2018 Volume 87 Issue 1 Pages 26-33
To apply the beneficial results obtained for potted citrus trees to field-grown ones, the effects of seawater application on soil electrical conductivity (EC), water relations, and fruit quality in field-grown satsuma mandarin (Citrus unshiu Marcow.) trees were determined. In 2010, periodical applications of smaller amounts of half-strength-diluted (1/2X) or undiluted seawater (1X) delayed the reduction of leaf water potential to the objective level at 0.3 to 0.5 MPa lower than that in the control, resulting in an insignificant increase in soluble solids content (SSC). Thus, half of the total amount of 1X per area applied in 2010 was irrigated once or twice in September in 2012 and 2013. Soil EC in 1X markedly increased after just the first application at above 1.8 dS·m−1 and was kept at a significantly higher level than in the control until harvest, although it gradually decreased by leaching due to rainfall. Leaf water potential at predawn was reduced by 1X and the objective value was achieved from early-October and mid-September to harvest in 2012 and 2013, respectively. SSC was higher in 1X than the control throughout the experimental period and the difference between 1X and the control at harvest was 1.4 and 1.2°Brix in 2012 and 2013, respectively. Other fruit quality parameters including titratable acidity (TA) were not significantly affected by seawater irrigation in either year, except for fruit size which was slightly inhibited in 1X. No difference was observed in the leaf chlorophyll index and abscission between 1X and the control, although the Na content in the leaves was increased in 1X. These results suggest that application of relatively higher amounts of undiluted seawater in the early stage of maturation could induce moderate salt or water stress through the inhibition of water absorption by roots and improve fruit quality by increasing SSC without any visible leaf injury in field-grown satsuma mandarin trees.
Since a non-destructive fruit grading system was developed and introduced in many packing houses, the production of high quality citrus fruit with higher soluble solids content (SSC) has been required by citrus growers for stable management in Japan. It is well known that SSC in juice is increased by applying water stress to citrus trees (Barry et al., 2004; Kadoya, 1973; Kallsen et al., 2011; Yakushiji et al., 1998). Therefore, mulch cultivation using plastic sheets that prevent rainfall from permeating the soil but allow water vapor from soil to evaporate has been widely used to produce high quality citrus fruit in higher rainfall regions, including Japan (Iwasaki et al., 2011; Jiang et al., 2014; Yakushiji et al., 1996).
Salinity is one of the most serious worldwide problems limiting crop growth and productivity, especially in arid and semi-arid regions. However, in the tomato, a moderately sensitive plant to salinity, salt stress in addition to drought stress has also been used to improve fruit quality by increasing SSC in various culture systems, although yield is often reduced (Mitchell et al., 1991; Niedziela et al., 1993; Saito et al., 2009). On the other hand, citrus trees are relatively sensitive to salinity stress (Levy and Syvertsen, 2004). Thus, only a few reports have been published about the positive use of salinity to improve fruit quality in citrus (Kawai et al., 2002; Levy et al., 1979; Yakushiji et al., 1997; Yamada et al., 2015), while a lot of investigations on the negative effects of salinity on growth and physiology have been conducted (Balal et al., 2012; Garcia-Sanchez and Syvertsen, 2009; Gonzalez et al., 2012; Lloyd et al., 1990; Storey and Walker, 1999). Since there is high rainfall above 1300 mm per year in southwestern Japan where most of citrus fruit is produced, salinity is a less important issue, except for the occasional typhoon that sometimes causes severe salty wind damage to citrus trees (Beppu et al., 1995). As most citrus orchards are located in coastal areas, seawater can be considered as a free local resource for possible use to improve citrus fruit quality (Yamada et al., 2015). Yamada et al. (2015) found that mild salt stress induced by half-strength-diluted seawater irrigation to the extent of 0.3 to 0.5 MPa lower water potential at predawn than in unstressed trees could improve the taste of fruit, giving a higher SSC and similar titratable acidity (TA) without inducing any obvious injuries like leaf abscission, in potted citrus trees.
The objective of this study was to determine (1) the application methods of diluted or undiluted seawater to induce and maintain moderate salt stress in field-grown citrus trees; (2) whether the moderate stress could improve fruit quality without any side effects, as observed in pot-grown trees. Based on the results of pot-grown trees (Yamada et al., 2015), the objective level of moderate stress was set at 0.3 to 0.5 MPa lower leaf water potential at predawn than that in the unstressed condition in this study.
