Jujube fruit water relations at fruit maturation in response to water deficits

1 Jujube fruit water relations at fruit maturation in response to 1 water deficits 2 3 A. Galindo a , Z.N. Cruz b , P. Rodríguez b , J. Collado-González a , M. Corell c,d , H. 4 Memmi e , F. Moreno d,f , A. Moriana c,d , A. Torrecillas a , D. Pérez-López e,∗ 5 6 a Dpto. Riego, Centro de Edafología y Biología Aplicada del Segura (CSIC), 7 P.O. Box 164, E-30100 Espinardo, Murcia, Spain 8 b Dpto. Fisiología y Bioquímica, Instituto Nacional de Ciencias Agrícolas (INCA), 9 Ctra. de Tapaste, km 3.5, San José de Las Lajas, Mayabeque, Cuba 10 c Escuela Técnica Superior de Ingeniería Agronómica. Universidad de Sevilla, 11 Carretera de Utrera, km 1, E-41013 Sevilla, Spain 12 d Unidad Asociada al CSIC de Uso sostenible del suelo y el agua en la 13 agricultura (US-IRNAS). Crta de Utrera Km 1, 41013, Sevilla, Spain. 14 e Departamento de Producción Vegetal. Fitotecnia. Escuela de Ingeniería 15 Técnica Agrícola, Technical University of Madrid, Ciudad Universitaria s/n, 16 28040 Madrid, Spain. 17 f Instituto de Recursos Naturales y Agrobiología (CSIC), P.O. Box 1052, E-18 41080 Sevilla, Spain. 19 20 * Corresponding author. Tel.: +34 91 336 54 56; fax: +34 91 336 37 27. E-mail 21 address: [email protected]. (D. Pérez-López). 22 23 24 25 2 Abstract 1 The fruit maturation stage is considered the optimal phenological stage 2 for implementing water deficit in jujube (Zizyphus jujuba Mill.), since a low, 3 moderate or severe water deficit at this time has no effect on yield, fruit volume 4 or eating quality. However, no information exists at fruit water relations level on 5 the mechanisms developed by Z. jujuba to confront drought. The purpose of the 6 present study was to increase our understanding of the relationship between 7 leaf and fruit water relations of jujube plants under different irrigation conditions 8 during fruit maturation, paying special attention to analysing whether fruit size 9 depends on fruit turgor. For this, adult jujube trees (cv. Grande de Albatera) 10 were subjected to five irrigation treatments. Control plants (T0) were irrigated 11 daily above their crop water requirements in order to attain non-limiting soil 12 water conditions in 2012 and 2013. T1 plants were subjected to deficit irrigation 13 throughout the 2012 season, according to the criteria frequently used by the 14 growers in the area. T2 (2012), T3 and T4 (2013) were irrigated as T0 except 15 during fruit maturation, in which irrigation was withheld for 32, 17 and 24 days, 16 respectively. The results indicated that the jujube fruit maturation period was 17 clearly sensitive to water deficit. During most of this stage water could enter the 18 fruits via the phloem rather than via the xylem. From the beginning of water 19 withholding to when maximum water stress levels were achieved, fruit and leaf 20 turgor were maintained in plants under water deficit. However, a direct relation 21 between turgor and fruit size was not found in jujube fruits, which could be due 22 to an enhancement of a cell elasticity mechanism (elastic adjustment) which 23 maintains fruit turgor by reducing fruit cells size or to the fact that jujube fruit 24 3 growth depends on the fruit growth-effective turgor rather than just turgor 1 pressure. 2 3 Keywords: Deficit irrigation; Gas exchange; Plant water relations; Zizyphus 4 jujuba 5 6 1. Introduction 7 Jujube tree (Zizyphus jujuba Mill), is native to temperate Asia and is mostly 8 cultivated in China, India, Central Asia and southwest Asia (Williams 2006b). 9 Although considered as a multipurpose plant, its fruits are the major focus of 10 interest (Bowe, 2006). Z. jujuba is considered a minor crop, but is an integral 11 part of the culture and way of life for millions of Asians and has also become so 12 for large regions of Africa (Williams, 2006a). This growing interest on jujube fruit 13 is due to its presumed health-promoting effects, and it is now considered a 14 functional food, since it has nutritional as well as medicinal uses (Heo et al., 15 2003; Huang et al., 2007; Li et al., 2007; Zhao et al., 2008; Mahajan and 16 Chopda, 2009; Choi et al., 2011; Collado-González et al., 2013, 2014). For all 17 this, the International Centre for Underutilized Crops has identified Z. jujuba as 18 a crop with substantial growth potential (Williams et al., 2006). 