2015 Volume 84 Issue 3 Pages 195-201
Seawater was applied to pot-grown citrus trees, namely, 2 cultivars of satsuma mandarin (Citrus unshiu Marcow.) and ‘Miyauchi-iyokan’ (Citrus iyo hort. ex Tanaka), to clarify the feasibility of salt stress to improve fruit quality and its physiological mechanism. The applications of undiluted seawater (1X) and half-strength-diluted seawater (1/2X) significantly increased soil electrical conductivity (EC) and decreased leaf water potential to different degrees compared with those in the control. The effect of seawater concentration on leaf water potential was more obvious when it was determined at predawn than at noon. Only a slight increase of leaf Na content and no significant leaf abscission occurred in 1/2X compared with those in the control, while significant increases of these parameters were observed in 1X. Seawater irrigation inhibited fruit growth, but did not affect fruit shape and flesh ratio. Soluble solids content (SSC) in the juice was significantly higher upon the seawater applications at about 0.8 to 2°Brix depending on the cultivar than in the control; reducing sugars and/or sucrose were also increased by the salinity treatments in satsuma mandarin. Seawater treatments tended to delay the decrease of titratable acidity, but no significant difference between 1/2X and the control was detected at harvest. These results indicate that the mild salt exposure induced by 1/2X at the extent of 0.3 to 0.5 MPa lower water potential at predawn than that in the unstressed control could improve citrus fruit quality without injury. The results also suggest that salt stress by seawater irrigation would affect fruit quality through a similar physiological process, osmoregulation, to that of drought stress.
Plants adapt to reduced water availability, caused by salinity, drought and extreme temperatures, through osmoregulation (Niedziela et al., 1993). In citrus, it is well known that drought stress increases SSC in fruit juice (Barry et al., 2004; Kadoya, 1973). The main cause of SSC increment under drought stress is sugar accumulation by active osmoregulation through an increase in the translocation of photosynthates into fruit (Kadoya, 1973; Yakushiji et al., 1998). Thus, 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 (Iwasaki et al., 2011; Jiang et al., 2014; Yakushiji et al., 1996).
In tomato, a plant that is moderately sensitive to salinity, salt stress has also been used to improve fruit quality in various culture systems (Mitchell et al., 1991; Niedziela et al., 1993; Saito et al., 2009). On the other hand, in salt-sensitive citrus, few investigations have been conducted to use salinity for improving fruit quality (Kawai et al., 2002), whereas a lot of reports on the negative effects of salinity and salty wind on growth and yield have been published (Balal et al., 2012; Beppu et al., 1995; Nakagawa et al., 1980; Ogasawara, 1971; Storey and Walker, 1999). Kawai et al. (2002) found that the application of diluted deep seawater to potted satsuma mandarin trees increased SSC without significant changes in leaf water potential. Therefore, the physiological mechanism of SSC increase due to salinity was not clarified in the paper.
In our preliminary study on potted citrus trees, a significant increase of SSC was observed, accompanied by a significant reduction of mid-day leaf water potential during the late stage of seawater treatment, although marked leaf fall occurred after harvest and some trees died the next year (Yamada and Sogabe, 2010). In the current study, we aim to study the effect of seawater irrigation on water relations and fruit quality in pot-grown citrus trees and to determine the extent of salinity stress to improve citrus fruit quality without injury.
Surface seawater taken at Takahama fishing port in Matsuyama, Ehime Prefecture, was used in the current study. The concentration of NaCl was 3.1% (540 mM).
