Soil salinity and sugarcane juice quality

ELSEVIER

Field Crops Research 54 (1997) 259-268

Field Crops Research

Soil salinity and sugarcane juice quality Sarah E. Lingle USDA-ARS Subtropical Agricultural

*,

Craig L. Wiegand

Research Laboratory,

2413 E. Highway 83. Weslaco, TX 78596, USA

Received 20 August 1996; accepted

18 April 1997

Abstract Sugarcane (Saccharum spp. hybrids) juice quality is reduced by soil salinity. If the effect of salinity in commercial production is predictable, it will be possible to estimate juice quality in salt-affected fields prior to harvest. Variation in the effects of soil salinity on sugarcane juice quality in commercial production was assessed using 15stalk samples harvested in 1992 and 1993 from a salt-affected commercial field of CP 70-321 sugarcane. Mean electrical conductivity (EC,) of saturated water extracts of 54 (1992) or 74 (1993) soil cores from O-30, 30-60, and 60-90 cm depths were used to calculate mean EC, at each site, which ranged from 0.5-17.0 dS m- ‘. Most of the increase in EC, was due to increases in Na+ and Cl -. Magnesium, Ca2 + and K+ were also present. Stalk samples were harvested from 29 (1992) or 30 (1993) sites, with EC, ranging from 0.5 to 9.5 dS m-l. Each dS mm ’ increase in EC, decreased Brix (% soluble solids in juice) and Pol (% sucrose in juice) by about 0.6%, decreased apparent purity (Pol as % of Brix) by 1.3% in 1993, increased juice conductivity 0.8 dS m-‘, and increased cane residue (fiber) 0.5%. Effects of EC, on Brix, Pol and conductivity were very similar between years, indicating that the response of juice quality to salinity is predictable. This should allow development of sugar quality maps of commercial sugarcane fields for site-specific management decisions. Recoverable sugar yield per ton cane and per hectare were reduced by EC, in both years. In 1992, juice osmolality was less at the higher EC, sites, but in 1993 it was unaffected by EC,. About 90% of the osmolality of the juice was accounted for by the solutes analyzed (total sugar, Na+, K+, Ca*+, Mg2+, and Cl-). Potassium was the most abundant cation in the sugarcane juice (52.5 to 107 mmol, 1-l). In 1992 there was a weak curvilinear (R2 = 0.35) increase in juice K+ as EC, increased, while in 1993 Kf tended to increase linearly with EC, (r2 = 0.20). Juice Na + increased with EC, from 4.9 to 37.4 mmol, 1-i. There were also increases in juice Mg+ (11.3 to 42.0 mmol, l- ‘1 and Ca 2f (2 .2 to 22.4 mmol, l-l), with increased EC,. Most of the increase in juice conductivity was due to increases in Cl- (30.8 to 106 mmol, l- ‘1. For most attributes there were no significant differences between years. This study shows in greater detail than most previous studies how soil salinity affects juice ionic composition, osmolality, and the accepted industry measurements of juice quality, Brix, Pol, apparent purity, and conductivity. 0 1997 Elsevier Science B.V. Keywords:

Sugarcane;

Salt stress; Juice quality; Yield

1. Introduction

* Corresponding author. Tel.: + l-210-969-4830; fax: + 1-210969-4800; e-mail: [email protected]. 0378-4290/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZZ SO378-4290(97)00058-O

Sugarcane grown under irrigation in arid or semiarid regions is frequently subjected to soil salinity. The crop is moderately sensitive to salinity (Maas,

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Field Crops Research 54 (1997) 259-268

