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Soil-Root Interface Water
Potential in Sweet Corn as
Affected by Organic Fertilizer
and a Microbial Inoculant
Hui-Lian Xu
a
a
International Nature Farming Research Center ,
5632 Hata, Nagano, 390-1401, Japan
Published online: 20 Oct 2008.
To cite this article: Hui-Lian Xu (2001) Soil-Root Interface Water Potential in Sweet
Corn as Affected by Organic Fertilizer and a Microbial Inoculant, Journal of Crop
Production, 3:1, 139-156, DOI: 10.1300/J144v03n01_13
To link to this article: http://dx.doi.org/10.1300/J144v03n01_13
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PART II:
MICROBIAL APPLICATIONS
Soil-Root Interface Water Potential
in Sweet Corn as Affected
by Organic Fertilizer
and a Microbial Inoculant
Hui-lian Xu
SUMMARY. Effects of organic fertilization and application of a mi-
crobial inoculant (with EM as commercial name) on soil-root interface
water potential (Ψ
s
−
r
) of sweet corn (Zea mays L. cv. Honey-Bantam)
were examined. The microbial inoculant includes about 80 species of
microbes, such as Lactobacillus, Rhodopseudomonas, Str eptomyces,and
Aspergillus. The contributions to Ψ
s
−
r
from root amount and root ac-
tivity were analyzed using the Ohm’s law. Plants were potted with an
Hui-lian Xu is Senior Crop Scientist, International Nature Farming Research
Center, 5632 Hata, Nagano 390-1401, Japan (E-mail: huilian@janis.or.jp).
The author thanks S. Kato, K. Yamada, M. Fujita, K. Katase and Dr. T. Higa for
their technical assistance and advice.
[Haworth co-indexing entry note]: ‘‘Soil-Root Interface Water Potential in Sweet Corn as Affected by
Organic Fertilizer and a Microbial Inoculant.’’ Xu, Hui-lian. Co-published simultaneously in Journal of
Crop Production (Food Products Press, an imprint of The Haworth Press, Inc.) Vol. 3, No. 1 (#5), 2000,
pp. 139-156; and: Nature Farming and Microbial Applications (ed: Hui-lian Xu, James F. Parr, and Hiroshi
Umemura) Food Products Press, an imprint of The Haworth Press, Inc., 2000, pp. 139-156. Single or
multiple copies of this article are available for a fee from The Haworth Document Delivery Service
[1-800-342-9678, 9:00 a.m. - 5:00 p.m. (EST). E-mail address: getinfo@haworthpressinc.com].
E 2000 by The Haworth Press, Inc. All rights reserved.
139
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NATURE FARMING AND MICROBIAL APPLICATIONS
140
Andosol soil fertilized using anaerobically- or aerobically-fermented
organic materials with or without EM application, and with chemical
fertilizers as control. One month after sowing, as soil matric water
potential decreased, Ψ
s
−
r
was higher in plants with organic fertilizer
than those with chemical fertilizer and was also higher in plants treated
with EM than in those without EM. The higher Ψ
s
−
r
was attributed to
the larger root volume and higher root activity shown by root respira-
tion rate. Consequently, photosynthetic rates under soil water deficits
were also maintained higher in these plants. This suggested that mainte-
nance of a high Ψ
s
−
r
favored plants to resist water deficits. The meth-
odology is a practical means of analyzing the soil-plant water status
under undisturbed conditions.
[Article copies available for a fee from The
Haworth Document Delivery Service: 1-800-342-9678. E-mail address:
getinfo@ haworthpressinc.com <Website: http://www.HaworthPress.com>]
KEYWORDS. Effective microorganisms, EM, microbial inoculant,
organic fertilizer, soil-root interface water potential, drought resistance,
Zea mays
ABBREVIATIONS AND SYMBOLS. A
L
, leaf area per plant; DM
R
,
root dry mass per plant; E, transpiration rate per plant; E
A
, average leaf
transpiration rate; g, leaf conductance; L
R
, total root length per plant;
P
C
, photosynthetic capacity; r, leaf resistance; r
p
, average radius of soil
particles; r
R
, average radius of the root; R
s
−
r
, soil-root interface hy-
draulic resistance; R/T, root to total ratio on dry mass basis; S
R
, root
surface area; S
s
−
r
, soil-root interface area; V
R
, root volume; Γ
s
−
r
,
soil-root interface hydraulic conductivity; r
s
−
r
, soil-root interface hy-
draulic resistivity; Ψ
s
−
r
, soil-root interface water potential; ι
s
−
r
,soil-
root interface distance
INTRODUCTION
Organic farming practices have been proposed as alternatives to chemical
agriculture to protect the environment, reduce production costs and improve
food quality. Organic farming and nature farming have been practiced by
farmers in Japan for many years. Recently, a technology of applications of a
microbial inoculant (with Effective Microorganisms or EM as the commer-
cial name, International Nature Farming Research Center, Atami, Japan) is
introduced to the organic or nature farming (Higa, 1994, 1996). The micro-
bial inoculant consists of a group of beneficial microbes containing about 80
species such as Lactobacillus, Rhodopseudomonas, Streptomyces,andAsper-
gillus. Research has shown that organic fertilizers improve soil physical
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Part II: Microbial Applications
141
properties, whereby the soil retains more water and the crops growing there
resist a stronger water stress compared with the cases of chemical fertilizer
(Letey, 1977). However, there are very few scientific reports to support the
mechanisms for effects of EM although farmers have found that applications
of this microbial inoculant with organic fertilizers improve crop yield and
quality (Higa, 1994, 1996). Some reports show that EM applications to crops
are more effective in drought regions (Li and Ni, 1996) than in humid areas.
