Agricultural Water Management 77 (2005) 296–307 www.elsevier.com/locate/agwat Comparison of changes in stem diameter and water potential values for detecting water stress in young almond trees P.A. Nortes a, A. Pérez-Pastor a,b, G. Egea a, W. Conejero a, R. Domingo a,b,* a Dpto. Producción Vegetal. ETSIA, Universidad Politécnica de Cartagena (UPCT), Paseo Alfonso XIII, 52, E-30203 Cartagena, Spain b Unidad Asociada al CSIC de Horticultura Sostenible de Zonas Áridas (UPCT-CEBAS), Spain Accepted 1 September 2004 Available online 16 March 2005 Abstract Trunk diameter fluctuations (TDF) and the leaf water relation parameters, predawn and midday leaf water potential (Cpd and Cmd), midday stem water potential (Cst) and midday leaf conductance (gl) were compared for use in detecting water stress and for helping with irrigation management in young almond trees. TDF were monitored throughout 2002 in three irrigation treatments: T1 (control), irrigated at 120% of crop evapotranspiration (ETc), T2, at 60% ETc for the whole year and T3, irrigated at 100% ETc except between 3 June and 15 September, when 40% ETc was supplied. The annual reference crop evapotranspiration and rainfall during the experiment were 1375 and 320 mm, respectively, while the amount of water applied in the treatments was 3165, 1525 and 1430 m3 ha 1 year 1 for T1, T2 and T3. These irrigation treatments had Cst that varied over the season from around 0.65 to 1.0; 0.9 to 1.3 and 0.7 to 1.5 MPa, respectively. Two parameters were obtained from the TDF measurements: (i) maximum daily trunk shrinkage (MDS) and (ii) trunk growth rate (TGR). The MDS values were influenced by the evaporative demand and varied greatly between trees (CVaround 10–35%). Although all the water status indicators showed a response to the water supply, the greatest changes were observed in Cpd, Cst and TGR. The results obtained indicate that MDS and TGR were sensitive to water stress and that TGR is the most useful parameter for quantifying water deficit intensity and duration, its behaviour being very similar to that of Cpd and Cst. * Corresponding author. Tel.: +34 968 325445; fax: +34 968 325435. E-mail address: rafael.domingo@upct.es (R. Domingo). 0378-3774/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2004.09.034 P.A. Nortes et al. / Agricultural Water Management 77 (2005) 296–307 297 It is concluded that TGR is useful as an indicator of stress and could serve for making irrigation decisions in young almond trees. # 2005 Elsevier B.V. All rights reserved. Keywords: Almond trees; Deficit irrigation; Plant water status; Trunk diameter fluctuations; Tree growth 1. Introduction In Spain almond is mainly cultivated in the Mediterranean area, where the scarcity and irregularity of the rainfall, in conjunction with the high evaporative demand, are the cause of pronounced seasonal water deficits. In these conditions, water becomes the main factor in growth and production. Almond is a nut tree species which is very adaptable to a wide range of water availability, although yields may be seriously affected. Girona and Marsal (1995) pointed to dryland/irrigated crop yield ratio of 1/10, which goes a long way to explaining the increased use drip irrigation systems in commercial almond tree orchards. Irrigation scheduling is very important for optimising the use of water resources but, before any decisions can be taken as regards the timing and amount of water to be supplied, the crop evapotranspiration (ETc) (Allen et al., 1998) and/or soil and plant water status (Campbell and Mulla, 1990; Hsiao, 1990) must be known, as should the vegetative and productive response of the crop in question to the provision of water. Plant water status monitoring offers an important source of information for irrigation management. Many researchers have found a strong correlation between predawn leaf water potential (Cpd) and transpiration (Shouse et al., 1982) and a dependence on soil water status (Ritchie and Hinckley, 1975). This information has led to several authors proposing a critical Cpd value for initiating irrigation. On the other hand, McCutchan and Shackel (1992), Shackel et al. (1997) and Naor (2000), recommend using Cst to determine plant water status in field conditions and as a useful index of irrigation needs in fruit trees. However, the inconvenience involved in measuring Cpd before sunrise or the need to cover the leaf for 2– 3 h before Cst measurements might have curtailed the adoption of these techniques for programming irrigation in commercial orchards. Accordingly and despite the variety of irrigation scheduling criteria that exists, none is totally satisfactory when managing high frequency irrigation systems in tree crops (Fernández et al., 2002). Hence, several groups in recent years have attempted to establish more precise irrigation protocols for these conditions. Within this research, the techniques that have seen the most notable improvements have been measurements of: (i) continuous trunk diameter fluctuations (TDF), using lineal variable displacement transducers (LVDT) and (ii) sap flow in the same organs using heatpulse or heat-balance techniques. The usefulness of different parameters derived from continuous trunk and main branch diameter measurements was analysed by Goldhamer and Fereres (2001). These authors found that TDF could be considered as a valid technique for detecting water stress in peach and almond and could perhaps be considered for establishing precise irrigation schedules (Goldhamer and Fereres, 2004). Furthermore and unlike Cpd and Cst, TDF-derived indices have the advantage that they can be considered of practical applicability in automatic irrigation scheduling. 298 P.A. Nortes et al. / Agricultural Water Management 77 (2005) 296–307 The type of scheduling based on plant indicators would be particularly suited to those cases in which water stress has to be avoided at all times. Such is the case with young plantations, where the objective is to maximise growth so that trees can mature as fast as possible, which implies the avoidance of even mild water deficits. Equally, it would be of great interest in controlled deficit irrigation, a practice that requires robust indicators capable of the early diagnosis both of the appearance and assessment of water stress situations and of recovery from these conditions. Recent research has identified the most sensitive and reliable plant water status indicators for young trees (Goldhamer and Fereres, 2001 and Remorini and Massai, 2003, in peach; Moriana and Fereres, 2002, in olive; Naor and Cohen, 2003, in apple) in response to withholding irrigation. These irrigation regimes are clearly different from the conditions described in this work that is why our results are not easily comparable. The aim of the study described in this paper was to evaluate the use of LVTD sensors for recording trunk diameter fluctuations compared with discrete measurements of plant water status for detecting water stress in young almond trees exposed to slight and moderate long term water deficit. 2. Materials and methods 2.1. Plot and irrigation treatments characteristics The experiment was carried out in 2002 in a 1 ha plot planted in December 1999 with ‘Marta’ almond trees (Prunus dulcis (Mill.) Webb) grafted on ‘Mayor’ rootstock. The trees were spaced 7 m  6 m apart and drip-irrigated by four pressure compensated drippers per tree, each with a flow rate of 4 L h 1. The orchard is located in the province of Murcia (SE Spain), where the climate is semiarid Mediterranean. The annual rainfall and reference crop evapotranspiration, ETo (Penman-Monteith) during the experiment were 320 and 1375 mm, respectively. The soil is a deep silt–clay–loam with an available water capacity of about 0.18 m m 1 and bulk density ranging from 1.3 to 1.55 Mg m 3. The soil is poor in both available potassium and organic matter, but rich in phosphorus and does not present salinity problems (EC of the saturation extract 1.4 dS m 1). The irrigation water used had an average electrical conductivity (EC25 8C) of 1.2 dS m 1, and an average chloride and sodium content of 4.6 and 4.41 meq L 1, respectively. Trees were fertilised with 30–30– 50 kg ha 1 year 1 of N, P2O5 and K2O, respectively. A routine pesticide programme was maintained. No weeds were allowed to develop within the orchard, resulting in a clean orchard floor for the duration of the experiment. Trees were pruned manually in December each year. Three irrigation treatments were applied according to a randomised block statistical design, with three blocks. These were: T1, irrigation at 120% of the crop evapotranspiration (ETc) measured in six drainage lysimeters planted with almond trees of similar size to the rest the orchard; T2, irrigated at 60% ETc all year and T3, irrigated at 100% ETc except between 3 June and 15 September when 40% ETc was supplied, coinciding with a high evaporative demand period, less water available for irrigation and high trunk growth rates. P.A. Nortes et al. / Agricultural Water Management 77 (2005) 296–307 299 Irrigation frequency was the same for all the treatments except between 1 July and 15 September, when frequency was 14, 7 and 6 times a week for T1, T2 and T3, respectively. The water amounts applied in the treatments were 3165, 1525 and 1430 m3 ha 1 year 1 for T1, T2 and T3, respectively. 