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PENELITIAN PERTANIAN TANAMAN PANGAN VOL. 25 NO. 1 2006

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ABSTRAK. Pengaruh Kekeringan terhadap Jumlah StomataDaun dan Status Air Tanaman pada Ubi Jalar ( Ipomoeabatatas(L). Penelitian dilaksanakan untuk melihat respon fisiologisubi jalar terhadap kekeringan. Beberapa aspek fisiologi tanamanseperti jumlah stomata dan status air tanaman ubi jalar (kandunganair daun relatif dan transpirasi) diamati pada kultivar Lole danWanmun, yang ditanam pada tiga tingkat perlakuan air tanah, yaitu20%, 40%, dan 80% kapasitas lapang. Penelitian menggunakanrancangan acak lengkap faktorial dengan empat ulangan. Hasilpenelitian menunjukkan bahwa kandungan air tanaman dantranspirasi pada kultivar Lole dan Wanmun dipengaruhi oleh statusair tanah. Transpirasi pada kultivar Lole lebih rendah daripada

Wanmun. Status air tanaman kultivar Lole lebih tinggi dibandingkandengan kultivar Wanmun dan jumlah stomata kultivar Lole lebihrendah dibandingkan dengan kultivar Wanmun. Hal ini menunjukkanbahwa Lole lebih efisien dalam memanfaatkan air tanah, sehinggalebih toleran terhadap kekeringan dibandingkan dengan kultivarWanmun.

Kata kunci: Ubi jalar, toleran kekeringan, stomata, kandungan airdaun relatif

ABSTRACT. Sweet potato is the primary food source for thehighlanders of Papua, Eastern Indonesia. However, due to theoccational prolong drought many crops including sweet potatoessuffered drought stress, especially when El Nio occurred. Thephysiology of sweet potato has been almost neglected in terms ofscientific research. The present research was aimed to observe

the physiological response of sweet potato to the water stress.Stomatal density and plant water relations represented thephysiological parameters were observed in Lole and Wanmun sweetpotato cultivar. Lole and Wanmun were subjected to three waterstress levels. The water stress levels were imposed by maintainingthe soil water content at 20 %, 40%, and 80% of field capacity. Thefactorial experiment used a complete randomised design with 4replications. The results showed that plant water status andtranspiration were both affected by soil water regimes. Lolerecorded greater plant water status and less transpiration than didWanmun in all soil water regimes, this was also shown by lowerstomatal number in Lole cultivar in spite of no effect on stomataldensity due to water stress. This indicated that Lole was moreefficient in consuming soil water and hence more tolerant to waterstress than Wanmun.

Keywords: Sweet potato (Ipomoea batatas (L), cultivar Lole andWanmun, drought tolerance, stomata, leaf relativewater content

More than 70% of available land in tropicalenvironments is under rainfed agriculture(Prakash and Ramachandran 2000). Because

water is the main limiting factor in this area, and rainfallis available in both space and time, drought is common.

Drought alters and modifies the physiology, anatomy,and morphology of plants, affects plant function, limitsplant growth, and reduces the productivity of the land(Boyer 1982). Water deficits affect leaf water potential,reduce total water use, and subsequently reducestomatal conductance, leaf area, root mass, tuberdevelopment, and total plant weight (Sivanet al.1995).

Drought causes plant water deficits that reduce cellturgor and cell enlargement, close stomata, thus

reducing the amount of productive foliage, decreasingthe rate of photosynthesis per unit of leaf area andshortening the vegetative period (Kramer 1980; van Loon1981; Bradford and Hsiao 1982).

Water plays an important role in sweet potato growthand yield. Sweet potato requires a constant water supplythroughout the growing season to produce high yields(Newell 1991). Improvement of plant productivity underwater stress needs understanding of physiologicalmechanisms by which water stress affects plant growth.To date, there have been few studies of the waterrelations and physiological aspects including anatomicalfeatures of sweet potato to water stress conditions.

The objective of the experiment was to observe theanatomical feature (stomatal density) and water relationtraits (leaf relative water content and transpiration) ofsweet potato (Ipomoea batatas L.) under water stressconditions. The information will be useful in determiningthe desirable plant characters and water relation traitsof sweet potato that provide tolerance to droughtconditions and which may be crucial information to plantphysiologists and breeders.

