALS AND METHODS

The soil core seedling test, described by Asady et al. (1985) was used
in these experiments which were conducted in the greenhouse. Charity clay
(illitic, calcareous, mesic, Aerie Haplaquept) soil was sieved to a range of
1.0-2.0 mm aggregates and was compacted into the middle layer (2.5 cm
high) of a three core assembly to bulk densities (B.D.) of 1.1, 1.4 and 1.7
Mg m"3 at a gravimetric soil moisture content of 18%. The top soil layer
(2.5 cm ) and bottom soil layer (7.5 cm) were filled with soil to a bulk densi-
ty of 1.1 Mg m‘3. The three layers were wrapped with 5 cm wide plastic im-
pregnated duct tape, saturated for 24 hours, and drained. Dry edible bean
seeds were surface sterilized with 0.1 % hyperchlorite solution and germinat-
ed on wet paper towels in the dark, at 25 C, for three days. Three seedlings
were transplanted to each core. The surface soil of the core assembly and
core container were covered with polyethylene film to prevent water loss by
evaporation. This experiment included two water treatments: 1) control,
which was maintained by adding water daily, according to the water lost
during the previous day, as determined by weighing, and 2) water deficits,
which were developed by not irrigating for 14 days. Plant nutrition was
maintained by the natural soil fertility. Plants were harvested 14 days after
transplanting by cuting them at the surface of the soil and leaf areas were
measured by a leaf area meter (Li-3000 Protable Area Meter, Licor, 4421
Superior Street PO. Box 4425 , Lincoln, Nebraska). Roots were removed
from the soil by washing manually to retain the intact root system for further
analyses. Root length and diameter were determined by computer image
processing (Smucker, et al. 1987). Plant tissue (leaves, stems and roots) was
dried, weighed and digested before determining the N, P and K contents by
the modified method of Parkinson (1975). Nitrogen, phosphorus and potassi-

40

um absorption efficiencies were calculated based on the total nutrients which
accumulated in the roots and shoots during the 14 day experiment and divid-
ed by the harvest root weight, length and surface area. Water absorption
efficiencies were calculated based on water lost during the last 24 hour
period before harvest. Analysis of variance with completely random block
design were performed to determine if the treatments were significantly dif-
ferent from each other. Four replications for each treatment are applied to
the analysis of N, P and K , and eight replications for each treatment are ap-
plied to the analysis of other parameters.

41

RESULTS AND DISCUSSION

Plant tissue dry weights decreased with increasing bulk density and
developing water deficits (Table 1). Compaction (1.7 Mg m'3) significantly
decreased the dry weight of leaves, stems and roots when water was not lim-
ited. Water deficit decreased dry weights of leaves, stems and roots at bulk
densities 1.1 and 1.4 Mg m'3. There were nansignificant decreases between
the water control and water deficit at a bulk density of 1.7 Mg m‘3. This im-
plies that soil compaction decreased dry weight only when soil water condi-
tions were favorable. The production of dry matter seemed more sensitive to
water deficit than to mechanical impedance. Once the plants were stressed
by a water deficit, the effect of compaction on the plant growth seemed to

have less influence on dry weight accumulations.

Leaf areas were significantly decreased by greater bulk density and wa-
ter deficits (Table 2). Soil compaction caused leaf areas to be significantly
smaller only at the greatest level (1.7 Mg m'3) of compaction, whereas
drought stress decreased leaf areas at all three of the compaction levels.
Therefore, it can be seen that the combined influences of compaction and
drought reduced leaf area by 49% as compared to the control plants (243.5

cm).

Specific transpiration rates were significantly greater for all the bulk
density treatments of the well-watered treatment. The highest bulk density
appeared to increase the specific transpiration by plants of drought stress
treatments (Table 2). This indicated that the compaction mediated increases
of specific transpiration rates were a function of the water deficit. In com-
parison to total plant dry weight, which decreased. with increasing bulk den-
sity (Table 1), specific transpiration rates increased with greater bulk

42

Table 1. Plant tissue dry weight responses to sail mechanical impedance

and water stresses at 14 days after transplanting.