Experiment in 2011Fifteen 4-year-old ‘Okitsu-wase’ satsuma mandarin trees grown in a rain shelter and 12 14-year-old ‘Ehime-nakate’ satsuma mandarins grown in a screen house, both grafted on trifoliate orange [Poncirus trifoliata (L.) Raf.] and potted in 60-L plastic pots, were used. Crop load was adjusted to about 30 leaves per fruit by fruit thinning on Aug. 30. Three irrigation treatments of undiluted seawater (1X), half-strength-diluted seawater (1/2X), and tap water (control) were established. Each treatment had 5 and 4 replicate trees in ‘Okitsu-wase’ and ‘Ehime-nakate’, respectively. Four liters of treatment solution was applied to the soil of each pot on Sep. 5 when an orange color started to develop at the stylar end. On the basis of the changes in leaf water potential, additional application was carried out at a rate of 2 L/pot/time to maintain significant differences of salt stress among the treatments. As a result, ‘Okitsu-wase’ and ‘Ehime-nakate’ received 2 and 5 additional applications, resulting in 8 L and 14 L in total, respectively. To minimize the leakage of salt in the seawater treatments and to avoid inducing drought stress in every treatment during the experimental period, daily irrigation of tap water was carried out, based on observation of the soil surface, at a volume that caused a little water to leak from the pot bottom.
Leaf water potential at predawn and noon was periodically determined using a pressure chamber (Model 1000; PMS Instrument Co., OR, USA). Leaf abscission and fruit growth were measured on the previously selected 2-year-old twigs and fruit, respectively. One fruit per tree was sampled at two-week intervals and SSC and acidity in the juice were measured using a handy refractometer and titration with 0.05 N NaOH, respectively. At harvest on Dec. 5, fruit quality characteristics such as fruit weight, pulp ratio and sugar composition were also determined. Sugars in the vesicle were extracted in 80% ethanol and sugar composition was analyzed by gas chromatography (GC-14B; Shimadzu Co., Kyoto, Japan). Seven spring-flushed leaves and 50 g of soil at a depth of 5 cm were sampled at 4 weeks after the first treatment and at final fruit harvest. 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. Soil electrical conductivity (EC) was measured using an EC meter (CM-55; Takemura-denki Co., Tokyo, Japan) after mixing dried soil with fivefold distilled water.
Experiment in 2012Six 5-year-old ‘Okitsu-wase’ satsuma mandarin trees in a rain shelter and 10 15-year-old ‘Miyauchi-iyokan’ trees in an open field, both on trifoliate orange, were grown in 60-L pots and used for the experiment. As ‘Okitsu-wase’ was in the off year, uniform trees with relatively high fruit load of 50 to 60 leaves per fruit were selected and not thinned. In ‘Miyauchi-iyokan’, fruit were thinned to 80 leaves per fruit in August. On the basis of the results in 2011, only the treatment of half-strength-diluted seawater (1/2X) was compared to the control. Each treatment contained 3 and 5 replicate trees in ‘Okitsu-wase’ and ‘Miyauchi-iyokan’, respectively. First irrigation of treatment solution at 4 L/pot was conducted on Sep. 12 and Oct. 2 in ‘Okitsu-wase’ and ‘Miyauchi-iyokan’, respectively. Additional seawater application was carried out to maintain the predawn water potential in 1/2X 0.3 to 0.5 MPa lower than that in the control, resulting in additional application at 4 L/pot in both cultivars. Measurement parameters and methods and daily irrigation management during the experimental period were the same as those in 2011, except for adding the determination of peel color in the equatorial region of fruit with a chromameter (CR-321; Minolta Co., Osaka, Japan). Mature fruit were harvested on Nov. 28 and Dec. 25 in ‘Okitsu-wase’ and ‘Miyauchi-iyokan’, respectively.
Statistical analysisData were subjected to analysis of variance and the means were separated by Tukey’s test and t-test at P < 0.05 in the experiments in 2011 and 2012, respectively. The percentage data of leaf abscission and pulp ratio were transformed using arcsine before statistical analysis.
As similar results for the changes in water potential, fruit growth index, SSC and titratable acidity during the experimental period and sugar composition at harvest were obtained in the two cultivars in each year, only the data of ‘Okitsu-wase’ in 2011 and ‘Miyauchi-iyokan’ in 2012 are presented as representatives. The data of soil EC, Na content, leaf abscission, and fruit quality characteristics at harvest are shown for all cultivars.
