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SEASONAL CHANGES IN LEAF NITROGENOF OLIVE TREES GROWN UNDERDIFFERENT IRRIGATION REGIMES ANDCROP LEVELRiccardo Gucci a , Giovanni Caruso a & Luca Sebastiani ba Dipartimento di Coltivazione e Difesa delle Specie Legnose ,University of Pisa , Pisa, Italyb Scuola Superiore di Studi Universitari e di PerfezionamentoSant’Anna , Pisa, ItalyPublished online: 18 Aug 2010.

To cite this article: Riccardo Gucci , Giovanni Caruso & Luca Sebastiani (2010) SEASONAL CHANGESIN LEAF NITROGEN OF OLIVE TREES GROWN UNDER DIFFERENT IRRIGATION REGIMES AND CROP LEVEL,Journal of Plant Nutrition, 33:12, 1849-1859

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Journal of Plant Nutrition, 33:1849–1859, 2010Copyright C© Taylor & Francis Group, LLCISSN: 0190-4167 print / 1532-4087 onlineDOI: 10.1080/01904167.2010.503833

SEASONAL CHANGES IN LEAF NITROGEN OF OLIVE TREES GROWN

UNDER DIFFERENT IRRIGATION REGIMES AND CROP LEVEL

Riccardo Gucci,1 Giovanni Caruso,1 and Luca Sebastiani2

1Dipartimento di Coltivazione e Difesa delle Specie Legnose, University of Pisa, Pisa, Italy2Scuola Superiore di Studi Universitari e di Perfezionamento Sant’Anna, Pisa, Italy

� An experiment was conducted over 12 months using field-grown olive trees (Olea europaea)to assess the combined effect of soil water availability and fruit number on seasonal changes inleaf nitrogen (N) concentration. Three irrigation regimes were established and three trees per irri-gation treatment were thinned to reduce their yield to about half that of unthinned trees. The Nconcentration of fully-expanded leaves from either the current-year growth or one-year old part offruiting shoots was determined every two months. Nitrogen concentration was higher in current-year leaves than in one-year old ones at most sampling dates. Maximum values of leaf N weremeasured in spring, minimum values in August. Leaf N concentrations were positively correlatedwith leaf water potential during fruit development. This relationship was weak at the onset of rapidoil accumulation in August and became more evident at harvest. There was no correlation betweenleaf N and crop level.

Keywords: deficit irrigation, leaf age, leaf water potential, nitrogen, Olea europaea L.

INTRODUCTION

Nitrogen (N) is a critical element for plant growth and fruiting in peren-nial crops and fruit trees, including olive (Olea europaea L.). Dynamics ofN are different between deciduous and evergreen trees. For instance, stor-age of N in deciduous trees occurs in woody roots and old stems (Munozet al., 1993), whereas in evergreen conifers N is mainly stored in old needles(Millard and Proe, 1992). In peach (Prunus persica L.) leaf N appears tobe mobilized from the leaf to perennial organs before leaf drop, stored inthe roots during winter and then remobilized to sustain growth after budbreak in the spring (Tagliavini et al., 1998). Leaves of olive trees accumulate

Received 8 December 2008; accepted 25 July 2009.Address correspondence to Riccardo Gucci, Dipartimento di Coltivazione e Difesa delle Specie

Legnose, University of Pisa, Via del Borghetto, 80, I-56124, Italy. E-mail: rgucci@agr.unipi.it

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substantial amounts of N that may be later used for growth of vegetativeorgans and fruits (Fernandez-Escobar et al., 2004).

Despite the importance of mineral nutrition there is little informationon nutrient requirements of olive trees. Bouranis et al. (2001) determined Npartitioning between the leaf and the shoot (Nleaf /Nshoot) in four olive cul-tivars during bud differentiation and reported that the Nleaf /Nshoot ratio in-creased or remained constant prior to bud differentiation, while it decreasedduring and after bud differentiation. In general, the leaf N concentration ofrain-fed olive trees decreases or remains relatively stable during bud differ-entiation and until emergence of the inflorescence (Bouranis et al., 2001),then decreases to a minimum in July or August, and recovers to initial levelsonce summer drought is over (Failla et al., 1997; Fernandez-Escobar et al.,1999, 2004; Perica, 2001).

