Particle film technology: A supplemental tool to save water

Agricultural Water Management 147 (2015) 154–162

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Particle film technology: A supplemental tool to save water Francesca Boari a,∗ , Antonio Donadio a , Maria Immacolata Schiattone b , Vito Cantore a a b

Institute of Sciences of Food Production, CNR, Via Amendola, 122/O, 70125 Bari, Italy CIHEAM–Mediterranean Agronomic Institute of Bari, Via Ceglie 9, 70010 Valenzano (BA), Italy

a r t i c l e

i n f o

Article history: Available online 11 August 2014 Keywords: Gas exchange Salt stress Vegetables Water productivity Water stress

a b s t r a c t This paper aims to evaluate kaolin application as an effective tool for controlling stomatal conductance (gs ) and transpiration rate (E), in order to enhance tolerance to water stress and salinity and to improve water productivity. Five experiments were carried out under open field conditions in Southern Italy in order to evaluate the effect of kaolin application on the physiological responses of well watered bean and Clementine, tomato under three salinity levels and two irrigation regimes and to try to overcome the vegetables’ transplant shock under two irrigation regimes. Kaolin reduced crop evapotranspiration in bean by 13% and increased photosynthetic water productivity (pWP) and yield water productivity (yWP) by 20 and 9.8%, respectively. In Clementine kaolin reduced E by 26.2% and increased pWP by 30.5%. In tomato kaolin improved the water status of the plant, and reduced gs , net photosynthesis (A) and E under low salinity or well watered conditions. By contrast, under salt stress or drought, kaolin was effective in limiting the reduction in A and reducing leaf temperature (Tl ). This increased pWP by 25.3 and 33.1%, respectively, in well watered and water stressed tomato, and by 9.2 and 24.9%, respectively, in the non saline and high saline treatment. Kaolin proved effective in limiting transplant shock under conditions of limited water supply, increasing dry biomass growth rate and biomass water productivity (bWP) by 18.4 and 19%, respectively. Kaolin can be efficiently utilized as an antitranspirant to alleviate the effects of drought and salinity, to reduce transplant shock and to save water in dry regions. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Water is an essential production factor in agriculture, but the imbalances between precipitation and water needs of crops have a significant impact on yields and quality of agricultural products. This problem has now become a real concern, made even more critical by the growing increase in arid conditions due to climate change (Bates et al., 2008). Therefore, the management of water resources is becoming one of the major geostrategic challenges of the 21st century. All strategies that can be used to help save water and improve its productivity thus assume growing importance. The use of kaolin in agriculture, principally to fight pests and for the mitigation of heat stress, may prove to be an opportunity to exploit in the field of water saving, due to its antitranspirant effect. Kaolin-based particle film technology (Pft) has been developed over the past 15 years as a multi-functional, environment-friendly material, that provides effective insect control, mitigates heat

∗ Corresponding author. Tel.:+39 080 5929300. E-mail address: [email protected] (F. Boari). http://dx.doi.org/10.1016/j.agwat.2014.07.014 0378-3774/© 2014 Elsevier B.V. All rights reserved.

stress, and contributes to the production of high-quality fruit and vegetables, as well as being suitable for organic farming (Glenn and Puterka, 2005). Originally, kaolin (Surround® WP, Engelhard Corporation, Iselin, NJ, Serbios S.r.l., Badia Polesine-RO, Italy) was developed for the suppression of pests in many crops (Pace and Cantore, 2009). In addition, it has been demonstrated that the white kaolin film formed on the leaf surface increases the reflection of incoming solar radiation, changing the radiation and heat balance and reducing the risk of leaf and fruit damage from high temperatures and solar injury (Glenn, 2012). Moreover, the reduction of sunburn in some fruits, such as pomegranate, apple, walnut, citrus and tomato has been widely proven (Cantore et al., 2009; Glenn, 2012; Weerakkody et al., 2010). The mitigation of fruit temperature by kaolin may also contribute to an increase in average fruit weight (Cantore et al., 2009; Lalancette et al., 2005; Saleh and El-Ashry, 2006) and to the improvement of some qualitative aspects of fruits as redness, total soluble solids and anthocyanins concentration (Chamchaiyaporn et al., 2013; Shellie and King, 2013a,b; Yazici and Kaynak, 2009). In addition to the effects mentioned above, Pft can affect stomatal conductance and gas exchange, as a consequence of the

