Australian Journal of Agricultural Research

CSIRO PUBLISHING Australian Journal of Agricultural Research Volume 51, 2 CSIRO Australia 2 A journal for the publication of original contributions towards the understanding of an agricultural system www.publish.csiro.au/journals/ajar All enquiries and manuscripts should be directed to Australian Journal of Agricultural Research CSIRO PUBLISHING PO Box 11 ( Oxford St) Collingwood Telephone: 1 2 728 Vic. Facsimile: 1 2 711 Australia Email: jenny.fegent@publish.csiro.au Published by CSIRO PUBLISHING for CSIRO Australia and the Australian Academy of Science

Aust. J. Agric. Res., 2, 51, 2 8 Response of sweet potato cultivars to acid soil infertility factors. I. Effects of solution ph on early growth V. P. Ila ava A, C. J. Asher B, and F. P. C. Blamey B A Environment Department, Ok Tedi Mining Limited, PO Box 1, Tabubil, Western Province, Papua New Guinea. B School of Land and Food, The University of Queensland, Brisbane, Qld 472, Australia. Abstract. Sweet potato [Ipomoea batatas (L.) Lam.] is cultivated on soils varying widely in chemical properties, but relatively little is known about the effects of ph on the growth of this crop. In commercial and subsistence agriculture, sweet potato is propagated mostly from stem cuttings. This paper reports effects of a range of ph treatments (.5 8.) in flowing solution culture on early growth from cuttings of sweet potato cultivars. Root growth was either greatly reduced or inhibited at ph.5. Increasing the ph to 4. markedly increased root development. Further increases in solution ph from 4. to 8. did not appear to affect root growth in most cultivars. Top growth in most cultivars showed a tendency to increase when ph was increased from.5 to 5.5 before declining with further increases in solution ph. The sweet potato cultivars studied differed widely in their tolerance to low ph, producing 1 48% of maximum top dry mass at ph.5. Tissue analysis from selected cultivars showed that K and Ca appeared to be limiting at ph.5, while P may have been deficient at ph 8.. Results of this study indicate that low ph per se does not appear to be a major factor responsible for poor sweet potato yields in acid soils. Introduction The tropical root crops sweet potato, cassava (Manihot esculenta Crantz.), yams (Dioscorea spp.), and edible aroids [especially Colocasia esculenta (L.) Schott.] are the most important tropical root crops produced and consumed in the world. All have high yield potentials (about 1 t fresh weight/ha, corresponding to 25 t/ha of edible dry matter). However, sweet potato has the advantage of more rapid food production by virtue of its shorter growing season (4 5 months compared with the other tropical root crops, typically >8 months). In common with cassava, propagation is via stem cuttings. In the tropics, sweet potato is grown extensively on unlimed moderately to strongly acidic soils, and it is believed to be a crop that is well adapted to soils of low to moderate fertility (Hahn 177). However, relatively little is known about the response of sweet potato cultivars to acid-soil infertility factors. Carefully controlled solution culture experiments suggest that the optimum ph for many plant species is in the range 5 (Asher 178), but plant species differ widely in their tolerance to low ph (Islam et al. 18). Furthermore, there is evidence of differential tolerance to low ph among cultivars of several plant species (Kim et al. 185; Johnson and Wilkinson 12; Tan et al. 1; Liu et al. 14). CSIRO 2 Little is known about the effects of ph per se on the growth of sweet potato. An experiment by Ila ava et al. (15) with 4 sweet potato cultivars grown for 14 days in non-flowing solution culture showed that root growth, top growth, and leaf area were severely depressed at ph values <4.. In the present study, flowing solution culture (FSC), was used to further examine the response of sweet potato cuttings to ph using cultivars that are commonly grown in the Pacific region. A subsequent paper (Ila va et al. 2) will describe the responses of the same cultivars to low external calcium concentrations, and to aluminium toxicity. Materials and methods A description of the FSC equipment used was given by Asher and Edwards (178). On the basis of the results from an earlier experiment (Ila ava et al. 15), the 8 ph treatments selected were.5, 4., 4.5, 5., 5.5,., 7., and 8.. The cultivars studied were: Lole, Hawaii, LO2, Beerwah Gold, Wanmun, L, L11, L18, L4, L4, L11, L15, Markham, Meriken, and NG757. These cultivars included some of the common cultivars in the South Pacific and others reported to be adapted to a wide range of environmental conditions in PNG. The ph treatments were randomly assigned to the 8 FSC units used. The 4 replications of the cultivars were randomised within each FSC unit, these positions being re-randomised 1 week after planting. The composition of the basal nutrient solution was as follows ( M): 7 N (as NO ), 4 Ca, K, 1 Cl, 14 S, 1 Mg, 1 Fe, B, 2 P,.25 Zn,.18 Mn,.7 Cu,.2 Mo, and.2 Ni. Elemental concen- 4-4//

