Water use of young ‘Fuji’ apple trees at three soil moisture regimes in drainage lysimeters

Agricultural Water Management 50 (2001) 185±196

Water use of young `Fuji' apple trees at three soil moisture regimes in drainage lysimeters Hee-Myong Ro* Department of Horticultural Environment, National Horticultural Research Institute, RDA 475, Imok-dong, Jangan-ku, Suwon 440-310, South Korea Accepted 1 March 2001

Abstract Studies were conducted during 4 months of each growing season in 1994 and 1995 to measure water use of young apple trees (Malus domestica Borkh. cv `Fuji') growing under different soil moisture regimes in temperate climate conditions and to evaluate monthly crop coef®cients of such conditions. To do so, double pot lysimeters under a transparent rain shield were designed and installed. The three soil moisture regimes in three replicates each were: (A) drip-irrigation at 50 kPa of soil matric potential (IR50); (B) drip-irrigation at 80 kPa of soil matric potential (IR80); and (C) constant shallow water table at 0.45 m below the soil surface (WT45). In each treatment, soil surface was maintained with or without turf grasses. Monthly water use was not different in drip-irrigated treatments (IR50 and IR80), but greatest in the WT45 treatment. Monthly crop coef®cients increased linearly in time for drip-irrigated apple trees (r2 values of 0.76 for IR50 and of 0.77 for IR80), while those obtained in the WT45 treatment ¯uctuated. Leaf water potential (LWP) of drip-irrigated trees was similar until 63 days after treatment (DAT), but the values for IR80 trees began to decline thereafter. The LWP of WT45 trees decreased from 48 DAT. Temporal variations in leaf water content (LWC) was similar to that of LWP, except for two abrupt decreases in IR80 trees. The LWC of WT45 trees began to decrease from 59 DAT, and this occurred 2 weeks after the reduction in LWP. Average shoot length of IR50 trees was greater than that of IR80 and WT45 trees. The results of this study provided water use and crop coef®cients for apple trees in relation to soil moisture regimes under temperate climate. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Automated irrigation scheduling; Crop coef®cient; Fuji (Malus domestica Borkh.)

* Tel.: ‡82-31-240-3716; fax: ‡82-31-240-3556. E-mail address: [email protected] (H.-M. Ro).

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 0 9 9 - 3

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1. Introduction Knowledge of crop water use is of practical importance in managing agricultural water resources efficiently. In particular, soil moisture regimes developed from agricultural practices have had a key role in understanding the fate of substances, from the environmental viewpoint, in relation to global warming and groundwater pollution. Where either excess or deficient moisture limits crop yields, response to nutrient is often limited. Moreover, excessive water in the whole root zone may inhibit the diffusion of oxygen in soil pores, thus resulting in root death, leaf drop, chlorophyll destruction, and a significant reduction in photosynthetic assimilation of the horticulture crops (Kramer, 1983). Accordingly, seeking a method to achieve the soil moisture regimes sufficient to sustain maximum crop growth and crop yield while minimizing leachate from the root zone has been a major concern (Pang et al., 1997). Irrigation level affects soil water availability and, thereby, plant water status, yield and fruit size (Naor et al., 1997). Therefore, excessive or inadequate irrigation at the wrong times may result in the reduction of fruit yield. However, deficit irrigation at selected times was introduced in semi-arid zones to save water, reduce shoot growth in favor of fruit growth, and thus improve fruit quality without reducing yield (Behboudian et al., 1998). There are several irrigation-control criteria based on soil water status available for several tree fruits (Williams and Ley, 1994). In recent years, the use of dataloggers to continuously monitor soil water status through in situ soil moisture sensors has facilitated automated irrigation scheduling based on the chosen irrigation level (Ro and Park, 2000). In Korea, soil matric potential of 50 kPa is currently recommended as appropriate for the drip-irrigated apple orchards. However, no quantitative measure of monthly water use was made. In addition, due to increasing demands for municipal and industrial water, more irrigation water will be needed to meet increasing demands for food for growing populations. This situation necessitates a testing of an alternative irrigation-control level that may save more water resources without sacrificing the productivity of apple orchards. On the other hand, high water tables often occur in poorly drained orchard soils and this situation may cause deficient aeration sometimes unfavorable to crop growth in terms of oxygen availability. During monsoon rain from July to August, high shallow water table often occurred around 0.45 m depth in orchard soils. However, most reports seldom accounted for the contribution of shallow groundwater to crop water use (Ayars and Hutmacher, 1994). As water use and the related crop coefficients are climate and region specific, various crop curves for tree fruits can be found in the literature (Chalmers et al., 1992; Doorenbos and Pruitt, 1977; Haman et al., 1997). In Korea, where the apple is the most widespread fruit crop, knowledge of water use by healthy apple trees under various soil moisture regimes would help establish appropriate irrigation-control criteria, but also obtain constant concentrations of liquid fertilizer based on the appropriate irrigationcontrol criteria chosen. Recently, the paradigm of apple orcharding in Korea has entered a period of major change to optimize dual goal of high fruit yield and low environmental degradation. A drainage-type lysimeter facility, whose design and

