Spinach yield and nutritional quality affected by controlled soil water matric head

Agricultural Water Management 51 (2001) 217±229

Spinach yield and nutritional quality affected by controlled soil water matric head E. Nishiharaa, M. Inoueb,*, K. Kondoc, K. Takahashia, N. Nakataa a Faculty of Agriculture, Tottori University, Tottori 680-8550, Japan Arid Land Research Center, Tottori University, Tottori 680-0001, Japan c The United Graduate School of Agricultural Sciences, Tottori University, Tottori 680-8550, Japan b

Accepted 9 May 2001

Abstract The yield and quality of spinach cultivar (Spinacia oleracea L. `Virginia') in sandy soil are improved by controlling soil water within the rhizosphere. An automated irrigation system using by a buried-type electric tensiometer connected to electromagnetic valves was shown to control soil water accurately. The soil water matric head was maintained in the ranges h ˆ 10 to 20 cm (treatment I), 20 to 30 cm (treatment II) and 30 to 40 cm (treatment III). According to commercial standards, treatment II could have yielded a lowest 20 days earlier than treatment I and III. The yield from treatment II was more than two times greater than that from the other treatments. Total ascorbic acid and sugar content, as well as the concentration of total N, P, K, Ca, Mg, and Na in the spinach of treatment II, were all signi®cantly higher than those from the other treatments. The water use ef®ciency (g dry weight/l water) and net water use ef®ciency (g fresh weight/l water) were better for treatment III. Only the yield and quality of spinach from treatment II reached commercial standards. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Automated irrigation system; Soil water matric head; Electric tensiometer; Water use ef®ciency; Spinacia oleracea L.

1. Introduction In general, the greenhouse cultivation of leafy vegetables in sandy soil is considered unsuitable because of its low water capacity compared to other soils. However, when leafy vegetables are grown in sandy soil, it has the advantage that the water in the root zone can be monitored and controlled very well. Automated irrigation systems play a *

Corresponding author. E-mail address: [email protected] (M. Inoue). 0378-3774/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 7 7 4 ( 0 1 ) 0 0 1 2 3 - 8

218

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

significant role in continuously monitoring and controlling soil water content within the root-zone and maintaining water tensions or contents at values that ensure early harvest and high commercial yields. Systems utilizing neutron thermalization (Inoue et al., 1981, 1983b; Goldhamer and Snyder, 1989), gamma densitometry (Miyazaki et al., 1985) or time domain reflectometry (TDR) (Wallach et al., 1992a,b; da Silva et al., 1998) can accurately control soil water content, but are expensive and labor-intensive. Therefore, comparatively cheaper tensiometer±pressure transducer systems have been designed for controlling soil water in the field (Phene et al., 1973, 1981, 1989; Inoue, 1994). The field performance of a manometer-type tensiometer, which generates an appropriate electrical signal for sensing values of soil matric head, has been reported to be better than that of a pressure-gauge type tensiometer (Trotter, 1984). Luthra et al. (1997) modified the design of a manometer-type tensiometer to automatically regulate irrigation and achieve a desired soil water level. Testezlaf et al. (1996) also demonstrated a real-time irrigation control system for a greenhouse. However, whenever the temperature fluctuated, the matric head measurements were strongly affected by air accumulation in the upper part of the tensiometer (Trotter, 1984). Moreover, because the water retention characteristic of sand tends to follow an S-shaped curve, a small measuring error of soil water matric head may cause a large error in soil water content. To overcome these difficulties, a high quality tensiometer for sandy soils was developed (Inoue and Nomura, 1983a). To reduce the temperature impact, a buried-type electric tensiometer system was developed with the pressure transducer inside the porous cup (Inoue et al., 1994). Inoue and Takeuchi (1997) demonstrated that the growth of leaf lettuce (Lactuca sativa L.) regulated from 26 to 30 cm matric heads with this tensiometer was better than that using a timer on a sand-bed culture in two automated irrigation systems. Also, unlike the timer system, the tensiometer achieved a stable soil moisture condition under prevailing temperature and weather variations. Because spinach is a highly desirable leaf vegetable with good cooking adaptability, high nutritive value and many important vitamins and minerals, its consumer demand in Japan increases annually. A comparative study of spinach (Spinacea oleracea L.) growth on sandy soil using the buried electric tensiometer in an automated irrigation system in a greenhouse has not been carried out before. Therefore, the objective of this study was to determine favorable ranges of soil water matric head, h, for spinach growth using this system to accurately control irrigation water and to develop a practical automated irrigation system for sandy soil in a greenhouse. The relationship between soil water matric head, yield, and nutritional quality of spinach were also examined while considering water use efficiency. 2. Materials and methods The experiment was conducted during autumn 1996 and spring 1997 at the Arid Land Research Center, Tottori University in Japan (358310 59.700 N, 1348120 54.500 E). Spinach cultivar (Spinacea oleracea L., `Virginia') was grown on sandy soil in a greenhouse. Within each experimental of 0:6 m  6 m, three drip irrigation lines with an emitter spacing of 0.1 m (T-type; T-Systems International Inc., USA) were installed 0.2 m apart.