24 V. P. Ila ava et al. trations were similar to those measured in soil solutions of tropical acid soils (Gillman and Bell 178; Bruce et al. 18; Menzies et al. 14). The concentrations of Ca, S, K, Mg, P, Fe, B, Zn, Cu, Mn, Mo, and Ni were measured daily by inductively coupled plasma atomic emission spectroscopy (ICPAES). Nitrate-N concentration in the nutrient solutions was also measured every 2 or days using a nitrate electrode. The concentrations of individual nutrients in the solutions were kept within 1% of the initial concentrations. Each cutting was 1 cm long with 2 or young leaves. One cutting from each cultivar was planted per plant support basket. The cuttings were placed in the baskets so that the basal quarter ( cm) of the cuttings was immersed in the nutrient solution. The plant-support baskets were then filled with black polyethylene beads to hold the cuttings in place and to prevent light reaching the nutrient solution. The plants were grown for 21 days in November 1. During the growing period, the glasshouse floor was kept wet constantly by a soaker hose to keep humidity levels >5%. The mean daily minimum and maximum temperatures in the glasshouse were 22 C (±2) and 8 C (±5), respectively. The solution temperature was maintained at about 25 C throughout the experiment. The ph of solutions in the FSC units was adjusted to their designated ph levels using either.25 M H 2 SO 4 or.5 M KOH. Although solution ph in the units was held constant by the automatic ph control equipment, daily checks on ph were also carried out using an independent ph meter (Picolo HI). The mean solution ph values during the growing period were.54, 4.5, 4.5, 5.2, 5.51,.5,.82, and 7.. The mean daily fluctuation of ph was <. in all treatments. At harvest, the roots were separated from the tops and the 7th to 1th leaf blades (referred to in the text as YLB) were removed from each plant. Considerable data on sweet potato nutritional status using similar leaves as the index tissue are now available (O Sullivan et al. 17). The YLB, tops, and the roots were then put into labelled paper bags, and dried in a dehydrator at 7 C for 2 days for dry mass measurements. On the basis of growth at ph 4., cvv. Wanmun, L4, Lole, Beerwah Gold, Meriken, and L11 were selected for chemical analysis. These cultivars represented the most tolerant to low ph (cv. Wanmun, 1% of maximum top growth) to the least tolerant (cv. L11, approximately 5% of maximum). The YLB from these cultivars were ground using a coffee grinder with a stainless steel blade and stored in paper bags. Samples were wet ashed by a nitric perchloric acid digestion technique (Islam et al. 12), and the resulting digests analysed by ICPAES to determine concentrations of Ca, K, Mg, S, P, Fe, B, Mn, Zn, and Cu. The data were analysed statistically using the Queensland University Agriculture Statistical Package (QUASP) program. Mathematical functions were fitted to the root and top yield data. No l.s.d. is shown for data where lines of best fit have been plotted. Results Root growth Solution ph had marked effects on root growth of the sweet potato cultivars tested (Fig. 1). In contrast with results from Ila ava et al. (15), limited root growth was evident on cuttings grown in the solution maintained at ph.5 (Fig. 1). These roots were short, had no lateral root development, and were brown in color. Root dry mass of most cultivars increased by >2% when solution ph was increased from.5 to 4. (Fig. 1). Roots of the plants grown in solution at ph 4. were normal and healthy in appearance. With most of the cultivars, root growth was not greatly affected when solution ph was increased from 4. to 8. (Fig. 1). Top growth Increasing solution ph from.5 to 4. markedly increased top growth of all cultivars tested (Fig. 2). However, increasing the solution ph from 4. to 7. had little effect on the top growth of most. In the present study, where the plants were grown for 1 week longer than in that of Ila ava et al. (15), top dry mass decreased by about -fold in most cultivars at ph 8. (Fig. 2). Lole and L11 were the cultivars least affected by solution ph (Fig. 2). Top dry mass at ph.5 ranged from 1% to 48% of the maximum yield attained. Nutrient concentrations in the YLB of selected cultivars When averaged over sweet potato cultivars, nutrient concentrations in the YLB were generally indicative of adequate levels for plant growth (data not shown). However, the mean P concentration at ph.5 was marginal and deficient, whereas at ph 8., it was below the published critical concentrations of O Sullivan et al. (17). At ph.5, K concentration in 5 cultivars and Ca concentration in 2 cultivars were below the levels considered adequate for healthy growth by O Sullivan et al. (17) (data not shown). Discussion Effects of solution ph on plant growth In the present experiment and in that of Ila ava et al. (15), all sweet potato cultivars produced normal and apparently healthy growth at ph 4.. At ph <4., root growth was either not evident (Ila ava et al. 15) or very poor (Fig. 1). These results are in agreement with previous solution culture studies involving other plant species (Arnon and Johnson 142; Islam et al. 18; Kim et al. 185). For example, Arnon and Johnson (142) found that roots of tomato, lettuce, and Bermuda grass seedlings were unable to grow in the nutrient solutions maintained at ph.. Similarly, Kim et al. (185) observed that roots of subterranean clover seedlings died in solutions maintained at ph.5. While root growth of plants grown in solutions maintained at ph <4. in the present experiment was poor, Fig. 2 suggests that top growth in most cultivars was more sensitive to both low and high ph. These results imply either that the sweet potato cultivars studied were sensitive to solution ph values of.5 and 8. or that the problems of maintaining adequate nutrient supply at these ph values were not fully overcome. Sweet potato tolerance to low solution ph Solution culture studies have shown that plant species differ widely in their tolerance to low solution ph. Thus, Islam et al. (18) found that crop species achieved between 8% and about 45% of maximum yield at ph., ginger and cassava being the most tolerant species while wheat and maize were the least tolerant. In a study involving several subterranean clover cultivars, Kim et al. (185) reported <5% of maximum yield at ph.5. In the present study, root and top growth at ph.5 were depressed by approximately