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performance is described here, was constructed to study this aspect of apple physiology. Accordingly, the primary objective of this study was to determine water use in applegrowing lysimeters in response to three different soil moisture regimes. Additionally, the monthly crop coefficients for apple trees in relation to different soil water management were estimated. 2. Materials and methods 2.1. Drainage lysimeters and trees This study was conducted under a removable translucent rain shield (5 m above ground) using a sandy loam soil (coarse, loamy, mesic family of Typic Dystrochrepts). The rain shield was constructed with polyethylene to close in rainy days with minimal changes in solar radiation regimes; otherwise it was open. A rain shield automatically closes as soon as rain is detected (not more than a maximum of 1.5 mm precipitation) via a rain gage (TR-525M, Texas Electronics, Inc., USA) from a weather station near the lysimeter facility. The soil had a pH(1:1) of 6.2, CEC of 13.6 cmolc kg 1, 4.0 g organic matter (OM) kg 1, 0.4, 5.8, and 2.4 cmolc kg 1 exchangeable K, Ca, and Mg, respectively, and 1.1 g Kjeldahl-N kg 1, 11.1 mg NH4±N kg 1, and 185.0 mg NO3±N kg 1. Eighteen aboveground double pot lysimeters were constructed. Each inner pot having an internal diameter of 1.2 m and a depth of 1.0 m depth was packed with soil to a bulk density of 1.3 Mg m 3. The outer pot had internal dimension of 1.5 m diameter and 1.2 m depth with tap water circulating in the annular space between the pots to minimize changes in soil temperature. Perforated plastic pipe was placed at the bottom of each inner pot to allow drainage. Eighteen 2-year-old young `Fuji' apple (Malus domestica) trees grafted on Malus domestica M.26 rootstock were transplanted individually into each of the 18 inner pots during late spring of 1994. One tree per each double pot lysimeter was completely randomized in three ways at a spacing of 2.5 m in the row and 4.0 m between rows, lest tree canopies should overlap and the ventilation between tree canopies should be hindered. However, no measure of relative ventilation effects was made. Throughout the study, soil surfaces of nine lysimeters were maintained with turf grasses, while those of the others were maintained bare. The inter-row spaces were maintained with grasses. During experiments, trees were grown under three soil moisture regimes: (A) dripirrigation at 50 kPa of soil matric potential (IR50); (B) drip-irrigation at 80 kPa (IR80), and (C) constant shallow water table at 0.45 m below the surface (WT45). Trunk cross-sectional area measured at 0.1 m above the graft union increased from 4.53 to 6.15 cm2 in 1995. At the end of experiment, tree canopies expanded to a modified prolate ellipsoid of 1.7 m high and 1.5 m wide. In each lysimeter, three tensiometers were installed in 0.15 m intervals from 0.15 m depth to monitor daily variations in soil water tension profile and to control dripirrigation. Four drip emitters (Netafim, Israel) were equally spaced in a circle 0.3 m from the tree trunk. The recommended N±P±K fertilization rates for apple trees were chosen to