1990) with a threshold for yield reduction at 1.7 dS m-l (Maas and Hoffman, 1977; Maas, 1990). Saline soil or irrigation water reduces sugarcane stalk yield (Bernstein et al., 1966a,b; Prothero, 1978; Ginoza and Moore, 1985) by reducing both stalk population (Rizk and Normand, 1969) and stalk weight (Syed and El-Swaify, 1972). Wiegand et al. (1996) found that each dS m-l increase in root zone salinity decreased stalk population by 0.6 stalk m-* and individual stalk weight by 0.15 kg, resulting in a stalk yield decrease of 13.7 t ha-‘. Juice quality is important in sucrose production. The sucrose content of the juice determines the maximum sucrose yield, while other sugars, organic acids, and minerals reduce the efficiency of sucrose recovery after milling. Most studies have shown that salinity reduces Pol, an estimate of sucrose content, and apparent purity (ratio of Pol to Brix where Brix is an estimate of total soluble solids). Salinity also increases juice conductivity, a measure of mineral content (Prothero, 1978; Thomas et al., 1981). Studies done in greenhouse pots or in small field plots may not extrapolate to field production, however. For instance, in a large-scale, multiyear field study, Thomas et al. (1981) found that saline irrigation water did not consistently decrease Brix, Pol, or apparent purity. They attributed this to the leaching of salt from the root zone by heavy rainfall prior to harvest, which allowed juice quality to improve. Remote sensing by aerial videography or satellite imagery enables the mapping of crop fields for sitespecific production management decisions. For instance, soil and stalk samples from a commercial sugarcane field were used to calibrate 3.3-m-resolution aerial videography to ground conditions so that a detailed soil salinity map of the field could be produced (Wiegand et al., 1996). Similar procedures were applied to map soil salinity and yield of wheat in a 43000-ha irrigation district in Mexico (PulidoMadrigal et al., 1995). Mapping of fields serves two purposes: preharvest yield estimates aid harvest scheduling and producers can manage production inputs to correct conditions in particular areas within fields (Richardson et al., 1996). In the case of sugarcane, juice quality contributes significantly to the yield of the final product, sucrose. If juice quality can be reliably predicted from soil salinity estimates, management decisions

can be made on sucrose yield rather than stalk yield only. For this report, effects of both electrical conductivity of saturated soil extracts and the individual ions on juice quality parameters were considered.

2. Materials

and methods

2.1. Field description

and soil sampling

This study was conducted in a 59-ha commercial sugarcane field near Monte Alto, Texas (26”22.5’N, 97”57.2’W, 18 m above sea level). The appearance of the field in 1991 satellite ‘Systeme Probatoire d’observation de la Terre’ (SPOT) images, and reconnaissance aerial videography of the plant (firstyear) crop indicated probable salt-affected areas within the field. When the study was initiated in 1992, the cultivar CP 70-321 was the first ratoon (regrowth) crop of a multiyear production cycle. The 372 m X 1586 m field contained 1039 rows centered 1.5 m apart, with the rows running east-west. The field had five soil types (Fig. 1; Jacobs, 1981). Approximately 30% of the area was Hidalgo sandy clay loam (mixed hyperthermic Typic Calciustoll; USDA taxonomy), and 55% of the area was Willacy fine sandy-loam (mixed, hyperthetmic Udic Argiustoll). Remaining areas were Racombes sandy clay loam (mixed, hyperthermic Pachic Argiustoll), and Rio clay loam (mixed, hyperthermic Typic Argiaquoll) in two areas filled during leveling, and an unnamed loamy Ustorthent that remained after leveling. The field’s owner followed the fertilization, furrow irrigation, and pesticide practices normal to the area. On 23 April 1992, when the crop was still short enough to be straddled by a tractor, soil cores were taken to 90 cm depth using a tractor-mounted hydraulic sampler. Samples were taken from the center of the rows at nine sites, 40 m apart, on each of six transects (total of 54 sites; Fig. 1). This sampling strategy crossed several of the areas that appeared salt-affected in the satellite image. Each core was subdivided into O-30, 30-60, and 60-90 cm depth increments which were bagged separately. Air-dried soil was ground to pass a 2-mm sieve. Electrical conductivities (EC,) of saturated-soil extracts were determined (Rhoades and Miyamoto, 1990), and val-

S.E. Lingle, C.L. Wiegand/Field

f

-0

200

400

600

Crops Research 54 (1997) 259-268

800

1000

1200

261

1400

Meters North of South End 1992 1993 i

0 A

m

Hidalgo sandy clay loam

0

loamy Ustorthents

Wllacy tine sandy loam

m

Rio clay loam

Racombes sandy clay loam

Fig. 1. Distribution of soil types and relative location of soil sampling sites in 59-ha sugarcane field. Soil samples were taken from 54 sites in 1992, and from 74 sites in 1993. The sites were chosen to transect areas influenced by soil salinity. Dotted symbols represent stalk sampling sites.