Availability and uptake of N are markedly affected by soil moisture(Neilsen et al., 2001). Although olive trees are typically grown under rain-fed conditions, modern high-density orchards require irrigation. Irrigationincreases yield, fruit size, and oil content (Costagli et al., 2003; Goldhameret al., 1994; Lavee et al., 2007). In recent years it has been shown that reg-ulated deficit irrigation (RDI), consisting in supplying water volumes lessthan 100% of the tree needs at specific phenological stages, is beneficial tooptimize water use, yield, and oil quality in olive trees (Ben Ahmed et al.,2007; Grattan et al., 2006; Gucci et al., 2007; Lavee et al., 2007; Moriana et al.,2003; Servili et al., 2007). Lavee et al. (2007) reported that leaf N, K, and Mgconcentrations of irrigated trees are higher than those of rain-fed ones, (butnot of deficit-irrigated trees), but little is known about seasonal changes inleaf nutrient concentrations of irrigated orchards and how mineral nutritionshould be adjusted when deficit irrigation strategies are used.

Besides soil moisture, factors affecting leaf nutrient concentrations in-clude leaf age, crop load and leaf position in the canopy (Fernandez-Escobar et al., 1999, 2004; Perica, 2001). Leaf N concentration is higherin young leaves than in leaves from either one- or two-year old fruitingshoots (Fernandez-Escobar et al., 1999, 2004). Alternate bearing may inter-fere with the seasonal course of leaf N concentration, which appears stable inold leaves of rain-fed olive trees during ‘off’ years and decreased from Aprilthrough October in “on” years. Moreover, new leaves accumulate N duringthe ‘off’ year and mobilize it during the “on” year (Fernandez-Escobar et al.,2004).

The objective of the present study was to determine seasonal changes inN leaf concentrations of olive trees subjected to different irrigation regimes,including a deficit irrigation treatment. Additionally, we investigated theeffect of pre-determined crop levels on N concentration of either current-year or one-year old leaves and how crop level interacted with tree waterstatus.

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MATERIALS AND METHODS

Plant Material

Field-grown, self-rooted olive trees (Olea europaea L. cv. ‘Leccino’) inan irrigated orchard (43◦16′ N; 10◦35′ E), planted in 1998 at a 6 × 3.8 mdistance in a sandy-clay soil (Table 1), were used in 2003 and 2004. Thetrees were trained to a single trunk and cultivated according to standardpractices for olive orchards in that area (Gucci et al., 2007). Eighteen treeswere selected based on similar canopy size and fruit number four weeks AFB,which occurred on 26 May 2003, from three adjacent rows of trees (six treesper row). Half of the trees were hand-thinned between five and six weeksAFB to reduce the number of fruits to about 20% of that of the unthinnedcontrols. In 2004 full bloom occurred on June 3.

Irrigation Treatments and Leaf Water Potential Measurements

Each tree was irrigated by four pressure-compensated drippers (4 L h −1)spaced at about 0.9 m distance along the irrigation lines. Irrigation regimeswere imposed on three adjacent rows of trees and tree water status assessed bymeasuring PLWP weekly from 6 through 19 weeks AFB, using a Scholander-type pressure chamber (Tecnogas, Pisa, Italy). One fully-expanded leaf perplant from fruiting shoots in the outer part of the canopy at about 1.5–1.8 m height was sampled from each of the experimental trees. Leaves wereexcised with a razor blade, immediately put in the chamber cylinder, whichwas then pressurized with nitrogen gas at a maximum rate of 0.02 MPa s−1

TABLE 1 Physical and chemical characteristics of the soil sampled at six locations (two per row) of theexperimental area at two different depths on 23 August 2003

Depth (m)

0.30 0.60

Sand (%) 48.3 48.2Silt (%) 12.6 12.7Clay (%) 39.0 39.2pH 7.35 7.43I.E.C. (meq/100g) 19.6 18.9Organic matter (%) 0.83 0.70N (%) 0.056 0.049P (ppm) 8 7.3K (ppm) 222 241Ca (ppm) 2566 2400Mg (ppm) 607 605Na (ppm) 194 191

I.E.C., ion exchange capacity.

