Agr. Med., Vol. 122. 225-235 (1992)
OLIVE-TREE ROOT DYNAMICS UNDER DIFFERENT SOIL WATER
REGIMES
J.E. Fernández1 , F. Moreno l ,
J. Martin-Aranda 1 , E. Fereres2
¡Instituto de Recursos Naturales y Agrobiología, (es/e), Sevilla; 2Departamento de Agronomía, Um'versidad de Cardaba, Spain.
SUMMARY - Tree roals develop in response to environmental conditions in the soil. The dynamics
of Aevelopment of olive-tree root systems (Olea europaea L., ev. Manzanillo) in response to soil water
conclítions were studied under three different soil water regimes in a 20-year old grave near Seville
(Spain). Minirhizotrons clown to 1 m were used to characterize changes in rooting density under rainfed,
flood and localized (d rip) irrigation. The results showed a clase association between root growth and
soil water status throughout the season. Root growth was observed far away from the tree trunk in
the rainfed trealment but it was concentrated only in the wetted zone in the case oF drip irrigation.
Root growth under F100d irrigation was more uniform over the whole area occupied by one tree. Root
growth periods in the rainfed treatment occured when the soil water content was high , and was very
limited during the summer (rainless period). In the irrigation treatments, the maximum root growth
was observed during the irrigation period (summer). Root appearance changed from paJe to a dark
brown colour in about one month after il was firsl observed in the rainfed treatment. However, il
took two to three months before ¡he same change in aspect was observed in the irrigated treatments.
Key words: irrigation, minirhizotrolTs, olive lree, rool dynamies, soil moisture.
!NTRODUCTION
Understanding raot system dynamics is important in studies of the soil-plant system.
This is particularly so in the case of tree root
systems where Httle is known of their dynamics of development in relation to the availability of soil resources, water in particular.
A mature tree establishes its root system over
many years and must have Iimited flexibility
in any one year to adjust it to variations in
soil conditions. In recent years, dryland olive
graves have been con verted to irrigated g,oves
in southern Spain, primarily by the drip
method. It is not known how the olive root
system performs under the new soil water regime created by irrigation.
Root dynamics studies in the field are Iimited by the lack of suitable techniques. $oil
sampling has the disadvantages of root system disruption and erfors introduced due to
the spatiaJ variation caused by periodic sam-
pling. The use of transparent tubes permanently installed in the soil for root system observations (minirhizotrons, Bóhm, 1979) offers
a viable alternative for studies of in si tu
changes of the root system. Several authors
have described the use of minirhizotrons
(Bates, 1937; Sanders and Brown, 1978; Bragg
et aL, 1983) and Upchurch and Ritchie (1983)
pro vide quantification techniques to estimate
rooting density based on root counts as seen
in the rhizotrons. A recent publication (Taylar, 1987) provides a comprehensive description of the instaIlation, use and data analysis
of minirhizotrons.
We present here observations of the changes
observed throughout the year in the rooting
density of olive trees under three different
water regimes carried out with the aim of assessing the influence of the soil water regime
and of the method of irrigafion on the behaviour of the olive tree root system.
225
AGRiCOLTURA MEDlTERRANEA 122 (1992)
60
50
40
E "
E
20
A
s
o
N
o
F
1989
1988
Fig. 1 - Rainfall recorded during the experimental periods.
MATERIAL AND METHODS
Experimentalsite, crop and treatments - Root
system dynamics was studied in a grove situated in the Aljarafe zone, sorne 13 km southwest of Seville (Spain). The experimental plot
was located on sandy loam soil (27.5 % eoarse
sand, 36.5% fine sand, 13.4% silt and 22.6%
elay) of 2 m depth (Moreno et al., 1983). The
hydrodynamic characteristics of the soil are
given by Moreno et aL, 1983. The climate
of the area is typically Mediterranean, with
mild, rainy winters and very hot, dry summers. Annual rainfall is about 550 mm which
falls mainly between October and April.
Figure 1 presents meteorologicaI data for the
experimental periodo Twenty-year oId olive
trees (Olea europaea L., varo Manzanillo)
planted with a 7x7 m spacing and loeated in
a l-ha plot were used in the experimento The
plot was tilled three times a year, in winter,
spring and summer, by means of a cultivator
and dise harrow down to 15-20 cm depth.
