Plant and Soil 260: 169–181, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.
169
Effects of regulated deficit irrigation under subsurface drip irrigation conditions on vegetative development and yield of mature almond trees Pascual Romero1 , Pablo Botia1,2 & Francisco Garcia1 1 Instituto
Murciano de Investigaci´on y Desarrollo Agrario y Alimentario. Estaci´on Seric´ıcola 30150. La Alberca, Murcia, Spain. 2 Corresponding author∗
Received 6 August 2003. Accepted 31 October 2003
Key words: Prunus dulcis, regulated deficit irrigation, root distribution, subsurface drip irrigation, vegetative development, water use efficiency, yield
Abstract The influence of several regulated deficit irrigation (RDI) strategies, applied under subsurface drip irrigation (SDI), on vegetative development and yield parameters in mature almond (Prunus dulcis (Mill.) D.A. Webb, cv. Cartagenera) trees was analysed during a 4-year field experiment. Five treatments were applied: T1 (100% crop evapotranspiration (ETc), full season); T2 (irrigated at 100% ETc except in the kernel-filling stage (20% ETc)); T3 (equal to T2 but in SDI); T4 (SDI, 100% ETc, except in the kernel-filling stage (20% ETc) and post-harvest (75% ETc)); T5 (SDI, 100% ETc except in the kernel-filling stage (20% ETc) and post-harvest (50% ETc). A close correlation between applied water, plant water status (pd) and tree growth parameters was observed. After four years, the vegetative development in T5 was reduced significantly due to a larger annual cumulative effect of water stress on growth processes, resulting in a smaller tree size (trunk and branch growth, canopy volume and pruning weight) compared to other treatments. Moreover, water stress during kernel-filling produced a significant reduction in the leaf expansion rate and a stimulation of premature leaf abscission, resulting in a smaller tree leaf area in this treatment. SDI produced a greater horizontal distribution of fine roots in the soil profile than surface drip system. The RDI practices applied under subsurface drip irrigation stimulated a deeper root development (40–80 cm) than surface treatments (0–40 cm), producing also a higher root density in the subsurface treatments watered the least (T4 and T5). Water stress during the pre- and post-harvest periods had no important effect on bud development, bloom, fruit growth or fruit abscission. Moreover, there were no significant reductions in kernel dry weight or kernel percentage. Reductions in kernel yield were significant compared to T1, being of 11% in T2, 15% in T3, 20% in T4 and 22% in T5. Water use efficiency (kg m−3 ) was increased significantly in the SDI treatments T4 and T5. A significant correlation between kernel yield and tree growth parameters was observed. We conclude that the application of overall reductions of water use of up to 50% during high water stress sensitivity periods (such as post-harvest) under SDI system, is a promising alternative for water management in semiarid regions in order to improve water use efficiency. Nevertheless prolonged water stress during kernel-filling and post-harvest can reduce excessively the vegetative development of almond, negatively affecting the long-term yield response.
Introduction Spain is the Mediterranean country having the greatest production of almonds (Prunus dulcis (Mill.) D.A. Webb) and is ranked second in the world, accounting for 17% of world production, being the main ∗ FAX No: +34 968 366792. E-mail:
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competitor of the USA (Alston et al., 1993). Although the area of land dedicated to almond is greater in Spain than in the USA, 629100 ha (AEA, 1999) as opposed to 165000 ha, the kernel yield is ten times higher in the USA than in Spain, with the USA having mean values of between 1200 and 1800 kg ha−1 , as opposed to 150 kg ha−1 in Spain (Tous Martí, 1995). In Spain, the traditional almond plantations are gen-
170 erally located in marginal areas, on poorly drained soils and with no irrigation supply. This fact has resulted in plantations of low profitability, that have not made use of new irrigation technologies and crop practices. To improve this situation in semiarid regions of Spain, studies are being carried out to increase the performance of these orchards, applying new practices (for example, regulated deficit irrigation, RDI) and irrigation technologies (for example, subsurface drip irrigation). In these areas, with limited water resources, high temperature and low relative humidity, RDI can be a promising alternative (Salazar and Melgarejo, 2002) and has stimulated research to determine the sensitivity of almond production to specific periods and levels of water stress. Some studies have determined that almond is a drought-tolerant crop, indicating that almond yield is relatively insensitive to mild or moderate water stress during the kernelfilling stage (dry weight accumulation in the kernel) (Girona and Marsal, 1995; Goldhamer and Shackel, 1990; Goldhamer, 1996). However, the same studies indicate that it is necessary to avoid water stress during active vegetative and fruit growth periods. Almond productivity may be also vulnerable to water stress during the post-harvest period because reproductive bud differentiation occurs late (early August–early