This suggests that effects of EM might be associated with increased water
stress resistance caused by root water uptake ability.
Moreover, in research on plant response to drought conditions, measure-
ments of plant water status such as leaf water potential (Ψ
leaf
) are suggested
to be more appropriate as a water stress indicator than measurements of soil
water conditions such as soil water potential (Ψ
soil
) and water content (Hsiao,
1973; Jackson, 1974). However, the fluctuation of environmental factors on a
hot-dry day can cause a short-term change in plant water status such as Ψ
leaf
.
On the other hand, some plants with very inadequate water supply can have
the same Ψ
leaf
as in well-watered plants if the stomata close efficiently in
response to changes in environmental factors. This leads to a disadvantage in
using Ψ
leaf
as an indicator of water stress. Fortunately, Jones (1983a) pro-
posed a method to estimate the Y
soil
at the root surface or, the so-called,
soil-root interface water potential (Ψ
s
−
r
) using the familiar Ohm’s law. The
Ψ
s
−
r
is associated with both soil physical properties and plant water root
activity and water consumption. It can be considered as an appropriate indi-
cator for the status of soil water that is available or ready to enter the plant.
Therefore, in the present research, Ψ
s
−
r
of sweet corn plants and the con-
tribution of root amount and root activity to Ψ
s
−
r
were examined and the
effects of organic fertilizations and microbial inoculation were investigated.
MATERIALS AND METHODS
Plant Materials
Sweet corn (Zea mays L. cv. Honey-Bantam) plants were grown in Wag-
ner’s pots (16 cm in diameter and 20 cm h igh) in June 1996. The pots, each
with one plant remaining after thinning, were placed in a Latin Square design
in a glasshouse.
Soil
A fine textured Andosol soil was collected from a field where soybean was
previously cultivated. The total soil nitrogen, available phosphorus, and po-
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NATURE FARMING AND MICROBIAL APPLICATIONS
142
tassium were 3.4, 0.025 and 0.44 g kg
−
1
, respectively, with a C:N ratio of 13.
The field capacity (or capillary capacity) was 80% on a gravimetric basis, i.e.,
there was 80 g water in 100 g dry soil when it was saturated with water. Each
pot was filled with 3 kg fresh soil with a water content of 38% of the field
capacity.
Fertilizers
Ammonium sulfate, superphosphate and potassium sulfate were used for
the chemical fertilizer treatments and the total quantities of N, P and K
applied to the chemical treatments were equivalent to the total contents of N,
P, K contained in the organic fertilizers applied to the organic treatments.
Organic materials such as oilseed sludge, rice bran and fish-processing by-
product were fermented anaerobically or aerobically with or without EM.
The total nitrogen, available phosphorus, and potassium concentrations were
58, 30 and 2 g kg
−
1
, respectively, for both anaerobic and aerobic organic
fertilizers.
Microbial Inoculant
The microbial inoculant used in this study, with a commercial name as
‘‘Effective Microorganisms’’ o r EM (International Nature Farming Research
Center, Atami, Japan), contains a group of beneficial microorganisms that
can co-exist under the same culture. About 80 species are reportedly present
in the product liquid of this microbial inoculant. The main species comprising
EM are summarized as follows: (1) Lactic acid bacteria: Lactobacillus plan-
tarum, Lactobacillus casei, Streptococcus lactis; (2) Photosynthetic bacteria:
Rhodopseudomonas palustris, Rhodobacter sphaer oides; (3) Yeasts: Sacchar-
omyces cerevisiae, Candida utilis; (4) Ray fungi: Streptomyces albus, Strep-
tomyces griseus; (5) Fungi: Aspergillus oryzae, Mucor hiemalis; (6) Others:
Some microorganisms that are naturally occurring and combined into the
inoculant in the manufacturing process and can survive in the inoculant liquid
at pH 3.5 o r below (Pathogenic microorganisms do not usually survive solu-
tion at pH below 3.5). The density of most of the above-mentioned microbes
are in the range of 1 × 10
6
to 1 × 10
8
per ml.
