Impaginato 112 Adv. Hort. Sci., 2011 25(2): 112-122 Received for publication 24 March 2011. Accepted for publication 3 June 2011. Root distribution in young Chétoui olive trees (Olea europaea L.) and agronomic applications C. Masmoudi-Charfi *, M. Masmoudi ** N. Ben Mechlia** * Institut de l’Olivier, Tunisie. **Institut National Agronomique de Tunisie. Key words: irrigation, overall root length, root-canopy ratio, root density, root volume, water requirement. Abbreviations: ETc (mm)= Crop evapotranspiration as determined by the FAO method (Allen et al., 1998). ET* (m3)= Evapotranspiration volume of an individual tree relative to its root area. Kc =Crop coefficient. Kr = Minorative coefficient introduced in the formulae of ETc - FAO to take into account the soil coverage. Ksupply = A supply ratio determined in order to link the water supplied to trees to the evaporative demand, it takes into account only the tree-related quantities. ETo (mm)= Reference evapotranspiration determined following to Penman-Monteith equation. Pe (mm)= Effective rainfall determined following the USDA-SCS method (FAO, 1976). I (mm)= Irrigation amount supplied during the irrigation period. I*(m3)= Irrigation amount supplied by localized system or in small basins around the trunk. P (mm)= Total rainfall. P* (m3)= Effective rainfall for a single tree received around the trunk. Sc (m2)= The maximum projected canopy area determined for each of the six trees assuming a circular shape. Sr (m2) = Area concerned by tree transpiration i.e. where roots are active. (t)= The number of years from planting. Lo, Lx= Dimension of interest respectively at planting and at maximum growth. α, β = Adjustment parameters within the logistic root and canopy growth curve. Abstract: The study was carried out to have a comprehensive view of the root system behavior of young olive trees cultivated under field conditions. The experiment involved irrigated trees (Olea europaea L., cv., Chétoui) cultivated at 6x6 m2 spacing in Mornag (36.5°N, 10.2°E), northern Tunisia. The way in which roots explore the soil volume during the first years after planting was explored through ‘in situ’ root system drawings and esti- mation of root densities. The relationship between canopy and root growth parameters was also investigated. The last section of this paper proposes a methodological approach for determining irrigation requirements of young olive trees and how water supply could be linked to the development of canopy and root system during the first years of cultivation when ground cover and the root system are not completely developed. Some agronomic appli- cations were then deduced concerning water and fertilizers for such orchards. Results show that the main deve- lopment of the olive root system occurs during the two to four first years of cultivation confining most roots (70%) to the top soil layers (20-40 cm). Maximum root densities were observed at this depth at a distance of 0.4 m from trunks. For young trees, water and fertilizers should be supplied at these depths and distances from trunk to allow easy and efficient root absorption. Obtained results also show a significant relationship between canopy and root areas which can be approximated by a linear model (r = 0.94). The root-canopy ratio estimated from their areas decreased rapidly beginning from the second year after planting, resulting from the establish- ment of competition between vegetative growth and fruiting. The optimum ratio root length/leaf canopy area of 2.3 km m-2 was found for the six-year-old tree indicating good equilibrium between the above and underground parts. The mathematical model developed on the basis of canopy cover and root extension allows precise esti- mation of water needs taking into account the actual root surface. However, while the canopy cover measurement was relatively easy to carry out, it was much more difficult to determine the surface covered by the root system. Results obtained in the present work also show an over-estimation of water needs when the FAO method is adop- ted to estimate the evapotranspiration of young trees. 113 1. Introduction The primary function of the root system, i.e. water absorption and acquisition of soil nutrients, has a great influence on many of the physiological processes in the tree (Doussan et al., 2003). However, despite of this importance, the root system is possibly the least explored area in crop physiology because of the diffi- culty involved in reaching it, in addition to the highly spatial-temporal variability which can generate many constraints to root extension. Amongst the first papers dealing with this area, are those of Yankovictch and Berthelot (1947), Vernet and Mousset (1963) and Abd- El-Rahman et al. (1966) which were carried out in North Africa, mainly on cultivars Chemlali and Picho- line Marocaine. Research conducted a few years later in Spain and Italy investigated the relationship between water and root extension (Pisanu and Corrias, 1971; Bohm, 1979; Nunuez-Aguilar et al., 1980; Mar- tin-Aranda et al., 1982; Michelakis and Vougioucalou 1988; Pastor et al., 1998; Smit et al., 1999; Palease et al., 2000). It was shown that apart from genetics and the origin of the plant (Ayachi-Mezghani, 2009) root distribution and extension can be markedly influenced by neighboring trees and soil texture and depth (Ben Rouina et al., 