September) (Goldhamer and Smith, 1995; Goldhamer, 1996; Goldhamer and Viveros, 2000). It is important also to consider long-term effects of water stress for spur-bearing species such as almond, due to reduced vegetative development and renewal of fruiting positions (Prichard et al., 1992; Esparza et al., 2001). The use of subsurface drip irrigation (SDI) systems, applying water below the soil surface directly to the root zone and minimising soil evaporation, has been used also to save water (Camp, 1998; Lamm, 1995; Phene, 1999) and improve water use efficiency in semiarid regions (El Gindy and El Araby, 1996; Phene, 1993). In almond, the use of SDI has produced a higher water application efficiency, for both soil and plants (Botía et al., 1998, 2000; Del Amor and Cerdá, 1997), and a better yield response than surface drip irrigation, achieving greater kernel yield and water use efficiency (Del Amor and Del Amor, 1999). However in different soil conditions, Schwankl et al. (1999) have proposed that the utilisation of other irrigation systems (surface drip or micro-sprinkler) is advantageous for growth and yield. Until now, there have been no studies in almond about the application of RDI under subsurface systems. Recently, Romero et al. (2004), combining both techniques (SDI and
RDI), reported that SDI promoted rapid recovery of soil water status after severe water stress, maintaining adequate levels of soil water content and plant water status post-harvest and producing a higher water application efficiency than surface systems. The aim of this work was to determine the effects of the application of several RDI strategies with buried drip irrigation on vegetative development and yield of mature almond trees in a four-year study. Specifically, we examined the long-term effects of water stress during the pre-harvest and post-harvest periods on leaf development, leaf abscission, shoot growth, root growth, yield determinants and the interrelationships between them and plant water status. Moreover, we evaluated the long–term yield response and the water use efficiency of the almond orchard under these irrigation conditions.
Material and methods Plant material, treatments and experimental conditions The experimental design has been described in Romero et al. (2004). Briefly, an irrigation experiment was carried out from 1997 to 2000, in a commercial orchard of 13-year-old almond trees (cv. Cartagenera, grafted on almond rootstock), on drip-line irrigation. The plantation is situated in the district of Aljorra, Murcia (Southern Spain). The orchard comprised rows of cv. Cartagenera (70%) planted alternately with polliniser rows of cv. Ramillete (30%). The weather is Mediterranean semi-arid, with mean annual precipitation at the site being 280 mm, confined to autumn and spring. The trial involved five irrigation treatments that were applied during four consecutive years, using two irrigation systems, surface and subsurface drip, and three different regulated deficit irrigation strategies: T1, 100% crop evapotranspiration (ETc) full season; T2 irrigated at 100% ETc except in the kernel-filling stage (20% ETc); T3 equal to T2 but in SDI; T4, SDI, 100% ETc, except in the kernel-filling stage (20% ETc) and post-harvest (75% ETc)); T5, SDI, 100% ETc except in the kernel-filling stage (20% ETc) and post-harvest (50% ETc). The lay-out of the experiment has been described in detail in Romero et al. (2004). Control (T1) was irrigated at 100% crop evapotranspiration (ETc) (ETo calculated via Pan evaporation Class-A method, U.S. Weather Bureau)
171 over the entire crop season. Irrigation was applied daily in short pulses, once or twice a day (high frequency irrigation) and was controlled and adjusted weekly according to soil matric potential and daily climatic data from a weather station in the vicinity (1 km) of the experiment. In both irrigation systems, a drip line was utilised with four self-compensating drips (type RAM, 3.5 L h−1 ) per tree, 1 m apart. In the subsurface drip irrigation system, the drip line was buried at 35 cm depth. A root growth-inhibiting chemical, trifluralin, (dinitro-N, N-dipropyl-4 trifluoro methylanidine) was used in the filtration system for avoiding root intrusion in the buried drips. Soil characteristics and climatic parameters (annual rainfall and reference evapotranspiration (ETo)) at the experimental site, annual applied water for each treatment, Kc (crop coefficients) and the fertilization program applied have been described in Romero et al. (2004). Water stress measurements Leaf water potential at dawn (pd) was measured before actual sunrise by using a pressure chamber (model 3000; Soil Moisture Equipment. Corp., Santa Barbara, California, USA), according to the Scholander et al. (1965) and Turner (1988) technique. Two mature, but not aged, leaves from the middle third of the tree were taken in twelve trees per treatment (six measurements per treatment and plot). Vegetative development measurements Trunk circumference was measured monthly for twelve trees per treatment during the experimental period. Six measurements per tree were taken at 50 cm above the soil surface (20 cm above the graft union). Transverse cross-sectional area was calculated according to Mitchell and Chalmers (1982). Tree canopy height and diameter (across and within rows (N-S, E-W)) were measured monthly for twelve trees per treatment. Canopy volume was calculated according to Hutchinson (1978): Volume = (width2× height)/2. Pruning weight was determined annually (end of November) in the same trees, independently for each tree. During 1997 and 1998, terminal growth of four young branches per tree, one from each compass direction, was measured on four trees per treatment (16 branches per treatment) every week (from March to November). The length and diameter were meas-