 

 

 

 

 

 

 

 

 

Water B.D. Leaves Stems Roots
Mg m'3 g
Control 1.1 0.287 0.272 0.126
1.4 0.266 0.255 0.121
1.7 0.217 0.174 0.103
Stress 1.1 0.213 0.185 0.101
1.4 0.198 0.193 0.104
1.7 0.178 0.131 0.098
LSDoas 0.045 0.045 0.001
Table 2. Leaf area and specific transpiration responses to soil
mechanical impedance and water deficit at 14 days
after transplanting.

Water B.D. Leaf area Specific
transpiration
ratio

Mg m‘3 cm2 mg/cm2

Control 1.1 243.5 96.71

1-4 225.5 91.88

1.7 173.2 107.40

Stress 1.1 143.6 38.27
1-4 138.8 49.29

1-7 125.1 72.28

LSDo.os 30.9 27.87

 

43

densities (Table 2). This suggests that transpiration may be correlated with
yield, because lower transpiration is important for a higher yield when wa-
ter is limited (Heen, 1980). In this study, lower transpiration ratios correlat-
ed to higher total plant dry weight when soil water was deficient
(R2=0.921). Heen (1980) also reported that plants with lower transpiration
rates usually occurred in those with good root adaptation. In this study, the
plants with larger root systems also had a lower transpiration under water
deficit condition (Table 2 and Table 6, R2 =0.999).

There were no significant effects of soil bulk density on leaf density, but
leaf densities were significantly greater on plants subjected to a water deficit
suggesting that water deficits had a greater influence on leaf thickness than
soil compaction (Table 3). In contrast, root densities increased with greater
soil bulk density, but did not change significantly between water treatments
suggesting that soil compaction influenced the thickness of roots more than
soil water deficits (Table 4) .

Table 5 compares the percentage distribution of root lengths having dif-
ferent diameters. The data shows that the percentage of root length with di-
ameters ranging from 0.1 -0.4 mm decreased with greater bulk density when
water was not limiting. Water deficit significantly decreased the percentage
of roots at this diameter at bulk densities of 1.1 and 1.4 Mg m'3. The per-
centage of root lengths with diameters of 0.5-0.8 mm increased with greater
bulk density. Water deficits caused a significantly greater percentage of
roots, in the medium diameter class ranging from 0.5-0.8 mm for all three
bulk densities. Greater percentages of large roots, 0.9-1.0 mm, increased
with greater bulk density and were decreased by water deficit (Table 5). In
summary, a greater percentage of larger roots were recovered from soil

Table 3. Leaf density of 14 days-old dry bean seedlings subjected

to soil mechanical impedance and water deficit.

 

 

 

 

 

 

Water B.D. ‘ Leaf density
Fresh weight Dry weight
Mg in

Control 1.1 13.31 1.183
1.4 13.58 1.178
1.7 13.99 1.253
Stress 1.1 14.69 1.484
1.4 14.67 1.424
1.7 14.22 1.421
15130.05 0.84 0.111

 

Table 4. Root density of 14 days-old dry bean seedlings subjected
to soil mechanical impedance and water deficit.

 

 

 

 

Water B.D. Root density
Fresh weight Dry weight ié
Mg rn‘3 mg/cm root

Control 1.1 2.921 0.106
1.4 4.349 0.156
1.7 3.955 0.160
Stress 1.1 2.105 0.093
1.4 3.010 0.131
1.7 3.617 0.145
LSDom 0.951 0.032

 

j/C’c

45

treatments at the highest bulk density. This is additional evidence which
demonstrates that mechanical impedance causes roots to become thicker.
Water deficit increased the percentage of roots with larger diameters prob-
ably because the mechanical impedance increased during drying, reducing
root penetration in the drought stressed soil.