Treatments
Six fertilization treatments were designed as follows: (1) EM-Organic 1--
anaerobically fermented o rganic materials 80 g, in which EM was added to
the materials before fermentation; (2) Organic 1--anaerobically fermented or-
ganic materials 80 g; (3) EM -Organic 2--aerobically fermented organic mate-
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Part II: Microbial Applications
143
rials 80 g, in which EM was added before fermentation; (4) Organic 2--aerobi-
cally fermented organic materials 80 g; (5) EM-Fertilizer--chemical fertilizers
(ammonium sulfate 5.3 g, long-period coated urea called LP coat-70 2.8 g,
superphosphate 13 g and potassium sulfate 4.95 g), with EM 80 ml, applied
into soil at the same time before sowing; the total amounts of nitrogen,
phosphorus and potassium were the same as in the above-mentioned organic
materials; and (6) Fertilizer--the same fertilizers as in (5) without EM applica-
tion.
Soil-Root Interface Water Potential
Figure 1 shows an electrical circuit analogue of the pathway of water flow
from soil through the plant to the atmosphere. The pathway is also called
soil-plant-air continuum (Nobel and Jordan, 1983). With the exception of the
tissue storage capacity (C
S
) and resistance (R
S
), parameters in the analogue
will be used to derive an equation to estimate Ψ
s
−
r
. In this pathway, Ψ
soil
plays a dominant role in controlling plant water status and Ψ
s
−
r
is proposed
as a better indicator for the status of the water available or ready to enter the
plant (Jones 1983a and b) because it is related not only to soil hydraulic
properties, which are determined by soil texture and physical properties, but
also to the plant root amount and water uptake ability. Usually, it is difficult to
FIGURE 1. An electric circuit analogue for water flow from the soil through the
plant to the air. Ψ
soil
, soil water potential; Ψ
s
−
r
, soil-root interface water poten-
tial; R
s
−
r
, soil-root interface hydraulic resistance; R
p
, the total hydraulic resist-
ance within the plants; Ψ
x
, leaf xylem water potential; r, leaf resistance; E, total
plant transpiration; R
s
, water storage resistance; C
s
, water storage capaci-
tance.
R
P
R
S
C
S
r
L
E
R
s
−
r
Ψ
s
−
r
Ψ
soil
Ψ
xylem
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NATURE FARMING AND MICROBIAL APPLICATIONS
144
measure Ψ
s
−
r
routinely. However, it can be estimated using the concept of an
electrical circuit analogue (Figure 1). In the present study we estimated Ψ
s
−
r
as proposed by Jones (1983a). According to the first equation of Ohm’s Law,
in a series-connected circuit,
V = V
1
+ V
2
+...+V
i
+ V
n
= I
1
R
1
+ I
2
R
2
+...+I
i
R
i
+...+I
n
R
n
,(1)
where V, I , and R are the electrical potential or voltage (Volt), current (Am-
pere) and resistance (Ohm) in an electric circuit. Accordingly, Ψ
soil
and Ψ
s
−
r
can be expressed as follows:
Ψ
soil
= Ψ
s
−
r
+ ER
s
−
r
,(2)
and
Ψ
s
−
r
= Ψ
X
+ ER
P
,(3)
where E is the plant transpiration rate (kg s
−
1
), corresponding to I
i
in (1);
R
s
−
r
and R
P
are the hydraulic resistance between the soil and root or called
soil-root interface hydraulic resistance (MPa s kg
−
1
) and the total plant
hydraulic resistance (MPa s kg
−
1
), respectively, corresponding to R
i
in (1);
Ψ
X
is leaf xylem water potential (MPa), corresponding to V
i
in (1). Because
R
s
−
r
cannot be measured directly, it is not possible to estimate Ψ
s
−
r
using
Equation (2), although Ψ
soil
and E can be determined. Therefore, Equation
(3) should be considered. It is known that transpiration rate per unit of leaf
area, E
A
(kg m
−
2
s
−
1
), is proportional to water vapor concentration differ-
ence, (e
s
− e
a
)(kgm
−
3
), between the evaporating surface and the ambient
air. According to the Ohm’s law, i t can be expressed as follows:
E
A
=(e
s
− e
a
)/r (4)
E
A
=(e
s
− e
a
)g (5)
where r (s m
−
1
) is the leaf resistance and g (m s
−
1
) is leaf conductance
shown as the reciprocal of r. Substituting (5) into (3) gives the following
equation:
Ψ
s
−
r
= Ψ
X
+(e
s
− e
a
)gAR
P
(6)
where A is the total leaf area (m
2
) of the plant. Because R
P
and A can not be
easily measured in undisturbed conditions, comparison with the well watered
control plant is used to eliminate these variables in the equation. First, for
convenience, let b =(e
s
− e
a
)AR
P
, and thus
Ψ
s
−
r
= Ψ
X
+ gb.(7)
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Part II: Microbial Applications
145
Ψ
s
−
r
for a well watered plant is 0.8 kPa in the present work and n egligible as
0 in comparison to the values (10-320 kPa) for plants with water deficit
treatments. Hence, equation (7) will be
Ψ
X[W]
= −bg
[W]
(8)
where the subscript [W] refers to a well-watered control plant. If the differ-
ences in (e
s
− e
a
), A and R
P
between moderately drought stressed [D]) and
well-watered plants change little during a short term treatment, b is assumed
the same for the former and latter (Jones, 1983a). Consequently, the follow-
ing equation is derived from (7) and (8):
Ψ
s
−
r[D]
= Ψ
X[D]
− Ψ
X[W]
g
[D]
/g
[W].