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

219

Fig. 1. Cross-section of buried-type electric tensiometer (dimensions in mm): (1) electric cord; (2) tube connecting to atmosphere; (3) pressure transducer; (4) porous ceramic cylinder; (5) air removal hole and screw.

2.1. Electric tensiometer system Soil water during the growth experiments was controlled by a buried-type electric tensiometer system (improved field-type tensiometer with a pressure transducer) (Fig. 1). The tensiometer system was installed in each treatment, 5 cm below the soil surface, centered between two drip lines, and 1 m from the front. Soil water matric head, h, was controlled to an accuracy of 2 cm even if temperature difference of 158C were present. The control soil water matric heads, h, were set at follows: treatment I: 10 to 20 cm, treatment II: 20 to 30 cm, and treatment III: 30 to 40 cm. An exact calibration and strict operational procedure was necessary for each tensiometer system to obtain an accurate relationship between the pressure inside the porous cup and the soil water matric potential outside the cup. Tensiometers with their air venting screw removed were submerged in a container of water placed in a desiccator. The container with tensiometers was depressurized to 960 cm by a vacuum pump for 24 h. When all the air was removed, the vent screws were replaced (Fig. 1). Each tensiometer was connected to an analog data logger (Green Kit 100; E.S.D Co. Ltd., Japan) and was calibrated by calculating the linear relationship between output voltage (X) and soil water matric head (h). The relation of each tensiometer had a high regression coefficient (r  0:9999). Values of a and b of the equation h ˆ aX ‡ b for each tensiometer were recorded in the data logger before starting the experiment. Installation involved connecting the tensiometers, data logger and electromagnetic valves to the drip irrigation system.

220

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

Fig. 2. Hydraulic property for sandy soil. Scanning drainage retention (field) (~). Main drainage retention (laboratory) (!). Drainage hydraulic conductivity (field) (*).

2.2. Soil water conditions The hydraulic property of the sandy soil in greenhouse (Inoue and Nomura, 1978; Fujimaki et al., 1999) is shown in Fig. 2. The availability of water on sandy soil is very low. The uniformity of soil water matric head and indirectly soil water content, was controlled by the electric tensiometers, was verified by monitoring additional regular tensiometers. These were installed in three different positions in each plot; at the front (1 m), center (3 m) and back (5 m from the front), and at three different soil depths; 0±5, 5±10, 10±15. Diurnal and cumulative amounts of irrigation applied to each plot were also measured. 2.3. Seeding and harvest As is customary before sowing spinach in sandy soil, fertilizer was applied in granular form (N:P2O5:K2 O ˆ 280:260:272 kg/ha). Three seeds were sown in 1.5 cm deep holes at 10 cm intervals along each drip line. On the 15th day after sowing, the seedlings were thinned to one plant per hole leaving a total of 242 spinach seedlings per treatment. At this time, each treatment was uniformly irrigated with 3 l water (0.83 mm). Thereafter, irrigation was automatically controlled by the buried-type electric tensiometers during the period 15±60 days after sowing in 1996 and 15±35 days after sowing in 1997. Before harvest, irrigation was stopped for 10 days in order to increase spinach growth and quality (Okayama and Arai, 1983). Ten days later, the spinach was harvested in 1996 and 1997 but analyzed for quality in 1996 only. 2.4. Growth measurement For nine samples from each experimental treatment, the height, maximum leaf and stem height, fresh and dry weight of leaves and stems, and leaf area were measured at 30 (15 days after controlling soil water matric head), 45, 60, and 70 (harvest) days after sowing