ues from the three depths averaged. Mean profile EC, for all sites ranged from 0.5- 13.4 dS m-l. On 13 April 1993, soil samples were taken from a total of 74 sites along 12 transects in the same field (Fig. 1). The number and positions of the sites were adjusted to increase the number of sites in the middle range of salinities, using a salinity map generated from video images taken in 1992 (Wiegand et al., 1996). At the soil core sites, three rows were removed from the 1992 locations to avoid any effects of trampling from the previous year’s plant sampling. Mean soil EC, at these sites ranged from 1.0-17.0 dS m-r. A comparison of EC, values in 1992 with nearby sites in 1993 indicated that soil salinities at the beginning of the growing season were similar in both years (data not shown). 2.2. Plant sampling In each year, 30 sites were selected for plant sampling, but in 1992 one of these (12.7 dS m-‘) produced no millable stalks, so no juice quality data were obtained. Plant sampling sites included the two rows on each side of the soil core row, and extended 1 m to the east and west of the soil core site, an area of 6 m*. On 9 December 1992, 15 millable stalks were taken from each of 29 sites by cutting at soil level. A millable stalk was one with solid stalk at about waist-height (110 cm). Each 15stalk sample was hand-stripped, weighed and crushed in a tlueeroller Squier-type (Cuban) mill. Industry-established

procedures (Chen, 1985, pp. 777-798) were used to determine conductivity and Brix. Pol was determined after the juice was clarified with aluminum chloride (Legendre and Clarke, 1991). Osmolality of unclarified juice was determined using a Wescor 5500 vapor pressure osmometer (Wescor Inc., Logan, Utah). Recoverable sucrose yields per ton cane and per hectare were estimated from juice quality factors (Legendre and H en d erson, 1972) and estimated cane yield (Wiegand et al., 1996). In the second year, the field burned accidentally on 26 October 1993, prior to the planned sampling date in early December. Samples of 15 stalks were taken on 27 October from each of 30 sites. These were weighed to calculate stalk yield, then finely chopped. A 500-g sample of the chopped material was pressed to 15.9 MPa for 2 min, and the collected juice used for standard juice quality analyses. Residue from the pressed sample was weighed, dried, then reweighed to calculate residue (fiber) content after applying a correction factor for the residual soluble solids. 2.3. Ion and sugar analyses Cations in the saturation extracts and sugarcane juice were determined by ion chromatography using either a Dionex IonPac CS3 cation exchange column (Dionex, Sunnyvale, California), with 35 mM HCl and 6.0 mM 2,3-diaminopropionic acid as the eluent, or a Dionex IonPac CS12 cation exchange column

262

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Field Crops Research 54 (1997) 259-268

with 22 mM methanesulfonic acid as the eluent. The major cations, Na+, K+, Mg2+ and Ca2+, were determined by suppressed conductivity quantified by external standards. Standards and samples run on both systems agreed well. Chloride concentrations in soil water extracts and juice were determined using a Cl--specific electrode. Simple sugars in juice were analyzed by ion chromatography using a CarboPac PA1 column (Dionex> with 190 mM NaOH as the eluent. Sugars detected by pulsed amperometry were quantified using external standards. 2.4. Statistical analyses Data were analyzed by regression using the GLM procedure of PC-SAS (SAS Institute, 1990). Linear and quadratic term contributions of EC, or soil ions to parameter estimates were tested by evaluating the significance of the F-value (P I 0.05) as each term was added. Simple (r 2, or multiple ( R2> coefficients of significant regressions are presented.

3. Results In 1992, the mean EC, of the O-90 cm depth at the plant sampling sites ranged from 0.5-6.6 dS m-’ (Table l), and in 1993 mean EC, ranged from 1.0-9.5 dS m-‘. At most of the sites EC, increased with depth, an indication of inadequate leaching (Rhoades, 1990). Sodium, Ca2+, Mg2+ and Clwere the major ions contributing to the EC, (Table 1). Potassium was a very minor component of EC,. The sodium absorption ratio @AR) of the saturation extracts, calculated according to Rhoades and Miyamoto (1990), increased from 1.4 to 10.3 across the range of EC, (data not shown). While, the EC of the irrigation water was not determined, the EC of water in the Rio Grande River, which supplied all of the irrigation water to the field, ranged from 1.l- 1.4 dS m-’ during 1992 and 1993 (John Sturgis, Texas Natural Resource Conservation Commission, personal communication). In both years, Brix and Pol declined linearly with increasing EC, (Fig. 2a,b), in agreement with the findings of others (Bernstein et al., 1966b; Tanimoto, 1969; Ginoza and Moore, 1985). Each dS m-l increase in salinity decreased Brix by 0.6 and 0.5% in