The l-ha pIot was split into three subplots,
each under a different water regime. Treatment R1 had the trees drip-irrigated with four,
4 l/h emitters, two on each side of the tree,
approximately 1 m apart. The trees were irrigated daily during the irrigation period,
which started near the end of May and ended in Oetober. The water applied was ealculated using the evaporation from a class A
pan having a erop coefficient of 0.4. T reat-
ment R2, was irrigated by flooding in 3 m
radius ponds around the tree trunk. One irrigation of approximately 150 mm was applied
every 4-5 days, keeping the soil near field capacity during the same period as in treatment
R1. Treatment S was rainfed, reeeiving 439
mm between March 1988 and March 1989 (35
mm during the irrigation period). The tree
height, trunk diameter anq eanopy diameter,
were respectively, 4.1±O.2 m, 0.085±O.009
m and 3.6 ± 0.3 m for the non-irrigated trees
and 4.4±O.2 m, O.085±O.008 m and 4.6±O.3
m for the irrigated trees.
Minirhizotrons - The minirhizotrons were
plexiglass tubes, 2 m long, with an outside
diameter of 8 mm and a walI thickness of
3 mm. Even though has been sorne controversy about the merits of either plexiglass
or glass tubes for use as minirhizotrons (Taylor and Bohm, 1976; Voorhees, 1976), it has
been shown that both types can satisfaetorily
be used (Brown and Upchurch, 1987).
A circular mark on the external wall of the
tubes was engraved at 10 cm intervals to provide a referenee for soil depth. AIso, another
linear mark alongside, covering all the length
of the tube, served as a reference for the annular section to be observed at each depth
interva1.
Two minirhizotrons per tree were instalIed
for three trees, each representing one treatment, aecording to the diagram of Pig. 2. The
226
l.E. FERNANDEZ, F. MOREN?,)- MARTlN·ARANDA, E. FERERES
number and position of the minirhizotrons
were chosen taking into account the results
about root distribution and root activity given
in a previous paper by Fernández et al. , 1991.
One of the minirhizotrons was situated in the
zone affected by irrigation (treatment R1) between the two drippers near the trunk, on
the east si de, at 1 m distance from the trunk
(position 1). The other was situated outside
the irrigation bulbs at 2 m from the tree trunk,
perpendicularly to the dripper line, on the
south side (position 2). The positions of the
minirhizotrons in treatments S and R2 were
similar to that described for treatment R1 (Fig.
would favour the absence of roots near the
tube at surface layers in the soil (Levan et
al., 1987; Mc Michael and Taylor, 1987). The
mouth of the tube was c10sed with a rubber
stopper, also covered with the same plastic
and material as the tube.
Observation equipment - The opticaI system
used to observe the roots consisted of an elliptical mirror iIluminated by a 50 w halogen
bulb, both located at the end of a 6 mmdiameter stai nl ess steel tube, 2.5 m long. The
lamp was wired to a 12 v, 5 A battery
through the tube in terior.
The mirror image was observed by means
of a magnifying lens made with a photographic lens (Schneider-Kreuznach Componar C
4/ 75) and an eye-piece of 1.25 mm connected together with PVC sliding tubes for focusing similarly to binoculars. Photographs were
also taken from the exterior of the minirhizotrons by means of a reflex camera and extensi~n tubes with a 70/ 200 mm objective, a blue
filter, using 800 ASA film.
2).
Dripptrs lint_
Field observations - Observations through the
minirhizotrons were carried out every 15 days
for 12 months. Measurements started two
months after installation, as recommended by
B6hm (1979), to aIlow time fer the roots to
extend normaIly around the observation tubes.
Roet density was determined using the Upchurch and Ritchie (1983) equa ti on:
Position'
Dr = N d / A d
Fig. 2 - Minirhizotrons position layout.
Minirhizotrons were installed in holes previously drilled in the soil, at an indination of
45 0 to prevent accumulation of roots at the
tube-soil interface, observed in vertical tubes
by sorne authors (Bragg et al., 1983). Once
the tube was introduced, the gap between the
soil and the tube wall was filIed with sorne
of the previously removed soil. as roots grow
more rapidly in empty voids than in the
proper soil (B6hm, 1979; van Noordwijk et
al., 1985) . The to tal length of buried tubes
was 1.4 m; the actual depth to prospect was
1 m. The part of the tube protruding from
the soil surface was covered with a black plastic sheet and an isolating material to prevent
Iight entering the tube and tube heating which
Dr
N
root density (cm root / cmJ soil)
number of roots observed in each tube
interval (10 cm)
A
area of the tube outer wall, in the given
interval (cm Z )
d
tube outer diameter (cm)
Root count ing was performed folIowing Upchurch and Ritchie's (1983) recommendations.
Measurement of soU water content - Soil water
content, was measured with a neutron probe
(Troxler modo 3333). Several trees including
the experimental S and R1 trees were equipped
with access tubes for neutron probe reading,
at given distances from the tree trunk, as
described in previous papers by Moreno et
al. (1987 and 1988). The tree of treatment R2
was similarly equipped.
