Pot-in-pot Reduces Salinity, Chloride Uptake, and Maintains Aesthetic Value in Euonymus japonicus Thunb. Under Saline Irrigation

The appropriate management of crop conditions can reduce the salt damage suffered by ornamental species and produce high-quality plants even when saline irrigation water is used. The aim of this study was to determine whether the pot-in-pot (PIP) cultivation system can improve the saline irrigation tolerance of Euonymus japonicus compared with aboveground potting (AGP) in terms of growth and development, aesthetic quality, ion accumulation, and leaf potentials. A 5-month experiment started on 6 Mar., and the interaction between the cultivation system (PIP or AGP) and water quality (fresh water and saline water, with 1.76 and 9.04 dS m, respectively) was assessed. The substrate used was a mixture of white peat, coconut fiber, and perlite (40/40/20, v/v/v). A soil moisture sensor-controlled system was used to irrigate all the treatments when the AGP treatment irrigated with fresh water reached a volume water content (u) of 0.33–0.35 m m. An interaction effect reduced the salinity effects in PIP and saline irrigation (PIP-s) compared with AGP and saline irrigation (AGP-s) in terms of damaged leaf area, plant dry weight (DW), and the compactness index. The PIP-s plants showed a survival rate of 93% compared with 57% in AGP-s. The substrate temperatures were milder in PIP regardless of the irrigation water, and the pore water electrical conductivity (EC) was 36% lower in PIP-s than in AGP-s. PIP reduced the Cl accumulated in leaves but did not influence Na, Ca, Mg, or the K/Na ratio. The lower amount of Cl accumulated increased leaf water potential (Co) in PIP. Saline irrigation produced a general accumulation of Cl and Na in leaves and decreased Ca, Mg, the K/Na ratio,Co, the shoot to root ratio, and height. In general, PIP reduced the salinity damage to Euonymus japonicus, the main effect being the lower Cl ion uptake, which improved its aesthetic value (less damage and greater compactness and growth). The supply of high-quality water has become increasingly limited in many areas of the world, especially in arid and semiarid regions such as the Mediterranean area. However, this area has a great potential for growing crops as a result of its high solar radiation, and many ornamental growers try to make the most of these climatic conditions although only low-quality water is available. However, the use of low-quality water for irrigation affects plants in different ways, depending on the degree of salt tolerance of the species (Alarcón et al., 1993), the level of water salinity, and the characteristics of the water itself. Typical plant responses to salinity include reduced shoot and root growth and reduced whole plant size (Munns, 2002). As salinity stress becomes more severe, foliar damage such as leaf burn, scorch, necrosis, and premature defoliation may occur (Niu et al., 2010a), negatively affecting plant quality, which is of serious concern because the visual quality of ornamental plants is more important than maximum growth (Niu et al., 2008). Growth reduction is related with the osmotic effect of salinity, which limits a plant’s ability to extract water from the soil (Rodriguez et al., 1997). Leaf damage is related to the Na and Cl accumulated in leaves which are not compartmentalized in vacuoles, making them metabolically toxic (Shannon and Grieve, 1999). Plant salt tolerance is the ability to withstand the effects of high salt concentrations in the root zone without a significant adverse effect (Shannon and Grieve, 1999). This tolerance may be determined by: 1) the ability to limit uptake and/or transport of Na and Cl to aerial parts, because these ions are retained in the root (Murillo-Amador et al., 2006); 2) the capacity to maintain nutrient uptake (Chaparzadeh et al., 2003); 3) the capacity of plants to maintain a high K/Na ratio in their tissues (Maathuis and Amtmann, 1999); and 4) the capacity to maintain a positive water balance through osmotic adjustment, which involves an active increase in tissue solute concentration (Torrecillas et al., 2003). As well as environmental conditions, the management of factors that affect humidity and temperature in the substrate can help produce good-quality plants depending on the level of exposure to salinity and the salt tolerance of the species in question. Niu et al. (2010b) reported that salt accumulation in the root zone is affected by the substrate properties, plant size, and environmental conditions because these factors influence the substrate moisture content and cation exchange capacity in the root zone. The factors that can reduce salt accumulation in the substrate may reduce the negative effect of salinity. In this respect, the PIP system compared with AGP reduces root zone temperature stress (Young and Bachman, 1996), which improves roots development (Mathers, 2003), and enhances efficient water use by decreasing container evapotranspiration (Martin et al., 1999), which will reduce salt accumulation as a result of lower substrate water evaporation (Miralles et al., 2009). The PIP crop system was introduced 1990 (Parkerson, 1990) in the United States and combines some of the benefits of both field and container production. In a PIP system, a holder or socket pot is permanently placed in the ground with the top rim remaining above. The container-grown plant is then placed within the holder pot for the production cycle (Ruter, 1998b). The Euonymus japonicus (Japanese Spindle) is a commercial woody perennial shrub with good aesthetic qualities, which is frequently planted in public areas such as streets, recreation areas, and car parks. Previous studies with this species before this experiment with different levels of salinity showed that it was quite tolerant to saline irrigation ( 6 dS m). Received for publication 21 Nov. 2011. Accepted for publication 6 Mar. 2012. This work was supported by project CICYT (CICYT AGL2008-05258-CO2-1-AGR and CICYT AGL2008-05258-CO2-2-AGR), the SENECA project (08669/PI/08), and by the Consejer ıa de Agricultura y Agua de la Región de Murcia program (UPCT-CEBAS-IMIDA 2008). We thank P. Cánovas for irrigating and supervising the shrubs while the experiment took place. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 47(5) MAY 2012 607 The objectives of this study were to evaluate the potential benefits of the PIP system for reducing the salt stress effect on Euonymus japonicus. To this end, plants of this species were grown in PIP and AGP and irrigated with fresh and saline water. The following points were studied: 1) substrate temperature and leachate EC and pH; 2) growth and development of the plant and any salt damage; and 3) pore water EC, leaf potentials, and ion concentrations. Material and Methods Plant material. One-year-old seedlings of commercial Euonymus japonicus var. Marieke were transplanted from 96-plug trays (each plug 56 cm) into 2.5-L black plastic pots (16 cm Ø · 15 height) in Jan. 2010. The substrate was a mixture of white peat, coconut fiber, and perlite (40/40/20, v/v/v) and was amended with 3 g L of a slowrelease fertilizer (Osmocote plus 14-14-14; 14N–6.1P–11.6K; release time 2–3 months at 21 C; The Scotts Company LLC, Marysville, OH). The plants were 5–7 cm in height at the beginning of the experiment. The fertilization was applied on 1 Mar. and 1 June with 7.5 g per pot of slow-release fertilizer (described previously). The experiment was conducted in an openair plot of 70 m at the ‘‘Tomás Ferro’’ Experimental Agro-Food Station of the Polytechnic University of Cartagena (lat. 37 35’ N, long. 0 59’ W) starting on 6 Mar. 2010 and ending the last week of July 2010. Weather conditions were taken from a meteorological station sited 100 m from the experimental plot. The mean, maximum, and minimum daily values of environmental temperature and daily precipitation were registered. Experimental design. The PIP system consisted of placing cultivation pots in pots already buried in the ground. The buried pots were made of black polyvinyl chloride with a grilled bottom to ensure drainage (5.5 L, 17 cm upper exterior diameter and 30 cm height). An air chamber of 15 cm separated the bases of both pots. Once the pots were buried in the ground, the plot was covered with a plastic permeable mulch (Horsol 140 g m; PROJAR S.A., Valencia, Spain), which was covered with a 4-cm layer of gravel ( 2 cm Ø). The cultivation pots were placed in 10 irrigation rows, each one 60 cm from the other. Five irrigation rows provided saline water (9.04 dS m) and the other five fresh water (1.76 dS m) in a random arrangement. A drip irrigation system was installed with one autocompensated 2 L h dripper (Netafim USA, Fresno, CA) per plant connected to one spaghetti tube. Each row had 22 cultivation pots [buried pots (PIP) in odd position alternating with aboveground pots (AGP) in even positions], which were placed 55 cm apart. This resulted in a total of four treatments: AGP with fresh water (AGP-c), PIP with fresh water (PIP-c), AGP with saline water (AGP-s), and PIP with saline water (PIP-s). A data logger, CR1000 (Campbell Scientific, Ltd., Logan, UT) was installed in the center of the plot. This registered data of substrate temperature with two temperature probes (Termistor 107; Campbell Scientific, Ltd.) per treatment and volume water content (q) with five EC-5 soil moisture sensors (Decagon Devices, Ltd., Pullman, WA) per treatment. The EC-5 probes were totally introduced in the pot at an angle of 45 at the side of the dripper. Soil moisture sensors were connected to the CR1000 through a multiplexer. The CR1000 was programmed to collect data every 10 min and to calculate average q and its SE per treatment. The q was obtained from the voltage readings of the soil moisture sensor using our own substratespecific calibration (q = 3.765 · voltage 0.451, R = 0.93) determined using the procedure of Nemali et al. (2007). The CR1000 determined the mean, maximum, and minimum daily temperatures of the substrate. The treatments were watered all at once for the same time ( 15 min and 550 mL) when the average q of the AGP-c treatment, measured by the EC-5 probes, reached the