ured with a digital calliper (Mitutoyo MTI, Corporation, City of Industry, CA). Leaf abscission was monitored in four trees per treatment, starting with the dry weight of fallen leaves (65 ◦ C, 24 h) collected periodically over the entire growth season in sieve boxes (mesh grade = 1 mm), quadrangular in shape (side = 2 m, height = 1 m), which occupied approximately one quarter of the total area spanned by the tree. Abscission over the stress period was expressed as a percentage of the total dry weight of accumulated fallen leaves during the year. In 1998 and 1999, leaf area and the number of canopy leaves per cubic metre, at the point of greatest stress, were determined via the sampling of a known volume of leaves (Boland et al., 1993), using cylinders of 20-cm diameter and 15-cm height, at various points throughout the entire three dimensions of the canopy and at different angles, at the rate of six per tree, with one tree per treatment per plot. Leaf area per tree (m2 m−3 ) was calculated taking into account the canopy volume. From May to July 1999 (a representative year), leaf growth rate (length and width) was measured weekly in eight young (of recent appearance) leaves per treatment and per plot, in the middle third of the tree, at different angles. The measurements of maximal width and length were recorded with a digital calliper (Mitutoyo, MTI Corporation, City of Industry, CA). Relative growth rate (RGR) was calculated according ln M2 − ln M1 ; M being the to the equation: RGR = t2 − t1 value of the growth parameter measured and t the time between measurements. Root distribution measurements After three years of the experiment (1999), root samples were taken by soil cores (100 cm3 ) using an auger. The samples were taken close to the tree, perpendicularly to an emitter at 25, 50 and 75 cm from the drip head, at 10-cm intervals to 100 cm depth, for a total of 4 trees per treatment (one per replicate). The roots were washed and shaken in water and sodium hexametaphosphate (30 g L−1 ) to separate the fine roots from the soil particles. Roots were filtered and dried in an oven at 80 ◦ C for 24 h. The density of fine roots (diameter < 1 mm) was expressed in dry weight per soil volume (mg cm−3 ) (Bielorai, 1982).
172 Bud development, flowering and fruit set In 1998 and 1999, in order to evaluate the effect of the different irrigation treatments on bud development, flowering and fruit set processes, the number of buds (vegetative and flower buds), flowers and fruits were recorded on four marked branches, of 20-mm diameter, per tree (16 branches per treatment). The density of buds, flowers and fruits was expressed as number per metre of branch length (Johnson et al., 1992). Fruit set was calculated 75 days after full bloom as the percentage of flowers set and grown into nuts. In 1997, 1998 and 1999, during the fruit growth phase (March–July), 40 fruits per treatment (10 per replicate) were sampled weekly to determine diameter, length and fresh and dry weights of the entire fruit (shell + kernel) and kernel (seed). Also in 1999 (from October 1998 to the harvest in 1999), the abscission of buds, flowers and fruits was periodically monitored in four trees per treatment, in the same sieve boxes (mesh grade = 1 mm) used for collecting the leaves. The number and the fresh and dry weights of fallen buds, flowers and fruits were determined. Yield parameters Individual tree yield was measured annually in three trees per treatment per replicate (twelve trees per treatment) during the experimental period 1997–2000. The harvest of each tree was separated in two fractions, ‘hull-tight’ (no evidence of hull split) and commercial (full hull split). Fresh and dry weights (in shell) were calculated to determine nut load. From the commercial fraction, a 1-kg nut sample was collected and the kernels were separated from the shells and hulls to determine kernel yield, single kernel weight and kernel percentage. Moreover the percentages of double, unfilled and faulty kernels (with visible kernel rot symptoms) were calculated. Regressions and statistical analysis Relationships between parameters were fitted to linear regressions. A variance analysis (ANOVA) was used in order to discern the main treatment effects.
Figure 1. (A), Trunk cross-sectional area growth response to irrigation treatments during the period 1997–2000. Each point is the mean of twelve measurements per treatment (3 trees/plot). The vertical bars indicate the standard error of the mean. (B) Relationship between annual trunk cross-sectional area growth rate and mean annual irrigation water for the period 1997–2000 (quadratic regression, y = −10.33 + 0.056x − 0.0001x2 , r = 0.98; P < 0.001, there was improvement in r 2 for the quadratic compared the linear model).