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

221

in 1996 and at 15, 20, 25, and 35 (harvest) days after sowing in 1997. After measuring leaf area with an automatic leaf area meter (Hayashi Denko Co. Ltd., Japan), the samples were dried at 808C for 48 h to determine total dry weight of the leaves and stems. 2.5. Spinach quality Spinach quality was only analyzed in 1996. Relative chlorophyll content of the most recent fully expanded leaves was measured 30, 45, 60 and 70 days after sowing with an in situ SPAD-502 chlorophyll meter (Minolta Co. Ltd., Japan). Actual chlorophyll content Y (mg/100 cm2) was calculated by substituting the SPAD reading for X in the standard formula Y ˆ 0:0996X 0:152 (Watanabe et al., 1994). Sixty and seventy days after sowing, 10 g of top fresh tissue were homogenized with a solution of 5 ml of 10% metaphosphoric acid and 4% thiourea and filtered through a filter paper (no. 6; Advantec Toyo Co. Ltd., Japan). After appropriate dilution, the total ascorbic acid content was measured at an absorbance of 410 nm according to the 2,4dinitrophenylhydrazine method. The total sugar content of 10 g of fresh sample was extracted according to the anthrone±sulfuric method and measured by spectrophotometer (UV-1200; Shimadzu Co. Ltd., Japan). Total oxalic acid content of fresh tissue from each treatment was also determined according to the method of Sugiyama and Hirooka (1992). Fresh material frozen by liquid nitrogen was homogenized and 10 g of the powder was extracted with 100 ml of 1 N HCl. The solution was filtered through a filter paper (no. 6; Advantec Toyo Co. Ltd., Japan) and total oxalic acid content determined by HPLC analysis on a Shimadzu gradient liquid chromatograph. Separations were achieved on a Wakose-II 5C-18±100 (4:8 mm  250 mm) using 50 mM NH4H2PO4 at 358C. The solvent flow rate was 1.0 ml/min. Spinach leaves including stems were analyzed for total N, NO3, P, K, Ca, Mg concentration in the same samples used for dry weight measurements at harvest. Total N and NO3 concentration were measured by the Kjeldahl and Gunning modified method, respectively. P concentration was measured by the molybdovanadophosphate colorimetric procedure (UV-1200, Simadzu Co. Ltd., Japan), and K, Ca and Mg concentration by atomic absorption spectrophotometry (AA-670, Simadzu Co. Ltd., Japan). These analyses were conducted only in 1996. 2.6. Statistical analysis Each data set was analyzed for mean, standard error, and significance using Duncan's multiple range test (P  0:05). 3. Results and discussion 3.1. Soil water conditions Changes of soil water matric head h, controlled by the tensiometer system, on day 50 of the cultivation period in 1996 are shown in Fig. 3. On average, treatment I was

222

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

Fig. 3. Variation on day 50 of soil water matric heads controlled in the ranges of 30 cm ( ) and 30 to 40 cm ( ).

10 to

20 cm (Ð),

20 to

irrigated 2.7 times/h, while treatment III was irrigated only once in 4 h, or about 11 times less frequent. The results with the additional tensiometers, installed in three different positions and at three different depths in each treatment, are not shown. They indicated that the soil water was maintained uniformly by just one control-tensiometer in each treatment. Diurnal and cumulative amounts of irrigation water applied for each treatment in 1996 are shown in Figs. 4 and 5, respectively. The average daily amount of irrigation water (mm/day) was 4.31, 2.78 and 0.98 mm in treatment I, II and III, respectively. The

Fig. 4. Daily amounts of irrigation for controlled soil water matric head ranges of 30 cm (&) and 30 to 40 cm (~).

10 to

20 cm (^),

20 to

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

Fig. 5. Cumulative irrigation for controlled soil water matric head ranges of 30 cm (&) and 30 to 40 cm (~).

