Sugar Tech (May-June 2016) 18(3):258–265 DOI 10.1007/s12355-015-0392-z
RESEARCH ARTICLE
Effects of Potassium Chloride and Potassium Sulfate on Sucrose Concentration in Sugarcane Juice Under Pot Conditions Kenta Watanabe1,2 • Yasunori Fukuzawa1,3 • Shun-Ichiro Kawasaki1,2 Masami Ueno1 • Yoshinobu Kawamitsu1
•
Received: 31 March 2015 / Accepted: 13 July 2015 / Published online: 2 August 2015 Ó Society for Sugar Research & Promotion 2015
Abstract Sugarcane is generally fertilized with potassium chloride (KCl). It was reported in Okinawa, Japan, that potassium (K) in sugarcane juice has a negative effect on juice sucrose concentration. However, in some experiments, increasing K levels with potassium sulfate (K2SO4) did not reduce sucrose concentration. We hypothesized that sugarcane quality is affected not only by K but also by other components of K fertilizers, such as Cl- and SO42-. To test this hypothesis, two pot experiments were performed using different K levels supplied by KCl and K2SO4. Juice K? and Cl- concentrations markedly increased with K levels irrespective of K type; however, there was little effect of the treatments on SO42- concentration. In the first experiment, as K levels increased, both KCl and K2SO4 application tended to increase sucrose concentration in August samples. In later periods, however, the relationships completely changed based on the K type: sucrose concentration tended to be reduced by KCl application but increased by K2SO4 application. Similar results were obtained in the second experiment, which resulted in lower sucrose concentration with higher levels of KCl; however, there was no negative effect of K2SO4 treatment. These results suggest that Cl- is the primary factor in sucrose reduction. K? concentration strongly affected Clconcentration in all K types; however, the rate of increase & Yoshinobu Kawamitsu
[email protected] 1
Faculty of Agriculture, University of the Ryukyus, Okinawa 903-0213, Japan
2
The United Graduate School of Agricultural Sciences, Kagoshima University, Kagoshima 890-8580, Japan
3
Present Address: Daitou Sugar Mill Co. Ltd., Okinawa 901-3805, Japan
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was lower when only K2SO4 was provided, thereby indicating that it is possible to lower Cl- concentration by K2SO4 application. This measure could lead to quality improvement. Keywords Sugarcane K fertilizer Chloride Juice quality Sucrose Abbreviations ANOVA Analysis of variance EC Electrical conductivity K Potassium KCl Potassium chloride K2SO4 Potassium sulfate N Nitrogen P Phosphorus TVD Top visible dewlap
Okinawa, which is located in southwest Japan, has a subtropical climate. Sugarcane is one of the most important agricultural products in this region, with an economic impact greater than that of any other crop (Iesaka 2001). Despite its essential role in the economy, the production, the harvested area, and the number of sugarcane growers in Okinawa are decreasing because of both internal and external factors. These include inappropriate cultivation management, labor shortage due to the decreasing number of younger farmers as well as the aging of sugarcane producers, and change in land use toward more marketable plants (Inoue 2006; Kikuchi 2009; Matsuoka 2006). Currently, the sugarcane price is determined by sugarcane quality based on sucrose concentration. Increasing the profit of producers requires appropriate fertilizer management.
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Potassium (K) is an essential nutrient for plant growth and plays numerous roles in plants, including turgor pressure regulation, photosynthesis, stomatal activity, transport of sugars, protein, and starch synthesis, and activation of more than 60 enzymes (Prajapati and Modi 2012). In Okinawa, however, it was revealed that there is a negative correlation between K and sucrose concentration in sugarcane juice (Kawamitsu et al. 1996). Furthermore, Ota et al. (2000) reported that exchangeable K is excessively accumulated in sugarcane fields in Okinawa. Considering that sugarcane producers generally use potassium chloride (KCl) as a K fertilizer, this accumulation may have been caused by the overuse of KCl. In contrast, pot experiments have shown that increasing K levels with potassium sulfate (K2SO4) did not adversely affect sucrose concentration (Nagae et al. 1997; Uehara et al. 2004; Kawamitsu et al. 2006; Azama et al. 2007). From these results, it was hypothesized that the subcomponents of K fertilizers, namely Cl- and SO42- as well as K itself, also affect sugarcane quality. In the present study, we thus evaluated the effects of K fertilizer subcomponents on sucrose concentration in sugarcane juice using two K fertilizers: KCl and K2SO4.