2.2. Measurements The soil matric potential (Cm) was measured using tensiometers at depths of 30, 60 and 90 cm, located perpendicular to the dripline and at 30 cm from a representative emitter on three blocks. Reading were taken between 8–9.00 h, depending on the time of the year, and just before irrigation. Predawn and midday leaf water potentials and midday stem water potential (Cpd, Cmd and Cst) were measured every two weeks on 12 mature leaves for each treatment (two leaves per tree on six trees per treatment), with a pressure chamber (Soil Moisture Equip. Corp, model 3000). Cst was measured taking single leaves near the trunk that had been covered with a small bag of black polyethylene covered by silver foil for at least 3 h prior to measurements. In all cases the recommendations of Hsiao (1990) were followed. Leaf conductance (gl) was measured, on a similar number of leaves as Cmd, using a LI-COR LI1600 steady-state porometer. Cmd and gl were measured on the same sun-exposed leaves. TDF was measured with LVDTs mounted on holders built of aluminium and ‘INVAR’ alloy. A single sensor was attached to the trunk of each tree 40 cm above the ground (six trees per treatment). The LVDTs were located on the northwest side of the trunk and the measurements were taken every 10 min by MicroIsis system. The maximum daily shrinkage (MDS) was calculated as the difference in diameter between the maximum, in the morning, and the minimum, in the afternoon (Goldhamer and Fereres, 2001). Trunk growth rate (TGR) was calculated by subtracting consecutive daily maximum diameters. The LVDTs had to be repositioned throughout the experiment, because the 5000 mm working range of the MicroIsis was exceeded by tree growth. The mean trunk diameters at the beginning of the experiment were about 5 cm. 3. Results and discussion The soil matric values (Cm) were similar in all three treatments until mid-April, when there was a change in the pattern recorded (data not shown). In T2 this was reflected by a pronounced fall in Cm until mid-May, after which it remained almost constant. T3 showed slightly lower values than T1 until the beginning of the deficit period, after which they were below the measurement range of the tensiometer at the three depths measured during the time that irrigation was at 40% ETc (June–mid September). In T1, the mean Cm values during the irrigation season were 12, 18 and 23 kPa at 30, 60 and 90 cm depth, respectively, which can be considered as reflecting a suitable level of water supply for maximum growth. The direction of flow determined by the hydraulic gradient (DCh/Dz) was downward, indicating that there was a certain degree of drainage. In T2 the mean Cm values during the period of high evaporative demand (June–October) were 60, 68 and < 80 kPa at 30, 60 and 90 cm depth, respectively, values considered by some authors 300 P.A. Nortes et al. / Agricultural Water Management 77 (2005) 296–307 (Fereres et al., 1981; Pogue and Pooly, 1985) as capable of producing a significant water stress, given the small portion of moistened soil (15%). After re-establishing irrigation at 100% T3 values recovered to reach those of T1 at the three depths within 10, 21 and 45 days, respectively. Seasonal patterns of predawn and midday leaf water potential (Cpd and Cmd), midday stem water potential (Cst,) and midday leaf conductance (gl) values in three irrigation treatments, are shown in Fig. 1. As can be seen, the Cpd values of T1 were close to 0.4 MPa throughout, values similar to those obtained by Torrecillas et al. (1996) and Marsal et al. (1997) in well irrigated almonds. From the middle of June (T2) and July (T3), Cpd values differed significantly from those recorded in the control, a difference that was maintained until the end of September (differences of 0.13 and 0.29 MPa in T2 and T3, respectively, with respect to the control). Clear differences in Cpd were also evident between both deficit treatments from mid-July to the end of September. The Cpd values during the growing season, together with the Cm values observed in the different treatments, show that leaf water potential at predawn depends on soil moisture conditions and is a good indicator of these conditions (Ritchie and Hinckley, 1975; Aussenac and Valette, 1982). The Cst values in T1 varied from 0.65 to 1.0 MPa (Fig. 1), levels similar to those obtained by Shackel et al. (1997) and Fereres and Golhamer (2003) in almonds under non-limiting soil water conditions. The T1 trees were irrigated in excess of their requirements to ensure their responses were affected only by the aerial environment. The two deficit treatments (T2 and T3) generated mild and moderate water stress respectively, as indicated by their Cst values, which varied over the season from around 0.9 to 1.3 