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(Costagli et al., 2003). At the end of the experiment the cumulated PLWPwas calculated by integrating actual values over the measurement period(Gucci et al., 2007).

Crop evapotraspiration was calculated using 0.50 as a crop coefficient,within the range commonly used for olive tree, and 1.0 as a coefficient ofground cover, determined from the mean diameter of the canopy projectedarea of each trees at solar noon (Gucci et al., 2007). Fully-irrigated (FI) treesreceived about 100% crop evapotraspiration, but the calculated volumes ofapplied water were adjusted based on the weekly measurements of PLWP.The PLWP for FI trees was higher than −1.1 MPa, that for deficit-irrigatedones (DI) and complementary-irrigated trees (SI) ranged between −1.0and −3.3 MPa and between −2.1 and −4.2 MPa, respectively. The deficitirrigation regime consisted in exposing trees to periods of water deficit andrelief from stress during fruit development (Gucci et al., 2007).

The irrigation period lasted from 30 June (6 weeks AFB) through 10September (16 weeks AFB). Fully-irrigated trees received approximately2732 L of water per tree, whereas deficit-irrigated trees and complementary-irrigated ones received an average of 1150 and 573 L of water, respec-tively. Precipitation over the 12-month study period (June 2003–June 2004)was 616 mm, similar to the 10-year (1994–2003) mean value (615 mm).Other details of irrigation management and climatic data were previously re-ported (Gucci et al., 2007). Fertilizers were not applied during the irrigationperiod.

Fruit Yield and Oil Determination

A sample of 100 fruits per tree was taken at harvest (21 weeks AFB) todetermine average fresh weight and ripening index (Gucci et al., 2007). Theoil content of the fruit mesocarp of five fruits per tree was measured in dupli-cate, after oven-drying at 70◦C, by nuclear magnetic resonance Oxford 4000analyzer (Oxford Analytical Instruments Ltd., Oxford, UK), as previouslyreported (Costagli et al., 2003).

Nitrogen Analyses

The N concentration was determined in either current-year leaves or one-year old leaves of fruiting shoots. Fully-expanded leaves were sampled fromthe median part of either current year or one-year old wood of the shootsfrom four branches with different orientations (N-S-E-W) in the canopyover a 12-month period. The shoots were tagged at the beginning of theexperiment and leaves were always excised from the same shoots. A total of48 leaves per tree were used at each of the six sampling dates (starting from27 June 2003) until 24 June 2004, that is immediately before the beginningof the 2004 irrigation period. Leaves were dried to constant weight in an

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oven at 60◦C, then ground and sieved. Total N was determined using theKjeldhal method, whereby 200 mg of dried material was digested at 370◦C for30 min in a digestor (Tecator 200, Foss Tecator, Hoganas, Sweden). Sampleswere distilled in a current of vapor using a Kjeltectm Foss 2002 (Foss Tecator,Hoganas, Sweden). The distillate was collected in a flask containing 25 mLof boric acid (H3BO4) and then titrated with hydrochloric acid (HCl) 0.1 Nuntil the solution turned grey.

Experimental Design and Statistical Analysis

A split plot design was used with irrigation as the main plot and crop loadas the subplot. Three replicate trees were used (each tree was one replicate).Means of treatments were separated by least significant differences (LSD)after analysis of variance (ANOVA) using CoStat software (CoHort software,Monterey, CA, USA).

RESULTS

Thinning fruits early determined a significant reduction in oil or fruityield (Table 2), which ranged from 41 to 49% of unthinned trees for fully-irrigated and deficit-irrigated trees, respectively, and from 58 to 75% fortrees that only received complementary irrigation. The discrepancy betweenthe pre-determined level of fruit removal (approx. 80%) and yield at harvest(approx. 50% of control) was due to the large initial fruit population (over10000 per tree at harvest) which made the counting of fruits at five weeksAFB difficult.