(9)
Equation (9) can be used to estimate Ψ
s
−
r[D]
using only g and Ψ
X
without
disturbing or damaging the plant and soil, except a half leaf blade is cut for
Ψ
X
measurement. However, leaf area would differ with a severe water deficit
or a long period of stress. In this case leaf area should be measured and a
factor of (A
[D]
/A
[W]
) should be included and (9) will be
Ψ
s
−
r[D]
= Ψ
X[D]
− Ψ
X[W]
g
[D]
A
[D]
/g
[W]
A
[W].
(10)
Moreover, (e
s
− e
a
) may also vary with water stress because stomatal closure
can increase leaf temperature and consequently increase e
s
. Therefore, values
of Ψ
s
−
r[D]
should be corrected using the data of e
a
and boundary layer
conductance (Jones, 1983b), when the leaf temperature difference is large.
Variance of another variable that has been neglected in the present work is R
P
because many documents have shown that the effect of short-term water deficit
on R
P
are usually small (Simmelsgaard, 1976; Jones, 1978; Passioura, 1980).
Soil -Root Interface Conductivity
Maintenance of Ψ
s
−
r
is, in a sense, attributed to the root volume on one
side and root water uptake activity and/or soil physical properties on the other
side. In order to elucidate the mechanism, I separately analyzed the contribu-
tions of root amount, shown here by total root surface area or soil-root
interface area (S
s
−
r
) and of the root activity and/or soil physical properties,
shown here by the soil-root interface conductivity (Γ
s
−
r
,gmMPa
−
1
m
−
2
s
−
1
). The determination of Γ
s
−
r
was made using data including those from
disturbed measurement, which was different from the determination of the
above-mentioned Ψ
s
−
r
. Soil-root interface hydraulic resistance, R
s
−
r
(MPa
m
2
skg
−
1
), in relations to S
s
−
r
(m
2
), soil -root interface resistivity, Ã
s
−
r
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NATURE FARMING AND MICROBIAL APPLICATIONS
146
(MPa m
2
mkg
−
1
), and soil-root interface distance, ι
s
−
r
(m), were analyzed
according the second equation of Ohm’s laws:
R = Ãι/S (11)
where Ã, ι,andS are the resistivity that is determined by the quality of the
conductor, conductor length, and conductor crosscut section surface area,
respectively. Therefore, in the case of soil-root interface,
R
s
−
r
= Ã ι
s
−
r
/S
s
−
r
(12)
where S
s
−
r
(m
2
) is the total contacting surface area between the soil and root
and equal to root surface area (S
R
). Therefore, (12) will be
R
s
−
r
= Ã
s
−
r
ι
s
−
r
/S
R.
(13)
In electrical physics, Ã is the resistivity of conductor determined by the
material property of that conductor. Therefore, the analogue variable Ã
s
−
r
is
the resistivity of the soil-root surface determined by root properties on one
side and soil properties on the other side. In convenience of proportion to
Ψ
s
−
r
, the reciprocal of Ã
s
−
r
, named as the soil-root interface conductivity,
Γ
s
−
r
(g m MPa
−
1
m
−
2
s
−
1
), was used to show the contribution to Ψ
s
−
r
from the soil-root interface properties (the root activity and soil physical
properties). It is expressed as follows:
Γ
s
−
r
=1/Ã
s
−
r
= ι
s
−
r
/R
s
−
r
S
R
. (14)
It must be mentioned h ere that Γ
s
−
r
is different in definition from both the
reciprocal of R
s
−
r
(hydraulic conductance) and the soil hydraulic conductiv-
ity obtained from the desorption curve (Hillel, 1980). Then Ψ
s
−
r
can be
written as
−Ψ
s
−
r
= ER
s
−
r
= Eι
s
−
r
/Γ
s
−
r
S
R
(15)
and Γ
s
−
r
can be calculated as
Γ
s
−
r
= −Eι
s
−
r
/Ψ
s
−
r
S
R.