Total root growth significantly decreased in soils packed to a bulk den-
sity of 1.7 Mg m'3, and dramatically decreased with water deficits (Table 6).
The data showed a 30% reduction of root length when bulk density was in-
creased from 1.1 to 1.7 Mg m'3 under the water control. Water deficits de-
creased total root length 23% and 24% at bulk densities 1.1 and 1.7 Mg m‘3,
respectively, but water deficit did not further reduce root length at the high
bulk densities of 1.7 Mg m'3. This indicated that the effects of water deficit
on root elongation depended on the compaction levels. Once the mechanical
impedance became great enough to inhibit root elongation, the effect of wa-
ter deficit on root elongation became less important. Tables 1 and 6 suggest
that soil compacted to a bulk density of 1.7 Mg rn‘3 reduced plant growth
similar to the drought stresses regardless of the level of soil compaction.
Carbon allocation to the shoots decreased with greater bulk density and
drought stress, so water deficit and greater bulk density (1.7 Mg m'3)
significantly decreased shoot to roots ratio (Table 6 and Huang and Smuck-
er, 1990).

Root surface areas having diameters ranging from 0.1-1.0 mm de-
creased with increasing bulk density and water deficit (Table 7). The root
surface areas decreased 27% and 18% for water control and water deficit,
respectively, when bulk densities were increased from 1.1 to 1.7 Mg m'3.
Water deficit decreased the root surface area 18% and 20% at bulk density

46

Table 5. Distribution of root lengths within three diameter classes for
dry edible beans subjected to soil mechanical impedance and

 

 

 

 

 

 

water deficit.
Water B.D. Root diameters
0.1 - 0.4 mm 0.5 - 0.8 mm 0.9 - 1.0 mm
Mg m'3 %
Control 1.1 88.0 10.9 1.1
1.4 87.1 1 1.5 1.4
1.7 85.5 12.8 1,7
Stress 1.1 84.7 14.5 0.9
1.4 84.1 14.4 1.2
1,7 84.2 14.1 1.8
LSD0.05 ~ 2.1 1.5 0,4

 

Table 6. Root lengths and shoot to root ratios of 14 days old dry
bean seedlings subjected to soil mechanical impedance

 

 

and water deficit.

Water B.D. Total root length Shoot/root ratio
Mg m‘3 cm Dry weight

Control 1.1 902.5 4.44

1-4 887.8 4.31

1.7 627.0 3.92
Stress 1.1 690.4 3.96

1.4 671.9 3.72

1.7 567.9 3.14

 

13130.05 115.2 0.482

47

1.1 and 1.4 Mg m‘3, respectively. Water deficit decreased root surface areas
by 7% at the highest bulk density of 1.7 Mg m’3, which was not significant.
It appears that the augmented responses of the root surface areas were sensi-
tive to both mechanical impedance and water deficit. The decreased root sur-
face areas of the thicker roots probably resulted in their lower capacities to
absorb water and ions. In this study, the total water, and the total N, P and K
absorbed by plant roots during the 14 days decreased with lower root sur-
face areas, which occurred at the greatest soil compaction level and water
deficit (Table 8). The decreased total consumption of water and nutrients
resulted in a reduction in the production of dry matter (Table 1).

Although there was less root growth by Phaseolus plants subjected to
mechanical impedance and water deficits, the average ion absorption
efficiencies by stressed roots were calculated for the duration of the experi-
ment. N, P and K ions absorption efficiencies were calculated based on the
total nutrients which accumulated in the shoots and roots during the 14 day
experiment divided by total root surface areas which had accumulated by
harvest. Consequently this experiment was not designed to measure specific
ion absorption efficiency. N, P and K ion absorption efficiencies decreased
with greater bulk density when soil water was sufficient (Fable 9). There
was a tendency for greater absorption of N and P by roots when plants were
subjected to a water deficit. There was a tendency for P absorption
efficiency (PAE) to decrease as bulk densities increased for both water treat-
ments (Table 9). It seemed that there were no combined effects of both com-
paction and water stress on the PAR. The greatest compaction (1.7 Mg m‘3)
decreased K absorption efficiency (KAE) for both water treatments. How-
ever, KAE did not respond to water deficit (Table 9).