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maintain optimum concentrations in leaves. All presented data were obtained in 1994 and 1995. 2.2. Water balances Soil water treatments in three replicates each were started in the beginning of July and lasted 4 months in 1994 and 1995. Irrigation was automatically scheduled using the scanned tensiometer reading at 0.3 m below the surface for each drip-irrigated apple trees. Each scanned tensiometer reading was multiplexed to a datalogger (CR21XL, Campbell Scientific Inc., USA) via pressure transducer (5301, Soilmoisture Equipment Corp., USA) and adjusted for gravitational potential to provide electrical control signal for automatic drip-irrigation through control module (SDM-CD16, Campbell Scientific Inc., USA). Irrigation was continued until soil matric potential rose equal to set value. Each lysimeter unit with a water table maintained at 0.45 m below the surface was connected to a Mariotte bottle water reservoir system. Volumes of irrigation water applied to each lysimeter were measured with a flow meter and those applied from the Mariotte bottle water reservoir system were determined from the daily difference in the water height of the Mariotte bottle. Drainage volumes from each lysimeter unit were negligible. Changes in the soil water storage were estimated daily by time domain reflectometry (TDR) measurements via a cable tester (Model 1502B, Tektronix, USA) made in 0.2 m intervals from 0.1 to 0.9 m below the soil surface. The TDR probes were installed horizontally with respect to roots. Water use in each lysimeter was determined using an equation of conservation of water, as previously done by Clark et al. (1996) and Chalmers et al. (1992). Lysimeter water use data are expressed as the equivalent depth over a cross-sectional area of tree canopy. Air temperature, relative humidity, solar radiation, rainfall, and evaporation data were real-time measured at the two sites (outside and inside the lysimeter facility). Pan evaporation (Epan) data were obtained from a Class A evaporation pan. Air temperature and relative humidity were measured using a combination thermistor/capacitive sensor (HMP35C, Campbell Scientific Inc., USA), solar radiation was measured using a silicon pyranometer (LI200S, Licor Inc., USA), and rainfall was measured using a rain gage (TR-525M, Texas Electronics, USA). The scanned weather data were summarized on a daily basis to provide maximum and minimum daily air temperature, average daily relative humidity, and total incoming solar radiation. Daily cumulative rainfall data were used to open and close the rain shield. 2.3. Leaf water-potential and -content From 9 July to the end of experiment in 1995, 12 measurements of leaf water potential was made periodically on the fully expanded, mature leaf of each test plant using a pressure chamber (DIK 7000, Daiki Rika Kogyo, Co., Ltd., Tokyo, Japan) and averaged. At every measurement, five leaves per tree were sampled between 1000 and 1200 h. The leaf was cut at the petiole base and immediately placed in the pressure chamber with approximately 0.5 cm of the cut end of the petiole protruding through the soft silicone