227
AGRICOLTURA MEDlTERRANEA 12Z (199Z)
Roo\ densi\y (cml cm l )
0.1
O;-~------'''--Position1
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¡
0.1
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E
oC
,<
60
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i
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.........
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1-9-88
80
l/i
\
100
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0.2
0.1
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40
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60
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0.1
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17-12- 88 1
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100
(a)
(b)
(e )
Fig. 3 - Root density on three representative dates of the experimental period: a) treatment S, b) treatment Rl, and
c) treatment Rz.
228
l.E. FERNANDEZ, F. MORENO, J. MARTlN-ARANDA, E. FERERES
e (cm3/cm3)
e (cm3/cm3)
(a)
0.3
0.3
SO
o 4/4/88
612/7/88
IJ
1/9188
SO
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+
7/12188
22/2/89
E
o
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'00
O~
O
, SO
'SO
Posit ion 2
Position
e
(cm 3/cm 3 )
(b)
9(cm3/cm 3)
O.,
0+-__~__OL.1__~__0~.1__- L_ _~0._3
so -
0.1
,
0.3
o 4/4/88
61217188
1/9/88
SO
•
7112/88
2212/89
E
óó
'00
..c: 100
O~
O
'SO
'SO
Position 1
e (cm 3/cm 3 )
Position 2
e (cm 3/cm 3 )
(e)
0~__~~0.1~~__~0.1~~__0~.3~
0.1
0·2
0.3
o
6
SO
IJ
SO
•
4/4/88
4/7/88
1/9/88
7/12/8~
22/2/89
E
óó
-;; 100
'00
O~
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'SO
'SO
Position 1
Position 2
Fig ..4 - Soil water eontent on various da tes of the experimental period: a) treatment S, b) treatment Rl, and e) treatment R2.
229
AGR¡COLTURA MEDITERRANEA 122 (1992)
RESULTS AND DISCUSSION
Fig. sa. As mentioned before, position 2
showed
the maximum growth during the
Differences in root density between the treatments - Figure 3 shows root density (Lv) pro- whole periad in the top soil layer (10-30 cm)
files for each treatment, corresponding to three and only from August in the deeper layer
different dates representative for the growing (50-70 cm). The slight decrease of root forseason. On these three dates, higher root den- mation and growth observed in this treatment
sities were observed in treatment S (Fig. 3a) at 10-30 cm depth Erom the middle oE July,
in the zone farther from the tree (position 2) is in agreement with the decrease of soil water
than in the nearer one, except in the interval content, as shown by the water profiles of
12-7-88 and 1-9-88 (fig. 4a; positions 1 and
between 70 and 80 cm depth.
Observations made in position 1 (Pig. 3b) 2). No rainfall was recorded during the sumof treatment R1 showed that the root density mer (see fig. 1). At 50-70 cm depth, the dyprofiles were very different from that of the namics of root density in position 1 follow
similar position in treatment S. Position 1 is a similar pattern to that of both positions in
situated within the zone affected by the water the top layer. A similar behaviour has also
been observed by Le Bourdelles (1977) in olive
applied by the emitters. The highest root dentrees and Roberts (1976) in pine trees during
sity was observed in the top soil layers (20-40
cm) (except at the beginning of the observa- the summer, when soil water content is
tion period, represented by the date 27-4-88) Iowest. On the contrary, an increase of root
density was observed in position 2, at 50-70
although it was also high at the 50-70 cm
depth. The root density profiles in position cm depth, Erom the middle oE August till the
2, outside the zone affected by irrigation on end of September; this could be attributed to
all dates considered, showed a very low root root system explaration of soil zones deeper
and farther from the tree trunk
formation and growth (Fig. 3b), Results from
Results from treatment R1 (Fig. 5b) showed
observations in treatment R2 (Fig. 3c) show
that increases in root density in position 1
an intermedia te situation between S and Rl.
are noticeable from the beginning of the irriDifferences between the treatments in L
were related to the variation in soil water con: gation period in the two soil layers at 10-30
cm and 50-70 cm depth. Increase of root dentent (Pig. 4) for the three dates considered.