At 7 days after the first application of seawater to ‘Okitsu-wase’ trees at 4 L/pot, leaf water potential at predawn was significantly lower in the seawater treatments than in the control, while no significant difference was observed at noon (Fig. 1). Therefore, additional applications at 2 L/pot were conducted at 7 and 14 days after the first treatment. As a result, significant reduction of water potential was observed in the salinity treatments even at noon, although no difference was detected between 1/2X and 1X until Oct. 27 and thereafter among the three treatments. On the other hand, the predawn water potential was maintained about 0.3 to 0.5 and 0.5 to 0.8 MPa lower in 1/2X and 1X than that in the control until Nov. 28, respectively. On the basis of the results in 2011, ‘Miyauchi-iyokan’ received only 1/2X solution at a total of 8 L/pot and the predawn leaf water potential was also maintained 0.3 to 0.6 MPa lower than in the control in 2012 (Fig. 2). These results suggest that the maximum water potential measured at predawn could detect a slight difference of water potential induced by salt stress and be a more useful index than that at noon. Kawai et al. (2002) did not detect any decrease in water potential after deep seawater application. Although they did not describe the time of determination, considering the values of water potential, they probably determined it during the day, resulting in a failure to detect a slight difference among the treatments.
Effect of seawater irrigation on leaf water potential at predawn and noon in potted ‘Okitsu-wase’ satsuma mandarin trees in 2011. Different letters indicate significant differences among the treatments at P < 0.05 by Tukey’s test (n = 5). Arrows indicate the dates of seawater application.
Effect of seawater irrigation on leaf water potential at predawn in potted ‘Miyauchi-iyokan’ trees in 2012. * indicates significant difference from the control at P < 0.05 by t-test (n = 5). Arrows indicate the dates of seawater application.
Seawater irrigation significantly increased soil EC on Oct. 2, particularly in 1X of ‘Okitsu-wase’, and higher EC was maintained at harvest on Dec. 5 in both cultivars in 2011 (Table 1). In spite of the greater volume of seawater application in ‘Ehime-nakate’ grown in the screen house, the EC values were lower than those of ‘Okitsu-wase’ in the rain shelter, probably due to the leakage of salt via precipitation. Similar results on soil EC were obtained in 1/2X of both cultivars in 2012. On the other hand, similar reductions of water potential were observed in 1/2X of ‘Okitsu-wase’ in 2011 and ‘Miyauchi-iyokan’ in 2012 (Figs. 1 and 2) in spite of the large difference in EC values (Table 1), indicating that the soil EC measured only at a depth of 5 cm could not be a reliable index of salt stress. Since the applied salt would gradually move to the subsoil by daily tap water irrigation and/or rain, EC measurement at different depths throughout the root zone might be required. Anyway, the higher soil EC would restrict water absorption by the roots and lead to a decrease of water potential (Garcia-Sanchez and Syvertsen, 2009; Levy and Syvertsen, 2004; Lloyd et al., 1990; Storey and Walker, 1999). With regard to the salt residue in the following seasons, soil EC was recovered to the control level within a month by a large volume of tap water irrigation equivalent to 100 mm of precipitation just after harvest in 1/2X of ‘Okitsu-wase’ in 2012 (data not shown).
Effect of seawater irrigation on soil EC, leaf Na content and leaf abscission in potted ‘Okitsu-wase’ and ‘Ehime-nakate’ satsuma mandarin trees in 2011 and ‘Okitsu-wase’ satsuma mandarin and ‘Miyauchi-iyokan’ trees in 2012.
