Agr. Med., Vol. 122. 225-235 (1992)

OLIVE-TREE ROOT DYNAMICS UNDER DIFFERENT SOIL WATER REGIMES

J.E. Fernández1, F. Morenol , J. Martin-Aranda1, 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

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-

A s o N o F

1989

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

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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. 2).

Dripptrs lint_

Position'

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

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, al so 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 nless 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.

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 tion:

Dr = N d / A d

Dr root density (cm root / cmJ soil) N 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

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AGR¡COLTURA MEDITERRANEA 122 (1992)

RESULTS AND DISCUSSION

Differences in root density between the treat­ments - Figure 3 shows root density (Lv) pro­files for each treatment, corresponding to three different dates representative for the growing season. On these three dates, higher root den­sities were observed in treatment S (Fig. 3a) in the zone farther from the tree (position 2) than in the nearer one, except in the interval between 70 and 80 cm depth.

Observations made in position 1 (Pig. 3b) of treatment R1 showed that the root density profiles were very different from that of the similar position in treatment S. Position 1 is situated within the zone affected by the water applied by the emitters. The highest root den­sity was observed in the top soil layers (20-40 cm) (except at the beginning of the observa­tion period, represented by the date 27-4-88) although it was also high at the 50-70 cm depth. The root density profiles in position 2, outside the zone affected by irrigation on all dates considered, showed a very low root formation and growth (Fig. 3b), Results from observations in treatment R2 (Fig. 3c) show an intermedia te situation between S and Rl.

Differences between the treatments in L were related to the variation in soil water con: tent (Pig. 4) for the three dates considered. For treatment S, differences between position 1 and 2 (Fig. 4a), eould be attributed to a larger volume of soil explored by the roots as the soil water content was similar. The differences between both positions, found in treatment R1, are in agreement with the large differences in the soil water profiles as shown i~ Fig. 4b (position 1 and 2). Root density dlfferences (Fig. 3c) between positions 1 and 2, in treatment R2, are only noticeable in the spring (27-4-88) although the soil water con­tent was adequate in a large zone around the tree in this period (Fig. 4c, 27-4-88), proba­bly due to the fact that irrigation applications in previous years were similar to those of treatment R1, giving rise to a root concentra­tion near the trunk.

Root dynamics - Results of root density dy­namics in treatment S, for the soil layers at 10-30 cm and 50-70 cm depth, are shown in

230

Fig. sa. As mentioned before, position 2 showed the maximum growth during the whole periad in the top soil layer (10-30 cm) and only from August in the deeper layer (50-70 cm). The slight decrease of root for­mation and growth observed in this treatment at 10-30 cm depth Erom the middle oE July, is in agreement with the decrease of soil water content, as shown by the water profiles of 12-7-88 and 1-9-88 (fig. 4a; positions 1 and 2). No rainfall was recorded during the sum­mer (see fig. 1). At 50-70 cm depth, the dy­namics of root density in position 1 follow a similar pattern to that of both positions in the top layer. A similar behaviour has also been observed by Le Bourdelles (1977) in olive trees and Roberts (1976) in pine trees during the summer, when soil water content is Iowest. On the contrary, an increase of root density was observed in position 2, at 50-70 cm depth, Erom the middle oE August till the end of September; this could be attributed to root system explaration of soil zones deeper and farther from the tree trunk

Results from treatment R1 (Fig. 5b) showed that increases in root density in position 1 are noticeable from the beginning of the irri­gation period in the two soil layers at 10-30 cm and 50-70 cm depth. Increase of root den­sity continued until soil water content reached a constant value, as shown in the profiles of 12-7-88 and 1-9-88 (fig. 4b, position 1); after that time, root density stayed constant in the top layer (10-30 cm). Thus, it seems, that a dynamic equilibrium is achieved, as suggest­ed by Ford and Deans (1977) in the case oE spruce. This periad of constant root density was maintained until the middle of Decem­ber, probably due to the frequent rains dur­ing that period which kept a water content status in this layer of soil similar (although a Httle lower) to that of the irrigation periodo The evolution of root density in position 1 at 50-70 cm depth is different from that of the top layer, showing the maximum value at the end of June. Fluctuations in L were observed after that date, decreasing sharply at the beginning of winter. Differences be­tween the two layers may be related to the fact that the most absarbing roots are situat-

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231

2

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

(cm) Position 1 Position 2

0·20 0.110 0.015 20-40 0.190 0.030 40-60 0.110 0.010 60·80 0.080 0.030 80-100 0.032 0.010

Minirhizotron

Posi tion 1 Position 2

0.042 0.010 0.086 0.018 0.092 0.012 0.074 0.016 0.015 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.

ACKNOWLEDGEMENTS

Thanks are due to Mr. J. Rodriguez for he!p with field measurements.

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soil-plastic container interface. Agronomy Jour­

nal, 68: 143. Address of the authors: P.O. Box 1052, 41080 Sevil­

la, Spain.

Accepted: March 1992

235