Nine 4-year-old ‘Nankan No. 20’ satsuma mandarin (Citrus unshiu Marcow.) trees on trifoliate orange [Poncirus trifoliata (L.) Raf.] grown in the experimental orchard of Ehime University at Tarumi in Matsuyama were used. The soil was well-drained sandy loam. Fruit were thinned to about 30 leaves per fruit on September 6 in 2010. Surface seawater was taken at Takahama fishing port in Matsuyama and used for the experiment. The electrical conductivity (EC) and Na concentration of the seawater were 66 dS·m−1 and 1.2% (520 mM), respectively.
2) Seawater treatmentThree irrigation treatments of undiluted seawater (1X), half-strength-diluted seawater (1/2X), and tap water (control: EC = 0.15 dS·m−1), each having three replicate trees, were established. A plastic frame at a size of 1 m2 (1 m × 1 m) was set on the soil surface around the trunk to allow uniform irrigation of the treatment solution under the canopy. The canopy diameter was about 1.4 m and it was set diagonal to the plastic flame. Forty liters of treatment solution was applied to the soil within the frame of each tree from September 10, when an orange color started to develop at the stylar end, to November 18 at about two-week intervals, resulting in 240 L·m−2 per tree in a total of six applications.
3) MeasurementsAbout 50 g of soil at depths of 5 and 20 cm was sampled at opposite sides of the trunk at two-week intervals from September 9, one day before the first seawater irrigation, and soil EC was measured using an EC meter (CM-55; Takemura-denki Co., Tokyo, Japan) after mixing dried soil with fivefold distilled water. Leaf water potential at predawn was periodically determined using a pressure chamber (Model 1000; PMS Instrument Co., OR, USA). Three fruit per tree were sampled every week and SSC and TA in the juice were measured using a handy refractometer and titration with 0.05 N NaOH, respectively.
At harvest on December 17, fruit quality characteristics such as fruit size and weight, pulp ratio, and peel color were also determined. The size of three fruit per tree selected and marked before the first irrigation treatment was measured both at the start of treatment and at harvest at the same equatorial region, and expressed as values relative to the size at the starting day set as 100. Peel color was measured at the equatorial region of fruit with a chromameter (CR-321; Minolta Co., Osaka, Japan). As an index of leaf chlorophyll, greenness of three spring-flushed leaves per tree was determined with a chlorophyll meter (SPAD-502; Konica Minolta, Inc., Osaka, Japan). For the three spring-flushed shoots selected, the leaf number was counted at the start of treatment, and the percentage of leaf abscission was determined at harvest.
2. Experiment in 2012 1) Plant materials and origin of seawaterSix 6-year-old ‘Nankan No. 20’ satsuma mandarin trees on trifoliate orange were used. As fruit set was relatively low, the number of leaves per fruit was adjusted to about 50 on September 11. The growing site and seawater origin were the same as those of the experiment in 2010.
2) Seawater treatmentTwo irrigation treatments of 1X and control were set and each treatment contained three replicate trees. The size of each frame around the trunk was widened to 1.21 m2 (1.1 m × 1.1 m) with the canopy expansion to 1.6 m diameter. Half of the total amount per unit area applied in 2010 at 120 L·m−2 or 145.2 L/tree was irrigated to each tree on September 12, when fruit coloring started. Based on the measurement of maximum leaf water potential at predawn, additional application of the same volume of treatment solution was conducted on September 24, resulting in 240 L·m−2 (290.4 L/tree) in total.
3) MeasurementsSoil EC at a depth of 10 cm was periodically determined using a portable handy EC meter (HI 98331; Hanna instruments Co., Tokyo, Japan). Spring-flushed leaves were sampled at harvest on December 5 and Na content in the leaves was analyzed by atomic-absorption spectrometry (Z-2310; Hitachi Co., Tokyo, Japan) at 589 nm after extraction with 1% HCl. Other parameters were determined in the same ways as 2010.
3. Experiment in 2013 1) Plant materials and origin of seawaterSix 30-year-old ‘Okitsu-wase’ satsuma mandarin trees on trifoliate orange grown at the experimental farm of Ehime University at Hojo in Matsuyama were used. Fruit were thinned to about 30 leaves per fruit in early-September. Surface seawater was taken at Doteuchi fishing port in Matsuyama and the EC and concentration of Na was the same as in 2010.