TABLE 2 Fruit yield, oil yield, and cumulated PLWP of olive trees subjected to different irrigationregimes. Each irrigation treatment included three trees with either a full crop (high) or a low croplevel. A low crop load was obtained by hand-thinning fruits between five and six weeks after full bloom.Least significant differences between treatments were determined within each date after analysis ofvariance for a split plot design with irrigation as the main plot and crop load as the subplot (P < 0.05).The level of probability for treatment effects and their interaction is also reported

Variable Fruit yield (kg/tree) Oil yield (kg/tree) PLWP (−MPa · d)

Irrigation (I)Full 13.960 1.630 79.61Deficit 12.299 1.800 167.32Complementary 7.340 1.242 235.45

Crop level (C)High 15.411 2.049 180.23Low 7.028 1.065 141.35

SignificanceI 0.066 0.153 0.036C 0.012 0.012 0.011I × C 0.176 0.085 0.099

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The different volumes of water applied determined wide differences inPLWP at harvest, but did not affect yield significantly (Table 2). Nevertheless,fully-irrigated, unthinned trees produced more than twice the fruit yield ofcomplementary-irrigated trees (19.501 and 9.375 kg per tree, respectively),but only slightly higher than deficit-irrigated trees (17.357 kg per tree). Theoil yield of unthinned trees was 2.181, 2.545, and 1.420 kg per tree for fully-irrigated, deficit irrigated and complementary irrigated trees, respectively.Trees with a low crop level had higher cumulated PLWP at harvest (Table 2).For instance, trees of DI and SI treatments that had had their fruit thinnedshowed higher cumulated PLWP values (−149.33 and −196.13 MPa · d,respectively) than unthinned trees (−185.30 and −274.77 MPa· d, respec-tively) at harvest, indicating that reducing the crop level improved tree waterstatus.

In 2003, there was hardly any effect of crop load on mesocarp oil contentat any of the sampling dates. The oil content in the mesocarp increased asthe level of water deficit decreased, i.e. oil content increased as daily PLWPincreased from about −3.5 to −1.5 MPa (Gucci et al., 2007). Differences inoil yield between irrigation treatments were less evident than those in fruityield. The interaction between irrigation treatments and crop level was notsignificant (P < 0.05) for any of the variables reported in Table 2.

The crop level did not have significant effects on N in either current-year or one-year old leaves at any of the dates of measurements (Tables 3and 4). Leaf age did not affect the seasonal course of N concentrations,but one-year-old leaves had lower concentrations than current-year ones.The N concentrations at the beginning and at the end of the 12-monthsampling period were similar in current-year leaves of fully-irrigated trees,

TABLE 3 Nitrogen concentration (% dry weight) in fully-expanded, current-year leaves of olive treessubjected to different irrigation regimes. Each irrigation treatment included three trees with either afull crop (high) or a low crop level. Statistical significance was determined within each date afteranalysis of variance for a split plot design with irrigation as the main plot and crop load as the subplot(P < 0.05). The level of probability for treatment effects and their interaction is also reported

26 June 28 Aug 16 Oct 22 Jan 23 Apr 24 JuneVariable 2003 2003 2003 2004 2004 2004

Irrigation (I)Full 1.79 1.78 2.00 2.28 2.18 1.76Deficit 1.74 1.66 1.85 2.26 2.43 1.92Complementary 1.82 1.63 1.68 2.14 2.41 1.93

Crop level (C)High 1.76 1.67 1.81 2.19 2.36 1.86Low 1.81 1.71 1.88 2.27 2.32 1.87

SignificanceI 0.475 0.099 0.009 0.199 0.200 0.052C 0.221 0.214 0.153 0.490 0.770 0.877I × C 0.054 0.401 0.071 0.360 0.760 0.023

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TABLE 4 Nitrogen concentration (% dry weight) in fully-expanded, one-year old leaves of olive treessubjected to different irrigation regimes. Each irrigation treatment included three trees with either afull crop (high) or a low crop level. Statistical significance was determined within each date afteranalysis of variance for a split plot design with irrigation as the main plot and crop load as the subplotdesign (P < 0.05). The level of probability for treatment effects and their interaction is also reported