Results and discussion Shoot growth response The growth rate in the trunk cross-sectional area showed significant differences between treatments (Table 1), revealing different trends in the trunk growth, with a tendency towards greater values of trunk cross-sectional area in the T1, T2 and T3 trees compared to T4 and T5 (Figure 1A). Treatments T2 (surface) and T3 (subsurface), with the same RDI strategy and the same amount of applied water, had similar trunk growth rate (Table 1). Schwankl et al. (1999) recorded similar patterns of trunk growth in almonds under surface and subsurface irrigation sys-
173 Table 1. Absolute growth rate (AGR) in the trunk cross-sectional area from 1997 to 2000. Mean values of branch diameter absolute and relative growth rate (AGR and RGR, respectively) in 1998. Canopy volume and cumulative pruning weight for the experimental period (1997–2000) Trunk cross-sectional area AGR (cm2 day−1 )
Branch diameter RGR (mm mm−1 day−1 )
Branch diameter AGR (µm day−1 )
Canopy volume (m3 tree−1 )
Cumulative pruning weight (kg tree−1 )
Treatments
1997–2000
1998
1998
1997
2000
1997–2000
T1 T2 T3 T4 T5 ANOVA
0.052a 0.047ab 0.050a 0.037bc 0.029c
0.0027ab 0.0035a 0.0039a 0.0018b 0.0020b
16.7ab 20.4a 23.2a 10.6b 10.7b
42.5a 41.5a 38.7ab 37.3ab 35.7b
23.44a 20.75a 21.63a 19.75ab 14.98b
∗∗∗
∗
∗
19.4 22.0 19.9 21.9 19.9 n.s.
∗
∗
n.s. not significant; ∗ P < 0.05; ∗∗∗ P < 0.001. For each column, mean values followed by distinct letters are significantly different; separation by Duncan’s multiple range test at the 95% confidence level.
tems in different soil conditions. A high correlation (r 2 = 0.94) was observed between trunk crosssectional area growth rate and the annual amount of water applied (Figure 1B). This relationship indicates that, in our experimental conditions, when the applied water was below 450 mm (threshold level) the annual trunk cross-sectional area growth rate was reduced considerably. Young branch growth, expressed as mean growth rate of branch diameter, showed significant differences between RDI treatments (Table 1). During kernelfilling (in 1998), T4 and T5 had branch growth rates significantly lower compared to T2 and T3, but similar to T1 (Table 1). In the same way, Hutmacher et al. (1994) reported that branch growth rates declined more in less-watered almonds, although different responses have been reported in almonds under water stress (Girona et al., 1993). The highest rate of growth occurred from March–May for all irrigation treatments (Figure 2A and B), as found also by Hutmacher et al. (1994). While the elongation growth period occurred principally in spring (March, April, May), in accordance with the active vegetative growth phases, the width growth continued during kernel-filling, although more slowly (Figure 2A and B). Similar patterns of branch growth were observed in 1997 (data not shown). Canopy volume growth was affected by irrigation treatments during the experimental period. Canopy volume at the end of the experimental period showed a significant reduction for T5, around 16 and 14% compared to T1 and T2, respectively (Table 1). T2 and T3 showed a similar growth in the canopy volume. Can-
Figure 2. Evolution of the length (A) and diameter (B) of young branches during 1999, for each treatment. Each point is the mean of sixteen repetitions per treatment. Vertical bars represent the standard error of the mean. Long dashed lines indicate the start and the end of the stress period in the kernel-filling stage (early June–early August).
174 into account that pruning costs for almond represent around 10% of total cultural costs (Klonsky and Blank, 1996), a reduced pruning weight could be profitable economically. Leaf growth, leaf abscission and leaf area
Figure 3. (A), Annual canopy volume growth rate as a function of mean annual water applied for the period 1997–2000. The line indicates the overall linear regression (y = 0.0047 + (27 × 10−6 )x, r = 0.95; P < 0.05). (B), Mean annual pruning weight as a function of annual water applied (y = −7.07 + 0.0047x + (3 × 10−6 )x2 , r = 0.98, P < 0.05). Points are based on treatment averages (12 measurements per treatment) for the different years (1997–2000).