1996). Also, roots proliferate within the potential root zone regardless of irrigation application and method (Fernandez et al., 1991, 1992, 2003; Fer- nandez and Moreno, 1999; Connor and Fereres, 2005). These last authors noted that localized irrigation increased root length density of Manzanilla olive trees but it decreased their spread, largely confining them within the wetted volume and nearby trunks. They reported also that except under the canopy, roots were less frequent in the top layers than in deeper strata. Root extension is also dependent on the available carbohydrate resources (Dichio et al., 2002) and growth stage (Michelakis, 2000). Rapid growth is observed in spring and autumn; it depends on water supply. Root growth precedes shoot growth and may be drastically limited by the previous year’s fruit load. In fact, when no competition for carbohydrates occur- red with other organs, for example for young olive trees or/and for vigorous canopy growth trees, impor- tant root extension and greater root densities were reported (Palease et al., 2000). In contrast, limited car- bohydrate resources led plants to reduce their canopy growth and root length and even could deteriorate the root-canopy ratio as a result of competition between shoots, flowers, fruits and roots (Dichio et al., 2002). This relationship between root growth and the above- ground development is complex because it integrates many other factors and physiological processes like temperature, radiation, hormones, variety and alterna- te bearing. Reduction of the root-canopy ratio implies syste- matic reduction of the capacity of the rooting system to absorb water. In terms of root balance, the impor- tance of the water collecting system resides in its capa- city to obtain water to support the transpiring leaf area (Connor and Fereres, 2005; Connor, 2006) and it can be determined via an estimation of total root length through monitoring of root density. These techniques are reported by Tennant (1975) and Fernandez and Moreno (1999). Such measurements could provide reliable estimates of comparative activities. For olive trees, Connor and Fereres (2005) reported root densi- ties ranging between 0.1 and 1.0 cm cm-3. These values are lower than those provided for herbaceous crops and some deciduous orchards, although olive root systems can be extensive and deep. It appears from this short review that fundamental research on this subject is of prime interest: when and where the roots grow is crucial to understanding the functioning of the root system and its relationship with the above-ground organs. In fact, without precise information on root distribution, we cannot expect to efficiently manage the irrigation of the orchard. For these purposes, we have carried out the present study in order to have a comprehensive view of the root system behavior of young olive trees cultivated under field conditions. In this work, we examine how roots explore the soil volume during the first years after plantation. The rela- tionship between root and canopy development was also investigated. The last section of this paper propo- ses a methodological approach to determine irrigation requirements of young olive trees and considers how water supply could be linked to the development of canopy and root system during the first years of culti- vation when ground cover and the root system are not yet completely developed. 2. Materials and Methods Olive orchard The study was carried out during the period 1998- 2003 at the experimental farm of the Institut National Agronomique de Tunisie, located 15 km south of the capital Tunis (36.5ºN, 10.2ºE), northern Tunisia. In this region, climate is Mediterranean with yearly ave- rages of 450 mm rainfall and 1200 mm reference eva- potranspiration. It is dry and hot from May to Septem- ber. The orchard, of 1.6 ha, was planted in 1998 at 6x6 m2 spacing on a textural clay soil (29%C, 49%L, 23%S) of about 2 m depth. The volumetric soil water content was measured in the laboratory at field capa- city (50%) and at the wilting point (26%). Crop mana- gement practices carried out in the orchard, i.e. pru- ning, fertilizer (Masmoudi-Charfi and Ben Mechlia, 2009) and pest management practices, were similar to those applied in intensive orchards (Masmoudi-Charfi, 2006; Masmoudi-Charfi et al., 2006). The trial con- cerned trees of cultivar Chétoui, which is the main oil variety of northern Tunisia. 114 Climatic data and irrigation management Daily crop evapotranspiration (ETc) was determinedaccording to Allen et al. (1998) for the non-standard conditions such as: ETc = ETo ×Kc ×Kr, Kc rangingbetween 0.3 and 0.5 according to age, while Kr valueswere determined experimentally and varied between 0.69 and 0.75. For this purpose, a large white grilled (10 cm/10 cm) sheet was used. It was placed below the tree and the shade squares were counted and compared to the total number of squares (those lighted by sun and those shaded by leaves). This percentage represents the Kr value. Daily reference evapotranspiration (ETo) wascomputed according to the Penman-Monteith equation, with maximum and minimum yearly values of 1320 mm (1999) and 1212 mm (2003), respectively. Data relative to rainfall, ETo and temperature are reported inTable 1. All climatic data were recorded continuously with an automatic weather station located about 150 m from the