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stopper used to seal the chamber. At the same time, duplicate leaf sample was sampled for direct determination of leaf water content. 2.4. Soil- and leaf-N analyses and shoot growth Soil and leaves were sampled for N analysis by Kjeldahl digestion at 31 July, 31 August, 30 September, and 31 October of 1995. Soil samples were taken from 0 to 0.45 m from four profiles per lysimeter, air-dried, and passed through 100-mesh sieve for Kjeldahl-N determinations. Leaf samples were collected from around each tree, washed with distilled water, dried at 608C for at least 48 h, and ground for Kjeldahl-N analysis. Both samples were digested using a block digester (B-435, BuÈchi, Switzerland). KjeldahlN was determined from the steam distillates by the NH4‡±NH3 acid±base titration. Other soil chemical variables were analyzed by standard methods but were excluded. Ten shoots per replicate were tagged in the beginning of experiment. Shoot length was measured at approximately 5-day intervals on these tagged shoots and averaged. 3. Results and discussion In general, daily water use from the lysimeter systems followed the distribution pattern of Epan throughout the experimental period and the amount of daily water use and Epan of 1994 was greater than that of 1995 (Fig. 1). Monthly mean of air temperature, solar radiation, and Epan from July to October of 1994 was higher than the average for the past 30 years, while that of 1995 and the average was virtually the same (Table 1). Mean solar radiation of 4 months was 14.9 MJ m 2 in 1994 and 12.2 in 1995, while the average for the past 30 years was 12.1. In contrast, mean relative humidity during the same period was 67.5% in 1994 and 77.3 in 1995. For the comparison, the average relative humidity for the past 30 years was 79.3%. Consequently, monthly water use and Epan were greater in 1994 than in 1995. Mean Epan of 4 months was 131.8 mm in 1994 and 72.9 in 1995, while the average for 30 years was 77.4. However, mean air temperature was not greatly different between 2 years. Throughout the study, air temperature, solar radiation, relative humidity, and Epan measured inside and outside the lysimeter were not different greatly, resulting in similar evaporation demand. Differences in the average values for 2 years between outside and inside the lysimeter were 0.68C for air temperature, 0.4 MJ m 2 for solar radiation, 2.3% for relative humidity, and 4 mm for Epan, respectively. From the measured weather variables, daily potential evapotranspiration (ETr) was estimated using Jensen±Haise model: 79 mm for July, 81 for August, 63 for September, and 41 for October of 1995. An excellent agreement between ETr and Epan (r ˆ ‡0:92 , n ˆ 110) was obtained by correlation analysis (data not shown). Chalmers et al. (1992) reported the correlation coefficient between them was higher than 0.9 in late summer and fall. Daily water use from the WT45 treatment was mostly higher than that of drip-irrigated lysimeter systems (IR50 and IR80) for 2 years. Because the lysimeter soils of the WT45 treatment were near saturation due to capillary rise from water table, soil evaporation was considerable. In WT45 treatment, soil±water tension at 0.15 m and soil±water content at

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Fig. 1. Daily variation in the water use of young apple trees (Malus domestica Borkh. cv `Fuji') in grass-covered drainage lysimeters.

0.1 m were maintained at less than 5 kPa and more than 0.25 m3 m 3, respectively, thus indicating near saturation at soil surface layer. Differences in water use between dripirrigated (IR50 and IR80) and Mariotte-bottle connected (WT45) lysimeter systems were greater from mid-July to August and the difference thereafter gradually diminished, as Epan decreased. The amount of monthly water use from the WT45 treatments was significantly higher than that of drip-irrigated lysimeters (IR 50 and IR80) for 2 years (Table 2). Monthly water use was lowest in July for 1994 and October for 1995, while that was highest in August for both years. The cumulative lysimeter water use from the drip-irrigated

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Table 1 Monthly average of air temperature, solar radiation and Class A pan evaporation Month

Year

Air temperature (8C)

Solar radiation (MJ m 2)

Relative humidity (%)

Epan (mm)

Outsidea

Insideb

Outside

Inside

Outside

Inside

Outside

Inside

July

1994 1995 Averagec

28.7 24.8 24.4

29.0 25.0

15.2 12.8 12.8

14.8 12.6

68 79 82

72 83

169.6 82.1 87.0

162.9 81.2

August

1994 1995 Average

27.6 26.6 25.4

28.1 26.4

14.7 12.2 12.2

14.4 12.0

70 80 81

73 84

148.0 82.6 93.2

142.5 81.7

September

1994 1995 Average

20.7 20.1 19.8

21.1 20.4

15.0 12.9 12.9

14.5 12.6

64 75 78

65 76

124.2 75.9 74.7

115.9 71.8

October

1994 1995 Average

14.8 15.1 13.1

15.1 15.5

14.6 10.7 10.3

14.2 10.4

68 75 76

69 75

85.5 50.9 54.8

80.2 50.5

a

These subheadings denote the measurement sites outside the lysimeter facility. These subheadings denote the measurement sites inside the lysimeter facility. c Numbers in these rows denote the averages for the past 30 years. b