opy volume growth rate was correlated linearly with the annual amount of water applied (Figure 3A), as has been described before in water-stressed almond by Torrecillas et al. (1989) and Fereres et al. (1981). Pruning weight also was reduced by RDI, with significant differences between T5 and the rest of the treatmens. There was a close relationship (r = 0.98) between amount of annual water applied and mean annual pruning weight (Figure 3B). Accumulated pruning weight, after four years, was significantly reduced by about 36% in T5 compared to T1 (Table 1). The reduction of pruning weight under RDI may indicate a lower annual tree growth and has been described previously in almond (Girona et al., 1993; Prichard, 1996), pear (Pyrus communis L.) (Mitchell et al., 1989), peach (Prunus persica L. Batsch) (Johnson et al., 1992; Boland et al., 1993) and apricot (Prunus armeniaca L.) (Pérez, 2001). These results indicate the potential of the more severe RDI strategy (T5) to limit tree size and vegetative development. Taking
Water stress during kernel-filling (early June-early August) produced a significant decrease in leaf expansion rate (length and width) in all RDI treatments, with a reduction compared to T1 of around 49% in T2, 39% in T3, 47% in T4 and 50% in T5 (Table 2). Among T2, T3 and T4 there were no significant differences in the leaf expansion rate or leaf size (Table 2). Only the SDI treatment T5 showed a leaf size (single leaf area) significantly lower than the rest of the treatments. We observed an important effect of water stress on leaf abscission, with a premature stimulation of leaf fall in the RDI treatments. There were two periods of the year in which there was a higher abscission of leaves, one at the end of kernel-filling (end of July, greatest stress period), mainly in the RDI treatments due to severe water stress in this period, and the other at the end of November (vegetative dormancy) during which there was a pronounced physiological leaf fall, mainly in the control treatment (Figure 4A). During kernel-filling, the percentage of leaf abscission with regard to the accumulated total was about 36, 30, 25 and 33%, in T2, T3, T4 and T5, respectively, as opposed to 15% in the control (Figure 4A). Leaf abscission was influenced by the level of water stress (pd ) reached in this period, a correlation (r 2 = 0.60) being observed between these two parameters (Figure 4B). Similar patterns have been recorded in stressed almond by Castel and Fereres (1982) and Goldhamer (1996). A smaller leaf size and a higher leaf abscission, due to water stress, resulted in a lesser development of the tree leaf area in the ‘driest’ treatment, T5. The mean reduction in the tree leaf area in T5, compared to T1, was 14% in 1999 (Table 2). In 1998, the reduction was of 5%. The differing environmental conditions in 1998 and 1999, that resulted, in general, in excessive vegetative development and very low yields in 1998 (an anomalous year due to the environmental conditions) compared to 1999, and a more severe water stress during kernel-filling in 1999 (pd < −2.5 MPa) with respect to 1998 (pd > −1.5 MPa), could explain the greater leaf reduction observed in 1999. There was a close linear correlation between applied water amount, tree water status and leaf area (data not shown), as has
175 Table 2. Relative growth rate (RGR) in leaf expansion during the kernel-filling stage in 1999. Annual mean values of single leaf area in 1998 and 1999. Leaf area tree−1 was estimated according to the canopy volume. Each value represents the mean of 24 measurements in three different periods of the year
Treatments T1 T2 T3 T4 T5 ANOVA
Leaf expansion rate RGR (mm mm−1 day−1 ) ×10−3 Length Width
Single leaf area (cm2 )
Leaf area tree−1 (m2 )
1999
1998
1999
1998
15.0a 8.9b 11.0b 8.5b 8.1b
28a 16b 17ab 17ab 13b
5.82a 6.25a 5.93a 5.96a 4.93b
5.83a 5.46a 5.76a 5.50a 5.07b
357a 346a 335a 327a 307b
353ab 340b 403a 323b 338b
∗∗∗
∗
∗∗∗
∗∗∗
∗
∗∗
n.s. not significant ∗ P < 0.05; ∗∗ P < 0.01; ∗∗∗ P < 0.001. For each column, mean values followed by distinct letters are significantly different; separation by Duncan’s multiple range test at the 95% confidence level.
been pointed out for this species by Torrecillas et al. (1989) and for other fruit trees by Levy et al. (1978). Root growth and distribution
Figure 4. (A), Seasonal patterns of leaf abscission in almond trees for each treatment during 1999. Each point is the mean of four measurements per treatment. Vertical bars represent the standard errors of the means. (B), Leaf abscission as a function of pre-dawn leaf water potential (y = 4.77 − 18.45x, r = 0.77; P < 0.001). Each point is a single measurement (one tree per treatment and plot) during the period of greatest stress (June–August) in 1999.