10 to

20 cm (^),

223

20 to

cumulative amount of irrigation (l/m2) in treatment I was 4.4 times as much as that in treatment III. 3.2. Yields Changes in total height, top fresh weight, and dry weight and leaf area in each treatment are presented in Fig. 6. At 30 days after sowing in 1996, the height in treatment II and III was significantly greater than that in treatment I. At about 50 days, the height in plot II reached more than 26 cm, the shipment standard for the Japanese vegetable market, while in treatment I and III this height was reached at 70 days. Differences in leaf area, top fresh and dry weight for each treatment were also similar to that of height. Leaf area, top fresh weight and dry weight in treatment II were about 2, 2.5 and 2 times greater, respectively, than those in the other treatments at both 60 days and harvest time. The differences in height, fresh weight, dry weight, and leaf area were the same in 1997 as in 1996. Controlling soil water before harvest in spinach is very important because stem and leaf growth are very sensitive to these conditions. We believe that spinach growth was poor under the moderate drought conditions of treatment III in sandy soil because volumetric water content was between 0.07 and 0.1 cm3/cm3 and hydraulic conductivity was between 10 7.8 and 10 5 cm/s (Fig. 2) and because of the fact that photosynthesis was depressed primarily from increased stomatal closure (Turner, 1974; Chaves, 1991). Moreover, water stress may have accelerated the leaf senescence gradient in spinach as has been shown for several other species (O'Neill, 1983; Olsson, 1995).

224

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

Fig. 6. Growth parameters for controlled soil water matric head ranges of 10 to 20 cm (^), 20 to 30 cm (&) and 30 to 40 cm (~) in 1996 (upper) and 1997 (lower). Vertical bars are the S.E. of the means of nine plants. (A) Start of irrigation water control; (B) time to stop irrigation system; and (C) harvest time. Different letters within a column are significantly different by Duncan's multiple range test. Day that shipment standard of vegetable market (more than 26 cm height) was reached in treatment 20 to 30 cm matric head.

3.3. Spinach quality The diurnal changes of chlorophyll content (mg/100 cm2) were significantly different between all treatments at 60 days (Fig. 7). The chlorophyll content of treatment I and II continued to increase until harvest time. However, the total increase of treatment I was

Fig. 7. Leaf chlorophyll content for controlled soil water matric head ranges of 10 to 20 cm (^), 30 cm (&) and 30 to 40 cm (~). Vertical bars are the S.E. of the means of 25 plants.

20 to

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

225

Fig. 8. Indicated qualities at end of irrigation (60 days after sowing) and at harvest (70 days after sowing) under controlled soil water matric head ranges of 10 to 20 cm (&), 20 to 30 cm ( ), and 30 to 40 cm (&). Letters within a column indicate a significant difference by Duncan's multiple range test. Vertical bars are the S.E. of the means of nine plants.

much smaller than that of treatment II at harvest time. Treatment III showed yellowing of leaves after 60 days. We assume that the low chlorophyll content in treatment I was caused by significant leaching of N and in treatment III by breakdown of chlorophyll b in the leaves due to water stress. Similar results have been demonstrated with other plants (Bharwaj and Singhal, 1981). At harvest time, the chlorophyll content of the spinach in treatment I, II and III was 3.72, 5.07 and 3.34 mg/100 cm2, respectively. According to Watanabe et al. (1994), the average chlorophyll content of spinach in Japan during autumn cultivation is 4.1 mg/100 cm2. Thus, in our experiment the chlorophyll content of the spinach in treatment II was better than average. The total ascorbic acid content at harvest was the highest in treatment II, 63 mg/100 g fresh weight at harvest, compared with 57 and 53 mg/100 g for treatment I and III, respectively (Fig. 8A). According to Japanese commercial standards, the total ascorbic acid content of spinach grown in a greenhouse in autumn must be about 50±60 mg/100 g top fresh weight. Thus, the spinach leaves in treatment II did not reached the required standard in 60 days. However, during the following 10-day period without further irrigation, the total ascorbic acid content increased to about 2±3-fold at harvest time. This study confirmed that the practice of stopping irrigation a few days before harvest causes the ascorbic acid content of spinach to increase. The high ascorbic acid content in treatment II may have been the result of increased chlorophyll content which is necessary for photosynthesis to function when too much N is absorbed (Iwanami, 1989). Certainly, chlorophyll content and total N concentration (Fig. 7and Table 1) were both very high in treatment II. At 60 days, total sugar content of the spinach was the highest under the driest condition (treatment III), 0.08 g/100 g top fresh weight (Fig. 8B). At harvest time, the total sugar content in treatment I, II and III was 0.16, 0.22 and 0.20 g/100 g top fresh weight,

226

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

Table 1 Total concentration of leaf and stem nutrients at harvest time of spinach controlled by different soil water matric head Soil water matric potential head (cm)