(16)
Plant total transpiration (E) was calculated as
E = E
A
A (17)
Here, E
A
is the average transpiration rate of the leaves at different positions
on the plant. The length o f the soil-root interface pathway is assumed as the
average distance from the soil particles to the root surface and calculated as
ι
s
−
r
= r
p
+ r
R
− 4r
p
/3π − [ƒ(r
R
)dr
R
]/r
p
(r
R
≥ r
p
) (18)
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Part II: Microbial Applications
147
where r
p
and r
R
are the average radii of the soil particles and root crosscut
section surfaces; and ƒ(r
R
) are the circle area function for the root crosscut
section surface with r
R
as the radius. Equation (18) was derived from Figure 2
on the assumption that spheres of soil particles and cylinders of roots are
tangent with each other. Actually, i
s
−
r
is equal to the average height of the
cubic space between the soil particle and the overlapped part of the root
cylinder. The magnitude of this cubic space is graphically shown by the
broken line shaded p arts in Figure 2. The S
R
was estimated from the root
volume (V
R
) and root length (L
R
) as follows:
S
R
=2πL
R
√V
R
/πL
R
(19)
Root volume was estimated with the water replacement method. Washed fresh
roots w ere placed in a volumetric cylinder with a recorded volume of water in.
The increase in volume of the cylinder content was recorded as the root volume.
Root length was measured with the cross-line method (Morita et al., 1992).
Photosynthesis, Transpiration and Leaf Conductance
Five weeks after sowing, photosynthetic rates (P
N
) in the leaf just above
the ear were measured using a gas exchange system (LI-6400, LI-COR, Inc.,
Lincoln, Nebraska, USA) at different photosynthetic photon flux densities (0,
100, 250, 500, 800, 1200, 1600, 2000 µmol m
−
2
s
−
1
) under different soil
water conditions. The maximum photosynthetic potential or photosynthetic
capacity (P
C
) was obtained from the light-response curve as
P
N
=P
C
(1 − e
−
Ki
) − R
D
(20)
FIGURE 2. Crosscut section (left) and vertical section (right) of the root in
connection with a soil particle. The broken line sheltered parts show the areas
of the space between the root and the soil particle.
Root cylinder
O
P
O
R
O
P
r
P
r
R
r
P
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NATURE FARMING AND MICROBIAL APPLICATIONS
148
where P
N
is the net photosynthetic rate; K, the half-time constant; i, the
photosynthetic photon flux; and R
D
, the dark respiration rate. Leaf conduc-
tance and transpiration rate used for calculation of Ψ
s
−
r
and Γ
s
−
r
were
measured with this gas exchange system under natural light condition (about
1600 µmol m
−
2
s
−
1
)whenΨ
s
−
r
was measured.
Root Respiration Rate
Tip parts of the root with a length of 5 cm were cut and collected as root
samples (each with 1 g of fresh mass) for respiration measurement. The fresh
root sample was inserted in the assimilation chamber and measured under a
relative humidity of 85% at 25_C using the same gas exchange system as for
photosynthetic measurement. After measurement, the root sample was dried
and the respiration rate was expressed on a dry mass basis. The root respira-
tion was used to indicate the root physiological activity.
Leaf Xylem Water Potential
Because the leaf blade of corn is much broader and longer than those of
other cereal crops such as wheat and rice, a particular procedure was adopted
for the measurement. The tip half part of the leaf blade after photosynthetic
measurement was cut for measurement of leaf xylem water p otential. The
leaf blade was aligned on the adhesive surface of a 60 cm wide Scotch tape
and folded along the main vein. The leaf blade was then enclosed in the
Scotch tape to prevent evaporation water loss and pressure gas penetration
into the tissue. The p repared leaf blade sample was set in a pressure chamber
(Model 3000, Soilmoisture Ltd., Santa Barbara, California, USA). The tip of
the leaf blade with the tape was cut and the cut end was left 1 cm over the
rubber stop. Other processes were the same as in usual cases as mentioned by
Turner (1988).
Soil Matric Water Potential
Soil matric water potential was measured using tensiometers (SPAD
PF --33 with sensors of SPAD 2124 and 2127, Fujiwara Corp., Tokyo) with a
capacity range up to 350 kPa. Three tensiometer sensors were placed with the
sensor heads in different places within the soil volume of the sampled pots.
The average value of the tensiometer readings was recorded as the soil matric
water potential. Pots were weighed at the time when transpiration and leaf
conductance were measured and the soil water content was calculated. The
correlation(y=7011.9e
−
0.0767x
;r
2
= 0.83; n = 28) between soil matric water
potential (y) and soil water content (x) was obtained and confirmed in the
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Part II: Microbial Applications
149
laboratory. It is noteworthy that the soil matric water potential measured in
situ here does not represent Ψ
soil
in the analogue of Figure 1 because Ψ
soil
includes osmotic potential and pressure potential in addition to the matric
potential. In the present study, we used the in situ measured soil matric water
potential just to show the decreasing extent of soil water. If soil water poten -
tial is involved in the calculation and analysis, the one measured with a
psychrometer should be used.