Materials and Methods We performed two pot experiments: one from April 2010 to February 2011 (experiment 1) and the other from January 2013 to January 2014 (experiment 2) under greenhouse conditions at the University of the Ryukyus, Okinawa, Japan (26°250 N, 127°770 E; 125 m a.s.l.). Seedlings of a commercial sugarcane cultivar (Saccharum spp. cv. NiF8) were collected from fields at the Subtropical Field Science Center of the University of the Ryukyus. One-bud seedlings were immersed in a solution of BenlateR (5 g L-1, Du Pont) and in tap water for 24 h each for sterilization and to improve germination rate. These seedlings were planted and grown in containers from April 15 to May 12, 2010 in experiment 1 and from January 27 to March 5, 2013 in experiment 2. After the first fully expanded leaves were confirmed, seedlings were transplanted into 1/2000a Wagner pots filled with mixed soil of three materials: dark red soil (Shimajiri mahji), sea sand, and peat moss (1:1:1, v v-1). Tillers were immediately removed after emergence. Irrigation was carefully performed with daily soil moisture evaluation to prevent water stress. Fertilization was performed approximately once a month after transplanting. The same amounts of nitrogen (N) and phosphorus (P) were supplied for all pots, and treatments consisted of changing types and levels of K fertilizer. In experiment 1, 2.5 g pot-1 of N and
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0.75 g pot-1 of P were applied with ammonium sulfate and magnesium multi-phosphate, respectively. Eight plots were established using two types of K fertilizer: KCl and K2SO4, and four levels of K: 0.19, 0.75, 1.5, and 7.5 g pot-1 (expressed as i, ii, iii, and iv, respectively). Fifteen pots for each of the plots were prepared. In experiment 2, 2.9 g pot-1 of N and 1.16 g pot-1 of P were applied as ammonium nitrate and ammonium phosphate to exclude the effects of SO42- and other nutrients from N and P fertilizers. Thirteen plots, including one without K fertilizer (expressed as K-0), were established with three types of K fertilizer: KCl, K2SO4, and a mixture of KCl and K2SO4 (1:1, based on K amount; expressed as mix) and four levels of K: 0.87, 2.61, 8.7, and 26.1 g pot-1 (expressed as I, II, III, and IV, respectively). Six pots for each of the plots were prepared. The names of the plots are described as combinations of the supplied K type and level; for example, KCl-i. After transplanting, stem height from the ground to the base of the top visible dewlap (TVD) leaf, number of green leaves, and SPAD value of the TVD leaf using a SPAD meter (SPAD-502, Minolta Camera Co., Ltd.) were measured every 4 weeks to evaluate the effects of the treatments on plant growth. To investigate how the treatments affected quality as plants matured, we performed sampling four times in experiment 1: three plants from each of the plots on August 20, October 31, and December 20, 2010 and the remaining four to six plants on February 22, 2011. In experiment 2, four to six healthy plants from each of the plots were sampled only once on January 9, 2014. Samples were cut at the ground level. After removal of parts unnecessary for sugar refining, millable stalks were weighed and squeezed. Juice samples were stored at -80 °C until used for juice analysis to prevent deterioration in quality. In experiment 2, approximately 20 g of soil was collected from each pot after harvesting plants and was air-dried for soil analysis. After juice samples were completely melted, electrical conductivity (EC) of juice was measured with an EC meter (CM-14P, Toa) only in experiment 2. Juice samples were diluted 50 times with extra-pure water and passed through a filter paper (No. 6, Advantec) and a 0.45 lm membrane filter (Advantec). In experiment 1, K? concentration was measured by inductively coupled plasma emission spectrometry (ICPS-8100, Shimadzu), and Cl- and SO42concentrations were measured with an ion analyzer (IA300, Toa). In experiment 2, an ion chromatograph (ICS1600, Thermo Fisher Scientific) was used to determine the concentrations of these ions. Sucrose concentration was determined by high-pressure liquid chromatography (LC10A, Shimadzu). Soil samples were passed through a 2-mm sieve, and 5 g of each sample was shaken with 25 mL of extra-pure water