2) Seawater treatmentTwo treatments of 1X and control with three replicate trees were arranged. A plastic frame of 7.84 m2 (2.8 m × 2.8 m) was set under the canopy with a diameter of about 4.0 m and 940 L of undiluted seawater or tap water was uniformly irrigated in the frame of each tree on September 10. The total amount of irrigated seawater per unit area was 120 L·m−2, half of that in previous years, because no additional application was carried out in this experiment.
3) MeasurementsThe leaf chlorophyll index was not determined this year. Fruit were harvested on November 26, and other measurement parameters and methods were the same as those in 2012.
4. Statistical analysisData were subjected to analysis of variance and the means were separated by Tukey’s test in 2010 and t-test in 2012 and 2013 at P < 0.05. The percentage data for leaf abscission and pulp ratio were transformed using arcsine before statistical analysis.
The changes in temperature and precipitation during the experiment period in 2010, 2012, and 2013 are shown in Figure 1. Daily average temperature gradually decreased from about 28°C in early-September to about 10°C in late-November in the three years. The total amounts of rainfall during the experimental period in 2010 and 2012 were 261 and 223 mm, respectively, and distributed relatively even over the period (Fig. 1A and B), while most of the higher precipitation of 384 mm was concentrated in October 2013 (Fig. 1C).
Changes in temperature and precipitation during the experimental period in 2010 (A), 2012 (B), and 2013 (C). Arrows indicate the dates of seawater application.
As similar changes were observed in soil EC at depths of 5 and 20 cm, only the data at 5 cm was presented as representative in 2010 (Fig. 2A). Soil EC was increased by seawater irrigation, particularly in 1X, and it fluctuated in a similar pattern with two peaks on September 24 and November 18 both in 1X and 1/2X, whereas it maintained a lower level in the control. Soil EC measured by a portable EC meter at a depth of 10 cm in 2012 and 2013 was markedly increased by undiluted seawater irrigation to 1.8 and 2.6 dS·m−1, respectively, on the first measurement day after the first application of seawater (Fig. 2B). Then, it gradually decreased to about 0.3 dS·m−1 in late-November in both years, although a temporal increase was observed in late-October in 2013. EC in 1X was significantly higher than that in the control throughout the experimental period in both years.
Effect of seawater application on soil EC in 2010 (A), 2012 (B), and 2013 (B). Different letters indicate significant differences among the treatments at P < 0.05 by Tukey’s test with three replications (A). * indicates significant difference from the control at P < 0.05 by t-test with three replications (B).
In 2010, maximum leaf water potential at predawn in the control was kept at about −0.3 MPa until the end of November, then decreased to −0.9 MPa in mid-December (Fig. 3A). Water potential in 1X started to decrease from early-October and reached a significantly lower level than in the control from late-October through mid-December, while the value in 1/2X was between 1X and the control. The objective level of water or salt stress at 0.3 to 0.5 MPa lower than that in the control was attained from mid-November and mid-December in 1X and 1/2X, respectively.
Effect of seawater application on leaf water potential at predawn in field-grown satsuma mandarin trees in 2010 (A), 2012 (B), and 2013 (B). Different letters indicate significant differences among the treatments at P < 0.05 by Tukey’s test with three replications (A). * indicates significant difference from the control at P < 0.05 by t-test with three replications (B).
In 2012, leaf water potential in 1X decreased to a significantly lower level than that in the control in mid-September after the first seawater treatment, then it slightly increased and the difference from the control became insignificant in late-September (Fig. 3B), leading to additional irrigation with seawater. After the second application, water potential in 1X was maintained the objective value from early-October to early-December.
In 2013, 1X reduced leaf water potential at predawn to a significantly lower level than the control in mid-September, just after the first application (Fig. 3B). As the difference in water potential between 1X and the control ranged from 0.3 to 0.5 MPa until harvest on November 27, no additional application was carried out.
Seasonal changes in SSC and TAIn 2010, SSC decreased from early-September to early-October, then it increased through mid-December in every treatment (Fig. 4A). SSC in 1X tended to be higher than in the control after mid-November, but the difference was not significant. Undiluted seawater (1X) significantly increased SSC at the first and last two samplings both in 2012 and 2013 (Fig. 4B). The difference in SSC at harvest between 1X and the control was 1.4 and 1.2°Brix in 2012 and 2013, respectively.