RESULTS
Soil-Root Interface Water Potential
As soil matric water potential decreased, soil-root interface water potential
(Ψ
s
−
r
) maintained higher in o rganic-fertilized sweet corn plants than in
chemical-fertilized ones (Figure 3 ). For example, at the soil matric water
potential of 70 kPa, Ψ
s
−
r
decreased to −0.61 MPa in chemical-fertilized
plants while it maintained at −0.45 and −0.43 MPa in plants fertilized with
anaerobic and aerobic organic materials, respectively (Table 1 ). There was no
difference in Ψ
s
−
r
found between anaerobic and aerobic organic fertilizer.
Under both organic and chemical fertilization conditions, Ψ
s
−
r
was 0.08
MPa higher in plants with EM application than in those without EM. This
result suggests that microbial inoculation diminished decreases in Ψ
s
−
r
un-
der soil water deficit conditions although the differences between plants with
EM and without EM application were not as large as those between chemical
and organic fertilizations. There were only simply additive effects without
synergistic interactions between the treatments of organic fertilization and
EM application.
Contributions of Root Amount and Root Activity to Ψ
s
−
r
The contributions to Ψ
s
−
r
from root amount and root physiological activi-
ty were analyzed using the second equation (R = Ãι/S)ofOhm’slaw.The
related variables are presented in Table 1. First, it was found that root/top
ratio of biomass was higher for organic-fertilized plants than chemical -fertil-
ized plants and also higher for plants with EM application than for those
without EM (Table 1). However, the absolute values of root dry mass, total
root length, total root volume, and total root surface area were higher in
chemical-fertilized plants than organic-fertilized plants because the chemi-
cal-fertilized plants grew better as a whole. The differences in above-men-
tioned variables between plants with and without EM application were clear
in both organic and fertilizer treatments. The calculated value of soil-root
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NATURE FARMING AND MICROBIAL APPLICATIONS
150
FIGURE 3. Maintenance of soil-root interface water potential (Ψ
s
−
r
) and rela-
tive photosynthetic capacity (P
C
) in sweet corn plants grown with different
fertilizations as soil matric water potential decreased.
--0.00
--0.20
--0.40
--0.60
--0.80
--1.00
--1.20
--1.40
--1.60
--1.80
100
80
60
40
20
0
EM--Organic 1
Organic 1
EM--Organic 2
Organic 2
EM--Chemical
Chemical
0 --50 --100 --150 --200 --250 --300 --350
Soil matric water potential (kPa)
Relative P
N
(%)
Ψ
s
−
r
(MPa)
interface resistance (R
s
−
r
) at the soil matric water potential at 70 kPa was
higher in chemical-fertilized than organic-fertilized plants and also higher for
non-inoculation than for microbial inoculation treatments. The differences
between treatments in root radius and the length of the soil-root interface
pathway are very small. The little differences in ι
s
−
r
was from the differences
in root amount. The soil-root interface hydraulic conductivity, shown as the
reciprocal of the resistivity, was higher in organic-fertilized than chemical-
fertilized plants and also higher in plants of microbial inoculation treatment
in plants without. This result suggests that Γ
s
−
r
contributes to Ψ
s
−
r
.
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Part II: Microbial Applications
151
Root Respiration Activity
The respiration rate in the tip part of the root measured at 25_ was higher
in organic-fertilized plants than chemical-fertilized plants and higher for
plants with EM application than those without EM (Table 2). This result is
consistent with that of Γ
s
−
r
analyzed using Ohm’s law. This suggests that the
relatively h igh Ψ
s
−
r
in organic-fertilized plants or in EM-applied plants is
attributed, at least in part, to the relatively high root physiological activity.
TABLE 1. Soil-root interface water potential (Ψ
s
−
r
) and related parameters at
the soil matric potential of 70 kPa under organic and chemical fertilizations with
(MI+) or without (MI−) MI applications.
Variable Organic 1 Organic 2 Chemical
MI+ MI− MI+ MI− MI+ MI−
Ψ
s
−
r
(MPa) −0.45 a −0.52 b −0.43 a −0.51 b −0.61 c −0.69 d
E
A
(10
−
5
kg m
−
2
s
−
1
) 3.31 c 3.03 d 3.59 b 3.18 c 2.64 a 2.34 b
A
L
(m
2
) 0.108 e 0.106 e 0.113 c 0.107 d 0.146 a 0.129 c
DM
R
(10
−
3
kg) 1.83 bc 1.57 d 1.99 b 1.68 c 2.30 a 1.91 b
R/T (%) 27 a 24 b 28 a 24 b 20 c 18 d
L
R
(m) 440 b 393 c 457 b 397 c 501 a 438 b
V
R
(10
−
5
m
3
) 1.91 a 1.72 b 2.29 a 1.88 b 2.65 c 2.16 d
S
R
(m
2
) 0.325 a 0.291 b 0.362 a 0.316 b 0.408 c 0.344 d
R
s
−
r
(10
5
MPa s kg
−
1
) 1.26 a 1.61 b 1.06 a 1.49 b 1.63 c 2.24 d
r
R
(10
−
4
m) 1.18 cc 1.18 cc 1.26 b 1.23 b 1.30 a 1.23 b
l
s
−
r
(10
−
5
m) 4.59 a 4.59 a 4.55 a 4.56 a 4.53 a 4.56 a
Ã
s
−
r
(10
8
MPa s m
2
m
−
1
kg
−
1
) 8.94 d 10.28 c 8.44 e 10.06 c 14.69 b 16.90 a
Γ
s
−
r
(10
−
8
kg m m
−
2
MPa
−
1
s
−
1
) 11.19 b 9.23 c 11.84 a 9.94 c 6.81 d 5.92 e
See ‘‘Abbreviations and symbols’’ for definitions of the variables. The data followed by the same letter are not
significantly different with each other according to Waller-Duncan comparison.