Sweet potato response to acid soil. I. 25 Beerwah Gold LO2 L11 2. y = 1.2-4.ex -1 r 2 =.8 1..5 y = 1.8-7.8ex -1 r 2 =.8 y = 1.8-5ex -1 r 2 =.5 2. L4 L15 y = -1. + x -1-7.5x -2 r 2 =.78 Meriken y =. -.45x -1-1.7x -2 r 2 =.7 1..5 r 2 =. y = -.72 +.x -1-8x -2 Root dry mass (g/plant) 2. 1..5 Hawaii y = 5.8 -.8x - 1.x -1 r 2 =.8 NG757 y = -.81 + 24.1x -1 -.x -2 r 2 =.55 Markham y =.2 + 14.5x -1-45.1x -2 r 2 =.5 2. Wanmun y = -.25 + 2.7x -1 -.x -2 r 2 =. L L4 y = -.28 + 1.2x -1 -.1x -2 r 2 =. 1..5 y = -2.82 + 5.x -1 - x -2 r 2 =.7 2. 1..5 L18 y =. + 4.71x -1-18.5x -2 r 2 =.72 Lole y =.77-14.1ex -1 r 2 =.47 L11 y =.5-11.1ex -1 r 2 =.71 4 5 7 8 4 5 7 8 4 5 7 8 Solution ph Fig. 1. Effects of solution ph on root dry mass of fifteen sweet potato cultivars grown in flowing solution culture for 21 days. Data plotted are means of four replicates. 5% in few of the cultivars tested. Since the growing conditions employed by both Islam et al. (18) and Kim et al. (185) were similar to those of the present study, it can be concluded that sweet potato is moderately to highly tolerant of low solution ph. The differential tolerance among the sweet potato cultivars to low solution ph in the present experiment could not be explained in terms of concentrations of the essential elements in the YLB of selected cultivars for which chemical analyses were performed (data not presented). In 5 of the

2 V. P. Ila ava et al. Beerwah Gold y = -2. + 77x -1-72x -2 r 2 =.85 LO2 L11 y = -1. + 42x -1-1x -2 r 2 =.87 y = 2.8-4.5x - 5x -1 r 2 =.85 L4 L15 y = -.2 +.x - 1.x -1 r 2 =.1 Meriken y = 5.4-5.25x - x -1 r 2 =.7 y = 1 -.x - 257x -1 r 2 =.75 Top dry mass (g/plant) Hawaii y = -1. + 2x -1-5x -2 r 2 =.87 NG757 y = 85.2-7.8x - 21x -1 r 2 =.77 Markham y = 54.8-4.52x - 11x -1 r 2 =.8 Wanmun y = -2. + 2x -1-71x -2 r 2 =.85 L y = 1 -.x - 257x -1 r 2 =.8 L4 y = -. + 18x -1-42x -2 r 2 =.78 L18 Lole y = 52.4-4.4x - 4x -1 r 2 =.85 y = -8.2 + 11x -1-28x -2 r 2 =.82 L11 y = 7.2 + 114x -1-27x -2 r 2 =.71 4 5 7 8 4 5 7 8 4 5 7 8 Solution ph Fig. 2. Effects of ph on top dry mass of fifteen sweet potato cultivars grown in flowing solution culture for 21 days. Data plotted are means of four replicates. cultivars analysed, K concentration in the YLB of plants grown at ph.5 was below the critical concentration reported for this element by O Sullivan et al. (17). Calcium concentration in the YLB of cvv. Lole and Wanmun at ph.5 was also below the critical concentration reported by O Sullivan et al. (17). It appears that the high H + ion concentration at ph.5 might have caused K or Ca deficiency in most of the cultivars studied. Perhaps tolerance by sweet

