Potato Research 38 (1995) 119- 123

Effect of saline irrigation and water deficit on tuber quality

A. N A D L E R and B R U R I A H E U E R

Institute of Soils and Water, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel

Accepted for publication: 16 February 1995

Additional keywords: fry colour, dry matter content, tuber size, ion and solute accumulation, Solanum tuberosum L.

Summary

The effect of salinity and water deficit on the quality of tubers for processing was investigated. Total tuber yield was not affected by the treatments, while the percentage of non-marketable tubers was significantly reduced by high salinity (EC i = 6dS m-I) and by water reduction. Accumulation of dry matter in the tubers was increased by all the treatments, that of proline by salinity only and the content of reducing sugars was increased only by water deficit. The colour of the french fries was similar in tubers from the various treatments,

Introduction

Potatoes are moderately salt-sensitive (Paliwal & Yadav, 1980) and very sensitive to water stress (Kle inkopf, 1983). The extent of tolerance to salt stress depends on the stress intensity, the cultivar involved and the stage of development of the crop (Levy, 1992): however, this information is limited and rather controversial (van Hoorn et al., 1993: Bilski etal., 1988). Reportson the effectsofearly-season water stress on tuber quality also differ. Painter & Augustine (1976) found that moisture stress during early tuber growth increased the number of malformed tubers in cv. Russet Burbank. Shock et al. (1992) reported that if plan ts were water-stressed before tuber ini tiation, their quality at harvest was improved, and fewer tubers with dark stem-end fry colours were produced. Iritani & Weller (1973) observed exactly the opposite, and Levy (1986) found that heat stress enhanced sprouting, rotting and tuber malformation. Fry colour is an important quality factor for the consumer of french fries and chips, and is there fore strongly emphasized by the potato processing industry (Yada & Coffin, 1987). There is little information on the quali tyoftubersfromplantsirr igatedwithsalinewater (LevyetaI,, 1988),

The purpose of this study was to provide information on the effect of salinity and water deficiton the processingqualityofpotatotubers.

Materials and methods

Experimental design. Potatoes (cv. D6sir6e) were planted on 23rd February 1993 at Kibbutz Nahal Oz, in the northern Negev, Israel. The soil was a clay loam with a field moisture capacity of 18.5% w/w. The potatoes were planted in beds of pairs of ridged rows 1.04 m apart and 9 m long, with four plants per metre of row. Before planting, 40 t

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cow manure, 225 kg P205 and 5 t gypsum were applied per ha. Urea as the nitrogen source was applied with each irrigation, giving a total of 450 kg N h a-t. Drip laterals were laid in the middle of each row with emitters every 60 cm. giving 2.21 h-I. Available water in a 1 m soil profile at the beginning of the experiment was estimated at 12-24 ram. and was sufficient to ensure good germination (emergence was scored on 23rd March). The differential irrigation treatments were started on the same day. The experiment consisted of completely randomized blocks with four replications. The data are presented with the least significant difference (LSD0.f~5) for all the treatments together.

Irrigation treatments. Three salinity levels ( treatments 1-3) and two amounts of water (treatments 4 and 5) were imposed. The salinity treatments included irrigation with water from the National Water Carrier (ECi= 1.5 dS m-l) and with two salinized waters from the Shaphdan (Dan region effluent from an activated sludge treatment plant) having EC i of 3 and 6 dS mq respectively. The soil salinity at the beginning of the experiment was 2.0 dS mq and increased with time and with increasing quality of the irrigation water. Salinization started on April 8th. Irrigation was applied twice weekly at a rate varying seasonally from 5.0 to 8.3 mm day-J, equivalent to evaporation from a screened USWB Class A evaporation pan for the months of March to July. The crop coefficient varied from 0.6-1.0, according to the soil coverage given by the canopy. The total amount of water applied from treatment initiation to harvest was 480 mm for all the salinity treatments. The first restricted-amount treatment received at each application only 60% of the above amount of water, i.e., 288 mm throughout the growth period (Treatment # 4): in the second such treatment, irrigation was withheld for 2 weeks, between May 5 and 20 (Treatment # 5). The total amount of water applied in this treatment was 386 m3. Treatments 4 and 5 both received the same water quality as t reatment 1, which served as a control.

Tuberanalvses. Tubers were harvested on 12th July, weighed and sorted by tuber length as follows: extra large. >13 cm: large, 10-12 cm: medium. 6-9 cm: small, <6 cm: non- marketable. Tubers were dried at 6(] oC to determine their dry matter concentration. Dry matter samples of 0.1 g were wet-ashed with H2SO 4 and H , O , for analysis of total N and P and with HNO 3 and HCIO 4 for the determination of Na, K, Ca and Mg. Total N was determined by a micro-Kjeldahl method (Stubblefield & de Turk. 1940): Ca and Mg with a Perkin Elmer 460 atomic absorption spectrophotometer: K and Na contents using a Corning 400 flame photometer and C1 with a Haake Buchler chloridometer. Proline content was determined according to Bates et al. (1973).