TABLE 2. The photosynthetic capacity under well-watered conditions (P
C[W]
)
and at soil matric water potential of 70 kPa (P
C[70]
) and dark respiration rate of
the root at 25_C(R
D
) in leaves of sweet corn plants with different fertilization
treatments.
Treatment P
C[W]
P
N[70]
(% of P
C[W]
) R
D[25]
(μmol m
−
2
s
−
1
)(μmol kg
−
2
s
−
1
)
MI-Organic 1 25.8 bc 14.1 b (54.5) 5.2 c
Organic 1 24.8 c 10.9 d (43.9) 5.8 b
MI-Organic 2 26.6 b 16.6 a (62.4) 6.0 b
Organic 2 23.8 c 11.3 cd (47.7) 6.8 a
MI-Chemicals 29.0 a 12.4 c (4.8) 4.8 dc
Chemicals 26.8 b 9.0 e (33.7) 5.4 c
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NATURE FARMING AND MICROBIAL APPLICATIONS
152
Photosynthetic Maintenance Under Soil Water Deficit Conditions
Since Ψ
s
−
r
maintained higher in organic-fertilized and EM-applied plants
as soil matric water potential decreased, photosynthetic capacity (P
C
)was
consequently higher in these plants with higher Ψ
s
−
r
on both absolute and
relative bases (Figure 3, Table 2). The proportional association between Ψ
s
−
r
and P
C
was only apparent when soil matric water potential decreased, i.e.,
this relationship does not hold under well -watered conditions.
DISCUSSION
In the pathway of water flow from soil through the plant to the atmosphere
or soil-plant-air continuum (SPAC), Ψ
soil
plays a dominant role in controlling
plant water status (Nobel and Jordan, 1983). However, in some specific
cases, whether or not plants can absorb sufficient water from the soil is not
only dependent on water amount in the soil but also dependent on the water
uptake ability and the ability of the soil to transfer water from the soil to the
root surface. The soil-root interface water potential (Ψ
s
−
r
) is not only related
to soil properties but also associated with plant root activity and plant water
consumption. That is why Ψ
s
−
r
is considered as a better indicator of soil
water available to the plants (Jones 1983a). As mentioned by Jones (1983a),
Ψ
s
−
r
is actually the average water potential or effective soil water potential at
the root surface. It shows the status of water that is available or ready to enter
the plant. In magnitude, Ψ
s
−
r
is close to the plant water potential at predawn
but far lower than Ψ
soil
, especially that measured with a tensiometer. This
suggests that there is a large resistance at the soil-root interface. The results of
the present study indicated that the calculation o f Ψ
s
−
r
can be a useful
additional method to estimate plant water stress although there are limitations
from the fluctuations of related variables. In every treatment, measurements
are made on comparable leaves of water-stressed and well-watered control
plants that errors from t he Ψ
X
and γ could be minimized. There is no denying
the fact that leaf temperature and leaf area (in case the plant is vegetatively
growing) might change in response to a severe and/or a long-term water
deficit. In this case, the estimated Ψ
s
−
r
should be corrected using data of leaf
temperature, boundary conductance and leaf area as described by Jones
(1978, 1983a, b). The results showed a good trend among the treatments. The
Ψ
s
−
r
was higher for both anaerobic and aerobic organic fertilizers than
chemical fertilizers. In both organic- and chemical-fertilized plots, EM ap-
plication increased Ψ
s
−
r
almost equally. The relatively high Ψ
s
−
r
in organic-
fertilized plants or in plants applied with EM was attributed to both the
relatively large root surface area and relatively high soil-root interface hy-
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Part II: Microbial Applications
153
draulic conductivity (Γ
s
−
r
). By inferences as shown in Equation (14) from
Ohm’s law, Ψ
s
−
r
is determined by four factors, plant transpiration, the length
of the soil-root interface conductor, the root surface area and the soil-root
interface hydraulic conductivity (its reciprocal is called resistivity). The
length of the soil -root interface is actually the distance from the soil to root
surface and associated with the contact between soil and root. It is affected by
the soil texture and root morphology. In the present study, an Andosol was
used with a soil particle diameter of 0.18 mm for all treatments. Therefore,
the distance between soil and root varied just slightly with treatments because
of the small difference in root diameter. From the result of root surface area it
is concluded that the relatively high Ψ
s
−
r
in organic-fertilized plants or in
EM-applied plants is attributed, on the one hand, to the promoted root
amount. Another main factor that contributes to Ψ
s
−
r
is Γ
s
−
r
,whichis
determined by the properties of the soil -root interface pathway. One of the
terminals of the conducting pathway of the soil-root interface is the soil and
the other is the root of the plant. Therefore, Γ
s
−
r
is determined by both soil
physical properties and root physiological activities. Compared with plants
under chemical fertilization and without EM application as control, Γ
s
−
r
is
larger for organic-fertilized plants than chemical-fertilized plants and also
larger for plants with EM applications than for those without EM. We mea-
sured root respiration rate, which is supposed to be an indicator of the physio -
logical activity in the root (Huck, 1982). It was found that root respiration
rate at 25_C was relatively high in organic-fertilized or EM-applied p lants.