MATERIALS AND METHODS

A pot experiment was conducted in a glass house atDouglas Campus, James Cook University, NorthQueensland, Australia from August to December 2002.Two sweet potato genotypes (Lole and Wanmun) weresubjected to three water stress levels. The water stresslevels were imposed by maintaining the soil watercontent at 20%, 40%, and 80% of field capacity. These

Leaf Stomatal Density and Plant Water Relations as Affectedby Soil Water Regimes on the Sweet Potato Genotypes

Saraswati Prabawardani

Faculty of Agriculture and Agricultural Technique, Papua State University, Manokwari, Irian Jaya Barat

email: [email protected]

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PRABAWARDANI: SOIL WATER REGIMES ON THE SWEET POTATO GENOTYPES

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levels represented the severe water stress (20%),moderate water stress (40%), and no water stress (80%),respectively. The levels of soil water regimes weredetermined, based on the evaluation that below 20% ofsoil field capacity the sweet potato plants did not survive,and above 80% the sweet potato plants grow poorly dueto poor soil aeration.

The factorial treatments were arranged on acomplete randomised design with 4 replications. Thetotal number of experimental units was 24 pots withone plant per pot. Tip cuttings of two sweet potatocultivars (Lole and Wanmun) with each cultivar 25 cmlong were planted in of 10-L volume of pots. To preventevaporation, the whole pots were covered withaluminium foil.

Soil medium consisted of the mixture of peat: perlite:vermiculate (hydrous silicates) with the ratio of 1:1:1.Fertilizer was given at rate of 1g/plant using a commercialsoluble osmocote. Osmocote consists of 28%, 1.8%, and

14% of total N, P, and K and the micronutrients. Potswere rotated every week, thus each pot had the samechance to occupy the experimental area.

Analyses of variances were conducted for allcharacters measured. Significant treatments orcombination of main effects were stated based on theDuncan Multiple Range Test at a 0.05 probability level.

RESULTS AND DISCUSSIONS

Water Relations

Plant water status was mostly affected by soil waterregimes and varied between Lole and Wanmun cultivars,except for relative water content recorded at 2 months

after planting which was not significant (Table 1). Nointeraction effects were observed in the relative watercontent.

Transpiration

The daily mean transpiration rate of the Lole andWanmun cultivars was determined from 1 month to 5months (Table 1), and Lole showed significantly lesstranspiration than Wanmun under all soil water regimes.Water stressed plants transpired less water comparedto the well watered plants of both cultivars. Theinteraction effects between soil water regimes andcultivars were also significant.

The mean daily transpiration rate sharply increasedfrom 1 month to 3 months after planting, especially inthe well-watered plants. The sharp increased intranspiration between 1 and 3 months after planting was

Table 2. Effects of soil water regimes (20%, 40%, and 80% of soilfield capacity) on the daily transpiration rate of Lole andWanmun cultivars.

Lole Wanmun(g of H

2O/day) (g of H

2O/day)

1 month after planting20% of soil FC 130.25b B 160.38c A40% of soil FC 130.13b B 180.00b A80% of soil FC 210.35a B 305.35a A

2 months after planting20% of soil FC 167.50c B 263.89c A40% of soil FC 330.53b B 450.01b A80% of soil FC 630.17a B 815.44a A


4 months after planting20% of soil FC 308.09c B 335.81c A

40% of soil FC 534.07b B 569.14b A80% of soil FC 921.82a B 1013.48a A


Values within a column followed by the same small capital letter,and values within a row followed by the same big capital letternot significantly different (P

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attributed to an active vegetative growth and rapid tuberdevelopment. At this stage, a good water supply wasneeded to cope with rapid biomass and leaf areaincreases, as was also found by Brown (1992).

Leaf Relative Water Content

Both Lole and Wanmun cultivars had lower leaf relativewater contents in the drier (20% of field capacity) thanin the wetter soil moisture regimes (Figure 1). Waterstress caused a significant decrease in the leaf relativewater content of both cultivars. Under 20% and 40% ofsoil field capacity, leaf relative water content of Loledeclined by 3.9 % and 1.4%, respectively compared tothat of Lole grown at 80% of soil field capacity. At thesame time, the relative water content of Wanmun

declined by 0.9% and 3.8% at 20% and 40% of soil waterregimes, respectively.

At midday, the leaf relative water contents werefurther decreased in each cultivar but they recoveredduring the late afternoon. Lole had significantly higherleaf relative water content than Wanmun, apart fromthe midday value in the 40% soil water content at 2months after planting. Nevertheless, the 20% soil waterregime consistently recorded lower values than thehigher soil water regimes.

These results suggest that the pattern of reductionin plant water status of Lole and Wanmun under waterstress were similar between leaf water potential(expressing the energetic status of water inside the leafcells) and relative water content (expressing the relativeamount of water in the plant tissue). Relative watercontent was higher in the plants grown at 80% of soilfield capacity than in those grown at lower soil waterlevels, which indicated that plants with higher relativewater content had higher photosynthetic rates (Siddiqueet al.2000).