For treatment S, differences between position sity continued until soil water content reached
a constant value, as shown in the profiles of
1 and 2 (Fig. 4a), eould be attributed to a
larger volume of soil explored by the roots 12-7-88 and 1-9-88 (fig. 4b, position 1); after
that time, root density stayed constant in the
as the soil water content was similar. The
top layer (10-30 cm). Thus, it seems, that a
differences between both positions, found in
treatment R1, are in agreement with the large dynamic equilibrium is achieved, as suggestdifferences in the soil water profiles as shown ed by Ford and Deans (1977) in the case oE
spruce. This periad of constant root density
i~ Fig. 4b (position 1 and 2). Root density
dlfferences (Fig. 3c) between positions 1 and was maintained until the middle of December, probably due to the frequent rains dur2, in treatment R2, are only noticeable in the
spring (27-4-88) although the soil water con- ing that period which kept a water content
tent was adequate in a large zone around the status in this layer of soil similar (although
tree in this period (Fig. 4c, 27-4-88), proba- a Httle lower) to that of the irrigation periodo
bly due to the fact that irrigation applications The evolution of root density in position 1
in previous years were similar to those of at 50-70 cm depth is different from that of
treatment R1, giving rise to a root concentra- the top layer, showing the maximum value
at the end of June. Fluctuations in L were
tion near the trunk.
observed after that date, decreasing sharply
Root dynamics - Results of root density dy- at the beginning of winter. Differences benamics in treatment S, for the soil layers at
tween the two layers may be related to the
10-30 cm and 50-70 cm depth, are shown in fact that the most absarbing roots are situat230
J.E. FERNANDEZ, F. MORENO, J. MARTl N·ARANDA, E. FERER ES
lO- 3D c m
O.,
.. • .'
m
E
u
t . .·
0.05
E
..........-_A..../
?:
"
;
,',\
.'
....-........ . . ......• . ..
"
,
- - Positio n
u
.. --- ... Posit ion 2
~
e
~
"
A
."
oo
O.OS
O
O J
N
F
.'.
50 - 70,m
O.,
S
A
"
A
"
A
(O )
....-."
ce
O
"
A
"
S
A
J
o
N
o
J
1988
F
1989
10 - 30cm
O,,
m
E
u
0 .05
Eu O
,.,
"
O "
,..-- '"
A
M
.... " ............
l--..
_
A
J
S
'
o
,A ,
'
o
N
F
M
A
,-
~
e
~
(b)
o,
."
oo
.,, 4. ......~
...._. . ... '....,...
.•...
O.OS
ce
'
M
A
M
JASO
N
D
F
1988
O.,
...... .... .
-
E o.os
u
........'
E
M
'Vi
e
," "
A
M
o
S
A
J
O
·_-. -c
F
O
N
M
A
( e)
SO-70 cm
o.,
."
(;
-
-
~
~
A
10 -3 0 cm
m
-,.,
M
1989
...--- ...,'-,
.
~
O.OS
. .....
ce
"
;
M
A
M
J
'-.. ............
"
o
A
S
o
N
o
F
1988
M
A
1989
Fig. 5 - Root density d ynamies: a) treatmenl S, b) Irealmenl Rl, el treatment R2,
231
AGRICQLTURA MEDlTERRANEA 122 (1992)
ed in the 10-30 cm depth, in position 1, as
shown by Fernández et al. (1991) for this
treatment (R1). In positíon 2, root formation
at 10-30 cm and 50-70 cm depth was very
limited during most of the growing period as
shown in Fig. 5b. Only in the top layer (l0-30
cm) did root density reach the maximum
values in position 2. There were few changes
of Lv in this position, in agreement with the
constant and low water content in that soil
layer during spring and summer, as shown
in the profiles of Fig. 4b (position 2).
Analysis of results in treatment R2 (Hg. 5c),
where the wet zone is at near field capacity
(Fig. 4c), during the irrigation period and over
an area coverlng approximately the tree
shadow, shows that root dynamics, in positions 1 and 2, foIIows a similar pattern to
that of position 1 of treatment R1. However,
root formation and growth, in position 2, at
the beginning of the irrigation period, were
limited and similar to those observed in the
same position of treatment Rl, although its
soil water content was higher. The similarity
of the water content in a wide area around
the tree causes homogeneous root dynamics
in both positions and soil layers. The maximum value of root density, in position 2, was
reached at the middle of September in the top
layer (l0-30 cm), and at the end of August
in the deeper layer (50-70 cm), being, from
this date, much higher than in the same position for treatment R1.
Tab. 1 - Comparison of root densities (cm/cm3 ) determined by minirhizotron and auger methods, in
treatment Rl (date: 12-7-88).