F. Boari et al. / Agricultural Water Management 147 (2015) 154–162

partial reflection of PAR (Photosynthetic Active Radiation) and partial stomatal occlusion. However, results from the literature on the beneficial effects of kaolin are sometimes conflicting and vary according to species, plant architecture, environmental conditions and scale of measurement (leaf or canopy scale). On a single leaf scale, some authors have observed an increase in net assimilation (A) determined by kaolin (Chamchaiyaporn et al., 2013; Jifon and Syvertsen, 2003; Privé et al., 2007), whereas others have observed a reduction attributed both to the reduction in PAR intercepted by leaves and to the partial obstruction of stomata (Cantore et al., 2009; Rosati et al., 2006; Shellie and King, 2013b). At plant or canopy scale, Glenn et al. (2003) reported that net assimilation in apple increases in kaolin-treated plants, but only under high temperature conditions. Wünsche et al. (2004), meanwhile, observed a contrast in behaviour between the two scales of measurement. In fact, while on the leaf scale, net assimilation was reduced in the presence of Pft, at the whole plant level, no variations were observed. The author argued that the lower assimilation of the outer leaves was compensated by an increase in assimilation rate of the inner leaves that intercept the radiation reflected by the Pft, which produces a better distribution of PAR within the canopy. For tomato plants, by contrast, a reduction in net assimilation at canopy scale has also been observed (Cantore et al., 2009). The difference, compared to the data for apple by Wünsche et al. (2004), is attributed to the architecture of the canopy, which differs from herbaceous to tree crops (Makus, 2005). The reduction in stomatal conductance (gs ) and transpiration caused by kaolin has been observed in well-watered crops such as apple, pecan, tea, bell pepper, bean and tomato (Cantore et al., 2009; Le Grange et al., 2004; Lombardini et al., 2005). In addition, when comparing different water regimes on grapevine, Shellie and Glenn (2008) found a decrease in gs due to kaolin application only in well watered conditions. Kaolin’s potential as an antitranspirant may be exploited to reduce water stress of seedlings during establishment stages, especially when they are bare-root. This has been shown in the strawberry where kaolin, in addition to promoting plant establishment, also reduced its water needs by 30% at that delicate phenological stage (Santos et al., 2012). The antitranspirant effect of kaolin, proven on many species (Angbabu et al., 2007; El-Khawaga, 2013; Schroeder and Johnson, 2004) can result in a considerable benefit in terms of water savings and, in arid and semi-dry environments, in an increase in yield for crops sensitive to water stress. The hypotheses tested here were that the use of kaolin-based particle film technology may be an effective tool to control gs and transpiration rate, thus mitigating the detrimental effects of water stress and/or salinity, transplanting stress, improving water productivity and, overall, saving water. Therefore, the objectives of our research were to verify these hypotheses by studying the effects of kaolin application on (i) stomatal conductance, transpiration, plant water status and water use, in field grown bean, Clementine, and tomato under salt and water stress, (ii) transplanting stress in tomato, bell pepper, eggplant and zucchini seedlings.

2. Material and methods Five experiments were performed in Southern Italy to assess the effects of kaolin application (Surround® WP) on (i) gas exchange, crop evapotranspiration (ETc ) and water productivity in bean (Exp. 1), (ii) gas exchange and water productivity in Clementine (Exp. 2), (iii) salt stress in tomato (Exp. 3), (iv) water stress in tomato (Exp. 4), (v) transplant stress in tomato, bell pepper, eggplant and zucchini seedlings (Exp. 5).

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2.1. Experimental sites and climatic data The experiments were carried out in different years: summer 2006 (Exp. 1), autumn 2007 (Exp. 2), summer 2008 (Exp. 3 and 4) and summer 2011 (Exp. 5). Experiments 1, 3, 4 and 5 were performed at the experimental farm “E. Pantanelli” of the University Aldo Moro of Bari in Policoro (MT) (40◦ 10 20 NL, 16◦ 39 04 EL, altitude 15 m), characterized by loam soil. Experiment 2 was carried out on a private farm, in the countryside near Terranova di Sibari (CS) (39◦ 39 25 NL, 16◦ 20 05 EL, altitude 107 m), characterized by sandy clay soil. The main physical and chemical characteristics of both sites are reported in Table 1. At both sites, the climate is sub-humid, according to the De Martonne classification (Cantore et al., 1987). For the experiments carried out in Policoro, meteorological data were collected from a standard agrometeorological weather station located near the experimental fields equipped with a pyranometer (model CM 4, Kipp and Zonen, Delft, The Netherlands), thermistor (model E001, Tecno.El, Rome, Italy), hygrometer (C-83 N Rotronic, Zurich, Switzerland), anemometer (model VT 0805B, SIAP Bologna, Villanova di Castelnaso-BO, Italy), ‘class A’ pan (NovaLynx Corporation Grass Valley, Auburn, Calif.), and tipping bucket rain gauge (Tecno.El, Rome, Italy), for measuring solar radiation, air temperature, relative humidity, wind speed, ‘class A’ pan evaporation and rainfall, respectively. The weather data were collected by the electronic system operated via a data-logger (model Kampus, Tecno.El, Rome, Italy) connected via modem to a PC. 2.2. Exp. 1 (bean) Two treatments on bean crop (Phaseolus vulgaris L.) cv Dragone, each disposed in one of two contiguous plots and in the corresponding lysimeters, were compared,: (i) crop sprayed with kaolin suspension (4% w/v) (K) every ten days (a total of 4 sprays), from the beginning of the flowering phase (31/7) until harvest, using a normal atomizer used for plant protection treatments, and (ii) a non-kaolin sprayed control (C), in which fresh water was sprayed using the same equipment. The bean was sown on June 18 in NorthSouth oriented rows, 0.5 m apart, in two contiguous plots, each with a square surface area of 2500 m2 . Two weighing lysimeters, with a surface area of 2 × 2 m2 and a depth of 1.3 m, were located in the centre of each plot. After emergence (June 26), the plant density was 26 plants m−2 . Water was applied with the drip method, restoring whole water losses by evapotranspiration (ETc ), when the soil reached 40% of available water depletion. Daily ETc of the bean crop was measured with the weighing lysimeters, by reading the weight each morning at 7:00 am. Harvesting took place on September 9 and the fresh pod yield was assessed. Leaf gas exchange (net CO2 assimilation—A, transpiration—E, stomatal conductance—gs ) was measured with a portable photosynthetic open-system ADC-LCA3 (Analytical Development Co., Hoddesdon, UK) equipped with an assimilation leaf chamber 6.2 cm2 large. The measurements were performed on clear-sky days (PAR > 1900 ␮mol m−2 s−1 ) between 11:30 and 14:30 h at the pod-setting stage (August 1 and 11). At each measurement time, two fully-expanded, healthy leaves well-exposed to the sun were selected for measurements on eight plants per plot chosen randomly. The kaolin-treated leaves were selected from those with the most uniform coating. The measuring order adopted for kaolin-treated leaves and untreated ones was completely random. Leaf temperature (Tl ) was assessed on the abaxial surface simultaneously with leaf gas-exchange measurements, using a fine-wire thermocouple mounted in the leaf chamber (PLC) of ADC-LCA3.