19 Jujube tree is admired for its multiple uses, easy management, early 20 bearing and wide adaptations to environmental conditions. In this sense, it is 21 tolerant to saline irrigation water, low winter temperatures during dormancy and 22 severe drought during the growing season (Dahiya et al., 1981; Ming and Sun, 23 1986; Jain and Dass, 1988). In this last respect, Cruz et al. (2012) showed that 24 Z. jujuba is able to withstand severe water deficits, while maintaining leaf turgor, 25 4 which allows good gas exchange rates and, as a consequence, good leaf 1 productivity. This leaf turgor maintenance was mainly due to two simultaneous 2 and complementary mechanisms: decreased leaf conductance and a shorter 3 period of maximum stomatal opening in order to control water loss via 4 transpiration (stress avoidance mechanisms). The gradual recovery of leaf 5 conductance after rewatering can also be considered as a mechanism for 6 promoting leaf rehydration. In addition, from the beginning of the stress period, 7 active osmotic adjustment operated, which can contribute to the maintenance of 8 leaf turgor (stress tolerance mechanism). The high relative apoplastic water 9 content levels and the possibility of increasing the accumulation of water in the 10 apoplasm in response to water stress, supporting a steeper gradient in water 11 potential between the leaf and the soil, which can be considered another 12 drought tolerance characteristic in pear-jujube leaves. 13 According to Cui et al. (2008), the phenological periods of jujube tree can 14 be divided into bud burst to leafing (stage I, early April - early May), flowering to 15 fruit set (stage II, mid May - late June), fruit growth (stage III, late June - late 16 July), fruit maturation (stage IV, early August - early September) and dormancy 17 (stage V, this October - next March) stages. Also, these authors indicated that 18 the fruit maturation stage is the optimal stage for implementing water deficit in 19 jujube, because low, moderate and severe water deficits have no effect on the 20 fruit weight and volume, the fruits taste sweeter and eating quality is improved. 21 In addition, the fruit maturation period is shortened, raising the market price of 22 the fruit, fruit firmness is enhanced and the percentage of rotten fruit after 23 storage is reduced. Despite the importance of the maturation period on jujube 24 fruit quality, to the best of our knowledge no information exists on jujube fruit 25 5 water relations. For these reasons, the aim of this study was to increase our 1 understanding of the relationship between leaf and fruit water relations of jujube 2 plants under different irrigation conditions during fruit maturation, paying special 3 attention to analysing whether fruit size depends on fruit turgor. 4 5 2. Materials and methods 6 2.1. Plant material, experimental conditions, and treatments 7 Two different but complementary experiments were performed with the 8 common goal of investigating if jujube fruit maturation period was clearly 9 sensitive to water deficit. In the first experiment (2012) control plants were 10 compared with plants subjected to moderate water deficit and with plants under 11 severe water deficit. In order to verify the results obtained in the first 12 experiment, in the second experiment (2013) control plants were compared with 13 other plants subjected to different water stress conditions. 14 Both experiments were carried out at a farm near the city of Albatera 15 (Alicante, Spain) (38º 12’ N, 0º 51’ W). The plant material consisted of 8-year-16 old jujube trees (Zizyphus jujuba Mill), cv. Grande de Albatera), planted at 2 m x 17 6 m. The soil of the orchard is a Torrifluvent with a sandy loam texture, very low 18 electrical conductivity (109 µS/cm, 1:10 w:v), high lime content (570 g/kg), very 19 low organic matter content (3 g/kg), low exchangeable potassium (40 mg/kg) 20 and available phosphorus (20 mg/kg) levels. The irrigation water had an 21 electrical conductivity of between 1.7 and 2.2 dS/m and a Cl − concentration 22 ranging from 36 to 48 mg l −1 . Pest control and fertilization practices were those 23 usually used by the growers, and no weeds were allowed to develop within the 24 orchard. 