young olive orchard. During the six years of the study, rainfall amounts varied from 327 mm (2001) to 790 mm (2003), while effective rainfall amounts ranged between 226 mm and 546 mm. These values were determined according to the USDA-SCS method (FAO, 1976). Accounting for these conditions, olive trees were irrigated every year during the spring-summer season. Water flows were programmed four times per season regardless of the critical stages and water availability. Irrigation was supplied by furrows (basin and drain) during the four first years and then by a drip system (2002 and 2003). Two parallel drip lines were fixed on the soil surface at about 0.5 m from trunks. There were four emitters per tree, two at each side of the tree trunk, separated 1 m from each other; each having a 4-L h-1 flow rate. The area wetted by irrigation application varied between 1 m2 (1st year) and 6 m2 (6th year). Wate- ring conditions for the whole period are given in Table 2. Water requirements were covered at levels varying between 0.3 ETc and 1.1 ETc according to year andwater availability. Measurements Soil water content. The volumetric water content of the soil was measured with a neutron probe (SOLO 25) which was previously calibrated for the soil in question (Masmoudi-Charfi, 2008). Twenty-eight access tubes, 1.5 m long, were placed at the corners of a square of 2 m2 below the canopy but also within the tree line and between tree lines. The soil moisture in each of these tubes was recorded frequently during the irrigation period every 0.3 m to 1.2 m depth, and the mean calcu- lated separately for each position: below the canopy, far from the emitters, along and between the lines of tree (unpublished data). For the top 0.2 m soil layer, soil water content was determined by gravimetry. More details are given in Masmoudi-Charfi (2008). Table 1 - Climatic data recorded during experimentation (1998-2003) Annual rainfall (mm) Effective annual rainfall (mm) Absolute Tmax (°C) Absolute Tmin (°C) Average Tmax (°C) Average Tmin (°C) Annual ETo (mm) 1998 1999 2000 2001 2002 2003 376 260 47.0 3.0 25.0 13.3 1313 440 304 41.0 1.0 23.7 15.0 1320 410 283 44.0 4.0 25.2 14.8 1293 327 226 42.0 3.0 25.8 15.8 1282 345 238 43.0 3.0 25.6 15.5 1231 790 546 46.0 3.0 24.9 14.9 1212 Table 2 - Water requirement and irrigation application for young olive trees of cultivar Chétoui during the experimental period Irrigation system First irrigation Last irrigation Dose (m3/tree) Irrigation amount (m3/tree/year) I + Pe (mm)* ETc (mm)* I+Pe / ETc 1998 1999 2000 2001 2002 2003 Basin March August 0.12 0.84 140 243 0.6 Basin May September 0.18 0.72 61 241 0.3 Drain April September 0.22 0.88 180 291 0.6 Drain April September 0.44 1.76 141 287 0.5 Drip March August 0.7-1.7 4.98 248 273 0.9 Drip May September 0.3-1.0 5.41 389 368 1.1 (*) indicates that values are determined for the irrigation period. Pe is the effective rainfall determined according to the USDA-SCS method (FAO, 1976) and I is the irrigation amount. The ratio I+Pe / ETc was calculated for the irrigation period. The tree downward projection canopy flat area varied between 2% (first year) and 33% (sixth year). 115 Root distribution. Distribution of the root system was studied during the rest period (November-Decem- ber) on the same Chétoui olive trees by extensive observations of their root system. The trench method was used as described by Fernandez et al. (1991). For this purpose, a large pit was opened at 0.4 m from the trunks and roots were counted on the internal trench wall, which was divided into five layers of 0.2 m width each and down to 1.0 - 1.2 m depth. Root diameter was measured by means of a caliper 1/100. Maximum distance of roots from trunk was determined at each soil layer in order to estimate lateral root extension. Total volume of soil and the area explored by the root system were determined assuming central symmetry to the trunk. Root density. Root densities were determined on the same Chétoui olive trees by using the cylinder method as described by Fernandez et al. (1991). Soil samples were taken during the rest period by a conventional auger at 0.4 m, 0.8 m and 1.2 m from trunks in order to quantitatively assess the importance of the root system through an estimation of root densities as described by Tennant (1975). Samples were taken within layers of 0.2 m width, down to 1.0 - 1.2 m depth, following east and south directions, along the line of drippers (south) as well as perpendicular to this. They were then washed out abundantly and sieved through a 0.5 mm screen. Extracted roots were counted by adopting a reference scale (Tennant, 1975). Root length was then derived from the average root density value for each of the six trees. Figure 1 presents details on both protocols. With this scheme, it was possible to obtain information on root distribution in the zones affected and not affected by irrigation. Canopy measurements. Canopy diameter measure- ments were monitored at the same time as the study of the root system and on the same experimented trees. The maximum projected canopy area (Sc) was determi-ned for each of the six trees assuming a circular shape. These measurements were used to set a typical model of growth and to examine the relationship between root and canopy development. Canopy leaf area was determined for the six-year- old olive tree by computing the number