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Table 1 Main physical, chemical and hydrological characteristics of the soil in the five experiments. Soil parameters

Unit

Policoro Exp. 1

Particle-size analysis Total sand (2 > ø > 0.02 mm) Silt (0.02 > ø > 0.002 mm) Clay (ø < 0.002 mm) Chemical properties Total nitrogen (Kjeldahl method) Available phosphorus (Olsen method) Exchangeable potassium (ammonium acetate method) Organic matter (Walkley Black method) Total limestone (Dietrich-Fruhling method) Active limestone ECe ESP pH (pH in H2 O) Hydrological properties Field capacity Wilting point (− 1.5 MPa) Bulk density

Terranova di Sibari Exp. 3 and 4

Exp. 5

Exp. 2

(g 100 g−1 ) (g 100 g−1 ) (g 100 g−1 )

39.5 36.8 23.7

40.0 37.1 22.9

38.2 37.4 24.4

51.2 39.4 9.4

(g kg−1 ) (mg kg−1 ) (mg kg−1 ) (g 100 g−1 ) (g 100 g−1 ) (g 100 g−1 ) (dS m−1 ) (%)

1.61 25.9 215.3 3.4 1.6 0.4 0.89 1.8 7.6

1.67 26.7 227.0 3.6 1.5 0.5 0.95 1.9 7.7

1.85 29.0 280.6 3.0 1.5 0.4 0.91 2.0 7.7

1.11 19.4 163.5 2.2 1.9 0.7 1.2 1.6 7.8

(g 100 g−1 ) (g 100 g−1 ) (kg dm−3 )

31.4 14.9 1.22

31.5 15.0 1.25

31.6 15.2 1.27

26.9 12.4 1.34

ECe, saturation extract electrical conductivity; ESP, exchangeable sodium percentage.

Photosynthetic water productivity (pWP) was computed from the A/E ratio, while yield water productivity (yWP) was calculated from the pod yield/total ETc ratio. 2.3. Exp. 2 (Clementine) In the 11-year-old Clementine (Citrus clementina Hort. ex Tan.) orchard, two treatments were compared: (i) kaolin (1.5% w/v) sprayed trees (K) (on September 13 and 22, and October 1) with a normal atomizer used for plant protection treatments, and (ii) untreated control (C). The experimental plan was a randomized blocks with four replications (10 trees per plot). The orchard was well watered by the drip method, according to the normal irrigation scheduling applied at the farm, which consisted in watering when soil water depletion reached 30–40% of available water. The latter was controlled by four tensiometers placed at a depth of 0.25 and 0.50 m, in correspondence of the drippers. Leaf gas exchange parameters and leaf temperature were measured at veraison stage (October 2 and 9) with the same equipment and criteria described for experiment 1. At each measurement time, two fully-expanded, healthy, and sun well-exposed leaves were chosen for measurements on two plants per plot. Leaf water potential ( l ) was measured simultaneously to leaf gas exchange, using the pressure bomb (Model 3005, Ecosearch, Città di Castello-PG, Italy). The measurements were performed on two fully expanded, healthy, young leaves of the same plants on which gas exchanges were monitored. pWP was computed from the A/E ratio. 2.4. Exp. 3 (tomato-salinity) Three salinity levels of irrigation water (ECw = 0.5, 5 and 10 dS m−1 , shown as S0, S1 and S2, respectively) and two kaolin treatments (kaolin treated and control untreated, shown as K and C, respectively), on tomato (Solanum Lycopersicum L.) crop cv Perfectpeel were compared. The experimental arrangement was a split plot design with three replications, and plots with a surface area of 40 m2 . Tomato seedlings were transplanted by hand, on soil mulched with black PVC, at the third true leaf stage on May 12, in single East–West oriented rows, 1.5 m apart, with 0.25 m between plants in the row, giving a plant density of 2.7 plants m−2 . Water was applied with the drip method every 2–3 days to keep soil water

content in the root zone close to readily available water (40% of available water depletion) and to restore 100% of ETc . Drip lines, with in-line emitters located 0.30 m apart and with an emitter flow rate of 4 L h−1 , were placed 10 cm away from the plants. ETc was calculated as follows: ETc = ETo Kc

(1)

where ETc = crop evapotranspiration (mm day−1 ); Kc = crop coefficients reported by Tarantino and Caliandro (1984), adjusted for saline treatments (Allen et al., 1998); ETo = reference evapotranspiration computed as follows: ETo = EV Kp

