Effects of Potassium Chloride and Potassium Sulfate ... - Springer Link

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

123

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.

Sugar Tech (May-June 2016) 18(3):258–265

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

259

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

123

260

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,

123


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


261

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.

123

262


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

123

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.


263

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.

123

264


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.

123

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


265

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.

References Hamid, A.M.A., and Y.M.I. Dagash. 2014. Effect of sulfur on sugarcane yield and quality at the heavy clay soil ‘‘Vertisols’’ of Sudan. Universal Journal of Applied Science 2: 68–71. Azama, T., Y. Kawamitsu, Y. Fukuzawa, N. Uehara, E. Taira, and M. Ueno. 2007. Effect of potassium on growth and sugar accumulation in sugarcane. Japanese Journal of Crop Science 76(1): 130–131. (in Japanese). Iesaka, M. 2001. Problem of work force on agriculture and productive structure of sugarcane in Okinawa, Japan. Okinawa Kansyatou Nenpou 32: 21–28. (in Japanese). Inoue, S. 2006. Present situation and challenges of the sugarcane and sugar industry in Okinawa prefecture. Journal of Agricultural Policy Research 12: 65–84. (in Japanese with English summary). Kafkafi, U. 2001. Introduction. In Potassium and chloride in crops and soils: the role of potassium chloride fertilizer in crop nutrition, 9–15. Basel: International Potash Institute. Kawamitsu, Y., M. Ueno, Y. Tokashiki, T. Nagae, N. Ohmi, L. Sun, Y. Asanuma, and M. Iritakenishi. 1996. Relationship between sugar content and elements in the stem of sugarcane. Journal of Okinawa Agriculture 31: 2–10. (in Japanese). Kawamitsu, Y., T. Azama, Y. Fukuzawa, E. Taira, and M. Ueno. 2006. Effects of soil potassium on leaf photosynthesis and sugar accumulation of sugarcane with different sugar content. Japanese Journal of Crop Science 75(1): 156–157. (in Japanese).

Kikuchi, K. 2009. Sugarcane production in Islands, ‘farm management and utilization of biomass’. Tokyo: Nourin Toukei Press. (in Japanese). Laties, G.G., I.R. MacDonald, and J. Dainty. 1964. Influence of the counter-ion on the absorption isotherm for chloride at low temperature. Plant Physiology 39: 254–262. Lingle, S.E., and C.L. Wiegand. 1997. Soil salinity and sugarcane juice quality. Field Crops Research 54: 259–268. Macklon, A.E.S., and I.R. MacDonald. 1966. The role of transmembrane electrical potential in determining the absorption isotherm for chloride in potato. Journal of Experimental Botany 17: 703–717. Matsuoka, M. 2006. Sugarcane cultivation and sugar industry in Japan. Sugar Tech 8: 3–9. Nagae, T., Y. Kawamitsu, N. Ohmi, T. Kawanaka, M. Ueno, and Y. Tokashiki. 1997. Improvement of sugar production in sugarcane with crop, soil and system engineering. Effects of potassium treatment on sugar content in sugarcane juice. Japanese Journal of Crop Science 66(1): 264–265. (in Japanese). Ota, M., M. Kuba, and C. Yara. 2000. Nutritional and soil diagnosis of sugarcane in Okinawa. Report of the Kyushu Branch of the Crop Science Society of Japan 66: 56–59. (in Japanese). Prajapati, K., and H.A. Modi. 2012. The importance of potassium in plant growth—a review. Indian Journal of Plant Sciences 1: 177–186. Stewart, C.M., J.F. Melvin, N. Ditehburne, S.H. Tham, and E. Zerdoner. 1973. The effect of season of growth on the chemical composition of cambial saps of Eucalyptus regnans trees. Oecologia 29: 349–372. Taiz, L., and E. Zeiger. 2010. Solute transport. In Plant physiology, 5th ed, ed. L. Taiz, and E. Zeiger, 131–159. Sunderland: Sinauer Associates, Inc. Uehara, N., H. Sasaki, Y. Kawamitsu, and R. Osugi. 2004. Effect of excess potassium on sugar-yield in sugarcane. Japanese Journal of Crop Science 73(1): 262–263. (in Japanese). U.S. Salinity Laboratory Staff. 1954. Chapter 2. Determination of the properties of saline and alkali soils. In Diagnosis and improvement of saline and alkali soils, 7–33. Washington, DC: U.S. Government Printing Office. White, P.J., and M.R. Broadley. 2001. Chloride in soils and its uptake and movement within the plant: a review. Annals of Botany 88: 967–988.

123