of leaves on representative shoots and estimating its specific leaf area. It reached 14 m2 on May 2003. This value was adopted to calculate the root length/leaf canopy ratio. Methodological approach to determine irrigation requirements of young olive trees. This section propo- ses a methodological approach to determine irrigation requirements of young olive trees and how the water supply can be linked to the development of the canopy and root system during the first years of cultivation when ground cover and root system are incompletely developed. Determination of water requirements accor- ding to the FAO method (Allen et al., 1998) is adequa- te for standard conditions, i.e. when soil coverage rea- ches 60% or more. However, when the coverage area is less, a reductive coefficient Kr is introduced (COI,1997; Allen et al., 1998). In some cases, particularly for young and new orchards (low tree canopy cover), this coefficient may not be precise enough to allow good estimation of water needs. In addition to pro- blems estimating Kr values, the Kc is strongly affectedby conditions that influence evaporation from the soil surface (Orgaz et al., 2006). Recently, Testi et al. (2004) proposed a simple linear relationship between the olive ground cover (and Leaf Area Index) and the average Kc of the summer months, valid for groundcover fractions up to 0.25, along with its variation when wet surface soil spots are present. These authors indicate that this relationship does not apply outside a rainless summer, and the contribution to soil evapora- tion from the drip system depends on the surface area and location of the wet spots and is not scalable. Thus, we developed the following approach which is designed to determine the consumptive use of olive trees in relation to their canopy growth and root deve- lopment during the first six years after planting. Before full development of the root system, only a fraction of rainfall water is accessible to trees. Thus, the water balance equation should consider the area concerned by tree transpiration i.e. where roots are active (Sr); Sr is assumed to be circular and to increasefollowing a logistic-shaped curve. Root extension, as well as canopy increase, seems to coincide with a logistic growth curve as given by the following equation: L(t)=Lo+ Lx-Lo1+exp[α(t-β)] where (t) is the number of years from planting; Lo, Lx dimensions of interest, respectively, at planting and at maximum growth; α, β are adjustment parameters. In order to link the water supplied to trees to the evaporative demand, a supply ratio (Ksupply) that takesinto account only the tree-related quantities is defined by this equation: Ksupply = (P* + I*) / ET* Fig. 1 - Scheme of sampling to determine root distribution and root densities for young olive trees aged one to six years. Root pro- files were mode following to NW direction while samples of root density determination were taken at 0.4 m, 0.8 m and 1.2 m from trunks to 1.2 depth following to SE direction. : Emitter. 116 Considering that irrigation (I*, m3) is supplied by a localized system or in small basins around the trunk, only a small surface is wetted and affected by soil eva- poration and transpiration. Irrigation water is therefore assumed to be fully accessible to the root system of the tree. On the other hand, effective rainfall for a single tree (P*) is taken as the volume of rainfall water avai- lable to the root system which could be approximated by the following equation: P* (m3) = P (m) x Sr (m2) P is rainfall, considered here as total rainfall. The evapotranspiration volume of an individual tree (ET*) can be estimated from the root area of the tree as: ET* (m3) = Kc x ETo (m) x Sr (m2). Different water supply ratios are determined as Kc -FAO, I/ETo, P*+I*/ET*and I*/ET*. The ratio I/ETo isthe irrigation supply, P*+I*/ET* is the volumetric total supply and I*/ET* is the volumetric irrigation supply. These ratios are for the period April-August over the first six years of olive tree cultivation. Values are repre- sented in the same figure to compare results. 3. Results Soil water status Simultaneous monitoring of soil moisture carried out during the 2003 campaign at the canopy limit and near the emitters showed that soil water contents vary from 15 to 39% according to depth and distance to trunk (Fig. 2). Low values of soil water content were observed in the upper layers, while minimums were recorded within the superficial strata (0-20 cm) as a result of soil water evaporation and root absorption. This result confirms the concordance between root development and soil water depletion. The results showed large variation between measurements at the limit of the canopy, while low variation of soil moistu- re was observed near the emitters with values ranging between 32 and 38% according to depth (Fig. 2). Root system drawings Root profiles for the tagged trees show two or three types of roots according to age (Fig. 3). During the first years after planting, trees developed fine roots in the upper 0.2 m of the soil layer, which then extended Fig. 2 - Soil water content (%) measured at two sites: on the left at 10 cm from the emitters and on the right at the limit of the canopy during the 2003 campaign. Table 3 - Maximum number of roots and root diameter emerging from the trench face for each soil layer for olive trees aged one to six years 