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for 1 h (1:5, soil:water; U.S. Salinity Laboratory Staff 1954). EC of saturated water extracts was measured by an EC meter (CM-14P, Toa). K?, Cl-, and SO42- concentrations were determined with an ion chromatograph (ICS1600, Thermo Fisher Scientific) after filtering with a 0.45lm membrane filter (Advantec). Means and standard deviations (SD) of the replications were calculated, and statistical analysis was performed using the software R (R Core Team 2014). As a result of two-way analysis of variance (ANOVA), there was a significant interaction between K type 9 K level for K? and Cl- concentration and EC in juice and soil samples. This required a separate analysis for each K type and K level (Tables 1, 2, 3). Data were subjected to a t test between two types of K fertilizer or one-way ANOVA between three types of K fertilizer and between K levels. When significances were found, the Tukey test or Tukey–Kramer test for groups with different sample sizes were conducted and significant differences were accepted based on a P value\0.05.
Results Visual observation revealed that all the plants appeared to be growing well. In experiment 1, stem height increased slowly in the early growth period; however, the speed of growth increased 10 weeks after transplanting and gradually decreased again (Fig. 1). All the plots showed similar changes, and stem heights finally reached approximately 230 cm. In experiment 2, the change of stem height was similar to that in experiment 1; growth was vigorous during 14–34 weeks after transplanting but sluggish at earlier and later periods of the growing season (Fig. 2). The final values ranged from 220 to 260 cm in experiment 2, which were wider than in experiment 1. In particular, mix-III and mix-IV had lower stem height, although there was no clear difference either in their SPAD values of the TVD leaf or in the numbers of green leaves (data not shown). In experiment 1, irrespective of sampling time, juice K? concentration increased with K level both in the KCl and K2SO4 plots and this tendency lasted until the final sampling in February (Table 1). K? concentrations varied widely from 200 to 3100 mg L-1 throughout the experiment. Juice Cl- concentration also increased with K levels, ranging from 400 to 2000 mg L-1. This tendency was observed even in the K2SO4 plots where no Cl- was administered. The Cl- concentration of the KCl-iv plot was significantly higher than that of the K2SO4-iv plot, although there were little effects of K type on Cl- concentration in the other K level plots. In contrast to Cl-, juice SO42- concentration seemed to show smaller effects of increasing K levels; K2SO4 application contributed little to increasing SO42- concentrations. In the last sampling,
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however, SO42- concentration tended to decrease in both the KCl and K2SO4 plots as K levels increased. Similar results were confirmed in experiment 2: juice K? and Clconcentrations responded positively to increasing K levels, irrespective of K type (Table 2). Compared to experiment 1, wider ranges of K? from 400 to 5700 mg L-1 and of Cl- from 400 to 3000 mg L-1 were obtained in experiment 2 because of the higher amounts of K fertilizer. There was no obvious difference in K? concentration between K types; however, Cl concentrations varied greatly depending on K type. K2SO4-II, III, and IV had a significantly lower Cl- concentration than plots of KCl and mix at the same K level. In the KCl plots, juice SO42- concentration decreased with an increase of K level, whereas there was no significant difference in SO42- in the other plots. EC of juice also increased significantly with K levels, and EC of juice of the K2SO4-IV plot was significantly lower than those of the KCl and mix-IV plots. In experiment 2, juice K? and Cl- were highly correlated in all the K types and all the correlations were significant at the 1 % level (Fig. 3). However, the slopes of the regression lines were markedly different; the