25 6 Jujube plants were drip-irrigated every night, using one lateral pipe 1 parallel to the tree row and 2 emitters per tree, each delivering 8 l h −1 . In-line 2 water meters were used to measure the water supplied to each experimental 3 unit. 4 5 Experiment 1 (2012) 6 During the 2012 experimental period (DOY 93-230), control plants 7 (treatment T0) were irrigated in order to guarantee non-limiting soil water 8 conditions (41 % daily crop reference evapotranspiration (ETo) during bud burst 9 and leafing (DOY 93-121), 52 % ETo during flowering and fruit set (stage I, 10 DOY 122-167), 69 % ETo during fruit growth (stage II, DOY 168-197) and 106 11 % ETo during fruit maturation (stage III, DOY 198-230). Such percentages were 12 applied according to the water needs obtained in previous results. T1 plants 13 were subjected to deficit irrigation throughout the season, according to the 14 criteria frequently used by the growers in the area (23 % ETo during bud burst 15 and leafing (DOY 93-121), 30 % ETo during flowering and fruit set (stage I, 16 DOY 122-167), 40 % ETo during fruit growth (stage II, DOY 168-197) and 61 % 17 ETo during fruit maturation (stage III, DOY 198-230). T2 treatment was irrigated 18 as T0 except during 32 days before harvest, in which irrigation was withheld 19 (from day of the year (DOY) 198 to 230). Total seasonal water amounts applied 20 were 440, 252 and 274 mm for T0, T1 and T2 treatments, respectively. 21 22 Experiment 2 (2013) 23 During the 2013 experimental period (DOY 101-242), control plants 24 (treatment T0) were irrigated with a similar criterion to that used in 2012 (42 % 25 7 ETo during bud burst and leafing (DOY 101-126), 53 % ETo during flowering 1 and fruit set (stage I, DOY 127-169), 76 % ETo during fruit growth (stage II, 2 DOY 170-198) and 110 % ETo during fruit maturation (stage III, DOY 199-242). 3 T3 and T4 plants were irrigated as T0 except during the last 17 and 24 days 4 before harvest in which irrigation was withheld (from day of the year (DOY) 225 5 (T3) and 218 (T4) to 242), respectively. Total seasonal water amounts applied 6 were 441, 360 and 322 mm for T0, T3 and T4 treatments, respectively. 7 8 2.2. Measurements 9 Meteorological data, namely air relative humidity, air temperature, solar 10 radiation, rainfall and wind speed 2 m above the soil surface, were collected by 11 an automatic weather station located near the experimental site. Mean daily air 12 vapour pressure deficit (VPD m ) and daily crop reference evapotranspiration 13 (ETo) were calculated according to Allen et al. (1998). 14 The water relations of the leaves and fruits were measured at midday (12 15 h solar time). Fruits and fully expanded leaves from the south facing side and 16 middle third of the tree of four trees per treatment were selected for 17 measurements. Leaf conductance (g leaf ) was measured with a porometer (Delta 18 T AP4, Delta-T Devices, Cambridge, UK) on the abaxial surface of two leaves 19 per tree. Leaf water potential (Ψ leaf ), and stem water potential (Ψ stem ) were 20 measured in a similar number and type of leaves as used for g leaf using a 21 pressure chamber (PMS 600-EXP, PMS Instruments Company, Albany, USA) 22 (Greenspan et al., 1994; Nobel and de la Barrera, 2000). Leaves for Ψ stem 23 measurements were enclosed in a small black plastic bag covered with 24 aluminium foil for at least 2 h before measurements. Fruit water potential (Ψ fruit ) 25 8 was measured with the pressure chamber (PMS 600-EXP, PMS Instruments 1 Company, Albany, USA) in two fruits per tree as described by McFadyen et al. 2 (1996) and Gelly et al. (2004). 