Assessment ofqualio,. Tuber samples from each treatment were assessed for specific gravity, pH, concentration of total reducing sugar and NaCI, and fry colour. The measurements were carried out at the Tapud potato processing plant, Saar HaNegev. The pH of mashed tuber samples was measured as soil pH (Peech, 1965). Salt (NaCI) concentration was determined by titrating with AgNO 3 (Reitemeier, 1943). Reducing sugar was determined colorimetricaly on a water extract of mashed potatoes (Pearson, 1976). Fry colour and its uniformity were assessed by frying sticks (french fries) at 180 oC for 3 and 6 rain, respectively. Colour was scored using the

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EFFECT OF SALINE IRRIGATION AND WATER DEFICIT ON TUBER QUALITY

U S D A s t anda rd (Anon . , 1972) for frozen French fried po ta toes : it has a seven-poin t scale from 000 to 4. The fol lowing formula was used to calculate fry colour:

scale: 000 00 0 1 2 3 4 mul t ip l ica t ion factor: 0 1 2 3 4 5 6 n u m b e r of sticks: a b c .... total sticks tested: 20

fry co lour score = (a x 1 +b x 2+c x 3+...)/20

The appra isa l of qual i ty was made 2 weeks af ter harvest .

Results and discussion

The total tube r yield was not affected by the t r ea tmen t s (Table 1). This is somewha t surpris ing, but is consis tent with r e p o r t e d to le rance to drought (Steckel & Gray . 1979) and sal ini ty (Levy. 1992). The total number of tubers as well as the number of s tems per p lant were not af fec ted by the t r ea tmen t s (da ta not shown). We bel ieve that the reasons for these results were that at the beginning of the season, i.e. 2-4 weeks af ter emergence , the soil wate r potent ia l was s imilar for all the t rea tments , and that sa l inizat ion had p robab ly s ta r ted af ter tube r ini t iat ion. I r r igat ion with ~aline water r educed the pe rcen tage of extra large tubers and increased the yield of large ones. which is beneficial . A similar t rend was r epo r t ed by Paliwal & Yadav (1980). The effect on size was more p ronounced with r educed water , when the total weight of large tubers was increased by 65% c o m p a r e d with the control t rea tment . It is impor t an t to emphas ize that high salinity, as well as res t r ic ted i rr igat ion water ( t r ea tmen t 4), significantly r educed the pe rcen tage of non -marke t ab l e tubers.

Table 1. Tuber yield (t ha-I ), size distribution and concentration of dry matter (%) as a function of irrigation regime.

Treatment* Total Size distribution (t ha -t) Dry matter yield concentration

Extra Large Medium Small Non-marketable (%) large 10-12 cm 6-9cm <6cm _>13cm

I 32.49 5.87 4.71 14.83 5.58 1.49 16.21 2 33.41 3.27 6.01 16.51 6.19 1.42 18.67 3 31.04 2.69 6.69 14.99 6.07 0.59 19.56 4 32.27 3.48 7.78 13.29 7.36 0.35 18.36 5 30.90 1.93 4.09 16.41 7.42 1.03 17.61

LSD0.c~ 5 2.16 1.42 1.53 3.80 1.87 0.25 0.60

* Irrigation treatments: 1=1.5 dS mq: 2=3 dS m-I: 3=6 dS m-I: 4=60% water: 5=irrigation withheld for 2 weeks.

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Table 2. Ion accumulation in potato tubers (in mg (g dry wt)-I) as a function of irrigation treatment.

Treatment* Na K C1 Ca Mg N P

1 1.0 36.8 5.8 0,34 10.5 12.0 3.2 2 1.4 33.8 7.6 0.52 11.0 13.0 3.0 3 1.6 34.0 8.4 0.29 9.5 11.0 2.7 4 1.0 31.7 6.3 0.36 8.5 11.0 2.7 5 0.9 30.0 5.9 0.38 13.0 10.5 2.9

LSDo.o5 0.17 1.38 0.23 0.10 3.75 0.76 0.22

* See Table 1.

Dry mat ter product ion and its accumulat ion in pota to tubers are important parameters for assessing adaptat ion to stress conditions. The percentage of dry mat ter was significantly increased by both salinity and water reduct ion (Table 1). All the t reatments led to a decrease in the K and N contents (Table 2). Modera te and high levels of saline water increased the accumulat ion of Na and C1 in the tubers compared with the non-saline controls. High salinity and water deficit reduced the P content by 16%.

Protine and sugars are known to participate in the increase in the osmotic potential under stress (Greenway & Munns, 1980). It was thus interesting to ascertain if compatible solutes were translocated from shoots to tubers. All the t reatments affected proline accumulat ion in the tubers: it was mainly increased as a result of salinity (Table 3). Salinity slightly reduced the already low level of reducing sugars. However , drought resulted in an increase in sugars.

Quali ty appraisal based on dry matter content and fry colour showed that the potatoes f rom all the t reatments were well suited for processing into French fries (Table 3). The content of dry mat ter in all the tubers, except f rom the non-saline

Table 3: The influence of irrigation regime on proline accumulation in potato tubers and on potato processing quality.

Treatment* Proline NaCI Reducing pH Fry lamole (g dry wt) -I (% dry wt) (% fresh wt) sugars colour**

1 42.93 0.12 0.10 5.80 1,0 2 50.80 0.23 0.07 5.94 1.0 3 63.76 0.27 0.05 5.97 1.0 4 47.12 0.21 0.16 5.75 1.5 5 46.40 0.20 0.18 5.69 1.6

LSDo.o5 5.81 0.097 0.008 0.60 0.10

* See Table 1. ** Assessed on scale of 000 to 4 (Anon,, 1972).

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EFFECT OF SALINE IRRIGATION AND WATER DEFICIT ON TUBER QUALITY

control, was sufficiently high and the colour of the chips was suitably pale. Neither water deficit nor irrigation with saline water affected the yield or processing quali ty of the tubers.

References

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