(2)

where EV = ‘class A’ pan evaporation (mm day−1 ), Kp = pan coefficient reported by Castrignanò et al. (1985). At the fruit enlargement stage, kaolin suspension (4% w/v) was sprayed, utilizing a backpack power sprayer (model MS073D, Maruyama Mfg. Co. Inc., Japan) in treatment K. Instead, in the control, fresh water was sprayed with the same sprayer. The kaolin applications were repeated every 7–10 days until the fruit-ripening stage. Leaf gas exchange parameters and leaf temperature were measured at the end of fruit enlargement stage (July 29 and August 6) with the same equipment and criteria described for experiment 1. At each measurement time, two fully-expanded, healthy, and sun well-exposed leaves were chosen for measurements on two plants per plot. Leaf water potential was measured simultaneously to the leaf gas exchanges, with the same equipment as in experiment 2. The measures were performed on two fully expanded, healthy, young leaves of the same plants on which gas exchanges were monitored. pWP was computed from the A/E ratio. On the same days as the gas exchange measurements, the electrical conductivity of saturated extract (ECe ) was measured in situ by an EC-probe (Eijkelkamp Agrisearch Equipment, Geisbeek, the Netherlands), according to the Rhoades and van Schilfgaarde (1976) method, as follows: in three places per plot, crosswise to the row at 0–0.25–0.50 m from the emitters, at three different depths (0.20–0.40 and 0.60 m). 2.5. Exp. 4 (tomato–water stress) Two water regimes (restore 100% and 50% of ETc , shown as WW and WS, respectively) and two kaolin treatments (kaolin treated

F. Boari et al. / Agricultural Water Management 147 (2015) 154–162

and control untreated, shown as K and C, respectively), on tomato cv Perfectpeel, were compared. The experimental arrangement was a split plot design with three replications, and plots with a surface area of 40 m2 . Tomato seedlings were transplanted the same day and in the same way as in experiment 3. Water was applied with the drip method every 2–3 days to keep soil water content in the root zone close to the readily available water (40% of available water depletion) and to restore 100 and 50% of ETc , respectively, in the WW and WS treatments. The irrigation equipment and the ETc calculation were also the same as in experiment 3. When the water content in the root zone dropped below the readily available water, ETc was adjusted for water stress through a dimensionless reduction coefficient (Ks), depending on the level of depletion (0–1) of water (Allen et al., 1998). The same criteria as used in experiment 3 were utilized for kaolin application. Physiological measurements (gas exchange, Tl and  l ) were also performed as in experiment 3, on July 22 and 25.

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Table 2 Effect of kaolin treatments on net assimilation (A, ␮mol m−2 s−1 ), transpiration (E, mmol m−2 s−1 ), stomatal conductance (gs , mol m−2 s−1 ), photosynthetic water productivity (pWP), and leaf temperature (Tl , ◦ C) of bean (mean values of two days: August 1 and 11). Treatments

A

E

gs

pWP

Tl

**

*

*

C K

ns 23.6 23.1

11.6 9.9

0.65 0.56

2.0 2.4

ns 29.6 30.2

ns, * , ** indicate F test not significant or significant at P < 0.05 and P < 0.01, respectively.

2.6. Exp. 5 (vegetable transplant stress) Two watering regimes (RI) (restoration of 50%-RI50 and 100%RI100 of ETc ), and two kaolin treatments (seedlings sprayed with kaolin-K and unsprayed control-C), on seedlings of tomato cv Tomito, eggplant (Solanum melongena L.) cv Danka F1, bell pepper (Capsicum annuum L.) cv Kent F1, and zucchini (Cucurbita pepo L.) cv Alexander F1, were compared. The experimental plan was a split plot with four replications, arranging the watering regimes in the main plots and species in the sub-subplots. Fifteen seedlings of zucchini and 25 of each other species were transplanted into each sub-subplot. The seedlings, previously grown in nurseries, in growing trays and including their rootball, were transplanted by hand on July 5, in rows 1 m apart, with 0.3 (3.3 plants m−2 ) and 0.2 m (5 plants m−2 ) between plants in the row, respectively, zucchini and the three Solanaceae. Before transplanting, the soil was fertilized by applying 50 and 40 kg ha−1 of nitrogen and phosphorus, respectively, as ammonium nitrate and perphosphate. Two kaolin (4% w/v) applications were performed: the first immediately before transplant, and the second after seven days, utilizing a backpack power sprayer (see Section 2.4). Water was applied by the drip method at transplanting time and when water losses by evapotranspiration in RI100 treatment had reached 30% of available water in the root zone, by restoring whole water losses in RI100 and 50% in RI50 . The ETc of each species was calculated as in experiment 3, utilizing the Kc reported by Allen et al. (1998). When the water content in the root zone dropped below readily available water, ETc was adjusted for water stress as reported in experiment 4. Immediately before transplanting, height and shoot (sum of leaves and stems) fresh/dry biomass were determined on 50 seedlings of each species. After 17 days, all seedlings were harvested at root collar level and, after measuring the height, the seedlings were washed to remove kaolin and soil residues in order to determine fresh and dry biomass. The latter was assessed by putting the plants to dry in a ventilated oven at 70 ◦ C until constant weight. Dry biomass growth rate (DBGR) was calculated as the ratio between above-ground dry biomass and the number of growing days (17). Midday leaf water potential was measured 13 days after transplanting, with the same equipment as in experiment 2. The measures were performed on two fully expanded, healthy, young leaves for each plot. Biomass water productivity (bWP) was