threshold of 0.33–0.35 m m, which was –2.5 kPa estimated according to the retention curve of the substrate. The AGP-c treatment had an average leaching of 25%, which was 40% more than that recommended for irrigation with fresh water in the area to avoid salt accumulation in the fresh water treatments. This irrigation criterion was established because previous studies with the PIP system concluded that it saves water (Miralles et al., 2009), whereas Navarro et al. (2007) observed less water consumption in the salinity irrigation treatments. The automated irrigation control was based on that described by Nemali and van Iersel (2006) but using a CR1000 instead of a CR10X with the 12 VDC switch connected to a relay, which controlled the fresh water and saline water pumps through an Agronic 3000 irrigation controller (Sistemes Electronics Progres, S.A., Lerida, Spain). The saline solution was obtained by adding 4 g L of sodium chloride to the irrigation water in a 1000-L water tank. Water quality and substrate electrical conductivity. The pH and EC of irrigation water and leachate was registered once a week with a pH meter (pHep 4; Hanna Instruments S.L., Eibar, Spain) and a conductivity meter (Dist 6; Hanna Instruments S.L., Eibar, Spain). The samples were collected from May to July. There were a total of five replicates for each treatment. Substrate pore water EC was measured in five replicates per treatment in the last week of July following the pour-through method (Wright, 1986). Measurements of bulk EC (EC of the substrate mass including, air, solids, salts, and water) were taken weekly with a WET sensor (Delta-T Devices Ltd., Cambridge, U.K.). A total of 10 measurements per replicate was taken. Each sample was taken near the dripper because the EC-5 sensors were placed in that position. This measurement were taken simply to control that bulk EC did not exceed 8 dS m, the maximum level of salinity that does not interfere with the q estimates of the EC-5 sensor. Measurements of plant morphology and salt damage. On 30 July 2010, the plant morphology (height and profile area) was determined with a flexometer and a side view picture was taken with a digital camera HP CW450 (Hewlett-Packard Española S.L., Madrid, Spain) of five plants per treatment. The compactness index was determined as follows: compactness index = profile area/ {[(p/4)*[(height + width)/2]}, where the profile area, and the height and width of the plant profile, were obtained from the picture and the software UTHSCSA Image Tool (University of Texas, San Antonio, TX). The closer the result was to unit, the more compact the plants. Leaf area and the DW of roots, stem, and leaves were also determined in the same plants. The leaf area of both healthy and damaged leaves was determined with a LI3100C (LI-COR Biosciences, Lincoln, NE). Individual leaf area was calculated as plant leaf area/total number of leaves. To calculate the DW, leaves (healthy, damaged, and fallen), stems, and roots were washed with distilled water and introduced in clearly identified envelopes and placed in a natural convection bacteriological stove at 60 C until constant weight was reached. Finally, the DW was determined by weighing with a GRAM ST series precision balance. The growth indices determined were the shoot DW/root DW (S:R), the specific leaf area (SLA) (leaf area/leaf DW) and leaf DW/total DW. The DW of fallen leaves, damaged leaves, and the damaged leaf area were determined. The percentage of damaged leaf area was calculated with the image analysis software for plant disease quantification ASSESS 2.0 (University of Manitoba, Winnipeg, Canada). Leaf potentials. Midday leaf water potential (Yhmd), midday leaf yS (Yomd), and midday leaf turgor potential (Ypmd) were measured in the last week of July 2010. A total of five repetitions per treatment were used. The leaf water potential was estimated according to the method described by Scholander et al. (1965) using a pressure chamber (Soil Moisture Equipment Co, Santa Barbara, CA) following the recommendations of Turner (1988). Leaves from the Yomd measurements were frozen in liquid nitrogen. The values of the yS (Yomd) was measured using a Wescor 5520 vapor pressure osmometer (Wescor Inc., Logan, UT) according to Gucci et al. (1991). Estimates of Ypmd were based on the difference between Yhmd and Yomd for each time. Determination of inorganic ions. The inorganic ion content (Cl, Na, Ca, Mg, and K) were determined in roots and healthy leaves of all treatments and in damaged leaves of AGP-s. A total of five samples per treatment were collected, each sample consisting of 40 g fresh weight. The samples were processed to be analyzed in the SAIT laboratory of the laboratory by ion chromatography, as described by Bañón et al. (2011). 608 HORTSCIENCE VOL. 47(5) MAY 2012 Statistical analysis. A split plot design was used: salinity as the main plot with five blocks and crop system as the subplot. Data were subjected to a split plot analysis of variance with a P # 0.05 using the software Statgraphics Plus 5.1 software (StatPoint Technologies Inc., Warrenton, VA).