P lysimeters was always lower than the cumulative pan evaporation ( Epan ), while that of WT45 treatments was lower in July, but higher in August, September, and October. In general, monthly water use from all treatments peaked August and decreased P thereafter. In October, monthly water use was virtually the same or even higher than Epan , even Table 2 Monthly water use of young apple trees (Malus domestica Borkh. cv `Fuji') in grass-covered lysimeters. As a reference, Class A pan evaporation (Epan) was included Month

Epan (mm)

Water use in the lysimeter per canopy area (mm) IR50

IR80

WT45

Year 1 (1994) July August September October

67.6ba 94.0b 85.2b 86.9b

71.0b 102.0b 97.7b 91.0b

111.9a 153.6a 131.4a 112.6a

Year 2 (1995) July August September October

47.9b 63.5b 60.8b 46.5b

(42.6)b (52.1) (49.6) (36.4)

55.7b 63.2b 58.1b 47.0b

(40.2) (49.8) (50.3) (35.8)

78.9a 113.4a 83.6a 64.8a

169.6 148.0 124.2 85.5 (70.8) (95.0) (68.5) (40.7)

82.7 82.6 75.9 50.9

a Different letters at each row denote significant differences between means calculated by Duncan's multiple range test (P ˆ 0:05). b Numbers in parentheses are the monthly water use from bare-surfaced lysimeter systems.

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though the solar radiation and Class A pan evaporation data were lowest. In October, solar radiation was 14.6 MJ m 2 for 1994 and 10.7 for 1995. During 4 months of each growing season, total cumulative lysimeter water use varied from 509.5 mm for 1994 to 292.1 for 1995 in grass-covered WT45 lysimeter system. In both grass-covered drip-irrigated treatments, the total amount of water use decreased from 333.7 mm (1994) to 218.7 (1995) for IR50 treatment, and from 361.7 mm (1994) to 224.0 (1995) for IR80 treatment. During the same period, the cumulative Epan totaled 527.3 mm in 1994 and 292.1 in 1994, respectively. Despite the insignificance in monthly water use between the two drip-irrigated treatments, the average monthly water use was somewhat higher in IR80 treatment than in IR50 treatment. However, the amplitude of daily fluctuation in soil water status was greater in IR80 treatment, which may leave the system susceptible to stress even with short-term soil water deficit (Kramer, 1983). For the comparison, in 1995, total cumulative water use in bare-surfaced lysimeters was 180.7 mm for IR50, 180.1 for IR80, and 271.8 for WT45 trees, respectively. Irrespective of treatment, monthly water use of bare-surfaced lysimeters was less than that of grass-covered lysimeters throughout the experiment. In general, differences in water use between grass-covered and bare-surfaced lysimeter systems were greatest for WT45 treatments. In particular, daily water use of bare-surfaced WT45 lysimeters was lowest from mid-October to the end of 1995, implying that the amount of water used by these trees was reduced during that time. This reduction in water absorption by roots may result from the dissolved oxygen (DO) deficit in the soil solution surrounding roots. During October in 1995, DO concentration of soil solution sampled between 15 and 30 cm depths was lower than 6.0 mg l 1. Ro et al. (1995) showed that the sustained DO concentration below 6.0 mg l 1 at surface region of sandy loam soil is enough to cause a rapid decrease in leaf water potential and root activity of `Tsugaru' apple trees. The leaf water potentials were the same until 31 days after treatment (DAT), except for the first two lower values of IR80 trees at 9 and 24 DAT (Fig. 2A). The values (MPa  S:E:) for all trees increased to 1:26  0:04 at 31 DAT, and those at 37 DAT were 1:59  0:05 for IR50, 1:97  0:11 for IR80, and 1:63  0:02 for WT45 trees. However, the values (MPa  S:E:) for IR50, IR80, and WT45 trees at 59 DAT were 1:76  0:05, 1:78  0:04, and 2:16  0:06, respectively. For the last three measurements, WT45 trees had lower values than IR50 and IR80 trees, and for the last measurement (109 DAT) the values (MPa  S:E:) were 1:70  0:05, 1:86  0:04, and 2:18  0:07, respectively. The overall pattern of leaf water potential of IR50 and IR80 trees was similar until 72 DAT, but the values for IR80 trees declined slightly thereafter. However, leaf water potential of WT45 trees decreased considerably from 48 DAT. Daily variation pattern of leaf water content was similar to that of leaf water potential, but the values (kg kg 1 fw  S:E:) of IR80 trees decreased abruptly: 0:53  0:02 at 24 and 0:45  0:04 at 48 DAT, and recovered shortly (Fig. 2B). Leaf water content (kg kg 1 fw  S:E:) of IR50 and WT45 trees at 48 DAT was 0:60  0:02 and 0:61  0:01, respectively. However, leaf water content of WT45 trees began to decrease considerably from 59 DAT (0:53  0:02 kg kg 1 fw). The decrease in leaf water content of IR80 and WT45 trees followed about a week after the occurrence of reduction in leaf water potential. Zhang and Archbold (1993) reported that the plant maintains to some extent its leaf tugor potential as leaf water potential declines. For the last measurement