The distribution of soil water content influenced the root distribution in both irrigation systems, a more dense concentration of fine roots in the area close to the drip (wetted soil zone) being observed (Figure 5). SDI produced a greater horizontal distribution of fine roots in the soil profile than the surface drip system (Figure 5). A deeper root distribution also was observed under SDI. While surface treatments (T1 and T2) had a higher density of fine roots in the first 30 cm depth, in the SDI treatments (T3, T4 and T5) there was a greater root development around 40–50 cm depth (Figure 5). These root patterns are similar to those found in other species cultivated under SDI (Phene et al., 1991; Ayars et al., 1999). Moreover in SDI treatments T4 and T5 there were a higher density of fine roots than in the other treatments (Figure 5). A stimulation of fine root development in response to a severe soil water deficit could play an important role in drought resistance and recovery (McCully, 1999). In all treatments, root density was practically nil below 80 cm depth. 75% of fine roots were in the upper 70 cm, as has been found frequently in drip-irrigated almond (Catlin, 1996; Franco and Abrisqueta, 1997). In our study, factors such as physical properties of the soil (fine texture and a hard calcareous layer below 80 cm) and a severe soil dehydration (high soil strength) could have affected root growth directly by restricting penetration. The more dense distribution of absorbing roots in the wetted volume of soil (close to the drip), observed
176 in the driest SDI treatment, could be advantageous, allowing plants to utilise more efficiently soil water (Brown, 1996), but can be unfavourable if it leads to root intrusion in the buried drips (Zoldoske, 1999). We observed (visually in some excavated drips) a high concentration of roots covering the buried drips, but our study demonstrated after 4 years (at the end of the experiment), a good SDI irrigation uniformity (97%), similar to the surface system, and no root intrusion in the buried drips (García et al., 2000; Romero, 2002). The use of a chemical barrier (trifluralin) and pressurecompensating emitters (physical barrier) can prevent root intrusion, as demonstrated previously by Ayars et al. (1999). The root/shoot ratio in 1999 (dry weight of fine roots (mg cm−3 )/dry weight of leaves (mg cm−3 )) increased with water stress in the RDI treatments, mainly in treatments T4 and T5, (data not shown), as reported in plants under water stress by Kramer and Boyer (1995) and Huang and Fry (1998). This suggests an alteration in the transport and distribution of carbohydrates and nutrients between root and shoot. Bud development, flowering and fruit growth No significant differences were found in bud density (flower buds per branch) between treatments, although there was a slight increase in the density of vegetative buds in T2 compared to T1 (Table 3). Flowering density and fruit set were also not significantly affected by irrigation treatments (Table 3). Other studies carried out with almond under moderate water deficit found no significant effects on flowering density (Ruiz-Sanchez et al., 1988), fruit density and fruit set (Esparza et al., 2001). Entire (hull+shell+kernel) fruit growth (length and diameter and dry matter accumulation) with the time was also similar in all treatments (data not shown). There was a tendency for the abscission of buds and fruits to increase in T5 compared to the other treatments (although not significantly so) (Table 3). However, since bud and fruit abscission were highly variable between trees, we could not establish a clear trend in these parameters with respect to water stress. Yield and water use efficiency Figure 5. Fine roots (diameter < 1 mm) distribution as a function of depth and lateral distance from the emitter for each treatment, in the post-harvest period in 1999. Solid lines represent iso-lines of equal density (mg dry weight cm−3 soil) of fine roots. Black arrows indicate the localization and depth of the emitters in both irrigation systems.
In this study we show yield data for 1999 and 2000 only, because we consider that 1997 (first year of the experiment) and 1998 (year of very low yields, 300– 400 kg ha−1 , due to bad weather during bloom and
177 Table 3. Effect of irrigation treatments on the number (per metre of branch) of different stages of reproduction of almond trees in 1999. Fruit set and abscission of accumulated buds, flowers and fruits during 1999 (single year of the experiment)
Treatments
Vegetative buds
T1 T2 T3 T4 T5 ANOVA
81a 105b 92ab 77a 78a ∗∗
Density (n◦ m−1 ) Flower buds Flowers 144 164 146 127 142 n.s.
124 133 130 107 126 n.s.
Fruits
Fruit set
66 59 70 58 67 n.s.
49 37 46 48 49 n.s.
Abscission (N◦ /sieve box, 4 m2 ) Total buds Flowers Fruits 1236 1548 1046 1200 1442 n.s.
7857 6739 5562 6341 5573 n.s.
127 102 89 98 158 n.s.
n.s. not significant; ∗∗ P < 0.01. For each column, mean values followed by distinct letters are significantly different; separation by Duncan’s multiple range test at the 95% confidence level. Table 4. Effect of different irrigation treatments on yield response of almond trees during two years of the experimental period (1999–2000)∗ % kernel rot. Parameters Mean annual shell yield (kg tree−1 ) Mean annual kernel yield (kg tree−1 ) Cumulative kernel yield 1999–2000 (kg tree−1 ) Mean number of fruits tree−1 Single kernel weight, mean (g) Percent kernel, mean (%) Yield water use efficiency, mean (kg m−3 ) Hull-tight, mean (%) Faulty kernels (%)∗ Annual applied water, mean (mm year−1 ) Cumulative applied water 1997–2000 (mm) Annual reduction of applied water, mean (%)
T1 17.22a 4.96a 9.91a 4407a 1.18 29.4 0.23a 2.33a 1.58a 603 2411 0
T2 15.31ab 4.39ab 8.78ab 3926ab 1.20 29.2 0.28b 4.24bc 0.92ab 436 1744 28
T3 14.78b 4.21b 8.42b 3839ab 1.18 29.3 0.26ab 3.71abc 0.50b 436 1743 28
T4 13.84b 3.97b 7.95b 3744b 1.15 29.5 0.29bc 3.11ab 0.96ab 382 1528 37
T5 13.66b 3.89b 7.78b 3637b 1.15 29.3 0.33c 5.18c 0.88b 330 1320 45
ANOVA * ∗ ∗ ∗
n.s. n.s. ∗∗∗ ∗ ∗
n.s. not significant; ∗ P < 0.05; ∗∗∗ P < 0.001. For each row, mean values followed by distinct letters are significantly different; separation by Duncan’s multiple range test at the 95% confidence level.