10 to 20 to 30 to a

20 30 40

Spinach nutrienta Total N (%)

NH4‡ (%)

NO3 (%)

P (%)

Mg (mg/g Ca (mg/g K (mg/g Na (mg/g dry weight) dry weight) dry weight) dry weight)

2.63 c 5.18 a 3.16 b

2.27 b 4.87 a 2.64 b

0.37 a 0.31 a 0.52 a

1.27 b 1.80 a 1.68 a

6.50 b 7.63 a 4.00 b

1.83 b 2.16 a 1.76 b

54.91 b 61.59 a 49.94 c

0.74 b 1.59 a 0.81 b

Different letters within a column indicate significant difference by Duncan's multiple range test.

respectively. These contents are about 3±4 times greater than those before the 10-day period without irrigation. From 60 days to harvest time, total oxalic acid content of spinach in treatment I, II and III decreased from 1.22 to 0.49, 1.45 to 1.17, and 0.73 to 0.51 g/100 g top fresh weight, respectively (Fig. 8C). Thus, in treatment II this content remained more than 1 g/100 g top fresh weight even though irrigation was stopped for 10 days. According to Bengtsson et al. (1966), there is a positive correlation between total oxalic acid content and total cation (K ‡ Mg ‡ Ca ‡ Na) concentration. This is also true for our data where the total cation (K ‡ Mg ‡ Ca ‡ Na) concentration was highest in treatment II (Fig. 8C and Table 1). Mineral nutrient concentration of each treatment at harvest is shown in Table 1. Total N concentration (NO3 and NH4) was the highest in treatment II. However, the NO3 concentration, which is sometimes injurious to the human body, was not significantly different for treatment II compared to that of the other treatments. In general, the NO3 concentration in spinach ranges from about 0.1±1%. In this study, the concentration was low in each treatment, especially in treatment II with 0.31%. In contrast, the other mineral nutrients (NH4, P, K, Ca and Mg) in treatment II were significantly different from those found in the other treatments according to Duncan's multiple range test (P  0:05) (Table 1). Thus, spinach cultivation controlled by the matric head (water content) range of treatment II yielded spinach of improved quality and quantity and a more commercially desirable and valuable product. 3.4. Relationship between yield and water use efficiency The relationship between yield and water use efficiency for different soil water matric heads is shown in Table 2. Spinach yields cultivated in autumn in Japan are generally about 15±20 Mg/ha. Judged by this standard, the yield of 19.1 Mg/ha from treatment II was perfectly acceptable. The yields from treatment I and III were 8 and 8.3 Mg/ha, respectively, less than half of that from treatment II. However, from the viewpoint of saving water, the water use efficiency (g top dry weight produced by 1 l irrigation water) (Jensen et al., 1990; Howell, 1990) was 0.532, 1.663 and 2.637 g top dry weight/l for treatment I, II and III, respectively. Thus, water use efficiency of treatment III was 1.59

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

227

Table 2 Total irrigation and yield of spinach in 70-day cultivation period for different ranges of controlled soil water matric head Soil water matric potential head (cm) 10 to 20 to 30 to

20 30 40

Amount of Total dry total irrigation weight water (g/m2) (l/3.6 m2)

Water use efficiency (g dry weight/l)

Yield Net water Bunches/haa Gradeb (fresh weight) use efficiency (a bunch/ha) (Mg/ha) (g fresh weight/l)

713.88 461.16 162.36

0.532 1.663 2.637

8.0 19.1 8.3

105.50 213.02 118.94

4.034 14.916 18.404

39800 95490 41250

LL LL L

a In Japanese vegetable markets, spinach is divided into three grades: (1) LL (above 26 cm); (2) L (22±26 cm); and (3) M (18±21 cm). b Spinach is tied up in bunches of 200 g for vegetable market.