Effect of seawater application on SSC in the flesh juice in field-grown satsuma mandarin trees in 2010 (A), 2012 (B), and 2013 (B). Each value indicates the mean of three replicates ± SE (A). * indicates significant difference from the control at P < 0.05 by t-test with three replications (B).
TA gradually decreased during the experimental period and no significant difference was detected among the treatments in the three years (Fig. 5A and B).
Effect of seawater application on TA in the flesh juice in field-grown satsuma mandarin trees in 2010 (A), 2012 (B), and 2013 (B). Each value indicates mean ± SE (n = 3).
Fruit growth expressed as a value relative to the size at the starting day set as 100 tended to be inhibited by the seawater treatments in the three years and fruit length in 2010 and fruit width in 2013 were significantly lower in 1X than those in the control (Table 1). Other fruit quality characteristics at harvest were not affected by the seawater irrigation.
Effect of seawater application on fruit quality characteristics other than SSC and TA at harvest in field-grown ‘Nankan No. 20’ and ‘Okitsu-wase’ satsuma mandarin trees.z
Na content in the leaves was increased by 1X and the difference between 1X and the control was significant in 2012 (Table 2). Leaf chlorophyll index and abscission at harvest were not influenced by seawater application.
Effect of seawater application on Na content, chlorophyll (SPAD) index, and abscission of the leaves at harvest in field-grown ‘Nankan No. 20’ and ‘Okitsu-wase’ satsuma mandarin trees.z
Using potted citrus trees, Yamada et al. (2015) found that the application of half-strength-seawater (1/2X) at a total of 8 L/pot, equivalent to about 48 mm irrigation per soil surface area, reduced leaf water potential at predawn from 0.3 to 0.5 MPa lower than that in unstressed control trees and increased fruit SSC without leaf abscission. On the other hand, undiluted seawater (1X) induced more severe salt stress at the level of >0.5 MPa lower water potential than in the control, resulting in the inhibition of fruit growth and/or the promotion of leaf abscission, depending on the cultivars. Therefore, we set our objective value of salt stress induced by seawater irrigation to 0.3 to 0.5 MPa lower leaf water potential at predawn than that in unstressed trees in this study with field-grown trees.
In the experiment in 2010, in spite of the increase in soil EC (Fig. 2A) by seawater application of 240 L·m−2 in total, which is equal to 240 mm and a several times higher level than that in the experiment using potted trees (Yamada et al., 2015), a significant reduction in water potential compared to the control occurred only after late-October even in 1X and the achievement of our objective value of salt stress was delayed after mid-November and mid-December in 1X and 1/2X, respectively (Fig. 3A). The marked decrease in water potential observed in December in every treatment, including the control, may result from cold-acclimating temperatures below 10°C (Barkataky et al., 2013). The failure to induce enough salt stress may be caused by some factors; (1) division of seawater application into six times smaller amounts at two-week intervals during the longer period of two and a half months could not induce early or strong inhibition of water absorption by the roots, (2) relatively higher rainfall during the early stage until early-October could leach salts to the subsoil, and (3) unaffected-roots distributed in the soil outside the frame could compensate for the reduction in the water absorbing function by salt-affected roots. In particular, the last factor could explain the difference from the results of potted trees, in which a relatively small amount of seawater applied could reach throughout the whole root zone in the pot, leading to the efficient induction of moderate salt stress even in 1/2X. In the field-grown satsuma mandarin trees on trifoliate orange, about 13% of the roots distributed in the soil outside the ground surface covered by the tree canopy (Yoshida and Iwasaki, 2014). As the diameter of the canopy was set diagonal to the plastic frame in this study, the area of each frame occupied about 65% of the ground area covered by the tree canopy. Therefore, some roots which were not, or less, affected by seawater and had normal function of water absorption could exist in the soil outside the frame and compensate for the function reduced by affected roots. The lighter and delayed salt stress resulted in insufficient improvement in fruit quality in both seawater treatments, although a tendency to increase SSC was detected in 1X (Fig. 4A; Table 1). In addition, as no side effect on the leaves was observed even in 1X (Table 2), only undiluted seawater (1X) was applied afterward.