Depth
Sampling by auger
Minirhizotron
(cm)
Position 1 Position 2
Posi tion 1 Position 2
0·20
20-40
40-60
60·80
80-100
0.110
0.190
0.110
0.080
0.032
0.015
0.030
0.010
0.030
0.010
0.042
0.086
0.092
0.074
0.015
0.010
0.018
0.012
0.016
0.000
Comparíson of root densities determíned by
minirhizotrons and auger methods - Results
from minirhizotrons and auger sampling, in
treatment R1, for a given date during the ir-
Photograph 1 - Young active root from position 1 in
treatment Rl.
rigation perlod, are compared in Table 1. In
both observation positions, the trends oE variation with depth are rather similar. However,
large differences are found between root densities in position 1, except for the 40-60 cm
and 60-80 cm depths. Other authors have also
found differences between resuIts from each
method (Sanders and Brown, 1978; Bragg et
aL, 1983). In our case, minirhizotrons values
are always lower than those obtained by auger sampling. These differences should not
only be assigned to the methodology but also
to spatial variability of the root system.
Change in root appearance effected by the irrigation regime - Observation carried out in
each treatment showed that the period of root
activity was largely affected by the soil water
status. The length of the perlod of maximum
root activity was determined considering the
external aspect of the root, from early root
growth until the time when roots became
dark. In the zone affected by irrigation (position 1 of treatment R1), an important root
activity was observed, with many fine, white
232
j.E. FERNANDEZ, F. MORENO, j. MARTIN-ARANDA, E. FERERES
Photograph 2 - Qlder roots from position 1 in treatment
Rl.
roots during the irrigation periodo On the contrary, roots in position 2 of this treatment
were less turgid and brownish than in position 1. While the same roots were white and
turgid for more than two months in position
1 of treatment R1, the same period lasted only
one month in treatment S, before the roots
reached the darkest brown colour. The influnce of soil water content on root activity
has also been found by Atkinson and Wilson
(1978) in apple trees and Kummerow wt al.
(1982) in cacao tree. Periods of root activity
in treatment RZ were very similar to those
of position 1 in treatment R1. The aspect of
a young active root from position 1 in treatment R1 can be observed in Photo 1. A light
cream colour and numerous hairs can be seen.
Another example, corresponding to older roots
of the same treatment, showing a light
brownish colour, is given in Photo 2. Roots
corresponding to treatment S are shown in
Photo 3, in which a very dark brown colour
can be observed that could be due to a high
degree of suberization. Although suberized
roots can absorb water and solutes through
their fissures, wounds and lenticels (Hayward
et al., 1942; Kramer, 1983), their ability to
do so is largely diminished (Hayward and
Spurr, 1943; Kramer and Bullock, 1966).
When roots showed the aspect represented in
Photo 3, root shrinking was observed in sorne
cases (Huck et al., 1970) with partial separation from the surrounding soil.
Photograph 3 - Roots corresponding to treatment S.
CONCLUSIONS
Our results show that root dynamics of olive
trees are significantly influenced by the water
regime in the soil. In the case of dry-farming,
where the soil water content is low, the period of root emergence and growth is limited
to the spring, while in treatments where soil
water content is higher due to irrigation, it
extends to all the summer periodo In this case,
sorne root activity can also be observed in
autumn.
In the case of the rainfed trees, the most
intense root dynamics is found in areas of
the soil far from the tree trunk, except between 70 and 80 cm depth, where the root
dynamics near the tree trunk is also noticeable. Under these conditions the roots explore
a large volume of soil around the tree. On
the contrary, in drip-irrigated trees, the root
dynamics is concent~ated within the wet irrigation bulbs and at shallower depths. When
the soil moisture conditions are those of treatment RZ, the emergence and growth of roots
also extends to a larger zone around the tree
trunk and during the total period of irrigation.
Application of irrigation, particularly by the
drip method, on an olive grove traditionally
under dry-farming, gives rise to large changes
233
AGRICOLTURA MEDITERRANEA 122 (1992)
in the situation, aspect (colour, turgescence),
growth and activity of the root system.
¡ology, 45: 529-530.
KRAMER P.J. (1983) - Water relations of plants.
Academic Press, Inc. New York.
KRAMER P.J., BULLOCK H.C. (1966) - Seasonal variations in the proportions of suberized and unsuberized roots of trees in relation to the
absorption of water. Amer. J. Bot., 53: 200-204.
ACKNOWLEDGEMENTS
Thanks are due to Mr.
field measurements.
J.
Rodriguez for he!p with
KUMMEROW J., KUMMEROW M., SOUZA DA SILVA
W. (1982) - Fine-root growth dynamics in cacao (Theobroma cacao). Plant and Soil, 65:
193-201.
LE BOUROELLES J. (1977) - Irrigation par goutte
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Address of the authors: P.O. Box 1052, 41080 Sevil-
Received: November 1991
la, Spain.
Accepted: March 1992
235