48

Table 7. Root surface area sizes of 14 days-old dry bean seedlings
subjected to soil mechanical impedance and water deficit.

 

 

 

 

 

Water B.D. Root surface area
0.1-0.4mm 0.5-0.8mm 0.9-1.0mm 0.1-1.0mm
Mg m“3 cm2

Control 1.1 133.8 40.12 5.75 179.7

1.4 130.0 41.67 7.34 179.1

1.7 90.9 32.79 6.67 130.3
Stress 1.1 102.4 40.84 3.38 146.6

1.4 98.2 40.58 4.52 143.3

1.7 81.9 32.69 5.80 120.5
LSDMS 16.8 7.99 1.83 f 24.2

 

Table 8. Total water, N, P, and K in plant shoots and root after
14 days of exposure to soil mechanical impedance and
water deficit.

 

Total water
Water 3.1). consumption Nitrogen Phosphorus Potassium

 

 

Mg m'3 8 —— mg
Control 1.1 215.6 45.64 6.59 55.56
1.4 208.3 44.50 6.16 51.51
1.7 149.1 36.27 5.58 40.34
Stress 1.1 128.7 36.59 5.81 42.27
1.4 126.5 34.09 5.49 39.81
1,7 94.5 32.00 4.63 28.41
LSDMS 19.4 13.88 1.15 10.00

 

49

Bulk density did not affect water absorption efficiency (W AE) based on
root surface area or root length when the water in the soil was optimum, but
soils that were highly compacted, significantly decreased water absorption
efficiency when soil water was limited (Table 10). As expected a water
deficit significantly decreased the absorption efficiency of water by the root
systems in all three levels of compaction. This indicated that compaction did
not affect the capability of roots to absorb water when water was not limit-
ing root growth, but compaction appeared to adversely influence water ab-

sorption when water was deficient ill the soil (Table 10).

Table 11 presents the water use efficiency (WU E) responses to soil
compaction and drought stresses. Shoot WUE was significantly increased by
water deficit with no influence by bulk density within each water treatment.
Whereas root WUE was significantly increased by both compaction and wa-
ter deficits. Water use efficiencies, based on whole plant fresh and dry
weights were primarily increased by water deficit. Bulk densities of 1.7 Mg
m’3 appears to increase the WUE on a whole plant fresh weight basis (Table
1 1).

50

Table 9. N, P, and K ion absorption efficiencies by14 days-old
dry bean seedlings subjected to soil mechanical

 

 

 

 

impedance and water deficit.
Water B.D. Nitrogen Phosphorus Potassium
Mg m‘3 7 mg in whole plantldm2 root
Control 1.1 12.84 1.84 15.46
1.4 11.90 1.64 13.78
1.7 10.14 1.56 11.26
Stress 1.1 12.48 1.98 14.53
1.4 13.14 2.11 15.31
1,7 12.05 1.75 10.70
LSDom 1.82 0.33 2.43

 

Table 10. Water absorption efficiency (W AE) for the 14th day
dry bean seedlings subjected to soil mechanical

 

 

 

 

impedance and water deficit.
Water ’ B.D. Water absorption efficiency
Mg rrr'3 mg HZO/cm 2 root mg HZO/cm root g H20/g roar
per day
Control 1.1 124 24.8 187.0
' 1.4 119 23.8 177.4
1.7 147 28.8 176.0
Stress 1.1 37 8.0 54.2
1.4 48 10.2 66.0
1.7 77 16.4 95.3

 

15130.05 32 7.3 46.1

 

51

Table 11. Water use efficiency (WUE) of dry bean plant
responses to soil mechanical impedance and

 

 

 

 

 

 

 