times better than treatment II. Also, net water use efficiency (top fresh weight instead of top dry weight/l irrigation water) (Sutton and Merit, 1993) of treatment I, II and III was 4.034, 14l.916 and 18.404 g/l, respectively. Thus, net water use efficiency of treatment III was 1.2 times better than that of treatment II. Apparently, the range of soil water matric head of treatment III ( 30 to 40 cm) was more effective in conserving water than that of treatment II ( 20 to 30 cm), but the commercial value of spinach from treatment III was inferior. If we consider that spinach is tied up in 200 g bunches for the vegetable market, spinach from treatment I, II and III produced 39 800, 95 490 and 41 250 bunches/ha, respectively (Table 2). Since the yields generally vary from 75 000±100 000 bunches/ha, we concluded that only treatment II reached a good commercial spinach yield. 4. Conclusion Spinach, a leaf vegetable commonly consumed in Japan, was grown in autumn 1996 and spring 1997 in sandy soil using an automated irrigation system with a buried electric tensiometer to control soil water in the root zone. In 1996, soil water matric heads controlled between 20 and 30 cm (treatment II), equivalent to a volumetric water content between about 0.1 and 0.16 cm3/cm3 and hydraulic conductivity was between 10 5 and 10 3.5 cm/s (Fig. 2), were more favorable than the other treatment with respect to earlier harvest time, yield, and quality. In brief, spinach growth was the same in 1997 as in 1996 and favorable soil water conditions were controlled accurately by the tensiometer in the automated irrigation system in sandy soil. The study also demonstrated that soil water management of spinach cultivation in sandy soils should not be based on the concept of field capacity but on optimal ranges of the soil water matric head. This is because the volumetric water content of field capacity was no different from that of the wilting point in sandy soil and the available water fraction between field capacity and the wilting point on sandy soil is very small (Fig. 2). Therefore, maintaining field capacity might have an adverse effect on spinach yield. With proper control of soil water using an automated irrigation system, high yields of good quality spinach can be

228

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

produced on sandy soil in a greenhouse. Future experiments will ascertain the best tensiometer-controlled soil water management schemes in relation to each growth stage of spinach for earlier harvest time and improved quality, in particular, a reduction of total oxalic acid. We suggest that optimal ranges of soil water matric head be identified for each combination of vegetable crop and every soil type. References Bengtsson, B.L., Bosund, I., Hylmo, B., 1966. Mineral salts and oxalate content in spinach leaves as a function of development stage. Z. Pflanzenernahr. Dung. Bodenk. 115, 192±199. Bharwaj, R., Singhal, G.S., 1981. Effect of water stress on photochemical activity of chloroplasts during greening of etiolated barely seedlings. Plant Cell Physiol. 22, 155±162. Chaves, M.M., 1991. Effects of water deficits on carbon assimilation. J. Exp. Bot. 42, 1±16. da Silva, F.F., Wallach, R., Polak, A., Chen, Y., 1998. Measuring water content is soil substitutes with timedomain reflactometry (TDR). J. Amer. Soc. Hart. Sci. 123, 734±737. Fujimaki, H., Inoue, M., Yamamoto, T., Togashi, T., 1999. Determining unsaturated hydraulic conductivity of dune sand at low pressure head from a steady-state evaporation experiment. Sand Dune Res. 46 (1), 15±26 (in Japanese with English summary). Goldhamer, D.A., Snyder, R.L., Irrigation Scheduling: A Guide for Efficient on Farm Water Management, Vol. 21454. University of California, CA, pp. 6±16. Howell, T.A., 1990. Relationships between crop production and transpiration evapotranspiration, and irrigation. In: Stewrt, B.A., D.R. Nielsen (Eds.), Irrigation of Agricultural Crops: ASA, CSSC, and SSSA, Agronomy, Vol. 30. pp. 391±434. Inoue, M., Nomura Y., 1978. Hydraulic conductivity and soil water diffusivity in a sand dune field. Bull. Sand Dune Res. Inst. 17, 25±30 (in Japanese with English summary). Inoue, M., Nomura, Y., Yano, T., Cho, T., 1981. Calibration curves of a depth-type neutron moisture meter in a sandy field. J. JSIDRE 95, 11±18 (in Japanese with English summary). Inoue, M., Nomura, Y., 1983a. Laboratory determination of dune sand water constants and soil water characteristic curve during the drying process. Sand Dune Res. 30, 15±25 (in Japanese with English summary). Inoue, M., Nomura, Y., Cho, T., 1983b. Characteristics and calibration curve of a surface-type neutron moisture meter in a sandy field. J. JSIDRE 105, 19±26 (in Japanese with English summary). Inoue, M., 1994. Underground soil suction gauge. Sand Dune Res. 41, 74±79 (in Japanese with English summary). Inoue, M., Takeuchi, Y., Yamamoto, T., 1994. Effect of temperature variation on soil moisture measurement and improvement of moisture sensors in a sand dune field. Sand Dune Res. 41, 49±55 (in Japanese with English summary). Inoue, M., Takeuchi, Y., 1997. Application of buried-type electrical tensiometers to control soil moisture in sand vegetable cultivation. Sand Dune Res. 44, 30±35 (in Japanese with English summary). Iwanami, H., 1989. Relationship between quality of spinach and soils, or fertilizer application. Kinki Chugoku Agric. Res. 77, 8±11 (in Japanese). Jensen, M.E., Rangeley, W.E.R., Dieleman P.J., 1990. Irrigation trends in world agriculture. In: Stewart, B.A., Nielsen, D.R. (Eds.), Irrigation of Agricultural Crops: ASA, CSSC, and SSSA, Agronomy, Vol. 30. pp. 31± 67. Luthra, S.K., Kaledhonkar, M.J., Tyagi, N.K., 1997. Design and development of an auto irrigation system. Agric. Water Manage. 33, 169±181. Miyazaki, T., Kasubuchi, T., Hasegawa, S., 1985. A Comparison of Single-, Dual-Gamma Ray Measurement of Soil and Water, Vol. 119. pp. 29±37 (in Japanese with English summary). Okayama, K., Arai, S., 1983. Studies of the transplanting cultivation of spinach on soil blocks. Part 2. Effect of soil moisture content on the growth and the yield of spinach. Bull. Nara Agric. Exp. Stn. 14, 29±39 (in Japanese with English summary).