Respiration rate in the root indicates the physiological activity for ion and
water uptakes (Huck, 1982; Lauchli, 1982). This result is consistent with the
result of Γ
s
−
r
with analysis of Ohm’s law. Another factor associated with
Γ
s
−
r
might be the physical property, which determines the ease of water to
flow onto the root surface. In the present study, the author did not measure the
physical properties of the soil. So, it is not known whether just one season of
fertilization with a small quantity of organic materials or application of EM
can change soil physical properties. From our results, however, it can be
concluded that Ψ
s
−
r
is attributed to the developed root system and promoted
root physiological activity. In methodology, the analysis with the second
equation of Ohm’s law showed a good results for the experiment. However,
the disadvantage was that many data used for calculations were obtained
from disturbed measurements. Here, it was different from the estimation of
Ψ
s
−
r
.
Because of higher Ψ
s
−
r
under soil water deficit conditions, plants fertil-
ized with organic materials and applied with EM maintained higher P
C
than
plants fertilized with chemicals and those without EM application. It is log-
ical that, compared to that of a small root system, a plant with a large root
system and high root activity shows a higher water stress resistance ability.
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NATURE FARMING AND MICROBIAL APPLICATIONS
154
Although the absolute values of root biomass were larger in chemical-fertil-
ized than organic-fertilized plants, the root/top ratio was lower in the former
than in the latter. The results in the present study also support observations
that EM is more effective to crops under conditions of water deficit and under
other stresses (Li and Ni, 1996).
It has been known that long-term fertilization with organic materials im-
proves soil physical, chemical and biological properties (Hillel, 1980).
Growth and activity of the root system are promoted by the improved soil
properties and as a consequence the plants become more resistant to soil
water deficits by their strong root system (Jones, 1983b). Up to now, there
have been considerable research on organic farming (USDA, 1980; Locker-
etz and Kohl, 1981; Harwood, 1984; Vogtmann, 1984). However, the exact
mechanisms of the effects of EM are not precisely or conclusively known.
For example, it is not known whether EM applications affect soil physical
properties and how they are affected if so. It is also not clear whether the
effects are due to microbes themselves or from the substances produced by
microbes during the product manufacture or after it is applied. There has been
extensive research on the individual microbes that comprise the EM used in
the present study. Some phytohormones and their derivatives can be pro-
duced by soil microbes including some species existing in EM used in this
study (Arshad and Frankenberger Jr, 1992). Barea et al. (1976) have found
that among 50 bacterial isolates obtained from the rhizosphere of various
plants, 86, 58, and 90% produce auxins, gibberellins, and kinetin-like sub-
stances, respectively. Another report has shown that 55% of bacteria and 86%
of fungi isolated from the rhizosphere of Pinus silvestris are capable of
producing g ibberellins and their derivatives (Kampert et al. 1975). There
have been many reports showing that Actinomyces and Streptomyces like
those included in the EM used in the present study, produce auxins and
similar substances (Purushothaman et al., 1974; Mahmoud et al., 1984),
gibberellins (Arshad and Frankenberger Jr, 1992), and cytokinins (Bermudez
de Castro et al., 1977; Henson and Wheeler, 1977). Some fungi, like those
included in EM of this study (Aspergillus niger) produce gibberellins (El-
Bahrawy, 1983). Therefore, the promotion of root development and activity
by EM applications might be due to the production of plant growth regulators
by inoculated microbes. However, there are not available data to support this
hypothesis. Further studies are needed to examine the mechanistic basis for
the effects of EM on plant growth, characteristics associated with root water
uptake, and soil physical properties. Another fact is that EM has been used in
agriculture, especially in organic farming systems, in advance of fundamental
researches. Therefore, crop scientists and agronomists are encouraged to
elucidate the problems that they may encounter using the EM technology.
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Part II: Microbial Applications
155
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