 

water deficit.
Water B.D. WUE
Whole plant
Shoot Root Fresh wt Dry Wt

Mg 111’3 g/kg H20
Control 1.1 2.63 0.59 45.05 3.22
1.4 2.49 0.58 44.04 3.07
1.7 2.63 0.69 47.48 3.32
Stress 1.1 3.08 0.79 50.36 3.87
1.4 3.06 0.82 . 50.55 3.88
1.7 3.25 1.04 61.66 4.29
LSDODS 0.39 0.10 6.11 0.45

 

52

SUMMARY AND CONCLUSIONS

Compaction and water deficit caused a reduction of dry matter, root
length and root surface area. Leaf density was influenced more by water
deficit than by soil compaction. While root density was influenced more by
soil compaction than by water deficit. The absorption efficiencies of N and P
ions decreased with greater bulk densities only when soil water was
sufficient. The absorption efficiency of K was decreased by greater bulk
density for both water control and water deficit. Water deficit decreased wa-
ter absorption efficiency for all three levels of bulk densities, and greater
bulk densities increased water absorption efficiency when soil water was
limited. Water use efficiencies were significantly increased by water deficit,
and there was a tendency for further increase of WUE with greater bulk-den-

sities.

The study of ion and water absorption efficiencies by the stressed see-
dling roots has a importance for the future life of the crops. The seedling
stage is the beginning of the developmental stage of growth which will have
a sequential influence on the final yield. And also, the results of that com-
paction increased WAE and WUE when soil water was limited give us a in-
formation how to use water wisely in those areas where water is in short
supply. This study was designed to determine the specific water absorption
efficiency, but did not provide adequate data for determining the specific ion
absorption efficiency at day 14. Another experiment utilizing multiple har-
vests and the use of ion isotopes would be required for measuring ion ab-

sorption efficiencies by plants subjected to greater drought and compaction
stresses.

LITERATURE CITED

Asady, G., A.J.M. Smucker and M.W. Adams. 1985. Seedling test for
the quantitative measurement of root tolerances to compacted sails. Crop
Sci. 25:802-806.

Burke, D.W., L. D. Holmes and A.W. Barber. 1972. Distribution of
Fusarium solani f. sp. phaseoli and bean roots ill relation to tillage and soil
compaction. Phytopath. 62:550-554.

Baligar, V.C., V.E. Nash, M.L. Hare and J .A. Price, Jr. 1975. Soybean
root anatomy as influenced by soil bulk density. Agron. J. 67:842-844.

Garcia,F., R.E. Cruse and A.M. Blackrner. 1988. Compaction and ni-
trogen placement effect on root growth, water depletion, and nitrogen up-
take. Soil Sci. Soc. Am]. 52:792-798.

Heen, A. 1980. Root growth, transpiration and leaf firing during water
stress in barley: Breeding implications for drought resistance. Personal com-
munication.

Huang, B. and A.J.M. Smucker. 1990. Carbon partitioning in the shoots
and roots of Phaseolus seedlings subjected to soil mechanical impedance
and water deficits. Agron. J. (in preparation).

Miller, DE. and D.W. Burke. 1974. Influence of soil bulk density and
water potential on Fusarium root rot of beans. Phytopath. 64:526-529.

Prihar, S.S., M.R. Chowdhary, and T.M. Varghese. 1971. Effect of
post-planting loosening of unstable soil on the anatomy of corn root. Plant
Soil 35:57-63.

Parkinson, J .A. and SE. Allen. 1975. A wet oxidation procedure suit-
able far the determination of nitrogen and mineral nutrients in biological
material. Soil Sci. and Plant Analysis, (1): 1-11.

Phillips, RE, and D. Kirkham. 1962. Soil compaction in the field and
com growth. Agron. J. 54:29-34.

Russell, R.S., R.Q. Cannell, and MJ. Goss. 1975. Effect of direct dril-
ling on soil conditions and root development. Outlook Agric. 8:227 -232.

Smucker, A.J.M., J .C. Ferguson, W. Debruyn, R.K. Belford, and J .T.

53

54

Ritchie. 1987. Image analysis of video - recorded root systems. In: ASA
Special Publication 50: 67-80. ASA, Madison, WI.

