Differential Nutrient Uptake and Its Transport in Tomato Plants on Different Fertilizer Regimens

Application of controlled-release fertilizer (CRF) to a root-proof capillary wick irrigation system (a type of subirrigation method) has both economical and environmental benefits, because it does not require any equipment for fertigation and minimizes water leaching. In this study, we examined the effects of CRF and liquid fertilizer (LF), a conventional fertigation method, on fruit production and nutrient uptake and transport in forcing tomato cultures for harvesting 15 trusses per plant from October to June. No significant difference was noted in marketable fruit yield between CRFand LF-treated plants. The quantity of nutrient uptake per plant and per fruit yield was lower with CRF than with LF, indicating that nutrients were used more efficiently for fruit production in plants grown with CRF. Analysis of the volume and mineral concentrations of xylem exudates indicated that the amount of nutrients absorbed was greater with LF than with CRF, particularly after the tenth truss was harvested. Mineral concentrations in the substrate solution of CRF-treated plants were initially higher than those in the substrate solution of LF-treated plants but extremely low after the second truss was harvested, whereas mineral concentrations in the xylem exudates were similar in CRF and LF plants until the eighth truss was harvested. Thus, the difference in mineral concentrations between the xylem exudates and substrate solution was much larger in the case of CRF than in the case of LF, indicating that the plants absorbed the bulk of nutrients immediately after their release from the CRF surface. Therefore, CRF is suitable in this system, because it combines high fruit production with high nutrient utilization efficiency. Efficient nutrient use in greenhouse vegetable production is essential for both economic and environmental reasons. In general, nutrient and water utilization efficiency is higher with subirrigation than with drip irrigation, because subirrigation greatly reduces water leaching (Goodwin et al., 2003; Incrocci et al., 2006; Santamaria et al., 2003). For uniform water distribution in potted ornamental plants, capillary wick irrigation, a subirrigation method, has become popular in Japan since the 1980s. It eliminates the need for irrigation equipment, because it uses capillary forces. This simple method is also labor-efficient and economical. However, for long-term cultivation, capillary wick irrigation is unpopular, because it results in a lower fruit yield. This occurs because roots penetrate the wick and decrease capillary action. To solve this problem, Masuda (2008) developed a root-proof capillary wick irrigation system for long-term vegetable production by making the wick impenetrable to the roots. In this irrigation system, water is stably supplied by capillary action from the side of the substrate without root invasion into the wick. Subsequent research focused on determining a suitable nutrient concentration for the fertilizer solution to improve tomato cultivation using this system (Masuda and Fukumoto, 2008; Morishige et al., 2009a, 2009b). Compared with LF, CRF is economical, because it does not require equipment for adjustment of nutrient concentration and fertilizer delivery. In our previous study, we established that for harvesting 15 trusses, the optimal nitrogen supply is 16.2 g per plant (Kinoshita et al., 2010a, 2010b) if the timing of nutrient release from the CRF was optimized and that nutrient utilization efficiency was higher with CRF than with LF (Kinoshita et al., 2010b). Therefore, further investigations were necessary to optimize nutrient application with CRF, because fruit yield from the upper trusses was still lower with CRF than with LF (Kinoshita et al., 2010b). In particular, fertilizer combinations needed to be optimized to ensure an adequate supply of nutrients during the later growth stages. Furthermore, mineral concentrations in the substrate solutions were extremely low after the first half of the experiment with CRF application (Kinoshita et al., 2010a, 2010b). Imano et al. (2011) have also reported that the electrical conductivity (EC) of the substrate solution is very low after the first half of the growth period with CRF application. These results indicate that plants absorb the bulk of nutrients immediately after release from the CRF surface, which translates into high nutrient utilization efficiency with respect to fruit production. Therefore, we investigated the differences in the modes of nutrient uptake in CRFand LFtreated plants. Xylem exudates were analyzed after stem decapitation to assess nutrient uptake by the plant. The volume of xylem exudates per unit of time can be used to measure the activity of water uptake (Yamaguchi et al., 1995). On the other hand, the mineral concentrations in xylem exudates within 1 h after stem decapitation reflect mineral transport in intact plants, because the concentration remains fairly constant during this time (Armstrong and Kirkby, 1979; Masuda, 1989; Masuda and Gomi, 1982; Widders and Lorenz, 1982). Furthermore, some studies have shown that the nutrient uptake capacity of roots can be measured using volume and mineral concentrations of xylem exudates (Ho et al., 1993; Ma et al., 2005; Noguchi et al., 2005; Sakaigaichi et al., 2005, 2007). Therefore, in this study, xylem exudates were analyzed during the first hour after stem decapitation to assess nutrient transport and uptake. The aim of this study was to examine fruit production in plants treated with CRF-modified combinations of fertilizers and to determine the difference between CRF and LF with respect to nutrient utilization efficiency for fruit production and seasonal nutrient uptake by plants. Materials and Methods Plant material and growth