26 June 28 Aug 16 Oct 22 Jan 23 Apr 24 JuneVariable 2003 2003 2003 2004 2004 2004

Irrigation (I)Full 1.51 1.43 1.68 1.96 2.11 1.75Deficit 1.56 1.34 1.55 1.88 2.06 1.82Complementary 1.55 1.40 1.38 1.83 2.15 1.85

Crop level (C)High 1.57 1.38 1.51 1.83 2.07 1.79Low 1.50 1.40 1.56 1.95 2.14 1.82

SignificanceI 0.560 0.374 0.055 0.076 0.856 0.562C 0.245 0.609 0.452 0.148 0.548 0.528I × C 0.255 0.040 0.035 0.165 0.597 0.046

but slightly higher for the other two irrigation treatments at the end of thestudy (Table 3).

Irrigation effects were significant or close to significance (P < 0.05) atharvest (16 October) and on 24 June 2004 for current-year leaves and at har-vest for one-year old leaves (Tables 3 and 4). The interaction between croplevel and irrigation on N concentration of current-year leaves was significantor close to significance (P < 0.05) at both sampling dates in June (Table 3)and at three dates for one-year old leaves (Table 4).

The N concentration of current-year leaves was significantly higher forfully-irrigated trees than complementary-irrigated ones on 16 October 2003;leaf N concentration of deficit-irrigated trees was intermediate between val-ues of the other two treatments (Table 3). Following harvest, leaf N concen-trations increased until the January sampling date in current year leaves (ofboth thinned and unthinned trees) and one-year old leaves of unthinnedtrees in fully-irrigated trees and through April 2004 for deficit and com-plementary irrigation treatments. There was a drastic reduction in leaf Nconcentrations for all irrigation treatments between April and June 2004(Tables 3 and 4).

There was a linear negative correlation between leaf N and tree waterstatus, expressed as cumulated PLWP, when data from all treatments werepooled together (Figure 1), showing that increasing the degree of waterdeficit decreased leaf N. The relationship was similar for both current-yearand one-year-old leaves (Figure 1). The relationships were weaker in Augustthan in October, when fruits were harvested.

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FIGURE 1 The relationships between cumulated PLWP and leaf N concentration in fully-expandedleaves from either current-year growth (A, B) or one-year old part of fruiting shoots (C, D) on 28August (A, C) and 16 October 2003 (B, D). Different symbols indicate different irrigation regimes (eachpoint represents one tree). Linear regression equations: (A) y = 1.8294–0.00013x; R2 = 0.5483; (B)y = 2.1368–0.00018x; R2 = 0.6795; (C) y = 1.4865–0.0009x; R2 = 0.2442; (D) y = 1.8371–0.0019x; R2 =0.4571; All regression equations are significant at P < 0.05. Legend: FI, fully irrigated; DI, deficit irrigated;SI, complementary irrigated.

DISCUSSION

The different degree of water deficit experienced by olive trees due todifferent irrigation regimes affected leaf N concentrations during the fruitdevelopment period. Leaf N concentration, determined at mid-summer andharvest, appeared positively correlated with the amount of applied water.The seasonal courses of leaf N concentration of DI and SI trees followedpatterns previously reported for trees grown under rain-fed conditions withminima in mid-summer and maxima in winter and early spring (Failla et al.,1997; Fernandez-Escobar et al., 1999, 2004; Perica, 2001). However, the leafN of FI trees did not decrease from the June to the August sampling date. Inaddition, we showed an interaction between the water status and the crop-ping condition of the tree. For instance, the leaf N concentration of eitherfully-irrigated or deficit-irrigated trees increased from after pit-hardening(August sampling) to harvest (October), whereas the leaf N concentrationof trees receiving only complementary irrigation and bearing a full cropdid not. During the same period decreasing leaf N concentrations werealso reported for non-irrigated, non N-fertilized trees in ‘on’ years, when