0-20 20-40 40-60 60-80 80-100 Total number of roots Maximum root diameter (mm) 2 3 4 5 6 6 2 0 0 0 8 2 2 3 8 2 0 15 6 16 6 3 4 0 29 23 10 5 1 4 0 20 32 9 5 3 5 3 25 24 51 91 116 97 81 472 27 1Soil layer (cm) Age (year) Fig. 3 - Drawings of the root system of young olive trees of cultivar Chétoui aged one to six years. Roots were counted on the inter- nal trench wall, down to 1.0 - 1.2 m depth depending on age. rapidly in lateral and vertical directions with inclina- tions varying from 30° to 60° depending on their size and position. For older plants, larger roots were obser- ved beyond the first 0.3 m and they developed hori- zontally with numerous fine roots. The number and diameter of roots which emerged from the lateral face of the trench are summarized in Table 3. Results indicate that most roots (70%) are localized in the first 0.6 m of soil. The maximum number is found in the top layers, with diameters ranging between 2 mm (one-year-old tree) and 32 mm (four-year-old tree). Some roots developed in deeper strata, reaching 1.0 m depth. Very few roots were found below this depth even for the oldest tree. 117 Extension of the root system Results presented in Table 4 show that the main development of the root system occurred during the first two to four years of cultivation, horizontally and within the top layers (0.2-0.3m). During this period, the soil volume explored by roots increased at a regular rate of about 1.0 m3 yearly. For the three-year-old tree, roots explored a volume of 3.65 m3. The soil volume explored by the root system of the five-year-old-tree represents 47% of that reached by the older tree (six- year-old tree). Root density Results relative to root density estimation are repor- ted in figure 4. A noticeable root concentration is obser- ved for both east and south directions and close to trunk in the top layers around each of the six trees. Average values varied between 0.001 cm cm-3 and 0.670 cm cm-3 depending on depth, distance to trunk, direction and tree age. Greater values, by up to 0.5 cm cm-3, were recorded in the first 60 cm and at 0.4 m from trunk. These values decreased significantly as the distance to trunk increa- sed (except some measurements for two- and three- year-old plants). Roots were less frequent at all depths outside the canopy limit and particularly for the deeper layers. At these depths, however, it should be mentio- ned that root densities rarely exceed 0.4 cm cm-3 for both directions, while average values ranged between 0.067 cm cm-3 and 0.303 cm cm-3 (Table 5). Root system length The overall length of the root system varied from 1.0 km to 33.9 km depending on age (Table 6). A significant increase of the overall length of the root system was observed for the six-year-old tree. It was 4.8 times greater than that recorded the previous year. The lowest value was recorded for the four-year- old tree. There was no apparent cause which could explain this result. Root development and canopy growth Results presented in figure 5 showed for tree aged one to four years that roots grew at higher rates than Table 4 - Maximum distance of roots to trunk (m) and volume of soil explored by the root system (m3) for olive trees aged one to six years 0-20 20-40 40-60 60-80 80-100 Explored soil volume (m3) 2 3 4 5 6 1.05 1.15 0 0 0 1.45 1.05 1.10 1.00 0.80 0 2.55 1.25 1.30 1.25 1.00 0 3.65 1.45 1.45 1.25 1.25 0 4.60 1.50 1.45 1.25 1.25 1.00 5.30 2.12 1.95 1.80 1.65 1.55 11.2 1 Depth (cm) Age (year) Fig. 4 - Root densities (cm cm-3) recorded for olive trees of cultivar Chétoui aged one to six years based on direction and depth. For each tree, three measurements were carried out for both direc- tions at different distances from trunk; the first observation was made at 0.4 m, the second at 0.8 m and the third at 1.2 m. Table 5 - Average root densities (Dr, cm cm-3) determined for trees aged one to six years Dr (cm cm-3) 2 3 4 5 6 0.067 0.079 0.196 0.075 0.133 0.303 1 Table 6 - The overall length of root system (Lr, km) for trees aged one to six years Lr 2 3 4 5 6 1.005 1.975 7.056 3.450 7.049 33.936 1 Fig. 5 - Maximum root distance from the trunk (m) and Maximum canopy radius (m) following to age for olive trees Chétoui ages one to six years. Di sta nc eo rr ad iu s( m ) 118 canopy radius. Then, differences between the canopy radius and the root-to-trunk-distance decreased. Roots reached for the six-year-old tree a maximum distance to trunk of 2.10 m, while the canopy limit was obser- ved at 1.95 m. The projected canopy area (Sc) increa-sed slowly after planting to reach 0.21 m2 for the one- year-old tree and 11.94 m2 for the six-year-old-tree (Table 7), while the root area progressed at a constant rate of 1.2 m2 per year to reach 13.8 m2 for the six-year-old-tree. A significant relationship was found between canopy (Sc, m2) and root (Sr, m2) areas, which can beapproximated by a linear model with a correlation coefficient r of 0.94, as illustrated by Figure 6, where Sc = 1.183 Sr - 3.602 (R2 = 0.876) The Sr/Sc ratio derived from both canopy and rootareas decreased significantly from 20 to 0.9 depending on tree age. For the four-, five- and six-year-old trees, this ratio approximated the unit. A decrease of the Sr/Sc ratio implies a tendency toequilibrium between the under-ground and above- ground organs beginning from the fourth year after planting, which apparently results from the