conditions. The research was conducted in the plastic greenhouse (area, 252 m) at the National Agricultural Research Center for Western Region, Zentsuji, Kagawa, Japan (lat. 34 13# N, long. 133 46# E). Seeds of large-fruit tomato plants, ‘House Momotaro’ (Takii Seed Co., Kyoto, Japan), were sown in 128-well plug trays filled with commercial growth medium (Metro-Mix 350; Sun Gro Horticulture Distribution Inc., British Columbia, Canada) on 11 Sept. 2009. Seedlings were transferred to 9-cm polyethylene pots filled with a mixed substrate [paddy soil:bark compost:perlite:peatmoss, 2:4:1:1 (v/v)] on 1 Oct. 2009. This substrate is reported to be suitable for the root-proof wick irrigation system (Kinoshita and Masuda, 2011). On 20 Oct. 2009, 39-d-old seedlings were transplanted at a plant density of 2.8 plants/m into the root-proof wick irrigation system (Fig. 1) containing the same substrate (3 L per plant). Plastic boxes (39 cm long · 22 cm wide · 16 cm high) were used with two plants per box. Received for publication 3 Mar. 2011. Accepted for publication 25 June 2011. We thank the technical support staff of the National Agricultural Center for Western Region for helping with the measurements and the crop management in this study. Graduate student of Okayama University. To whom reprint requests should be addressed; e-mail [email protected]. 1170 HORTSCIENCE VOL. 46(8) AUGUST 2011 The distance between plants was 20 cm. The plastic boxes were arranged in rows from south to north. Thirty plastic boxes were placed in each row, and the distance between the rows was 1.8 m. Each plant was allocated one wick (45 cm long · 4 cm wide). The substrate surface was covered with rice husks to prevent evaporation. As the plants grew, all lateral shoots were removed, and the remaining single stem was trained vertically on a string attached to a horizontal wire at a height of 2.5 m. The flowering trusses were treated with 15 ppm parachlorphenoxyacetic acid to promote fruit set. Trusses were thinned to contain no more than five fruits. On 1 May 2010, plants were topped with two leaves retained above the 15th truss. Fruit harvesting was initiated on 19 Jan. and continued until 29 June 2010. The greenhouse was heated at night to maintain a minimum temperature of 13 C, and ventilation was initiated for daytime temperatures higher than 28 C. Air and substrate temperatures inside the greenhouse were measured and 10-min averages were recorded using a data logger (ZR-RX40V; OMRON Corp., Kyoto, Japan). Solar radiation outside the greenhouse was measured every 10 s with a pyrheliometer (MS-802; EKO Instruments Co. Ltd., Tokyo, Japan) and integrated every 1 h. Treatments. Plants were supplied with CRF or LF. The nutrient composition of CRF was determined from our previous results (Kinoshita et al., 2010a, 2010b), and nutrient release from CRF was simulated using nutrient release simulation software (Sehi-meijin Version 2.0; JA Zen-noh, Tokyo, Japan). The nutrient components of CRF are shown in Table 1. The nitrogen supply ratio of NO3-N:NH4-N was 76:24. Ash of chicken droppings was applied to adjust pH and to supply macroand microminerals, except nitrogen. All fertilizers were mixed with the substrate before planting. A half-strength commercial nutrient solution with an EC of 1.4 dS m (Otsuka Chemical Co. Ltd., Osaka, Japan) and containing nitrogen (N) (NO3-N:NH4-N = 9:1), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) at concentrations of 130, 26, 168, 82, and 18 mg L, respectively, was applied to the plants as LF. The experimental design was a randomized complete block with three replications. Each block was placed in each row. Each elementary plot comprised 30 plants. Measurements of plant growth and fruit yield. Individual mature fruits of eight plants were harvested from each plot once a week, and the fresh weight of each fruit was measured. Marketable fruit was defined as fruit weighing over 80 g with no physiological damage. The fruit Brix was measured using a digital refractometer (PAL-1; ATAGO Co., Ltd., Tokyo, Japan). As an indicator of plant growth, the diameter of the stem below each fruit truss was measured for six plants from each plot at the end of the experiment. The various parts of four plants (i.e., leaves, stem, and fruits) from each plot were separated. The plant parts were dried in an open-air oven at 80 C for 1 week and measured. The parts were ground into a powder, and the total N content of the dried plants was measured using a NC analyzer (Vario MAX CN; Elementar Analysensysteme, Germany). After wet digestion of the dried plants, P concentration was measured using the vanadomolybdophosphoric yellow color method; K, Ca, and Mg concentrations were measured using inductively coupled plasma atomic emission spectroscopy (SPS-1500NR; Seiko Instrument Inc., Chiba, Japan). Measurement of nutrients in substrate solution and xylem exudates. Substrate solution was collected at 2-week intervals from two spots, which were at 5-cm depth from the soil surface per plot using a soil moisture sampler (DIK-300B; Daiki Rika Kogyo Co. Ltd., Saitama, Japan). Xylem exudates were collected at 4-week intervals on sunny days from plants in each plot. For this purpose, two plants from each plot were decapitated at 5 cm above the ground at 1000 HR. The first few drops of exudates were discarded, and the exudates were collected in a vial for 1 h. The samples were stored at –20 C until analysis. The EC values of the substrate solutions were measured using an EC meter (B-173; HORIBA, Ltd., Kyoto, Japan). Concentrations of NO3-N, NH4-N, PO4-P, K, Ca, and Mg in the substrate solutions and exudates were determined by ion chromatography (DX-AQ; Nippon Dionex K.K., Osaka, Japan). Data analysis. Analysis of variance was carried out using the statistical software (Microsoft Office Excel 2007; Microsoft Co., Redmond, WA).