and 0.7 to 1.5 MPa, respectively (Fig. 1). These values and their low degree of variability are similar to those obtained in other trials where similar water restrictions were imposed Fig. 1. Mean values of predawn (Cpd) midday stem (Cst) and midday (Cmd) leaf water potential and leaf conductance (gl) for the different irrigation treatments. Asterisks indicate the beginning and end of 40% ETc period in T3. Vertical bars represent S.E. of the mean (not shown when smaller than the symbols); n = 12. P.A. Nortes et al. / Agricultural Water Management 77 (2005) 296–307 301 (Intrigliolo and Castel, 2004). Cst behaved in a similar way to Cpd, although the relative values were slightly lower (Fig. 2). This similarity in the behaviour of both parameters confirms the interest of the midday Cst as an indicator of water stress in young almonds, as has been suggested by some researchers (McCutchan and Shackel, 1992; Naor, 2000) for fruit trees, because of its high sensitivity to the irrigation regime. Shackel et al. (1997) indicated that midday stem water potential can be used to reliably quantify stress. As regards Cmd, the high degree of variability did not produce clearly significant differences between treatments, except during August, when the values fell to 1.92, 2.6 and 2.7 MPa in T1, T2 and T3, respectively, with no significant differences between the two deficit treatments (Fig. 1). Besides, relative (Cmd) values were the lowest and weakly related with the values corresponding to leaf conductance (Fig. 2). Leaf conductance (gl) of fully-illuminated spur leaves showed mean values of 215 and 180 mmol m 2 s 1 in T1 and T2, respectively, during the irrigation season and 136 mmol m 2 s 1 in T3 during the deficit period (Fig. 1). Although relative gl values (control/deficit) were similar to those of Cpd and Cst (Fig. 2), differences between T1 and T3 were only significant during the final period (July–September) as a consequence of the greater variability in the measurements. To illustrate the data obtained by the LVTDs, Fig. 3 shows the mean TDF for each treatment recorded during six consecutive days in each month from February to October. Fig. 2. Relative changes of predawn (Cpd) midday stem (Cst) and midday (Cmd) leaf water potential and maximum daily shrinkage, MDS (Tn/T1), and leaf conductance (gl) and trunk growth rate, TGR (T1/Tn). The vertical bars represent rainfall. Asterisks indicate the beginning and end of 40% ETc period in T3. 302 P.A. Nortes et al. / Agricultural Water Management 77 (2005) 296–307 Fig. 3. Mean trunk diameter fluctuation for the six days indicated between parentheses, for T1 (—),T2 (—) and T3 (– – –). As can be seen, cambium activity begins at the end of February, coinciding with maximal flowering and the emergence of new shoots and leaves. Cambial expansion continues until the end of October, with a relatively constant rate of growth until the end of August. Growth is reduced in September and to an even greater extent in October, coinciding with the first symptoms of leaf senescence (Table 1 and Figs. 3 and 4). TGR values for the different Table 1 Monthly mean trunk growth rate for the different irrigation treatments TGR (mm day 1) T2 T1 February March April May June July August September October 0.07 0.15 0.19 0.23 0.21 0.22 0.22 0.14 0.02 a a a a a a a a a 0.05 0.18 0.18 0.20 0.22 0.17 0.16 0.14 0.01 T3 a a a a a b b a a 0.05 0.19 0.18 0.20 0.21 0.16 0.13 0.09 0.01 a a a a a b b b a Values followed by different letters indicate statistically significant differences between treatments according to Tukey0.05 test. P.A. Nortes et al. / Agricultural Water Management 77 (2005) 296–307 303 Fig. 4. Mean trunk diameters recorded by LVDT sensors during the experiment for the different treatments. irrigation treatments were practically identical from March to June, with mean monthly values of 0.2 mm day 1 (Table 1). There was a clear diminution in these values in July and August in T2 as a result of the irrigation regime, a diminution that extended to September in T3 (Table 1). During the high evaporative demand period, the TGR values relative to the control were similar to those of Cpd and Cst, with statistically significant differences between the control and the deficit treatments for most days (Fig. 2). However, there were no statistically significant differences between the deficit treatments except on certain days in September, when T2 values showed similar growth to T1. The pattern of trunk diameter growth was characterised by a sigmoid curve type growth function. Overall trunk growth was greater in the control treatment than in the deficit treatments, the mean annual growth in diameter according to the LVTD sensors being 41.6, Fig. 5. Seasonal patterns of maximum daily shrinkage (MDS) in young almond trees for the different irrigation treatments. 