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Fig. 2. Temporal variations in (A) leaf water potential and (B) leaf water content measured at 1100 h. Vertical bar at each data point denotes  standard error of the mean when larger than symbol size.

(109 DAT) the values for IR50, IR80, and WT45 trees were 0:59  0:02, 0:57  0:02, and 0:46  0:02, respectively. The slopes from linear regression analyses of daily lysimeter water use versus Epan for each month and treatment showed the average monthly crop coefficients from each lysimeter treatment (Table 3). All the values were significant (P ˆ 0:001). Monthly crop coefficients determined in drip-irrigated lysimeter treatments increased linearly from July to October of 1994±1995, while those of WT45 fluctuated. Unfortunately, we did not measure either periodic increase in canopy size and rooting volume or leaf senescence, which may help support the increase in monthly crop coefficient of drip-irrigated trees Table 3 Slopes of linear regression equations of water use on Class A pan evaporationa Treatment

Year

Slopes of linear regressionb July

IR50 IR80 WT45 a b

1994 1995 1994 1995 1994 1995

0.41 0.56 0.43 0.60 0.73 0.88

(0.93) (0.81) (0.92) (0.67) (0.96) (0.82)

August

September

October

Average

0.62 0.71 0.65 0.69 1.07 1.28

0.66 0.75 0.75 0.73 1.00 1.05

0.98 0.88 1.03 0.89 1.25 1.22

0.58 0.69 0.62 0.69 0.94 1.09

(0.89) (0.89) (0.90) (0.82) (0.92) (0.94)

(0.90) (0.84) (0.83) (0.83) (0.87) (0.93)

(0.89) (0.82) (0.86) (0.86) (0.86) (0.94)

Linear model: Y ˆ aX, where Y ˆ water requirements and X ˆ Class A pan evaporation. Numbers in parentheses are the coefficients of determination at P ˆ 0:001.

(0.83) (0.83) (0.82) (0.78) (0.87) (0.89)