1992 and 1993, respectively. Each dS m-’ increase in salinity decreased Pol by 0.6% in both years. Sucrose concentrations determined by ion chromatography were highly correlated with Pol ( r = 0.88 in 1992, and r = 0.95 in 1993), and are not given in this report. Salinity had no effect on the relation between Pol and sucrose determined by chromatography. EC, had no significant effect on apparent purity in 1992, but in 1993 each dS m-’ increase in EC, decreased apparent purity 1.3% (Fig. 2~). In both years, juice conductivity increased 0.8 dS m-l perdSm -’ increase in EC, (Fig. 2d). Juice conductivity was highly correlated with total juice ion concentration (I- = 0.94 and 0.97 in 1992 and 1993, respectively). Most of the variation in juice conductivity was explained by Cl- alone (r2 = 0.88 in 1992, r2 = 0.90 in 1993). Sodium, Mg2+, Ca2+, and Cl- concentrations in juice increased with EC, (Fig. 3). In 1992, juice K+ concentration increased with EC, at low to moderate salinities, then decreased with salinities greater than about 4 dS m- ‘. In 1993, a linear regression of EC, with juice K+ was small but significant ( r2 = 0.20). When one especially high value (indicated by arrow on Fig. 3) was eliminated, the 1993 regression was also curvilinear CR2 = 0.22). However, there was no obvious reason to exclude this point. Concentrations of ions in soil extracts were usually highly correlated with concentrations of the same ions in juice (Table 2). All juice ions except K+ were correlated with all ions in soil extracts both years. Juice osmolality averaged ,841 mm01 kg - ’ in 1992, and 884 mmol kg in 1993 (Fig. 3e). Osmolality decreased with increased EC, in 1992, but was not significantly affected by EC, in 1993. Juice yield was low in 1992, constituting only 40% of fresh weight (data not shown). This was due to the inefficiency of the three-roller mill used that year. Juice yield in 1992 was not significantly affected by EC,. In 1993, juice yield was much higher, averaging 68% of fresh weight, and was significantly reduced by about 6% per dS m-’ increase in EC,. Water content of the stalks at harvest, calculated from juice yield and moisture content of residue in 1993, was not affected by increasing EC, (data not shown). We did not calculate water content of stalks in 1992. Percent residue in the stalk increased about 0.5% per dS m-l increase in EC, in 1993, although

S.E. Lingle, C.L. Wiegand / Field Crops Research 54 (19971 259-268 Table 1 Electrical conductivity (EC,) and ion concentrations CWO-321 sugarcane in 1992 and 1993

in saturated

extracts

from soil sampled

1992

263

fieldof

to three depths from a salt-affected

1993

O-30 cm

30-60

cm

60-90

cm

Mean

O-30 cm

30-60

cm

60-90

cm

Mean

dS m-’

EC, Minimum Maximum

0.3 4.3

0.3 6.3

0.5 9.3

Na’ Minimum Maximum

1.5 31.7

2.6 39.9

K+ Minimum Maximum

0.1 1.4

lVlgz+ Minimum Maximum

0.5 6.6

0.9 1.2

1.0 10.4

0.8 12.4

1.0 9.5

3.1 53.6

L-’ 2.8 39.0

6.9 60.7

5.0 80.1

5.8 99.6

6.7 75.1

0.0 1.5

0.0 2.7

0.0 1.6

0.4 2.1

0.0 1.2

0.0 1.2

0.2 1.3

0.5 9.8

0.3 14.7

0.2 32.0

0.4 16.9

1.6 12.1

0.7 14.0

0.6 18.9

I 1 13.3

Ca2+ Minimum Maximum

2.0 43.2

1.9 48.2

1.7 55.3

2.3 45.0

6.2 46.8

2.1 45.2

4.0 44.1

5.4 42.1

ClMaximum Minimum

1.0 44.2

1.6 58.6

1.2 106.0

1.4 62.8

3.8 49.1

4.3 86.0

3.9 136.0

4.1 90.4

mmol,

1.2

(a) I

.,,,.I

63

W 21.0.

2 ] ; o.8 t

1992

pur=82.6

q-

1993

v=89.2-1.3x

Iv

yj;-;--v:

5 0.6 -

?=0.53

1992

y=830.1+18.6x-5.9x2 R*=O.45

0

I 1

v-

1993

0.4

_

y=O.884

”

”

t

6

8

60 v

18 8

16

!i E 14 $ IY 12

0

2

4

6

a

10

0

2

4

6

a

10

0

2

4

EC, (dS/m) Fig. 2. Quality factors of CP 70-321 sugarcane

as affected by soil salinity (EC,) in two harvest years.

10

264

S.E. Lingle,

T --*-

Na R2=0.84 K R*=0.42 Mg R2=0.64

C.L.

Wiegand/Field

-T--

Ca +=0.45

+

Cl R*=O.72

Crops

Research

54 (1997)

259-268

Table 2 Simple correlations of ions in saturated soil extracts with ions in juice of CWO-321 sugarcane in two harvest seasons Juice

100 80

.

1993

.