To advance the time of salt stress induction, half the total amount of undiluted seawater per area applied in 2010 was irrigated once or twice in early to late-September, resulting in a marked increase in soil EC above 1.8 dS·m−1 at the first determination after the first application in both 2012 and 2013 (Fig. 2B). The higher EC significantly reduced leaf water potential just after the application in both years (Fig. 3B). However, as the water potential in 1X slightly increased and the difference from that in the control became insignificant after some rainfall during mid-September in 2012, additional application of the same volume of seawater was conducted in late-September (Figs. 1B and 3B). On the other hand, in 2013 when we had only a little rainfall in September (Fig. 1C), higher EC and lower water potential than in the control were maintained in 1X until late-November (Figs. 2B and 3B), resulting in no additional application. These early treatments with a large amount of seawater successfully reduced leaf water potential to the objective value of 0.3 to 0.5 MPa lower than that in unstressed controls from early-October and mid-September to harvest in 2012 and 2013, respectively (Fig. 3B). The reduction in maximum leaf water potential in 1X was relatively stable during the experimental period, while soil EC gradually decreased from above 1.8 to 0.3 dS·m−1, suggesting that soil EC determined only at a depth of 10 cm could not be a reliable index of salt stress as discussed in our previous article (Yamada et al., 2015). As described above, the subtle balance between severely salt-affected roots and less, or not, affected ones might result in moderate reduction in leaf water potential to the objective value.
The early achievement of objective salinity stress increased SSC without any significant changes in fruit quality parameters except for the fruit size, which was slightly decreased in 1X (Figs. 4B and 5B; Table 1). In recent years, internal quality involving SSC and TA have been considered more important characteristics than fruit size in citrus in the Japanese market due to the consumer demand for better tasting fruit and the introduction of a non-destructive SSC measuring system in many packing houses. Thus, the slight inhibition in fruit growth observed in this study may be negligible. In addition, a similar phenomenon was also detected in a plastic mulch cultivation system which has been commonly employed to give moderate drought stress and increase SSC (Iwasaki et al., 2011; Yakushiji et al., 1998).
Salinity affects citrus in two ways: osmotic stress and toxic ion stress (Levy and Syvertsen, 2004). Salinity water irrigation slightly increased SSC and TA in grapefruit grown in semi-arid Israel without any accumulation of salts or visible injury symptoms in the leaves (Bielorai et al., 1978; Levy et al., 1979). They concluded that the changes in fruit quality may be due mainly to osmotic stress and not to specific effects of Na or Cl on tree physiology (Levy et al., 1979). Our previous study with potted citrus trees also suggested that the inhibition of root water absorption by higher soil EC through diluted seawater irrigation (1/2X) could induce a reduction in water potential and the accumulation of sugars by osmoregulation via a physiological mechanism similar to drought stress (Yamada et al., 2015). In the present study, Na content was increased to 0.13% DW by undiluted seawater application (Table 2), but the level was similar to that in 1/2X in the experiment with potted trees, in which fruit taste was improved by higher SSC and a similar TA without inducing any leaf abscission (Yamada et al., 2015). The Na level was lower than the threshold values to induce leaf injury at above 0.5 to 1% DW (Beppu et al., 1995; Gonzalez et al., 2012; Yamada et al., 2015). Since trifoliate orange was found to be an efficient Na excluder at low salinities, but a poor Cl excluder (Storey and Walker, 1999), Cl content was considered as a better index of leaf injury (Levy and Syvertsen, 2004; Lopez-Climent et al., 2008; Syvertsen et al., 2010). No changes in leaf chlorophyll index or abscission in 1X (Table 2) suggested, however, that both the Na and Cl levels may be lower than the threshold value of each ion to induce leaf injury, although only Na was determined in the current study in the absence of a Cl analyzer. In the following year after each treatment, soil EC was restored to a similar level to the control by the spring and no visible difference in vegetative growth or flowering was observed between the seawater treatments and the control (data not available).
In conclusion, the results indicated that the application of a larger amount of undiluted seawater to the soil at the early stage of maturation reduced maximum leaf water potential to the objective level 0.3 to 0.5 MPa lower than that in unstressed trees and improved fruit quality with a higher SSC and similar TA and other quality characteristics without inducing any visible leaf injury in field-grown satsuma mandarin trees. These results suggest the feasible use of moderate salt stress by modified seawater irrigation to improve fruit quality, even in field-grown trees. As this study provided data on a one year basis, further study on the cumulative effect of seawater application in successive years on salts accumulation in the soil and trees, fruit quality, and yield are required as the next step. Since there is a high demand for better tasting fruit in Japan, seawater treatment once in a few seasons is a possible strategy, even though there could be negative effects with consecutive year application.