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and fall decrease in WT45 trees. However, the relations between leaf water potential and leaf water content shown in Fig. 2 would support linear increase in monthly crop coefficients of IR50 and IR80 trees and fall decrease in WT45 trees. With year, average monthly crop coefficients increased from 0.58 to 0.69 for IR50, from 0.62 to 0.69 for IR80, and from 0.94 to 1.09 for WT45 treatments. Compared to the results of 1994, the average monthly cumulative water use from the lysimeter in 1995 was smaller, but monthly crop coefficient was greater due in part to increase in canopy size or rooting volume. This suggests that, in 1994, lysimeter water use was greater in response to larger evaporation demand, but water absorption by roots was not so great. Despite lower evaporation demand (Table 1), higher monthly crop coefficients in 1995 were attributable to larger water absorption by roots. In general, the amount of water absorption by roots varies in accordance with the solar radiation under well-watered conditions; however, it increases with increased canopy size or rooting volumes of perennial trees (Kozlowski and Pallardy, 1997). However, since no measure of the mass flow rate of sap or rooting volume was made, this study lacks supporting information to explain the effect of water absorption by roots on water use and the resultant crop coefficient. Regression analyses of the crop coefficients versus month for drip-irrigated apple trees in this experiment site yielded model with 0.60 (r 2 ˆ 0:82 ) for IR50 and 0.64 (r 2 ˆ 0:79 ) for IR80 treatments for both years. With respect to alfalfa reference, Doorenbos and Pruitt (1977) reported crop coefficient values (per canopy area) of 0.7±0.9 for apples of similar size under their experimental conditions, but this range is slightly greater than other measurements (Chalmers et al., 1992). Clark et al. (1996) observed crop coefficient values increased linearly in drip-irrigated strawberry plants in a humid climate. However, Chalmers et al. (1992) found crop coefficients fluctuated during the well-watered period and declined variably during the fall for Asian pears growing in lysimeters under New Zealand conditions. Average shoot length (cm  S:E:) of IR80 trees was virtually the same as that of IR50 trees (19:7  1:23) until 38 DAT, but decreased thereafter (Fig. 3). However, the values of WT45 trees were significantly greater than that of drip-irrigated trees throughout the experiment. Differentiation of average shoot length with time showed that the rate of shoot growth decreased greatly at 49 DAT for IR50, 44 for IR80, and 38 for WT45 trees. This retardation in the rate of shoot growth may result from the earlier reduction in leaf water potential for IR80 and WT45 trees or physiological senescence of all trees. For the last measurement (70 DAT), the values (cm  S:E:) for IR50, IR80, and WT45 trees were 28:2  1:01, 24:3  0:88, and 34:3  1:74, respectively. Hong et al. (1997) reported that the average weight, the soluble solid content, and the number of shoots of `Fuji'/M.26 apples peaked when the average shoot length is ranged from 27 to 30 cm in this climate region. Soil Kjeldahl-N at the end of experiment was slightly reduced from its initial value and midseason-leaf Kjeldahl-N of 1995 at 10 August was not significantly different: 26.1 for IR50, 25.7 for IR80, and 23.3 g kg 1 for WT45 trees. Considering the water use, leaf water relations, shoot length, and leaf-N status, water use of young apple trees from two drip-irrigated soil moisture regimes and leaf-N concentration did not differ significantly, but IR50 trees finally had longer shoot length than IR80 trees. Moreover, IR80 trees showed a sign of dehydration of water from leaf

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195

Fig. 3. Temporal variations in average shoot length of trees in grass-covered lysimeters. Vertical bar at each data point denotes  standard error of the mean when larger than symbol size.

tissues, but it was recovered shortly. The experimental results of this study clearly supported that a soil matric potential of 50 kPa would be appropriate for drip-irrigated apples, but did not show the feasibility of soil matric potential of 80 kPa as an alternative. Without abrupt decrease in leaf water potential, the average shoot growth would be longer than measured. However, it should be noted that the results from this study show the short-term effects of soil moisture regimes on water use of young apple trees in the lysimeters. We also lack supporting information to explain either increase in crop coefficient of drip-irrigated trees or its decrease of WT45 trees in fall. This fall reduction in water use of WT45 trees could be explained in part by the fact that the sustained soil saturation greatly blocks O2 diffusion in soils. However, the results of this study would not only help field extension personnel prepare guidelines for drip-irrigation to the farmers, but also provide monthly water use of growing season for apple trees in response to different soil water regimes under temperate climate region. More specifically, the result of this study was used to establish an appropriate N concentration of liquid fertilizer at each occasion of irrigation to maximize uptake without overfertilizing in relation to tree ages (Ro and Park, 2000). However, the long-term effects of soil moisture regimes on water use of fruit bearing apple trees and the related productivity need further study. References Ayars, J.E., Hutmacher, R.B., 1994. Crop coefficients for irrigating cotton in the presence of groundwater. Irr. Sci. 15, 45±52.

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