!+

Ca2+

Cl

K+

Mg2+

1992 (n = 29) 0.82 b Na + Kf 0.79 b Mg*+ 0.83 b 0.66 b Ca*+ 0.90 b cl-

0.10 0.03 0.02 0.33 -0.10

0.77 0.65 0.74 0.76 0.64

b b b b b

0.67 0.62 0.64 0.62 0.58

b b b b b

0.84 0.73 0.81 0.79 0.76

b b b b b

1993 (n = 30) 0.79 b Na+ 0.58 b K+ 0.80 b Mg*+ Ca*+ 0.72 b Cl_ 0.79 b

0.39 0.61 0.42 0.40 0.38

0.69 0.57 0.68 0.54 0.72

b b b b b

0.56 0.65 0.58 0.45 0.62

b b b b b

0.62 0.72 0.65 0.59 0.65

b b b b b

Soil

Na+

a b a a =

a P 5 0.05, b P IO.01

(Wiegand et al., 1996). As a result of the decreased juice quality and reduced cane yield, each 1 dS m- ’ increase in EC, decreased estimated recoverable sugar yield by 2.2 t ha-’ in 1992 (r2 = 0.60; Fig. 4b) and by 1.1 t ha-’ in 1993. Higher cane yields in 1993 partially offset a lower sugar yield per ton cane

60

6

4

8

10

110

EC,(dS/m) v 7 -*-

Na ?=0.68 K 12=0.20 Mg RZ=0.56

-+ _

Ca P=O.33 C, Rz=0,43

Fig. 3. Concentrations of ions in juice of CWO-321 sugarcane as affected by soil salinity (EC,) in two harvest years. Dotted curve in bottom graph indicates response of juice K+ to soil EC, if data point indicated by arrow is removed from the analysis ( R2 = 0.22).

--c

o21

there was considerable variability (r2 = 0.48) (Fig. 3f). The juice extraction procedures used in 1992 did not permit a calculation of residue. Recoverable sugar yield, the sucrose yield after milling, is influenced by sucrose concentration in the juice, apparent purity of the juice, and fiber of the stalk, and yield calculations reflect these factors (Legendre and Henderson, 1972). Increased EC, decreased estimated sugar yield by 4.1 kg t-’ cane per dS m-l in 1992 (r2 = 0.43; Fig. 3a) and by 5.3 kg t-’ [dS rn-l]-l in 1993 (r2 = 0.61). Cane yield was in 1992 and by reduced by 22.2 t ha-’ [dS m-i]-’ 12.1 t ha-’ [dS m-1]-1 increase in EC, in 1993

t -B-

18

1992

y=93.5-3.6x

P=o.44

1993 y=85.8-4.9x

?=0.65

1992 1993

f=O.63 bo.50

y=13.2-2.0x v=10.7-1.0x

(b)

15 3 F

12

r"g -6 0 -.

3

. . .

0 0

2

4

6

a

lo

EC,(dSlm) Fig. 4. Estimated recoverable sugar yield (a) per ton cane and (b) per hectare of CP70-321 sugarcane as affected by soil salinity (EC,) in two harvest years.

S.E. Lingle, C.L. Wiegand/

than in 1992, resulting yield per hectare.

in a smaller

Field Crops Research 54 (1997) 259-268

loss of sugar

4. Discussion There were differences between years that may account for some of variability in response of juice quality factors to increasing EC, There was a longer time between irrigations in 1992 than in 1993, growth appeared to be slower, and cane yield was less (Wiegand et al., 1996). There was more rain during early growth in 1993 than in 1992, including over 100 mm on one day from a tropical storm. This occurred after we had taken soil samples from the field. The rain may have caused some leaching of salt from the root zone, and the additional water may have reduced the effect of the soil salinity that year. Additionally, the 27 October 1993 sampling occurred early in the harvest season, after the field was accidentally burned. Because CP 70-321 is an earlyripening cultivar, early harvesting did not have much effect on Brix or Pol (Fig. 2a,b). None of the individual soil ions consistently had a larger regression coefficient than EC, with any juice quality parameter (data not shown), nor was total soil ion concentration more highly correlated with any juice quality factor than was EC,. This indicates that EC,, which is easily measured, is as good or better than any specific ion concentration in predicting juice quality in a saline field. It also indicates that the effect of EC, may be an osmotic effect rather than a specific ion effect. There was large variability in the response of juice quality factors to EC,. Variation in soil type within the field may be a confounding effect through differences in cation exchange capacity, the pH, and plant root growth. Although regressions of EC, on each quality factor varied with soil type, the 95% confidence limits all overlapped. The saturation extract used in this study removed only water-soluble ions; we do not have an estimate of exchangeable cations which would also be available to the plant. Concentrations of exchangeable K+, Mg*+, and Cazt are usually determined on soil samples extracted with 1 M ammonium acetate (Haby et al., 1990), rather than the saturation extracts used to assess soil salinity (Rhoades and Miyamoto, 1990).