304 P.A. Nortes et al. / Agricultural Water Management 77 (2005) 296–307 37.0 and 33.2 mm for T1, T2 and T3, respectively (Fig. 4). This represents a reduction in growth of 11% in T2 and 20% in T3 with respect to the control. T2 and T3, which used similar volumes of water, showed clearly different reductions in trunk growth, underlining the more pronounced effect of the intensity of water stress rather than its duration. MDS values increased steadily from the beginning of March to the middle of June, were relatively stable during the summer months and decreased from September onwards (Fig. 5). The steady increase in MDS, which coincided with a growth in shoot length, was probably due to the increase in leaf area, higher temperatures and greater transpiration during this period of the year. Ginestar and Castel (1996) in young clementine citrus and Huguet et al. (1992) in apple trees observed a similar effect. Furthermore, the MDS under non-limiting soil water conditions was moderately correlated with daily ETo (R2 = 0.479), although to a lesser degree than was observed between Cst and daily ETo (R2 = 0.827) in identical conditions (Fig. 6). The highest values coincided with greater evaporative demand. The deficit treatments showed higher MDS value from June to August (Fig. 5), Fig. 6. Relationship between midday stem water potential (Cst) and maximum daily trunk shrinkage (MDS) with 0.35 (R2 = 0.827); reference evapotranspiration (ETo) for the control treatment. Cst = 0.091 ETo MCDT1 = 25.2 + 22.7 ETo (R2 = 0.479). P.A. Nortes et al. / Agricultural Water Management 77 (2005) 296–307 305 although the differences were not significant most days, perhaps as a result of the variability of the determinations. During the tree growth period the coefficients of variation varied between 10 and 35% for the three treatments, similar to the variations observed by Ginestar and Castel (1996). Analysis of the behaviour of TGR and MDS pointed to a closer relationship with water stress condition in the case of the former, which agrees with the findings of Moriana and Fereres (2002) in young olive trees, but differs from the findings of Goldhamer and Fereres (2001) in mature almond trees. Intrigliolo and Castel (2004) found which MDS was more sensitive than TGR during the fruit growth period, but the reverse during post-harvest period in productive plum trees, indicating the important influence of phenology on trunk growth. Hence, the greater or lesser sensitivity of one or the other parameter to water stress will depend on the trunk growth rates. The change in the tendency of the relative TGR values at the end of June coincided when the Cpd and Cst values reached about 0.55 and 1.2 MPa, respectively (Figs. 1 and 2). These water potential values differed significantly from the control values ( 0.4 and 0.93 MPa in Cpd and Cst, respectively), which remained within the range indicated by Shackel et al. (1997) for continuously irrigated almond trees. 4. Conclusions During the study period predawn leaf water potential and the midday stem water potential showed a clear response to the soil water status changes caused by the different irrigation treatments, confirming their great sensitivity to the irrigation regime. Both parameters, therefore, may be considered suitable for quantifying water stress and as useful tools in irrigation management. The parameters, midday leaf water potential, leaf conductance and maximum daily shrinkage, showed the highest degree of variability. However, both the leaf conductance and maximum daily shrinkage data indicated relatively consistent differences in plant water status in all the irrigation treatments. Maximum daily shrinkage was seen to depend on evaporative demand and water stress, although the possible influence of other growth-related factors makes it a less reliable tool for differentiating moderate levels of stress in young almond trees. The trunk growth rate was more indicative of the level of stress than maximum daily shrinkage since, as the soil and plant water status decreased, its values clearly decreased with respect to the control in the deficit treatments. It can therefore be considered as a useful indicator of stress for making irrigation decisions in almond trees during the period when the trunk grows rapidly. Acknowledgements The study was supported by CICYT (AGL2000-0387-C05-05) and F. Séneca (AGR-20FS-02) grant to the authors. P.A. Nortes was a recipient of a research fellowship from the Ministerio de Educación, Cultura y Deporte of Spain. 306 P.A. 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