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developing fruits are considered the main sink for N of the bearing shoot(Fernandez-Escobar et al., 2004). In our study the leaf N concentration ofcomplementary-irrigated trees bearing a low crop increased from Augustto October similarly to other irrigation treatments. Therefore, the crop-ping condition of the tree did not affect leaf N concentration when thedegree of stress was moderate or absent (e.g., deficit irrigation or full irri-gation), which may indicate that the N demand by the fruit in the final twomonths of development does not affect leaf N reserves. On the other hand,when water was scarce and the degree of stress severe (e.g., complementary-irrigated trees bearing a full crop) it is likely that the N demand for fruitdevelopment exceeded the low supply, resulting in source limitations anda decrease in leaf N concentration. The decrease in leaf N from June toAugust in water-limited treatments (both DI and SI) regardless of the crop-ping condition seems to indicate that, during that period of fruit develop-ment, leaf N is used to meet the N demand for maintenance and growth oftissues.

It should be noted that the levels of water deficit reached bycomplementary-irrigated trees were comparable to those experienced bytrees receiving minimum or no irrigation in other studies (Costagli et al.,2003; Grattan et al., 2006; Gomez-Rico et al., 2007), also due to the extremelyhigh temperatures of the summer 2003, when temperatures in July and Au-gust were 3.6◦C higher than the 10-year average for that area (Gucci et al.,2007).

The seasonal course of N concentration in current-year leaves paral-leled, but with higher values, that of one-year old leaves of the fruit bearingshoot, similarly to what reported in other studies (Fernandez-Escobar et al.,1999). At all dates of measurement N concentrations of current-year leaveswere above adequacy thresholds reported for olive trees grown in California(Freeman et al., 1994) and within the range reported for trees in Tuscany(Failla et al., 1997). Values were similar to those previously reported forleaves of similar age (Bouranis et al., 2001; Fernandez-Escobar et al., 1999;Perica, 2001).

Differences in leaf N concentration between irrigation regimes disap-peared the following spring, when leaf N of the deficit-irrigated treatmentreached the same values as those of fully-irrigated trees. Moreover, theprevious-year cropping condition of the tree did not affect leaf N concentra-tions in the following spring, which were generally higher than at harvest,similarly to what reported for leaves of similar age and position in otherstudies (Failla et al., 1997; Fernandez-Escobar et al., 1999; Perica, 2001). It islikely that the higher values of leaf N concentration in spring reflected morefavorable conditions for N uptake (soil temperature and humidity).

In conclusion, deficit irrigation allowed high yields while maintainingadequate levels of N in the olive leaf over a 12-month period. The degreeof regulated deficit compatible with high yields ranges from 35 to 50% of

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full irrigation (Ben Ahmed et al., 2007; Grattan et al. 2006; Gucci et al.,2007; Moriana et al., 2003), while higher volumes did not yield significantadditional fruits. Although deficit irrigation increases water use efficiencyof olive trees, the best schedules and strategies for deficit irrigation of olivetrees still remain to be determined (Gucci et al., 2007; Moriana et al., 2003),as they likely depend on cultivar (Tognetti et al., 2006), soil and climaticconditions. Beneficial effects of demand-controlled irrigation have beenshown in apple trees where RDI increased N fertilizer use efficiency withrespect to fixed-rate irrigation (Neilsen et al., 2001). A recent article showedthat, despite the effects of irrigation schedules appeared more expressed inthe ‘on’ than in the ‘off’ years, no consistent effect was measured on thedegree of alternate bearing and mineral content of the leaves (Lavee et al.,2007). However, we showed that the water status condition of the tree doeschange leaf N concentration and that the tree cropping condition stronglyinteracts with tree water status and can affect the leaf N status of olive treesthat undergo long periods of drought. Although our results on dynamicsof leaf N in trees under different irrigation regimes remain to be validatedover periods longer than one year, they represent a novel contribution tounderstand how to save water and nutrients in irrigated, high-density oliveorchards and complete the existing information on mineral nutrition underrain-fed conditions.

ACKNOWLEDGMENTS

We are grateful to Gaia Monteforti and Michele Bernardini for excel-lent technical assistance and Regional Agency for Development and Inno-vation in Agriculture and Forestry (A.R.S.I.A.) for providing meteorologicalrecords.

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