265

A survey of several of the soil series found in this field indicates that the exchangeable cation concentrations, especially of sodium, calcium and magnesium, are much higher than the soluble concentrations (Heilman et al., 1966). The similarities in the regression for Brix, Pol and juice conductivity between years (Fig. 2a,b,d) occurred despite significant differences in crop yield between years (Wiegand et al., 1996). Although much of the variation in these parameters was unexplained by the regression, the similarity between years indicates that the effects of EC, on Brix, Pol and conductivity are predictable. This predictability indicates that salinity maps of fields generated by remote sensing can be used to create sucrose yield maps. The sucrose yield maps would allow growers to decide when to take a field out of production and to calculate the economic feasibility of salinity-reducing technologies such as installing drainage. With the price of raw sugar in USA at $400 t-‘, the loss of 2 t sucrose ha- ’ per dS m- ’ increase in salinity in 1992 (Fig. 4b) represents a loss of $800 per dS m- ‘. A sugar quality map would also allow a sugar mill to schedule harvesting of various fields in order to blend poor-quality juice with high-quality juice. In 1992, the reduction in Pol by EC, (Fig. 2b) was the result of a decrease in Brix (Fig. 2a), not a specific decrease in sucrose, because apparent purity (Fig. 2c) was unaffected. In 1993, there appeared to be a specific reduction in sucrose as well as a general reduction in total soluble solids, as apparent purity was also decreased by increased EC, in that year. This fits with Tanimoto (1969) conclusion that increasing salt concentration in nutrient solution caused withdrawal of sucrose from lower internodes of sugarcane. Concentrations of sugars and ions in the juice accounted for 88% of the juice osmolality in 1992, and 91% of juice osmolality in 1993. Assuming a charge balance of cations and ions, non-Cl_ anion concentrations were estimated to be 37 f 9 mM in 1992, and 73 _t 10 mM in 1993. Taken together, all sugars, ions, and non-Cl_ anions accounted for 93% of juice osmolality in 1992, and over 99% in 1993. These proportions were not affected by EC, (data not shown). In 1993, but not in 1992, the calculated non-Cl_ anion concentration increased with EC, (data not shown).

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Field Crops Research 54 (1997) 259-268

In 1992 the increase in juice ion concentration (Fig. 3) with increasing EC, did not osmotically compensate for the loss of sucrose. As a result, juice osmolality decreased at the highest salinities (Fig. 2e). In 1993, however, juice osmolality was not significantly affected by salinity. This indicates that when the ion concentration in the juice increased with EC,, the crop did not accumulate as many of sugars and other solutes while maintaining the osmolality of the solution. The presumed increase in non-Cl_ anion concentration in 1993 may be one reason juice osmolality was not significantly affected by EC, in that year. The non-Cl_ anions are likely to be organic acids, especially aconitic acid, which can be present in significant concentrations in sugarcane juice (Chen, 1985, pp. 32-33). Juice ion concentrations were generally higher in 1992 than at comparable EC, in 1993 (Fig. 3). This may have been due to the later harvest date in 1992, which allowed the stalks to accumulate more salts, or for the cane to lose more water, since irrigation was withheld after October 1. If the cane had a lower water content in 1992, however, the Brix and Pol would also have been greater than in 1993, and they were about the same (Fig. 2a,b). The mean K+ concentrations, 66 mM in 1992 and 82 mM in 1993, are slightly less than the 100 mM cytoplasmic concentrations considered optimum for plant cells (Leigh and Wyn Jones, 1984), but agree with concentrations reported by Welbaum and Meinzer (1990) for symplast and apoplast of sugarcane internodes. Potassium concentrations tended to increase with salinity at least to about 4 dS m-i (Fig. 3). However, increased Na+ concentrations in the root zone are generally believed to decrease K+ uptake by roots (Lauchli and Epstein, 1990). A recent study indicated that Na+ may enhance root uptake of Kf at very low soil K+ concentrations (Rubio et al., 1995), although there is some doubt that this has physiological significance (Maathuis et al., 1996). We cannot state that there was increased Kt uptake by roots in this study, since we did not determine ion concentrations in the rest of the plant. It is possible that soil salinity changed the distribution of ions among the different organs, and K+ accumulated preferentially in the stalk. Bernstein et al. (1966a) also examined the cation concentration of juice from sugarcane grown at dif-