3 Midday leaf (Ψ π leaf ) and fruit (Ψ π fruit ) osmotic potentials were determined 4 in the same leaves and fruits as used for Ψ leaf and Ψ fruit measurements, 5 respectively. Leaves and fruits were covered with aluminium foil and 6 immediately frozen in liquid nitrogen and stored at -80 ºC. The osmotic potential 7 was measured after thawing the samples and expressing the sap, using a 8 vapour pressure osmometer (Wescor 5600, Logan, USA). Estimated midday 9 leaf (Ψ p leaf ) and fruit (Ψ p fruit ) turgor potentials were derived as the difference 10 between osmotic and water potentials (Milad and Shackel, 1992; Mills et al., 11 1997; Yamada et al., 2004; Galindo et al., 2014). 12 Marketable jujube fruits were harvested on 18 August 2012 (DOY 230) 13 and 30 August 2013 (DOY 242). The mean weight of jujube fruit was 14 determined according to the weight and number of fruits per box in randomly 15 selected boxes per replicate. 16 17 2.3. Statistical design and analysis 18 The design of the experiment was completely randomized with four 19 replications, each replication consisting of three adjacent tree rows, each with 20 eleven trees. Measurements were taken on the inner tree of the central row of 21 each replicate, which were very similar in appearance (leaf area, trunk cross 22 sectional area, height, ground shaded area, etc.), while the other trees served 23 as border trees. Statistical analysis was performed by an analysis of variance 24 using the general linear model (GLM) of SPSS v. 12.0 (SPSS Inc., 2002). 25 9 To check the regression model hypothesis (linearity, homoscedasticity, 1 normality and independency) Kolmogorov–Smirnov with the Liliefors correction 2 was used. Normality and homoscedasticity on the typified residuals were 3 evaluated using Shapiro–Wilk and Levene tests, respectively. Linearity was 4 observed in the graphics and independency was assumed due to the way data 5 were obtained. 6 All the measurements were taken on the same tree in each replicate. 7 Values for each replicate were averaged before the mean and the standard 8 error of each treatment were calculated. 9 10 3. Results 11 3.1. Meteorological conditions and leaf and fruit water relations 12 During the 2012 and 2013 experimental periods, meteorological conditions were 13 very similar. In this sense, average daily maximum and minimum air 14 temperatures were 29.7 and 15.1 ºC and 29.2 and 14.9 ºC, respectively. VPD m 15 ranged from 0.39 to 2.35 kPa in 2012 and from 0.22 to 2.27 kPa in 2013, and 16 accumulated ETo were 654 mm and 643 mm in 2012 and 2013, respectively 17 (Fig. 1). In the 2012 and 2013 experimental periods total rainfall were 30 and 63 18 mm, respectively, which took place mainly on DOY 95 (18 mm) and 103 (9 mm) 19 in 2012 season and on DOY 115 (15 mm), DOY 117 (5 mm), DOY 118 (23 20 mm), DOY 240 (8 mm) and DOY 241 (9 mm) in 2013 season (Fig. 1). 21 The Ψ leaf and Ψ stem values in T0 plants were high and almost constant 22 throughout both experimental periods (Figs. 2A-D). 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Different letters indicate significant differences between treatments in the same year according to LSD 0.05 test. Season Treatment Yield NF FW 2012 T0 34.78a 1105.65a 31.46a T1 26.09b 899.98b 28.99a T2 10.89c 803.28b 13.54b 2013 T0 30.78a 946.00a 32.55a T3 18.91b 794.50b 23.81b T4 17.73b 797.75b 22.21b Fig. 1. Daily mean air temperature (Tm, solid thick line), daily crop reference evapotranspiration (ETo, thin line), mean daily air vapour pressure deficit (VPDm) (dashed thick line) and daily rainfall (vertical bars) during both experimental periods Fig. 2. Leaf water potential (Ψ leaf , A, B), stem water potential (Ψ stem , C, D), fruit water potential (Ψ fruit , E, F) and leaf conductance (g leaf , G, H) values (mean ± SE, not shown when smaller than symbols, n = 4) at midday for jujube plants in T0 (closed circles and solid line), T1 (open triangles up and dotted line), T2 (open squares and short dash line), T3 (closed diamonds in grey and dash dot line) and T4 (closed triangles down in grey and long dash line) treatments during the experimental periods (2012 and 2013). Asterisks indicate significant differences between treatments according to LSD 0.05 test.