pollinization) are not representative of the experiment from a production point of view. There were significant differences in the almond nut yield components (shell yield, kernel yield and fruit load) between theT1 and RDI treatments (Table 4). Mean kernel yield per tree (for the period 1999–2000) was reduced by 11% in T2, 15% in T3, 20% in T4 and 22% in T5 compared to T1 (Table 4). However there were no significant differences between deficit treatments (SDI T3, T4, T5 and T2). The application of the same RDI strategy (with the same amount of water) under surface and subsurface systems (T2 and T3) produced a similar kernel yield response in both treatments (Table 4). Schwankl et al. (1999) observed a similar yield response in almonds under surface and subsurface irrigation systems in different soil conditions. However, other studies in the
Murcia region have shown a higher kernel yield under buried drip irrigation (Del Amor et al., 1999). There were no significant reductions in kernel percentage or kernel dry weight, being less than 3% in T4 and T5 (Table 4). Greater reductions in kernel weight (around 10%) have been recorded in other almond varieties under RDI (Goldhamer, 1996). These results show that, even under severe water stress during kernel-filling (pd < −2.3 MPa in 1999), the accumulation of photo-assimilates in the seed was not altered, indicating the strong sink activity of the fruit in this period. Kernel dry weight gain with time was nearly identical for all RDI treatments and T1 through early June-mid to mid-July (data not shown). Only from mid-July through harvest (three weeks) did kernel dry matter accumulation slow slightly in all RDI treatments, as pointed out previously by Goldhamer (1996) and Girona et al. (1997).
178 We observed that yield reductions were principally due to reductions in the fruit load (n◦ fruits tree−1 ). The number of fruits per tree in T4 and T5 was significantly reduced by 15 and 17%, respectively, compared to T1 (Table 4). Other studies in almonds under RDI during kernel-filling (irrigated at 50% or at 20% ETc) and late post-harvest showed lower reductions of fruit load (between 5% and 9%), indicating that the principal yield component affected was kernel weight (Girona and Marsal, 1995; Goldhamer, 1996). This different yield response in almonds under water stress could be due to different reasons: the variety of almond, differences in edaphoclimatic conditions and the pattern or level of the stress. Water use efficiency (WUE) increased significantly in the RDI treatments T2, T4 and T5 (between 20% and 30%) compared to T1 (Table 4). Water use efficiency increased linearly with the decrease in the irrigation water amount, as found by Torrecillas et al. (1989) and Hutmacher et al. (1994). Subsurface treatment T5 showed also a WUE, significantly higher than T2 and T3 (Table 4). In economic terms, these efficiency values can indicate that, for a fixed mean price per kg of kernel in SE Spain (around 2.90 ¤ kg−1 for the period 1998–2002) and taking into account the variability in the price of water in this region, between 0.13 and 0.35 ¤ m−3 , depending on the water quality, the gross profit margin estimated (income from production – water price, without taking into account other costs) increases for RDI treatments as water price increases, especially in the more restrictive strategies T4 and T5, compared to T1 (Figure 6). The price of water, together with the annual water saving achieved, basically decides the extent of this increased efficiency. Normally, in the Murcia region the price of water is much higher than that of the water used in this study (from irrigation canals), since major amounts of irrigation water are obtained (more expensively) principally from wells used for irrigation. Currently, with these actual prices of water, there are slight differences favouring deficit SDI treatments, but the foreseeable increase in the market price of water in these zones will make these strategies even more interesting from an economic point of view. In our experimental conditions, in general, we observed under RDI a slightly earlier onset of hull split than in the control (between 3 and 7 days). Also, hull-tight percentage increased significantly in RDI treatments. The subsurface treatment T5 and the surface treatment T2 had hull-tight percentages significantly higher compared to T1 (Table 4). Hull-tight
Figure 6. Gross profit margin (income from production – water price) as a function of water price for each treatment.