‘Okitsu-wase’ trees in 1X accumulated significantly higher Na in the leaves, resulting in 100% leaf abscission on Jan. 25 (Table 1). Although a similar tendency was observed in ‘Ehime-nakate’, the Na content and leaf abscission in 1X were lower than those in ‘Okitsu-wase’, suggesting the milder salt stress by lower soil EC in ‘Ehime-nakate’. With regard to salty wind injury by a typhoon, Na content above 1% DW (Beppu et al., 1995), 0.1–0.15 mg NaCl·cm−2 (Nakagawa et al., 1980), and 0.4–0.5 g Cl·m−2 (Ogasawara, 1971) were suggested as threshold values to induce leaf fall. Although Na content detected in 1X of ‘Okitsu-wase’ was lower than 1% DW, the combined effect of long-term exposure to toxic ions of Na and Cl and severe water stress might have led to the marked leaf abscission. Trifoliate orange was found to be an efficient Na+ excluder at low salinities but a poor Cl− excluder (Storey and Walker, 1999). Thus, Cl content in the leaves might be a better index of leaf injury, although only Na content was determined in this study. On the other hand, only a slight increase of leaf Na and no significant increase of leaf abscission were observed in 1/2X of the two cultivars in both years (Table 1). The abscission of old leaves was variable and ranged from 0 to 73.8%, even in the control, depending on the cultivar and year, probably due to age and nutritional conditions. Thus, ‘Okitsu-wase’ trees in 1/2X in 2011 showed relatively high old leaf abscission above 40%, but the comparison to the control showed no significant difference. The abscission rate of new leaves was much lower than that of old leaves and no difference in this variable was detected between the control and 1/2X, indicating that Na level accumulated in 1/2X did not have any toxic effect involving the induction of new leaf abscission. In the year after each experiment, no visible difference in vegetative growth or flowering was observed between the trees of 1/2X and the control, while many shoots and some twigs died and vegetative and reproductive growth was low in 1X (data not available). These observations indicate that the application of diluted seawater such as 1/2X to the soil could induce moderate water stress without any negative effect on tree growth in the following years.
Since the size of selected fruit for monitoring growth was slightly different among the treatments at the start of the experiment, the changes in fruit length and width were expressed as values relative to the size at the starting day set as 100. In ‘Okitsu-wase’, longitudinal growth was significantly affected by the extent of salt stress, while transverse diameter was significantly reduced by the seawater irrigation, without a significant difference between 1/2X and 1X (Fig. 3). In ‘Miyauchi-iyokan’, 1/2X inhibited only growth in the length compared with the control (Fig. 4). It is well known that water or salt stress decreases fruit size and yield in many fruits including citrus (Iwasaki et al., 2011; Kawai et al., 2002; Maotani and Machida, 1980; Yakushiji et al., 1998). These results indicate that salt stress by seawater irrigation also inhibits citrus fruit growth.
Effect of seawater irrigation on fruit growth in potted ‘Okitsu-wase’ satsuma mandarin trees in 2011. Different letters indicate significant differences among the treatments at P < 0.05 by Tukey’s test (n = 5).
Effect of seawater irrigation on fruit growth in potted ‘Miyauchi-iyokan’ trees in 2012. * indicates significant difference from the control at P < 0.05 by t-test (n = 5).
The actual values of fruit size and other quality characteristics at harvest are shown in Table 2. Significant reductions of actual fruit size were observed in both seawater treatments in ‘Okitsu-wase’ and only for width in 1/2X of ‘Ehime-nakate’ in 2011, while the differences of fruit size in both cultivars in 2012 and fruit weight in both cultivars and years were not significant. The periodical sampling data revealed that fruit weight tended to be lower in salt stress treatments than in the control during the treatment period in every cultivar and year, although a significant difference was detected only on certain dates (data not shown). These results suggest that the effect of seawater irrigation on fruit growth may be relatively small, particularly upon moderate stress like 1/2X, and sometimes hidden by relatively high variability in the raw data (Table 2). On the other hand, the periodical monitoring of fruit size using the same fruit on a tree and its expression as a value relative to the first size can eliminate the factors causing variability and detect a small effect (Figs. 3 and 4). Shape index and pulp ratio were not influenced by seawater irrigation. The colorimetric a value was significantly higher in 1/2X than in the control in ‘Okitsu-wase’ in 2012, although no significant difference was detected for all color parameters in ‘Miyauchi-iyokan’. The improvement of color was also found in drought-stressed citrus fruit (Iwasaki et al., 2011; Kallsen et al., 2011).
Effect of seawater irrigation on fruit quality at harvest in potted ‘Okitsu-wase’ and ‘Ehime-nakate’ satsuma mandarin trees in 2011 and ‘Okitsu-wase’ satsuma mandarin and ‘Miyauchi-iyokan’ trees in 2012z.