Sweet potato response to acid soil. I. 27 potato cultivars to low ph may be linked with their ability to absorb K and Ca under acidic conditions. Effects of high solution ph on growth of sweet potato Low P uptake by the plant roots at ph 8. appears to have been a major factor responsible for inhibiting growth of sweet potato in the present experiment. Evidence for this comes from the low P concentrations in the YLB of the cultivars analysed (data not shown). Indeed, P concentration in the YLB of the cultivars analysed decreased from >.% between ph 4. and 7. to <.22% (the published critical value by O Sullivan et al. 17) at ph 8. (data not presented). This result is in general agreement with that of Hendrix (17) who found that accumulation of P in the tops of bean plants increased with increasing ph from 4. to 5.5 and then declined with further increase to ph 8.7. The reasons for the decrease in P concentration in the YLB at ph 8. in the present study are not clear. However, Hendrix (17) showed that the accumulation of P in the plant tops at ph >. followed closely that of H 2 PO 4 concentration in solution. In the present study, K concentration in the YLB of cvv. Beerwah Gold and Meriken (data not presented) at ph 8. was also below the level considered adequate for good growth by O Sullivan et al. (17). It would appear that for Beerwah Gold and Meriken, K uptake by the plant roots was impaired at ph 8.. In the case of cv. Wanmun, K concentration in the YLB at all the ph treatments in the present experiment was <4.% (data not presented), suggesting that for Wanmun K concentration in the nutrient solution ( M) may have been inadequate for this cultivar. Conclusions Results of the present study showed that sweet potato is moderately to highly tolerant of low solution ph. Given that the nutrient solution used was similar to that reported for many tropical soils, it can be concluded that sweet potato will do well in acid soils. Increasing the solution ph from 4. to 7. generally had no effect on the growth of most sweet potato cultivars studied. This result would suggest that liming strongly acid soils (ph 4. 5.) may not greatly increase yields of many sweet potato cultivars. At ph.5, K concentration in the YLB of most cultivars analysed was in the deficient range. It appears that with sweet potato, K deficiency may be induced or exacerbated under strongly acid conditions. On the other hand, uptake of P is seriously inhibited under high ph conditions. Acknowledgments The authors are grateful to Ms J. Mercer and Mr G. Walters for technical assistance rendered during the project. The authors also acknowledge the contribution of Mr. J. Oweczkin who assisted with plant analysis. Thanks also to Dr J. O Sullivan for her support and advice during the research. Funding for this research came from the Department of Agriculture, The University of Queensland, and the Australian Center for International Agricultural Research (ACIAR) to whom the authors are greatly indebted. The senior author is very grateful to the following organisations: Australian Agency for International Development (AusAID) for their sponsorship of his studies at The University of Queensland, his former employers (The Papua New Guinea University of Technology) for their support during his studies, and his current employers (Ok Tedi Mining Limited, PNG) for their support in preparing these documents for publication. References Arnon, DI, Johnson, CM (142) Influence of hydrogen ion concentration on the growth of higher plants under controlled conditions. Plant Physiology 17, 525 5. Asher, CJ (178). Natural and synthetic culture media for spermatophytes. In CRC handbook series in nutrition and food. Section G: Diets, Culture Media, Food Supplements. Vol.. 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(Department of Agriculture, The University of Queensland: Brisbane, Qld) Johnson JW, Wilkinson RE (12). Wheat growth responses of cultivars to H + concentration. Plant Soil 14, 55 5. Kim M, Edwards DG, Asher, CJ (185). Tolerance of Trifolium subterraneum cultivars to low ph. Australian Journal of Agricultural Research, 5 578. Liu A, Latimer JG, Wilkinson, RE (14). Effect of ph on seedling growth of six cultivars of watermelon. Journal of Plant Nutrition 17, 57 548.

28 V. P. Ila ava et al. Menzies NW, Bell LC, Edwards DG (14) Exchange and solution phase chemistry of acid, highly weathered soils. I. characteristics of soils and the effects of lime and gypsum amendments. Australian Journal of Soil Research 2, 251 27. O Sullivan JN, Asher CJ, Blamey, FPC (17). Nutrient disorders of sweet potato. ACIAR Monograph No. 48, 1 pp. (Australian Centre for International Agricultural Research: Canberra) Tan K, Keltjens WG, Findenegg, GR (1) Aluminum toxicity in sorghum genotypes as influenced by solution acidity. Soil Science and Plant Nutrition Journal, 21 28. Manuscript received 27 January 1, accepted 1 August 1 http://www.publish.csiro.au/journals/ajar