percentage was correlated closely (r 2 = 0.70) with the level of water stress reached during kernel-filling (Figure 7A), as pointed out by Goldhamer and Viveros (2000). This characteristic had no economic repercussion in our study, but a high percentage of hull-tight nuts (as obtained in other varieties of almond in California) can reduce considerably the value of kernels (Goldhamer and Smith, 1995). There were no significant differences in the percentage of empty and double fruits between treatments (data not shown). However, the percentage of faulty fruits (visual shell and kernel rot) was significantly higher in T1 compared to T5 and T3 (Table 4). A low humidity during seed development can reduce some internal fungal diseases associated with high orchard humidity under deficit irrigation (Goldhamer and Viveros, 2000) and subsurface drip irrigation (Goldhamer, 1997b; Michailides, 1996). In our study significant linear correlations between kernel yield and several growth parameters were observed. A simple regression model indicated that trunk cross- sectional area and canopy volume (two parameters indicative of the overall growth of the tree) were the factors most closely correlated (positively) with the kernel yield (Table 5). Most studies of almond under water stress have associated yield reductions with reduced canopy development and tree size (Fereres et al., 1981; Castel and Fereres, 1982; Torrecillas et al., 1989; Hutmacher et al., 1994). The fruit load reduction observed in our study could be due to several factors: A cumulative effect of water stress on shoot growth due to reduced canopy volume and terminal shoot and branch growth. This effect was observed principally in the subsurface treatment
179 Table 5. Simple regression analysis between kernel yield (1999–2000) and two growth parameters (canopy volume and trunk cross-sectional area) measured in the year 2000. ∗ A stepwise multiple regression model was made between kernel yield and several growth parameters. Trunk-cross sectional area was the only one parameter that added significantly to the model. The other growth parameters not improved the model Model fitting results Parameter
Constant
Slope
Determination coefficient (r)
Canopy volume (m3 tree−1 ) Trunk-cross sectional area (cm2 )∗
−0859 −2.158
0.107 0.031
0.69∗∗∗ 0.78∗∗∗
Significance level: ∗∗∗ P < 0.001.
Figure 7. (A), ‘Hull-tight’ percentage as a function of pre-dawn leaf water potential during the kernel-filling stage, for the period 1998–2000 (y = 0.0713 − 2.4451x, r = 0.84, P < 0.001). Each point represents the mean of twelve trees per treatment. (B), Kernel yield as a function of the level of water stress reached in the period from the end of kernel–filling to early post-harvest (y = 1438 + 237.6x, r = 0.45, P < 0.05). Each point represents the mean of twelve trees per treatment for the period 1998–2000.
T5 (receiving the least water), resulting in a smaller tree size and less fruitwood growth (fewer fruiting positions), with more severe water stress, as found by Goldhamer and Viveros (1991), Prichard (1996) and Esparza et al. (2001). Also, a premature leaf
defoliation during kernel-filling (severe water stress period) and a significant reduction of photosynthesis in this phase (Romero et al., 2004) would result in a lower tree leaf area, and less carbon gain and carbohydrate accumulation, reducing the necessary reserves for shoot growth in the following year. In this regard, we observed a significant linear correlation between tree water status during this critical period (the end of the kernel-filling stage and early post-harvest and the kernel yield in the following year (Figure 7B). Based on the results from Romero et al. (2004) and from these data, we conclude that the application of the same amount of water with the different irrigation systems (T2 and T3) produced no significant differences in the tree water relations, vegetative development or yield. RDI strategies T4 and T5 (with a severe decrease of irrigation water, 80% ETc, during kernel-filling and up to 50% post-harvest), under subsurface drip irrigation conditions, did result in a significant reduction of kernel yield compared to the well-irrigated treatment (T1). The primary yield component affected was fruit load (number of fruits per tree), which was reduced significantly in T4 and T5. These treatments had no important effects on tree water relations parameters, bud development, flowering or fruit set processes and showed a significantly higher water use efficiency post-harvest. These irrigation strategies could be a good alternative in semi-arid regions, with strictly limited water supplies. In this situation, the grower could save significant amounts of irrigation water (up to 300 mm per year), improving WUE and maintaining a mean annual kernel yield above 1000 kg ha−1 , compared with the 450 kg ha−1 currently produced in Murcia (AEA, 2001). Nevertheless, long-term application of T5 strategy impacted shoot growth, reducing leaf area and fruitwood, which could influence negatively the longterm yield response. The optimisation of these RDI
180 strategies under SDI would require an increase in irrigation two weeks before harvest in order to maintain an adequate soil and plant water status at the end of kernel-filling and early post-harvest (pd − 1 MPa). Other aspects, such as salts, soil and irrigation management in RDI under a subsurface drip system and almond profitability in these irrigation conditions, need more investigation.
Acknowledgements The authors would like to thank Dr David Walker for correction of the English. This work has been supported partially by a grant of The Institute Euromediterraneo de Hidrotecnica Foundation, awarded to Pascual Romero Azorín.
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