regression coefficient was 0.44 in the KCl plots and 0.42 in the mix plots, whereas the value in the K2SO4 plots was 0.2. The same tendency was observed in experiment 1 also (data not shown). With any type of K fertilizer, similar to those in the juice samples, soil K? concentration and EC increased with K levels (Table 3). Soil K? was less than 2 mg 100 g-1 in the K-0, I, and II plots but rose to approximately 10 mg 100 g-1 in the III plots, although the values were not significantly different from those of the lower K plots. K? and EC of the K2SO4-IV plot were significantly higher than those of the KCl-IV plot. Soil Cl- concentrations increased with K levels in the KCl and mix plots and reached 48.3 and 30.4 mg 100 g-1 in the KCl-IV and mix-IV plots, respectively, resulting in a significant difference from that of the K2SO4IV plot. Similarly, increasing K levels increased soil SO42concentrations in the K2SO4 and mix plots. In experiment 1, sucrose concentration in the August sampling tended to increase with K levels and both KCland K2SO4-iv plots showed a significantly higher sucrose concentration than the lower K level plots (Fig. 4). At the October sampling, however, the relationships were markedly different depending on K type. As K levels increased, the sucrose concentration in the KCl plots tended to decrease from 16.5 to 11.4 %, whereas that in the K2SO4 plots increased from 6.3 to 16.2 %. In December, sucrose concentration spanned a smaller range; however, the difference in sucrose concentration between the K types still remained. As a result, the KCl-iv and K2SO4-i plots showed significantly lower sucrose concentrations than the other plots. At the last sampling in February, sucrose concentration rose above 21 % and the variation among
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Table 1 Effects of K type and level on juice K?, Cl-, and SO42- concentrations (experiment 1) K? (mg L-1)
Cl- (mg L-1)
SO42- (mg L-1)
Sampling time
K type
K level
August
KCl
i
273Ab
403Ac
1954Aa
ii
742Ab
673Abc
1950Aa
iii
1130Ab
784Ab
1770Aa
iv
3090Aa
1743Aa
1871Aa
i
387Ac
473Ab
1833Aa
K2SO4
October
KCl
K2SO4
December
KCl
K2SO4
February
KCl
K2SO4
ii
608Ac
543Ab
1591Aa
iii
1372Ab
957Aab
1956Aa
iv
2886Aa
1385Ba
2008Aa 1852Aa
i
356Ac
414Bc
ii
416Abc
520Bbc
1765Aa
iii
832Ab
695Ab
1838Aa
iv
2559Aa
1408Aa
1610Aa
i
241Bc
764Ab
2067Aa
ii
603Ab
836Ab
1788Aa
iii
909Ab
763Ab
1881Aa
iv
2497Aa
1259Aa
1774Aa
i
525Ac
544Ab
2208Aa
ii
652Abc
700Ab
2361Aa
iii
912Ab
870Ab
1968Aab
iv
3053Aa
1918Aa
1783Ab
i
309Bc
719Ab
2328Aa
ii
443Ac
678Ab
1898Ba
iii
892Ab
831Ab
2059Aa
iv
2496Ba
1208Ba
1973Aa
i
427Ac
501Ab
2294Aa
ii
470Abc
749Ab
2388Aa
iii
725Ab
898Ab
2130Aa
iv
2805Aa
1837Aa
1835Aa
i
204Bd
649Ab
2528Aa
ii
408Ac
729Ab
2351Aab
iii
749Ab
758Ab
2124Aab
iv
2572Aa
1276Ba
2049Aa
Values are means of each of the plots (August, October, and November, n = 3; February, n = 4–6). Within a sampling time, means followed by different uppercase letters and lowercase letters mean significant differences at the 5 % level among K types by t test and levels by Tukey test or Tukey–Kramer test, respectively
plots diminished greatly compared to the previous samplings. The KCl-iv plot seemed to have caught up with the other plots, whereas the concentration of the K2SO4-i plot stayed relatively low. At the end of the experiment, sucrose concentration was highest in the KCl-i plot and lowest in the K2SO4-i plot. In experiment 2, the range of sucrose concentration was narrower than in experiment 1 (Fig. 5). Sucrose concentration was lowest in K-0 plot and, irrespective of K type, tended to increase with K levels up to the II plots. However, when more K was
supplied, different effects of K fertilizer appeared, which were similar to experiment 1, thereby indicating that sucrose concentration decreased significantly in the KCl plots but remained stable or even slightly increased in the K2SO4 plots, whereas that in the mix plots tended to decrease similarly to that in the KCl plots. As a result of increasing K levels from II to IV, 1.6 and 0.6 % of sucrose reduction was observed in the KCl and mix plots, respectively. The highest sucrose concentration was observed in the KCl-II plot.