establish- ment of competition between shoots, roots and fruits (and explains the decrease of this ratio). In fact, trees began to produce olives within the second year after planting and the first commercial crop arrived in year four (6.5 kg / tree). Results indicate also that plants seem to be able to adjust their root systems to the larger above-ground development during the winter rest. This feature is well represented by the root length/leaf canopy area ratio. A value of 2.3 km m-2 of leaves for the six-year-old tree was found in the present study, a value which is consi- dered optimum for such conditions. Irrigation supply as a function of canopy and root development In order to link the water supplied to trees to the evaporative demand, a supply ratio (Ksupply) that takesinto account only the tree-related quantities is defined as developed in section ‘Measurements. Methodologi- cal approach to determine irrigation requirements of young olive trees’. This ratio could be considered as a crop coefficient for young trees when reference evapo- transpiration, rainfall and irrigation amounts are com- puted according to the previous equations and expres- sed in m3/tree. Adoption of such a ratio allows estima- tion of irrigation requirements for different rainfall and evapotranspiration regimes. The different water supply ratios, Kc - FAO, I/ETo, P*+I*/ET* and I*/ET*, deter-mined for each of the six olive trees are given in Figu- re 7 for comparative purposes. Results show that the ratio of applied irrigation (I, mm) to reference evapotranspiration (ETo, mm) duringthe dry season from April to August was very low. It increased from 0.02 to 0.14 when trees grew from one to six years. When using the volume method to calcu- late the irrigation and precipitation falling on the area covered by roots, Ksupply comes very close to the Kc-FAO. Estimation of effective precipitation remains however big challenge for using the proposed method. Table 7 - Canopy and root area estimations (m2) of olive trees aged one to six years Root area (Sr) Canopy area (Sc) Sr / Sc 4.20 0.21 20.00 1 3.80 0.82 4.60 2 5.30 1.86 2.80 3 6.60 3.79 1.70 4 7.10 8.04 0.90 5 13.80 11.94 1.20 6 Fig. 6 - Relationship between canopy and root areas for young olive trees aged one to six years. Fig. 7 - Variation of Kc-FAO, irrigation supply (I/ETo), volumetric total supply (P*+I*/ET*) and volumetric irrigation supply (I*/ET*) ratios calculated for the period April-August over the first six years of olive tree cultivation, 1998-2003, Mor- nag - Tunisia. 119 4. Discussion and Conclusions This study provides preliminary results on root distribution of young olive trees of cultivar Chétoui, which could be exploited to manage young olive orchards efficiently. Root profiles for trees aged one to six years show rapid extension of the root system during the first two to four years of cultivation fol- lowing to horizontal direction. Most roots (70%) are localized in the first 0.6 m of soil with a maximum number developed in the top layers. Some roots deve- loped in deeper strata, reaching 1.0 m depth, but at this age very few roots were found below this depth. The largest roots were observed beyond the first 0.3 m with maximum diameters between 2 mm and 32 mm accor- ding to age. Results indicate also that lateral fine roots are abundant and give rise to a fibrous root system which represents the main absorbing surface as it was reported by Palease et al. (2000). These roots originate from the branching of a parent root and constitute their ramifications, generally at right angles, as indicated by Doussan et al. (2003), who classified the roots into three main categories according to their ontogenesis: primary, adventitious and lateral. In our case, the pri- mary root constitutes the main root of the cutting. It was not dominated at the outset by a principal axis as it occurs in trees grown from seedlings. Rather, many adventitious roots are produced from the base of the cutting. Similar results were found in the literature for young olive trees, although studies were carried out under different conditions. Abd-El-Rahman et al. (1966) reported for young trees, aged seven years and grown under 150 mm of rainfall, that roots are contai- ned in the shallow tillage (0.15-0.30 m) to approxima- tely 0.3 m from trunk. In Sardinia, Pisanu and Corrias (1971) observed a very shallow root system in the roots of excavated trees. Their photographs and drawings clearly illustrate the horizontal development of the roots and the fact that roots of contiguous trees avoid competition by developing outwards from the tree row. In Spain, Nunuez-Aguilar et al. (1980) observed for 12-year-old ‘Manzanilla’ olive trees, that most roots are localized in the outer layers at 0.45 m from the trunk with diameter less than 0.5 mm. Mickelakis and Vougioucalou (1988) observed for five-year-old ‘Kala- mon’ olive trees cultivated in Create, a maximum num- ber of roots at a depth of 0.4 m. Later, Bongi and Pal- liotti (1994) indicated that the root system of young olive trees is mainly confined to the top meter of soil, growing at depths between 0.15 and 0.40 m at a maxi- mum distance of 0.30-0.40 m from the trunk. Results relative to soil volume exploration showed a regular increase of about 1.0 m3 yearly but this rate is apparently lower than that reported in other studies. For three-year-old