Fig. 1. Trends (a) of daily crop evapotranspiration (ETc ) of bean sprayed (K) and not with kaolin (C), and (b) of the percentage difference of ETc of crop sprayed with kaolin compared to the unsprayed one. The arrows indicate the dates of treatment with kaolin. Horizontal lines in Fig. 3b indicate the average value of the difference between the ETc of two lysimeters before and after the first spraying with kaolin.

computed from the ratio between shoot dry biomass and total evapotranspiration. 2.7. Statistical analysis In all experiments, analysis of variance of treatment effects was performed, and means were separated according to the post-hoc SNK test, using the facility supplied with the statistical package SPSS 12.0 (SPSS Inc., Chicago, IL). 3. Results 3.1. Exp. 1 (bean) Kaolin application on bean reduced gs and E, respectively, by 14.8 and 13.6%, and improved pWP by 20% (Table 2). Total water use for the bean crop was 529 and 520 mm for C and K, respectively. Daily ETc ranged from 2 to 10.8 mm for C and from 2.1 to 10.4 mm for K (Fig. 1a). The fluctuations observed during the crop cycle are related to the phenological stage, leaf area index and climatic variables. From sowing time to the first spraying with kaolin, the ETc of K, on average, was 5.5% higher than C; thereafter, instead, ETc was

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Table 3 Effect of kaolin treatments on net assimilation (A, ␮mol m−2 s−1 ), transpiration (E, mmol m−2 s−1 ), stomatal conductance (gs , mol m−2 s−1 ), photosynthetic water use efficiency (pWP), leaf water potential ( l , MPa), and leaf temperature (Tl , ◦ C) of Clementine (mean values of two days: October 2 and 9 2007). Treatments

A

E

gs

pWP

l

Tl

*

*

*

*

C K

ns 8.1 7.8

4.2 3.1

0.16 0.11

1.9 2.5

−1.08 −0.91

ns 27.6 27.9

ns, * , ** indicate F test not significant or significant at P < 0.05 and P < 0.01, respectively.

Table 4 Physiological measures carried out on tomato plants (mean values of two days: July 28 and August 6 2008): main effects of salinity level and kaolin application on net assimilation (A, ␮mol m−2 s−1 ), transpiration (T, mmol m−2 s−1 ), stomatal conductance (gs , mol m−2 s−1 ), photosynthetic water productivity (pWP), leaf water potential ( l , MPa), leaf temperature (Tl , ◦ C). Treatments

A

E

gs

pWP

l

Salinity S0 S1 S2 Kaolin C K

**

**

**

*

*

17.3 a 14.4 b 11.1 c ns 14.0 14.4

7.6 a 6.2 b 5.1 c

0.31 a 0.23 b 0.17 c

ns 2.3 2.3 2.2

−0.99 a −1.13 b −1.47 c

*

*

*

*

6.6 6.0

0.25 0.22

2.1 2.4

−1.28 −1.11

31.5 c 32.6 b 34.4 a ns 32.8 32.9

Tl

ns, * , ** indicate F test not significant or significant at P < 0.05 and P < 0.01, respectively. Mean separation within columns by SNK test (P < 0.05).

lower by an average of 7.5% (Fig. 1b). Therefore, an overall reduction of 13% in ETc was recorded when kaolin was applied to the bean crop. In practice, in the 38 days between the first spraying with kaolin and harvesting, the sprayed crop showed 37 mm less water use than the control (about 1 mm d−1 ). As for pWP, yWP was also higher in the kaolin-sprayed crop (3.7 kg m−3 ) than in the control (4.1 kg m−3 ).

Table 5 Physiological measures carried out on tomato plants (mean values of two days: July 22 and 25 2008): main effects of water regime, and kaolin application on net assimilation (A, ␮mol m−2 s−1 ), transpiration (E, mmol m−2 s−1 ), stomatal conductance (gs , mol m−2 s−1 ), photosynthetic water productivity (pWP), leaf water potential ( l , MPa), leaf temperature (Tl , ◦ C). Treatments

A

E

gs

pWP

l

Tl

Water regime WW WS Kaolin C K

**

**

**

*

**

*

16.5 11.1

6.7 5.1 ns 6.0 5.8

0.33 0.17 ns 0.24 0.26

2.5 2.1

−1.11 −1.56

*

*

2.1 2.6

−1.47 −1.20

32.4 34.0 ns 33.6 32.8

*

12.8 14.8

ns, * , ** indicate F test not significant or significant at P < 0.05 and P < 0.01, respectively.

S0 to S2, A, E, gs and  l reduced, respectively, by 36.0, 33.8, 44.2 and 49.2%, Tl increased by 9.2%, while pWP remained unchanged (Table 4). Kaolin, on average for the salinity treatments, reduced E and gs , respectively, by 9.4 and 14.8%, increased pWP and  l , respectively, by 14.5 and 12.7%, while it did not significantly affect A and Tl . However, response to kaolin application was different in relation to the saline treatments. In fact, there was a significant interaction between salinity and treatment with kaolin for all variables (Fig. 2). While in S0 kaolin showed a significant reduction in variables related to gas exchange, the decrease was slowed down in S1, and almost annulled in S2. Kaolin improved  l with increasing salinity level. In fact, the increase in this variable after kaolin application varied from 4.9% for S0 to 16.1% for S2. As a consequence of applying kaolin, Tl rose by 2.2% in S0, corresponding to about 0.7 ◦ C, while an inversion occurred with increasing salinity. In fact, in K these parameters were reduced by an average of 0.5% in S1 and of 1.4% in S2, corresponding to about 0.2 and 0.5 ◦ C, respectively (Fig. 2). 3.4. Exp. 4 (tomato–water stress)