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Table 2 Effects of K type and level on juice K?, Cl-, and SO42- concentrations and EC (experiment 2) K type
K level
K-0 KCl
K2SO4
Mix
K? (mg L-1)
Cl- (mg L-1)
SO42- (mg L-1)
EC (mS m-1)
435e
481d
1736a
251d
i
758Ad
579Ad
1695Aa
297Ad
ii
1435Ac
856Ac
1334Bab
347Acd
iii
3334Ab
1592Ab
1272Ab
607Ab
iv
5488Aa
2725Aa
1069Ba
935Aa
i
854Ad
582Ad
1545Aa
277Ad
ii
1633Ac
733Bc
1505ABa
367Ac
iii iv
3277Ab 4909Aa
1108Bb 1381Ba
1767Aa 1547Aa
626Ab 772Ba 270Ad
i
722Ad
526Ad
1624Aa
ii
1611Ac
844Ac
1616Aa
376Ac
iii
3262Ab
1385ABb
1664Aa
597Ab
iv
5635Aa
2670Aa
1575Aa
972Aa
Values are means of each of the plots (n = 4–6). Means followed by different uppercase letters and lowercase letters mean significant differences at the 5 % level among K types and levels, respectively (Tukey–Kramer test)
Table 3 Effects of K types and levels on soil K?, Cl-, and SO42- concentrations and EC (experiment 2) K type
K level
K-0 KCl
K2SO4
Mix
K? (mg L-1)
Cl- (mg L-1)
SO42- (mg L-1)
EC (mS m-1)
0.6b
4.9c
8.0c
29.4c
i
0.6Ab
5.0Bc
9.3Ac
30.4Ac
ii
1.6Ab
9.7Bc
9.1Bc
32.6Bc
iii
9.2Ab
25.6Ac
10.1Bc
41.9Abc
iv
46.2Aa
48.3Ab
5.4Bc
58.4Ab
i
0.5Ab
6.5Bc
11.3Ac
32.4Ac
ii
0.7Ab
8.6Bc
22.0Abc
iii iv
10.3Ab 68.0Aa
10.4Ac 10.6Ac
84.5Ab 166.4Aa
60.1Ab 97.8Aa
i
1.0Ab
12.3Abc
11.2Ac
36.5Ac
ii
1.1Ab
14.9Abc
17.1ABc
39.3Ac
iii
11.6Ab
15.8Abc
46.6ABbc
53.8Ac
iv
55.2Aa
30.4Aa
92.1ABb
85.1Ab
37.4ABbc
Values are means of each of the plots (n = 4–6). Means followed by different uppercase letters and lowercase letters mean significant differences at the 5 % level among K types and levels, respectively (Tukey–Kramer test)
Discussion In the present study, we observed a reduction of sucrose concentration in sugarcane juice with increasing KCl levels; however, there was no negative effect of increasing K2SO4 levels, a result also consistently observed in previous studies (Nagae et al. 1997; Uehara et al. 2004; Kawamitsu et al. 2006; Azama et al. 2007). The mix plots also showed a tendency of sucrose reduction as K levels rose, although the supplied Cl- levels were half of those in the KCl plots. These findings support our hypothesis that
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Cl- and not K? is the factor most responsible for sucrose reduction. In addition to the sucrose reduction with KCl application, we also confirmed the positive effects of K2SO4 on sucrose concentration. Although there is no clear explanation for this finding, it may have resulted from the positive effects of SO42- because sulfur plays an important role in plant metabolism (for example, in photosynthesis and synthesis of amino acids and proteins) as an essential macronutrient (Hamid and Dagash 2014), though SO42concentration in juice was not increased by K2SO4 application.