trees cultivated on loamy soil in southern Italy, Dichio et al. (2002) found volumes of about 8.6 m3 for the irrigated trees and 5.1 m3 for those cultivated under rain-fed conditions (670 mm/year of rainfall). In our case and for trees of the same age, roots explored a volume of 3.65 m3 only. This extension represents, according to Fernandez and Moreno (1999), Doussan et al. (2003), Fernandez et al. (2003) and Connor and Fereres (2005), the plants’ evolutionary response to the spatio- temporal variability. It explains, in our case, the lateral spread of roots and the depths they achieve; soil characteristics (clay) and its mecha- nical resistance may adversely affected root explora- tion. Increases in soil strength during the summer months, as a consequence of occasional water shortage (interval between irrigations varying between 20 and 50 days), may have reduced the average number of laterals developed on the primary axes. During the fol- lowing years (fifth and sixth years) the application of drip irrigation led trees to limit their root development, confining most roots to the upper layers with a noti- ceable root concentration observed for both east and south directions close to the trunk. An increase of root density is however observed with average values varying between 0.001 and 0.670 cm cm-3 depending on depth, distance to trunk, direction and tree age. Similar values ranging between 0.1 and 1.0 cm cm-3 were reported by Connor and Fereres (2005). Greater values of up to 0.5 cm cm-3 were recorded in the first 60 cm and at 0.4 m from the trunk. Values of root den- sity then decreased, significantly as distance to trunk increased (except some measurements for two- and three-year-old trees). Roots were less frequent at all depths outside the canopy limit and particularly for the deeper layers. Nunez-Aguilar et al. (1980) observed similar results for 12-year-old ‘Manzanilla’ olive trees with highest values of about 0.7 cm cm-3 at 0.45 m from the trunk. For seven-year-old olive trees growing with only 150 mm mean annual rainfall, Abd-El-Rah- man et al. (1966) also found maximum root densities in the top layers at 0.15 - 0.30 m and up to 0.3 m from the trunk. These results show good concordance between soil profiles made for the six experimental trees and their root density distribution, having agronomic appli- cations, since they could be used to manage more effi- ciently irrigation and also fertilization. Water and ferti- lizer supplies should be given at these distances from trunks for young trees to guarantee their efficacy. Many factors are cited to explain root density distri- bution (Fernandez and Moreno, 1999) amongst the cul- tural practices are reported in most papers. In our case, the six-year-old tree provided the highest values with average density of 0.303 cm cm-3, however the root densities recorded for the three-year-old plant were greater than those observed for the older trees. Genetic factors inherent to the potentialities of that tree may be involved (Michelakis and Vougioucalou, 1988). Howe- ver, it seems that the most influential factor that affec- ted root density is the heterogeneous distribution of water in the orchard. Results showed spatial variability of soil moisture with lower differences between mea- surements near the emitters (values ranging between 32 120 and 38% depending on depth) and larger variation between measurements at the limit of the canopy. This result was unexpected, but it may indicate lower rates of root absorption around the emitter despite the high densities observed at this distance from trunk (0.4 m). For such a situation, Fernandez and Moreno (1999) indicated that sites of maximum root density may coin- cide with low root activity as a compensation mechani- sm; thus root activity may be higher in zones of low root density than in zones of high density. The influence of soil water content on root distribu- tion is reported by Fernandez et al. (1991), who obser- ved that adequate watering makes roots continue to grow during the dry season, thus, increasing the period of their activity and preventing their shrinking during this period. Palease et al. (2000) and Bongi and Pal- liotti, (1994) indicated that root extension depends lar- gely on the distributed water amounts and the irrigation frequency. Larger volumes of water would favor the existence of wider wet bulbs and could increase root length density. In opposite, low water availability can slow down root growth because roots are able to sense the soil dryness and order stomata to close; thereby reducing water losses and preventing excessive water stress. Water shortage may also increase mortality of fine roots even in the irrigated orchards; roots develo- ped outside the wetted area during the rainy period may die. Root distribution and densities are also highly dependent on leaf area and canopy development. In fact, this trial shows that olive tree establishes equili- brium between root and canopy development rapidly, around the fourth year after planting despite the larger extension of roots observed during the first two years in comparison to canopy growth. Such increases in root area could be explained as a need to adequate the root system to a more vigorous canopy development. Inver- sely, greater leaf area could provide greater total car- bohydrates reserve for root activity. This relationship