Trouse, AC 1971. Soil conditions as they affect plant establishment,
root development and yield. pp. 253-278. In: K.K. Barnes, W.M. Carleton,
HM. Taylor, R.I. Trockmorton, and G.E.Vanden Berg, (eds). Compaction
of Agricultural Soil. Am. Soc. of Agric. Eng, St. Joseph, Michigan.

Wiersum, L.K. 1979. A comparison of the behavior of some root sys-
tems under restricted aeration. Neth. J. Agric. Sci. 27:92-98.

Zimmerman, RP. and LT. Kardos. 1961. Effect of bulk density on root
growth. Soil Sci. 91:280-288.

CHAPTER 4

SUMMARY AND CONCLUSIONS

Short term experiments were designed to determine the (separate and
combined effects of soil compaction and water deficit on the root growth of
dry edible bean seedlings and the associated uptake of water and ions as
well as the relative distribution of photosynthates in the plants and soil.

Leaf area, shoot and root dry weight, root length and root penetration
ratio, as the parameters to indicate the growth of plants, were significantly
decreased by greater bulk densities and water deficits. The leaf area of the
stressed plants decreased about 50% below the nonstressed plants. The

14C02 fixation rates decreased primarily because of the smaller leaf areas.
Nearly 92% of the 14C-label was recovered following the 3 hours of pulse
labeling for nonstressed plants having leaf areas of 220 cm2. Whereas 74%,
47% and 41% were recovered from the stressed plants with the leaf areas of
176, 141 and 131 cm2, respectively. Consequently, plant biomass decreased
with less fixation and the greater losses by the stressed plants.

. Leaf and root densities collected 14 days after transplanting showed that
leaf density increased with the development of water deficit, whereas, root
density increased with greater bulk densities. This suggested that water
deficit had a greater influence on leaf thickness and compaction had a
greater influence on the thickness of roots than did water deficits. A larger

55

56

percentage of roots with bigger diameters in the 0.9-1.0 mm size were found
in the greater bulk density treatments which is additional evidence that
demonstrates mechanical impedance causes roots to become thicker. Root
surface areas also decreased with increasing bulk density and water deficit.
The decreased root surface area probably resulted in a lower capacity for up-

take of water and ions.

The investigation of total water and ions absorbed during the 14 day ex- .
periment showed that total absorption decreased with decreasing root sur-
face areas. After 14 days of drought, the water absorption efficiency (W AB)
was decreased by water deficits. When water was not limiting plant growth,
soil compaction had little influence on WAE. Greater soil bulk densities
seemed to improve WAE when soil water contents were deficient. Ion ab-
sorption efficiencies were adversely influenced by the greater bulk density.
K ion absorption seemed to be influenced more than N and P, by the soil
bulk density and drought stresses.

In conclusion, soil mechanical impedance and soil water deficit stresses
altered the root morphology resulting in a series of changes in their func-
tions. The decreased total root length and surface area resulted in a lower
capacity for root absorption of ions and water which caused a decrease in
plant biomass. Additionally, a greater percentage of the limited amount of
photoassimulates were lost to the soil by stressed seedlings. As plant and
soil biological systems mature, however, this dynamic and interactive sys-
tems becoming more complex. Generally, environmental stresses inhibit
root system development and alter the root morphology resulting in root dis-
function. Plant growth studies under controlled conditions such as the soil

core seedling test, can produce relative results and their interpretations can

57

be extended beyond the seedling stage to other temporal and spatial con-
texts. Since the continuous growth at all developmental stages has a big
influence on final yield, plants stressed at the seedling stage will affect their
future growth resulting in a lower tolerance of environmental stresses
throughout their life.

Although the studies reported here have shown that water and mechani-
cal impedance stresses adversely influence plant morphology, carbon fixa-
tion and retention, as well as ions and water absorption, additional studies
are needed to elucidate specific changes in root branching and root thickness

correlate to the assimilation and loss of carbon.

 

"Illllllllllllllll“