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Fig. 1 Change of stem height in the KCl and K2SO4 plots (experiment 1). Vertical bars indicate SD
Surprisingly, the close relationship between K? and Clwas also confirmed in the K2SO4 plots, although the plants in the K2SO4 plots were not given Cl- by fertilization. This result indicates that they positively absorbed Cl- from irrigation water with the uptake of K, which is luxuriously consumed by sugarcane (Hunsigi 2011). This absorption most likely follows the principle of electrical neutrality by which a bulk solution always contains equal numbers of anions and cations (Taiz and Zeiger 2010). However, the increase in Cl- concentration was smaller with the K2SO4 than with the KCl treatments. These results suggest that it is possible to make sugarcane absorb less Cl- and accumulate more sucrose by K2SO4 application instead of KCl, although currently a great majority of the crops are fertilized with KCl (Kafkafi 2001). White and Broadley (2001) defined Cl- transport as ‘‘passive’’ when Cl- moves in the direction of its electrochemical gradient and as ‘‘active’’ when Cl- is accumulated against its electrochemical gradient. They also stated that active Cl- transport dominates Cl- influx to root cells at low Cl- concentrations in the external medium and that passive Cl- influx to root cells occurs under more saline conditions. Cl- influx to root cells was passive at Cl- concentrations in the external medium between 1 and 40 mM when Cl- was supplied as KCl (Laties et al. 1964; Macklon and MacDonald 1966). Taken together with our results, these reports suggest that
Fig. 2 Change of stem height in the KCl, K2SO4, and mix plots (experiment 2). Vertical bars indicate SD
sucrose reduction occurs via passive Cl- influx, for example when KCl is used, and that actively absorbed Cldoes not lead to negative effects. Lingle and Wiegand (1997) investigated the effects of soil salinity on juice quality of sugarcane from a salt-affected field and reported that EC of soil increased EC of juice and markedly lowered sugarcane quality such as Pol, Brix, and apparent purity, and that most of the increase of EC in juice was explained by Cl-. These findings partly support ours; however, the authors also speculated that the effect of EC of soil on sugarcane quality is an osmotic rather than a specific ion effect. In our study, sucrose reduction was observed only when KCl was supplied, whereas increases of EC of juice and soil were observed for all the K types, indicating a possibility of Cl- ionic stress.
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Fig. 3 Relationships between juice K? and Cl- concentrations with different K types (experiment 2). Horizontal and vertical bars indicate SD of juice K? and Cl- concentrations, respectively. The expressions of the linear lines are described in the figures
Because the ion concentrations varied widely among the treatments, the change of ion composition may have affected physiological functions associated with sucrose accumulation. Considering that sucrose is both a product of photosynthesis and a reserve substance after translocation of photosynthate (Stewart et al. 1973), the reduction of sucrose concentration may have resulted from photosynthesis inhibition or sucrose allocation. In addition, sucrose reduction by Cl- is likely to occur during the progress of sucrose accumulation as the weather becomes colder because the negative effects of higher KCl applications appeared at the October sampling in experiment 1. Elucidation of the mechanism, however, awaits further study.
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Fig. 4 Effects of K type and level on sucrose concentration (experiment 1). Vertical bars indicate SD. Different letters mean significant differences at the 5 % level
Because we focused on sugarcane quality, we have not discussed quantitative parameters in this paper. In fact, we saw no consistent effect on millable stalk weight between
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Fig. 5 Effects of K type and level on sucrose concentration (experiment 2). Vertical bars indicate SD. Different letters mean significant differences at the 5 % level
the two experiments (data not shown); therefore, the effects of Cl- on sugarcane yield remain unclear. However, these parameters are also worth considering for a better K management aimed at improving sugar production. Furthermore, it is unknown whether or not we can observe the same results under field conditions, including tillers as well as mother stems. To answer this question, a field study assessing the effects of types and levels of K fertilizer on sugarcane yield and quality is currently under way. Acknowledgments We would like to thank Dr. Ryuichi Suwa, Dr. Shin Yabuta, and Dr. Jun Tominaga for providing valuable advice and technical assistance.
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