Leaf Anatomy (Stomatal Density)

An analysis of stomatal density in both the Lole andWanmun cultivars is presented in Table 1 and Figures 2and 3. There was no significant effect of water stress onthe stomatal density on the adaxial (upper) and abaxial(lower) leaf surfaces of either cultivar.

Lole or Wanmun produced 21 and 27 stomata/mm2

in their adaxial surfaces irrespective of soil water content.These results were lower than the correspondingstomata number on the abaxial surface where Lole hadan average of 43 stomata/mm2, whereas Wanmun had

73 stomata/mm2

, irrespective of the soil water regime.However, plants grown at 80 % of soil field capacity hadan abaxial stomatal density not significantly differentfrom that of plants grown under water stressedconditions. This suggests that stomatal density did notrespond to soil water stress conditions.

Nevertheless, other physiological parameters suchas time of sampling (2 months after planting) may affectstomatal density. Sung (1981) reported that it is radiation

Figure 1. Leaf relative water content of Lole and Wanmun, affected by three soil water levels (20, 40, and 40 % of field capacity) at twomonths (left) and three months after planting (right). Error bars represent standard errors of means with four replications.

Figure 2. Means of stomatal density on adaxial (upper) and abaxial(lower) surfaces of leaves of Lole and Wanmun sweetpotato cultivars as affected by soil water regimes (20%,40%, and 80% of soil field capacity).

Lole Wanmun

0102030405060708090

20 40 80 20 40 80Soil field capacity (%)

Stomataldensity(mm)2 adaxial surface abaxial surface

Lole Wanmun

657075808590

06.00 am 01.00 pm 06.00 pm 06.00 am 01.00 pm 06.00 pmTime of measurement

20% 40% 80%of soil field capacity

3 months after plating

L

eafrelativewatercontent

(%)

60657075808590

06.00 am 01.00 pm 06.00 pm 06.00 am 01.00 pm 06.00 pmTime of measurement

Leafrelativewatercontent

(%)


2 months after planting

Lole Wanmun

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Figure 3. Stomatal density of the Lole cultivar in 20X of magnification grown at 20% (A), 40% (B), and 80% (C) of soil field capacity and inWanmun cultivar under the same conditions 20% (D), 40% (E), and 80% (F).

E

F

D

B

C

A

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instead of leaf water potential that regulate the stomatalactivity of sweet potato. However, the sensitivity ofstomata to water stress varied among species (Ackersonand Krieg 1977). Ackerson et al. (1980) reported thatstomatal density varied with stage of growth.

As the growth stage sampled in the present study,water stress was not likely to be strongly evident and

may not have influenced the density of stomata.However, cultivars vary in their responses to water stressconditions. Wanmun had significantly greater stomataldensity than Lole, which suggests that less stomata inLole could be the mechanism by which the transpirationof Lole was more efficient than that in Wanmun.

Tuber Yield Components

Tuber Weight

With decreasing soil water levels, tuber yields decreased,

particularly in Wanmun (Figure 3).The measurement oftuber yields was determined by progressive destructivesampling each month from 2 to 6 months after planting,as tubers started to develop only 1 month after planting.When the plants were harvested 1 month after planting,the primary roots of Lole and Wanmun had notdeveloped into the tubers yet. When the plants wereharvested at 2 months after planting, small tubers hadstarted swelling.

The tuber yield of Lole declined by 62% and 28%under 20% and 40% soil field capacity, respectively, incomparison to yields under 80% soil field capacity at 6months after planting. At the same time, tuber yield of

Wanmun also declined by 69% and 41% under 20% and40% soil field capacity. At harvest (6 months afterplanting) the maximum tuber yields per plant wasrecorded in Wanmun (1228.5 g per plant). AlthoughWanmun produced about twice the mass of tubers/plant than Lole under well watered conditions, Wanmuntuber production was severely affected by water stressand produced an equivalent mass of tubers/plant to thatof Lole under 20% soil field capacity (Figure 4). The abilityto sustain tuber production under severe water stressconfirms a high drought tolerance on Lole.

The present results clearly showed that, irrespective

of soil water regimes, the growth of sweet potato occursin three phases: an initial phase, when the fibrous rootgrow extensively and vine grow moderately; a middlephase, when the vines grow extensively, tubers areinitiated, and leaf area increase remarkably; and a finalphase, when tuber bulking occurs and vines, and totalleaf area and fibrous root growth declines. However thisduration of growth stage may vary among cultivars andenvironmental conditions (Onwueme and Charles 1994).

Figure 5. The effect of soil water regimes (20%, 40%, and 80%field capacity) on the number of tubers produced per

plant by Lole and Wanmun cultivars at harvest time (6months after planting). Error bars represent standarderrors of means with four replications.

Figure 4. The effect of soil water levels (20%, 40%, and 80% fieldcapacity) on tuber yield of two sweet potato cultivars(Lole and Wanmun) at different times after planting. Errorbars represent standard errors of means with fourreplications.