ferent salinities. The K+ concentration in juice of our field study did not increase as much, and the Na+ concentration increased more with increased salinity than in their study, perhaps because additional K+ was added to their greenhouse pots and field plots, but not to our field. A consequence of decreased K+ concentrations at higher salinities, as observed in 1992, may be a salinity-induced K+ deficiency. Leigh and Wyn Jones (1984) asserted that cytoplasmic concentrations of K+ should be 100 to 200 n&I. Most of the K+ concentrations in sugarcane juice (which includes apoplastic solution) were less than this (Fig. 3). Therefore, additions of K+ fertilizer might alleviate some of the affects of salinity on sugarcane. Lopez and Satti (1996) showed that additions of Ca*+ and/or K+ to the soil solution somewhat ameliorated the effects of increased Naf in tomato. The reduction in juice yield in 1993 (data not shown) was not due to a decrease in water content in the stalks, which was not affected by increasing EC, (data not shown). The reduction in juice yield was more likely related to the increase in residue content (Fig. 2f). Assuming that most of the residue is fiber, the increase in residue may be the result of a decrease in internode diameter with increased salt concentration (Rizk and Normand, 1969). Because most of the fiber is in the outermost portion of the stalk (Artschwager, 19251, a decrease in internode diameter would increase the proportion of the tissue that was fiber. However, Bernstein et al. (1966b) found that while sugarcane grown in pots in the greenhouse had smaller stalk diameter under salinity, those in the field had greater diameters under salinity. Internode diameters were not measured in the present study. Another possibility for the increase in residue is a reduction in internode length with salinity, which we observed (unpublished data). Because nodes have higher fiber percentage than internodes, and their length is more constant than internode length (Lingle, unpublished data), reducing internode length would also increase the node/internode ratio, and thus the percent fiber. Results of this study reveal in greater detail than most previous studies that soil salinity decreases juice quality in commercial sugarcane and has a severe impact on sugar yield. In this study, each 1 dS m- ’ increase in conductivity of the root zone de-

S.E. Lingle, C.L. Wiegand/

Field Crops Research 54 (1997) 259-268

creased both total soluble solids and sucrose in juice by about OS%, and increased juice conductivity by about 0.9 dS m-l. An increase in juice Cl- appeared to contribute the most to this increase in conductivity. It is yet to be determined whether the increase in juice ions are a cause of the reduction in sucrose concentration in the juice with salinity, or if both sucrose and ion concentrations are influenced by some other response of the plant to salinity. The similarity of juice osmolality across the range of EC, studied indicate suggest a direct effect of increased ion concentration on sucrose, and this is currently being examined.

Acknowledgements We are grateful to Mr. Robert Fletcher of Monte Alto, Texas, and the Rio Grande Valley Sugar Growers for the use of this field and their cooperation in the experiment. We also acknowledge the technical efforts of Romeo Rodriguez, Jean Anne Pearcy, Paul Thompson and Roland0 Mireles.

References Artschwager, E., 1925. Anatomy of the vegetative organs of sugar cane. J. Agric. Res. 30, 197-241. Bernstein, L., Clark, R.A., Francois, L.E., Derderian, M.D., 1966a. Salt tolerance of N, Co varieties of sugar cane: II. Effects of soil salinity and sprinkling on chemical composition. Agron. J. 58, 503-507. Bernstein, L., Francois, L.E., Clark, R.A., 1966b. Salt tolerance of N, Co varieties of sugar cane: I. Sprouting, growth, and yield. Agron. J. 58, 489-493. Chen, J.C.P., 1985. Meade-Chen Cane Sugar Handbook, 1 lth edn. John Wiley and Sons, New York. Ginoza, H., Moore, P.H., 1985. Screening sugarcane varieties for tolerance to salinity. Report of the Hawaiian Sugar Technologists (43). FE5-FE6. Haby, V.A., Russelle, M.P., Skogley, E.O., 1990. Testing soils for potassium, calcium and magnesium. In: Westerman, R.L. (Ed.), Soil Testing and Plant Analysis. Soil Sci. Sot. Am., Madison, Wisconsin, pp. 18 l-227. Heilman, M.D., Thomas, J.R., Carter, D.L., Thompson, C.M., 1966. Chemical, physical, and mineralogical characteristics of eight irrigated soils of the lower Rio Grande Valley of Texas. SWC Research Report 382. USDA, Agricultural Research Service, Weslaco, Texas, 63 pp. Jacobs, J.L. 1981. Soil survey of Hidalgo County, Texas. USDA, Soil Conservation Service. U.S. Government Printing Office, Washington, D.C.