In ‘Okitsu-wase’ in 2011, seawater irrigation significantly increased SSC compared with that in the control from Sep. 18, about 2 weeks after the first application, and the difference between them reached about 2°Brix on Dec. 5 (Fig. 5). A significant increase of SSC at a similar level in the seawater treatments was also observed at harvest in ‘Ehime-nakate’ (Table 2). There was no significant difference between 1/2X and 1X in both cultivars. In 2012, the diluted seawater application also increased SSC of ‘Okitsu-wase’ and ‘Miyauchi-iyokan’ fruit at 1.2 and 0.8°Brix higher than in the control at harvest, respectively (Table 2; Fig. 6). For titratable acidity, salt stress tended to delay its decrease, but the difference between 1/2X and the control was not significant at harvest in both cultivars and years (Table 2; Figs. 5 and 6). The seawater irrigation promoted the accumulation of reducing sugars, fructose and glucose, and 1/2X also increased sucrose content compared with that in the control in ‘Okitsu-wase’ (Table 3). A similar tendency was observed in ‘Miyauchi-iyokan’, although the difference between the treatments was not significant.
Effect of seawater irrigation on SSC and acidity in the flesh juice in potted ‘Okitsu-wase’ satsuma mandarin trees in 2011. Different letters indicate significant differences among the treatments at P < 0.05 by Tukey’s test (n = 5).
Effect of seawater irrigation on SSC and acidity in the flesh juice in potted ‘Miyauchi-iyokan’ trees in 2012. * indicates significant difference from the control at P < 0.05 by t-test (n = 5).
Effect of seawater irrigation on sugar composition in the flesh of ‘Okitsu-wase’ satsuma mandarin and ‘Miyauchi-iyokan’ fruit harvested on Dec. 5, 2011, and Dec. 25, 2012, respectively.
In citrus, a lot of reports have demonstrated that drought stress increased SSC and acidity through a decrease in water potential (Barry et al., 2004; Kallsen et al., 2011; Maotani and Machida, 1980) and mulch cultivation has recently been used to produce high-quality fruit by Japanese citrus growers (Iwasaki et al., 2011; Yakushiji et al., 1996). In response to water stress, citrus tree exhibited an enhanced rate of photosynthate partitioning in fruit rather than in other organs, even though total photosynthate production was decreased (Asakura et al., 1991; Kadoya, 1973; Yakushiji et al., 1998). Sucrose, fructose and glucose were accumulated by active osmoregulation in order to maintain cell turgor under drought conditions (Yakushiji et al., 1996; 1998). In tomato, which is moderately sensitive to salinity, salt stress has been used to improve fruit quality in addition to drought stress (Mitchell et al., 1991; Niedziela et al., 1993; Saito et al., 2009). On the other hand, few trials for the use of salt stress to produce high-quality fruit have been conducted in salt-sensitive citrus. Kawai et al. (2002) found that the application of diluted deep seawater to potted ‘Miyagawa-wase’ satsuma mandarin trees increased SSC at harvest without any reduction in water potential during the experimental period. In the present study, the application of seawater to potted citrus trees decreased leaf water potential and increased sugar content in the fruit with depression of fruit growth. Since sucrose is the primary substance for translocation in citrus and degradation of sucrose to fructose and glucose was promoted during the process of active osmotic adjustment induced by drought stress (Yakushiji et al., 1996), the accumulation of sugars, particularly reducing sugars, observed in the seawater irrigation would also contribute to active osmoregulation. These results suggest that the inhibition of root water absorption by higher soil EC through seawater irrigation could induce the reduction of water potential and the accumulation of sugars by osmoregulation as a physiological mechanism similar to drought stress. In addition, mild salt or water stress at the extent of 0.3 to 0.5 MPa lower water potential at predawn than in unstressed trees could improve the taste of fruit with higher SSC and similar acidity without inducing any obvious injuries like leaf abscission. As most citrus orchards in Ehime Prefecture are located in coastal areas and on islands in the Seto Inland Sea, citrus trees have sometimes suffered severe salty wind damage during typhoons (Beppu et al., 1995). In contrast to such negative features, seawater can be considered as a free local resource for possible use to improve citrus fruit quality, as suggested in the current study. Further study is needed to apply these results obtained with pot-grown trees to field-grown ones.