E. Nishihara et al. / Agricultural Water Management 51 (2001) 217±229

229

Olsson, M., 1995. Alterations in lipid composition, lipid peroxidation and anti-oxidative protection during senescence in drought stressed plants and non-droughted stressed plants of Pisum sativum. Plant Physiol. Biochem. 33, 547±553. O'Neill, S.D., 1983. Role of osmatic potential gradients during ceswater stress and leaf senescence in Fragaria virginiana. Plant Physiol. 72, 931±937. Phene, C.J., Hoffman, O.J., Austin, R.S., 1973. Controlling automated irrigation with soil matric potential sensor. Trans. ASAE 16, 773±776. Phene, C.J., Fouss, J.L., Howell, T.A., Patton, S.H., Fisher, M.W., Bratcher, J.O., Rose, J.L., 1981. Scheduling and monitoring irrigation with the new soil matric potential sensor. In: Proceedings of the Conference on Irrigation Scheduling for Water and Energy Conservation in the 1980s. ASAE Publications, pp. 23±81, 91± 105. Phene, C.J., Allee, C., Pierro, J., 1989. Soil matric potential sensor measurements in real-time irrigation scheduling. Agric. Water Manage. 16, 173±185. Sugiyama N., Hirooka, M., 1992. Relationships between oxalate and reduced N concentrations in spinach leaves. J. Jpn. Soc. Hortic. Sci. 61, 569±574 (in Japanese with English summary). Sutton, B.G., Merit, N., 1993. Maintenance of lettuce root zone at field capacity gives best yields with drip irrigation. Sci. Hortic. 56, 1±11. Testezlaf, R., Zazueta, F.S., Yeager, T.H., A real-time irrigation control system for greenhouse. In: Proceedings of the Sixth International Conference on Computers in Agriculture. pp. 204±211. Trotter, C.M., 1984. Errors in reading tensiometer vacua with pressure transducer. Soil Sci. 138, 314±316. Turner, N.C., 1974. Stomatal behavior and water status of maize, sorgum and tobacco under field conditions. II. At low soil water potential. Plant Physiol. 55, 360±365. Wallach, R., da Silva, F.F., Chen, Y., 1992a. Hydraulic characteristics of tuff (scoria) used as a container medium. J. Am. Soc. Hortic. Sci. 117, 415±421. Wallach, R., da Silva, F.F., Chen, Y., 1992b. Unsaturated hydraulic characteristics of composted agricultural wastes, tuff and their mixtures. J. Am. Soc. Hortic. Sci. 53, 434±441. Watanabe, Y., Uchiyama, F., Yoshida, K., 1994. Compositional changes in spinach (Spinacia oleracea L.) grown in the summer and in the fall. J. Jpn. Soc. Hortic. Sci. 62, 889±895 (in Japanese with English summary).