between leaf and root is very important to consider because dry soil conditions determine a cumulative effect over the years which indirectly affected root acti- vity through an integrated chemical and hydraulic signaling mechanism controlling leaf water relationshi- ps, as stated by Fernandez and Moreno (1999). It could be represented by the under-/above-ground ratio. Our results show high values of this ratio during the first year after planting, indicating a greater availability of water per unit of leaf area. However, beginning from the second year after planting this ratio decreased rapidly to attain a minimum value of 0.9. Dichio et al. (2002) explains that a decrease of root-canopy ratio is a consequence of lack of water during the growing phase, which led plants to several physiological modi- fications; thus it can be used as an indicator of tree adaptation to water shortage. Other reasons could be evoked to explain the decrease of this ratio such as the establishment of com- petition for nutrients between shoots, roots and fruits, which are considered the strongest sinks. The establi- shment of such competition is important to insure a balanced development of the tree, once it begins to set fruits. During this period of youth and first-fruit-set, the tree re-orientates the mobilization of carbohydrates (Proietti and Tombesi, 1996; Palease et al., 2002) and high amounts of assimilates are drain to growing oli- ves against the competing demand of the growing roots and shoots. As a results, the number of roots and their length could be reduced because their growth remain highly dependent of the available assimilates. Under adequate watering conditions, olive trees seem to be able to adjust their root systems to the lar- ger above-ground development during the winter rest essentially when no (or low) competition with other organs occurs. Such result was reported by Palease et al. (2002) and Connor and Fereres (2005) who indica- te that this feature is well represented by the root length /leaf canopy area ratio. In our experiment we found a value of 2.3 km m-2 of leaves for the six-year-old tree. This ratio is concordant with the optimum values of 2.2 - 2.9 km m-2 which were determined for intensive plan- tations (Connor and Fereres, 2005), and this result is very important for the current work because it indica- tes that olive trees were adequately irrigated. Such management of water ensured good development of the root system and the canopy despite the difficulties involved in fixing the irrigation amounts for such young trees with regard to the incomplete soil covera- ge and root development. For such young orchards, it is known that only a fraction of rainfall water is accessible to trees. Thus, the water balance equation should consider only the area concerned by tree transpiration i.e. where roots are active. This area is assumed to be circular and results show that it increases following a logistic-shaped curve (Masmoudi et al., 2007). This factor was taken into account to develop a mathematical model which allows estimation of irrigation needs of young trees, based on the study of the root/canopy over a long period of time. In this model a supply ratio was determined as shown previously in order to link the water supplied to the evaporative demand that takes into account only the tree-related quantities. Results show that the ratio of applied irrigation to reference evapotranspiration during the dry season from April to August was very low. It increased from 0.02 to 0.14 when trees grew from one to six years. When using the volume method to calculate the irrigation and precipitation falling on the area covered by roots, the supply ratio comes very close to Kc-FAO. Estimation of effective precipitationremains, however, a big challenge for using the propo- sed method. The present study provides preliminary results on root distribution of young olive trees of cultivar Ché- toui, which could give insight into the efficient mana- gement of intensive orchards. However, these results 121 should be enhanced by root activity observations. Furthermore, development of a more detailed study on young trees would be useful to get more information on the relationship between root activity and root distribu- tion because sites of heavy root density may present lower root activity, even in young trees. In such study, the root system should be viewed as a population of roots with varying, although coordinated, morphologi- cal and physiological properties. Measurements of car- bohydrate status at different stages of development at both root and canopy levels would also improve these results and give us more valuable information on the relationship between the rooting system distribution and canopy development, essential for irrigation requi- rement estimation as they determine evapotranspiration and water available for the root system. More know- ledge is needed on root growth in young trees because they are more vulnerable to water shortages. For such trees, and particularly plants obtained from rooted cut- tings, water uptake remains highly dependent on the effective areas of transpiration and water absorption. Thus, it is probably more convenient to consider eva- potranspiration, rainfall and irrigation in terms of volu- me of water/tree instead of mm. 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