3.2. Exp. 2 (Clementine) There were no changes in the variables examined between the measurement dates; therefore, the following are the average values for the two dates. Pft application resulted in the reduction of E by 26.2%, mainly due to partial obstruction of the stomata by the kaolin film and the consequent reduction in gs (−32.3%) (Table 3). The reduction of E caused by kaolin contributed to the preservation of water status in the plants, as evidenced by  l which, in the kaolin-sprayed plants, was 15.7% higher than in the control (Table 3). The lower water losses by plants treated with kaolin were reflected in the 30.5% increase in pWP. 3.3. Exp. 3 (tomato-salinity) Irrigation with brackish water led to an increase in soil salinity, according to the salinity levels. In fact, on the dates of the gasexchange measurements, as an average of three soil layers (0–20, 20–40 and 40–60 cm) and three distances from the dripper, ECe was almost unchanged (1.1–1.2 dS m−1 ) in the treatment irrigated with fresh water (S0), while in the two saline treatments ECe reached values ranging between 4.5 and 4.7 dS m−1 in S1 and between 6.5 and 6.9 dS m−1 in S2, respectively, on July 28 and August 6. No differences were observed between the two dates in the measured parameters. Below are therefore reported the averages for the two measurement dates. In general, salinity levels and kaolin application affected A, E, gs , pWP,  l and Tl (Table 4). With increasing salinity, both gas exchanges and  l reduced, while Tl increased. Particularly, from

As in experiment 3, no differences were observed between the two dates in the measured variables, so the average values measured are reported. In general, the parameters related to gas exchange were influenced both by the irrigation regime and the kaolin treatments (Table 5). Water stress reduced gas exchange and  l . In particular, A, E, gs , pWP,  l were lower in WS than WW, respectively, by 33, 23, 50, 14.9 and 40.5%, whereas Tl increased by 5.1% as a consequence of a lower transpirational cooling effect. Kaolin application for all the irrigation regimes led to an average increase in A and pWP of 16.1 and 24.3%, respectively, as is evident from  l which was 18.4% higher than for the control (Table 5). In addition, there was significant interaction between the irrigation regimes and treatments with kaolin for some of the variables examined (Fig. 3). Kaolin application improved the water status of plants, mainly in WS, as is evident from the  l , which corresponds to higher values of A, E and gs . Kaolin influenced Tl in a different way in relation to the irrigation regime. For instance, kaolin made no significant changes in the well watered crop, while in the water-stressed one it reduced Tl by 1.8 ◦ C (Fig. 3). 3.5. Exp. 5 (vegetable transplant stress) The seedlings grew to different extents with the various treatments. Reducing the water supply caused a 30% drop in  l . In contrast, with the reduction in water availability (RI50 ), the increase in fresh and dry biomass, height, DBGR and bWP during the 17 days

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Fig. 2. Physiological measures carried out on tomato plants (mean values of two days: July 28 and August 6 2008): interaction between salinity levels (S0–S2) and kaolin treatments (C-control without kaolin; K-kaolin sprayed plants) on net assimilation (A), transpiration (E), leaf water potential ( l ), stomatal conductance (gs ), photosynthetic water productivity (pWP), and leaf temperature (Tl ). The vertical bars indicate the standard deviation. ns, * indicate F test not significant or significant at P ≤ 0.05.

of growth, fell, respectively, by 18, 13, 26, 10 and 25% compared to the crops under the optimum water regime (RI100 ) (Table 6). The increase in the biometric parameters was significantly higher in zucchini and lower in eggplant and bell pepper. Kaolin application led to an overall average increase of 17% in  l , while it had no effect on the biometric variables and bWP. In addition, a significant interaction was observed between treatment with kaolin and irrigation regime (Table 6, Fig. 4). Seedlings subjected to the optimum irrigation regime grew the most in C, whereas, in the case of deficit irrigation, plants sprayed with kaolin grew the most. Indeed, with optimum irrigation, kaolin

treatment compared to the control resulted in a lower increase in fresh and dry biomass, height, DBGR and bWP, by 8, 7, 9, 8 and 7%, respectively. In contrast, with deficit irrigation, kaolin increased the same variables, respectively, by 17, 17, 39, 18 and 19% compared to the untreated control. 4. Discussion In this work, we aimed to prove that kaolin, normally used for the control of some pests as an alternative to agrochemicals, and sometimes also to protect fruits from heat stress (e.g. sunburn),

Fig. 3. Physiological measures carried out on tomato plants (mean values of two days: July 22 and 25 2008): interaction between water regimes (WW-well watered; WS-water stressed) and kaolin treatments (C-control without kaolin; K-kaolin sprayed plants) on net assimilation (A), transpiration (E), leaf water potential ( l ), stomatal conductance (gs ), photosynthetic water productivity (pWP), and leaf temperature (Tl ). The vertical bars indicate the standard deviation. ns, * indicate F test not significant or significant at P ≤ 0.05.