267

Lauchli, A., Epstein, E., 1990. Plant responses to saline and sodic conditions. In: Tanji, K.K. (Ed.), Agricultural Salinity Assessment and Management. Am. Sot. of Civil Engi., New York, pp. 18-41. Legendre, B.L., Clarke, M.A., 1991. Comparison of clarification reagents for polarization analysis of sugarcane juice. J. Am. Sot. Sugar Cane Technol. 11, 75-81. Legendre, B.L., Henderson, M.T., 1972. The history and calculations of sugar yield calculations. Proc. Am. Sot. Sugar Cane Technol. 2 (NS), 10-18. Leigh, R.A., Wyn Jones, R.G., 1984. A hypothesis relating critical potassium concentrations for growth to the distribution and functions of this ion in the plant cell. New Phytol. 97, 1-13. Lopez, M.V., Satti, S.M.E., 1996. Calcium and potassium-enhanced growth and yield of tomato under sodium chloride stress. Plant Sci. 114, 19-27. Maas, E.V., 1990. Crop salt tolerance. In: Tanji K.K. (Ed.), or include after book Tl in Lauchli, Maas and Rhoades, Agricultural Salinity Assessment and Management. Am. Sot. Civil Eng., New York, pp. 262-304. Maas, E.V., Hoffman, G.J., 1977. Crop salt tolerance - current assessment. J. brig. Drainage Div., ASCE 103, 115-134. Maathuis, F.J.M., Verlin, D., Smith, F.A., Sanders, D., Femandez, J.A., Walker, N.A., 1996. The physiological relevance of Na+-coupled K+-transport. Plant Physiol. 112, 1609-1616. Prothero, G., 1978. The effect of saline irrigation water on sugarcane ripening. Report of the Annual Conference of the Hawaiian Sugar Technologists (371, pp. 69-71 Pulido-Madrigal, L., Sanvicente-Sanchez, H., Wiegand, C.L., Anderson, G.L., 1995. Estimation de perdidas de cosecha en suelos ensalitrados mediante imagenes de satelite. Proc. VII Latin American Symposium on Remote Sensing, November 1995, Puerto Vallarta, Mexico, pp. 880-891. Rhoades, J.D., Miyamoto, S., 1990. Testing soils for salinity and sodicity. In: Westerman, R.L. (Ed.), Soil Testing and Plant Analysis, 3rd edn. Soil Sci. Sot. Am., Madison, Wisconsin, pp. 299-336. Rhoades, J.D., 1990. Overview: Diagnosis of salinity problems and selection of control practices. In: Tanji, K.K. (Ed.), Agricultural Salinity Assessment and Management. Am. Sot. Civil Eng., New York, pp. 18-41. Richardson, A.J., Wiegand, C.L., Anderson, G.L., Gerbermann, A.H., Bray, M., Summy, K.R., Sugden, E.A., 1996. Six exemplary applications of GIS technology to subtropical Texas agriculture and natural resources. Geocarto lnt. 11, 49-60. Rizk, T.Y., Normand, W.C., 1969. Effects of salinity on Louisiana sugar cane. lnt. Sugar J. 71, 227-230. Rubio, F., Gassmann, W., Schroeder, J.I., 1995. Sodium-driven potassium uptake by the plant potassium transporter HKTl and mutations conferring salt tolerance. Science 270, 16601663. SAS Institute, Inc., 1990. SAS/STAT User’s Guide, Version 6, 4th edn., vol. 2, SAS Inst., Inc., Cary, North Carolina. Syed, M.M., El-Swaify, S.A., 1972. Effect of saline water irrigation on N. Co. 310 and H50-7209 cultivars of sugar-cane: I. Growth parameters. Trop. Agric. 49, 337-346. Tanimoto, T.T., 1969. Differential physiological response of sug-

268

SE. Lingle, CL. Wiegand/

Field Crops Research 54 (1997) 259-268

arcane varieties to osmotic pressures of saline media. Crop Sci. 9, 683-688. Thomas, J.R., Salinas, F.G., Oerther, G.F., 1981. Use of saline water for supplemental irrigation of sugarcane. Agron. J. 73, 1011-1017. Welbaum, G.E., Meinzer, F.C., 1990. Compartmentation of so-

lutes and water in developing sugarcane stalk tissue. Plant Physiol. 93, 1147-1153. Wiegand, C., Anderson, G., Lingle, S., Escobar, D., 1996. Soil salinity effects on crop growth and yield - illustration of an analysis and mapping methodology for sugarcane. J. Plant Physiol. 148, 418-424.