Tuber Numbers

The number of sweet potato tubers produced per plantwas significantly affected by soil water stress (Figure 4).Under 80% soil field capacity, Lole produced significantlymore tuber numbers than did Wanmun (Figure 5).However, the size of tubers produced by Lole at 80% ofsoil water level was much smaller as compared to thoseproduced by either Wanmun or Lole at lower soil watercontents.

Lower soil water levels depressed the number oftubers produced by Lole. For instance, Lole under the40% soil water regime produced no more than two orthree tubers per plant. On the other hand, the numberof tubers in Wanmun was unaffected by different soil. Interms of average tuber size (not less than 150 g/tuber),Lole grown at 40% soil water content produced

Lole Wanmun

02004006008001000

12001400

2 3 4 5 6 2 3 4 5 6Time after planting (months)

Tuberyield(g/plant)


0246810

12

20 40 80Soil field capacity (%)

Tubernumber/plan

t Lole Wanmun

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comparable average tuber size (247.5 g/tuber) to thoseof Wanmun grown at 80% soil water level (243.8 g/tuber)(Figures 5). However, Lole yielded a much lower totalweight of tubers per plant than Wanmun.

CONCLUSIONS

Lole cultivar regulated water more efficiently thanWanmun cultivar. This was associated with less stomataldensity in Lole, and hence less water efflux or trans-piration from the leaf surfaces. This was also supportedby greater leaf relative water content in Lole than that inWanmun cultivar.

ACKNOWLEDGEMENT

The author expresses appreciation and gratefully

acknowledge support from Dr. M Johnston, Assoc. ProfR Coventry, and Dr J Holtum as the supervisors for thisproject from which this work is reported.

REFERENCES

Ackerson, R.C. and D.R. Krieg. 1977. Stomatal and nonstomatalregulation of water use in cotton, corn, sorghum. PlantPhysiology 60, 850-853.

Ackerson, R.C, D.R. Krieg, and F.J.M. Sung. 1980. Leaf conductanceand osmoregulation of field-grown sorghum genotypes. CropScience 20, 10-14.

Boyer, J.S. 1982. Plant productivity and environment. Science 219,443-448.

Bradford, K.J. and T.C. Hsiao. 1982. Physiological response tomoderate stress.In: OL Lange, P.S. Nobel, C.B. Osmond, andH. Ziegler (Eds.). Encyclopaedia of plant physiology.Physiological plant ecology. II. Water relations and carbonassimilation. Heidelberg Springer; New York, Berlin. 263-324 pp.

Brown, R.H. 1992. Photosynthesis and plant productivity in sweetpotato.In: WA Hill, CK Bonsi, PA Loretan (Eds.). Sweet potatotechnology for the 21st century. Tuskegee University; Tuskegee,

Alabama. 273-281 pp.

Kramer, P.J. 1980. Drought stress and the origin of adaptation. In:NC Turner and PJ Kramer (Eds.). Adaptation of plants to waterand high temperature stress. John Wiley & Sons; New York.7-20 pp.

Newell, L.L. 1991. Screening sweet potato genotypes (Ipomoeabatatas L. Lam) for drought tolerance. Abstract of MS thesis.Mississippi State University, Mississippi.

Prakash, M. and K. Ramachandran. 2000. Effects of chemicalameliorants on stomatal frequency and water relations inbrinjal (Sol anum melongena L.) under moisture stressconditions. Journal of Agronomy and Crop Science 185, 273-239.

Siddique, M.R.B., A. Hamid, and M.S. Islam. 2000. Drought stresseffects on water relations of wheat. Botany Bull. Acad. Sinica41, 35-39.

Singh, T.N., D. Aspenall, and L.G. Paleg. 1972. Proline accumulationand varietal adaptability to drought in barley: a potentialmetabolic measure of drought resistance. Nature New Biology236, 188-189.

Sivan, P., C.J.Asher, and F.P.C. Blamey. 1996. Effects of potassiumon drought tolerance of taro and tannia. In: ET Craswell, CJAsher and J.N. O Sullivan (Eds.). Mineral nutrient disorder ofroot crops in the Pacific. Proceedings of a workshop,Nukualofa, Kingdom of Tonga, 17-20 April 1995. TheAustral ian Center for International Agr icul tura l Research(ACIAR); Canberra, Australia.100-104 pp.

Sung, F.J.M. 1981. The effect of leaf water status on stomatal activity,transpiration and nitrate reductase of sweet potato. Agri-cultural Water Management 4, 465-470.

Van Loon, C.D. 1981. The effect of water stress on potato growth,development and yield. American Potato Journal 58, 51-69.

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