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Table 6 Biometric and physiological parameters of seedlings of vegetables species: main effects of water regime, kaolin application and species on the increase in fresh biomass, dry biomass and height to 17 days after transplanting, dry biomass growth rate (DBGR), dry biomass water productivity (bWP) and leaf water potential ( l ) of seedlings. Treatments Water Regime (WR) RI100 RI50 Kaolin (Ka) C K Species (S) Eggplant Pepper Tomato Zucchini Significance (1) WR Ka S WR × Ka WR × S Ka × S

Fresh biomass (g plant−1 )

Dry biomass (g plant−1 )

Height (cm)

DBGR (g day−1 plant−1 )

bWP (kg m−3 )

 l (MPa)

35.8 a 29.3 b

3.3 a 2.9 b

10.9 a 8.1 b

0.19 a 0.17 b

0.28 b 0.35 a

−1.05 a −1.36 b

32.9 32.2

3.0 3.1

9.1 9.9

0.18 0.18

0.31 0.31

−1.32 b −1.10 a

10.4 c 6.3 c 35.9 b 77.6 a

1.6 c 0.9 c 3.6 b 6.3 a

5.0 c 8.1 b 11.8 a 13.0 a

0.09 c 0.05 c 0.21 b 0.37 a

0.16 c 0.09 c 0.35 b 0.62 a

−1.18 −1.25 −1.19 −1.22

**

**

**

*

*

*

ns

ns

ns

ns

ns

**

**

**

**

**

**

*

*

*

*

*

ns ns

ns ns

ns ns

ns ns

ns ns

ns ns ns ns

(1) Separation of the averages within columns with the SNK test (P < 0.05). F-test not significant (ns) or significant (* ) (P < 0.05).

may also be a useful tool for saving water and improving crop performances in the presence of salinity or water stress. The results from the five experiments confirm the starting hypothesis. In particular, kaolin was effective in reducing gs and E, probably mainly due to partial obstruction of the stomata by the particle film (Cantore et al., 2009; Boari et al., 2014), thus improving the water status of the plant, according to the findings of other authors (Le Grange et al., 2004; Lombardini et al., 2005; Shellie and King, 2013a,b). In the well watered bean crop, kaolin reduced gs and E, leading to a reduction in ETc . These results are in agreement with those obtained for bean (Tworkoski et al., 2002), as well as for other species, such as tomato and tea (Anandacoomaraswamy et al., 2000; Cantore et al., 2009) at a leaf and/or whole plant scale. However, they are contrary to the findings of Glenn et al. (1999), who

found that kaolin, marketed as Surround® WP, due to its particular physical characteristics, does not impede gs and gas exchange in apple. By reducing evapotranspiration and water use without affecting net assimilation, kaolin increased bWP and yWP in bean crop, in agreement with the results reported by Lukic et al. (2012) who obtained similar results on tomato crop. Similarly to the bean crop, the well watered Clementine orchard recorded a positive effect of kaolin on the control of gas exchange which led to an improvement in pWP. In the tomato crop, as expected, with the increase in salinity both gas exchanges and  l reduced, whereas Tl rose. In particular, the increase in salts in the soil lowers soil osmotic potential ( ␲ ) and, consequently, total soil water potential ( t ) (Caruso, 1993). This implies a decrease in water availability for the plant (Munns, 2002),

Fig. 4. Increase in fresh biomass, dry biomass and height of seedlings to 17 days after transplanting, and dry biomass growth rate (DBGR), leaf water potential ( l ) and dry biomass water productivity (bWP) of seedlings: interaction between irrigation regime and treatments with kaolin. The vertical bars indicate SD.

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reducing its  l and, consequently, stomatal conductance and gas exchanges (Brugnoli and Lauteri, 1991). This also led to a reduction in the transpirational cooling effect with a leaf temperature increase (Sharma et al., 2005). Similarly to the previous crops, taking all the salinity treatments together, kaolin reduced E and gs values in tomato, increasing pWP and  l , but not affecting A and Tl . However, response to kaolin application varied among saline treatments. The interaction between salinity and kaolin treatment is in agreement with the observations for the grapevine, where kaolin application reduced gs on crops receiving an optimal water supply while it did not affect gs in water-stressed crops (Shellie and Glenn, 2008). Indeed, water stress, in many senses, causes the same effects as salt stress in terms of reducing soil  t (Yeo, 1983). In practice, with moderate or no water stress, the stomatal limitation caused by kaolin prevails, reducing gas exchange. Under salt stress, when stomatal limitation in untreated crops is high, kaolin shows its effectiveness as an antitranspirant (Schroeder and Johnson, 2004; Angbabu et al., 2007; El-Khawaga, 2013) allowing crops to preserve good water status by limiting water loss (Boari et al., 2014). The variations in Tl related to kaolin application and salinity levels is in agreement with transpiration data. Indeed, kaolin influences the heat balance of vegetation mainly due to the dual effect of reflection of incoming radiation and partial occlusion of the stomata which, in turn, reduces transpiration and the resulting cooling effect (Cantore et al., 2009; Boari et al., 2014). In water-stressed tomato, a reduction in gas exchange and  l was observed. Indeed, reduction in water supply decreases water availability for the plant, reducing its  l and, consequently, gs and gas exchange (Brugnoli and Lauteri, 1991). This also led to a reduction in the transpirational cooling effect with an increase in Tl (Sharma et al., 2005). By reducing stomatal conductance and, in turn, improving the plant water status, mainly in water stressed plants, kaolin improved the response of tomato to the water stress, by mitigating the harmful effect of heat and drought. Similar results to the above were also obtained on well watered and water-stressed grapevine (Shellie and Glenn, 2008). In practice, the reduction in gs determined by kaolin, as demonstrated in other species (De Boer et al., 1983; Schroeder and Johnson, 2004; Angbabu et al., 2007), would help to save water, which would benefit more crops with limited water supplies and, therefore, over time would maintain a higher assimilation rate than crops without kaolin, as also observed by Denaxa et al. (2012) on olive trees. Kaolin affected Tl in tomato plants in different ways for each irrigation regime as observed also by Shellie and King (2013a,b) for grapevine. In particular, kaolin did not lead to any changes in the well watered crop, while in the water-stressed one, with very low water availability for transpiration and thermoregulation, reflection of incoming radiation due to kaolin prevailed, with a consequent reduction in Tl . Kaolin proved effective in limiting transplant stress in water shortage conditions. With optimum irrigation, kaolin application resulted in a lower increase in biometric parameters and bWP. In contrast, under deficit irrigation, kaolin improved the same variables. This shows that, when water supply is good, the negative effect of kaolin on the accumulation of biomass prevails, due to the reduction in net assimilation (Cantore et al., 2009). By contrast, the positive effect of kaolin in water-stressed conditions confirms the starting hypothesis. In this case, kaolin acts as an antitranspirant (Angbabu et al., 2007; El-Khawaga, 2013), improves plant water status, and mitigates the leaf temperature increase, thus protecting the plants from water stress. In the case of the four vegetables studied, use of kaolin cannot be

161

recommended in conditions of good water availability, as stated by Santos et al. (2012), who report an improvement in plant establishment in kaolin-sprayed strawberry plants, and revealed a 30% reduction in water requirements during plant establishment. 5. Conclusions From the results shown, it can be underlined that the main effects observed as a result of applying kaolin to various crops concern the reduction in stomatal conductance, which entails a reduction in transpiration and an improvement in plant water status. The reduction in stomatal conductance also leads to a reduction in net assimilation, though to a lesser extent than for transpiration. A general increase in water productivity therefore follows. On tomato, although kaolin reduces net photosynthesis, it also reduces stomatal conductance and transpiration and improves plant water status, thus improving salt and water stress tolerance by helping to maintain a good level of photosynthesis under stress conditions. Kaolin has proved effective in limiting transplant stress under water shortage conditions. Therefore, kaolin could be used to reduce transplant stress when bare-root transplants are used. The reductions in stomatal conductance and water loss, and the increase in water productivity, indicates that kaolin, in addition to any benefits for pest control and mitigation of heat stress, can be efficiently utilized as an antitranspirant to alleviate the effects of drought and salinity and to save water in dry regions such as those found in the Mediterranean. However, it should be underlined that kaolin should not be used specifically to save water, as its main purposes must remain to limit heat stress and to control pests. Acknowledgements This research was supported by (i) Research Project funded by Serbios S.r.l., Badia Polesine (RO), Italy, and (ii) Research Project funded by the Italian Ministries of Finance and Economy, of Education, University and Research, for the Environment and Territory, of Agricultural, Food and Forestry Policies (CLIMESCO contract N. 285–20/02/2006; Coordinator: Dr. D. Ventrella). We thank Egidio De Palma (CNR-ISPA) for providing technical support. References Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration. FAO Irrigation and Drainage Paper. Paper 56, 1. FAO, Rome, pp. 300. Anandacoomaraswamy, A., De Costa, W.A.J.M., Shyamalie, H.W., Campbell, G.S., 2000. Factors controlling transpiration of mature field-grown tea and its relationship with yield. Agric. Forest Meteorol. 103, 375–386. Angbabu, S., Pravat, P., Sarkar Asit, K., Sinha Ashim, C., 2007. Growth and yield of wheat as influenced by evapotranspiration control measures and levels of fertilizer under rainfed condition. Indian J. Plant Physiol. 12 (2), 194–198. Bates, B.C., Kundzewicz, Z.W., Wu, S., Palutikof, J.P. (Eds.), 2008. Climate Change and Water. Technical Paper of the Intergovernmental Panel on Climate Change. IPCC Secretariat, Geneva, p. 210. Boari, F., Cucci, G., Donadio, A., Schiattone, M.I., Cantore, V., 2014. Kaolin influences tomato response to salinity: physiological aspects. Acta Agric. Scand., Sect. B: Plant Soil Sci., http://dx.doi.org/10.1080/09064710.2014.930509. Brugnoli, E., Lauteri, M., 1991. Effect of salinity stomatal conductance, photosynthetic capacity, and carbon isotope discrimination of salt-tolerant (Gossypium hirsutum L.) and salt-sensitive (Phaseolus vulgaris L.) C3 non-halophytes. Plant Physiol. 95, 628–635. Cantore, V., Iovino, F., Pontecorvo, G., 1987. Aspetti climatici e zone fitoclimatiche della Basilicata. CNR-IEIF 2, 49. Cantore, V., Pace, B., Albrizio, R., 2009. Kaolin-based particle film technology affects tomato physiology, yield and quality. Environ. Exp. Bot. 66, 279–288. Caruso, G., 1993. Risposta fisiologica del pomodoro (Lycopersicon esculentum Mill.) alla concentrazione salina dell’acqua di irrigazione. Italus Hortus 1, 32–37. Castrignanò, A., de Caro, A., Tarantino, E., 1985. Verifica sulla validità di alcuni metodi empirici di stima dell’evapotraspirazione potenziale nel Metapontino. L’Irrigazione 32 (4), 23–28.

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