EFFECT OF PHOSPHORUS SOURCES ON PHOSPHORUS AND NITROGEN UTILIZATION BY THREE SWEET POTATO CULTTVARS

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1 EFFECT OF PHOSPHORUS SOURCES ON PHOSPHORUS AND NITROGEN UTILIZATION BY THREE SWEET POTATO CULTTVARS A. MONTANEZ, F. ZAPATA Soils Science Unit, FAO/IAEA Agriculture and Biotechnology Laboratory, International Atomic Energy Agency, Seibersdorf K.S. KUMARASINGHE Soil Fertility, Irrigation and Crop Production Section, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Intenational Atomic Energy Agency, Vienna XA Abstract EFFECT OF PHOSPHORUS SOURCES ON PHOSPHORUS AND NITROGEN UTILIZATION BY THREE SWEET POTATO CULTWARS. A greenhouse experiment was conducted at the FAO/IAEA Agriculture and Biotechnology Laboratory in Seibersdorf, Austria using three sweet potato cultivars,, and. The three sweet potato cultivars were grown at two levels of phosphorus (0 kg P/kg soil and 60 kg P/kg soil). The fertilizer treatments consisted of two sources of phosphorus, Gafza rock Phosphate and triple super phosphate with and 19.76% total phosphorus, respectively. I5 N labelled urea was used to study the nitrogen recovery in tubers from the applied nitrogen fertilizer. The results from these preliminary studies indicate that there is considerable genotypic variation among cultivars in the efficiency with which phosphorus and nitrogen are taken up and used to produce biomass. Their response to different sources of phosphorus are also variable. TIS-2 and TIS-1487 have a greater ability to absorb phosphorus from Gafza rock phosphate and produce higher tuber yields indicating their greater potential for using alternative sources of natural phosphate fertilizers more effectively. Gafza rock phosphate also increased accumulation of nitrogen in TIS-1487, a characteristic which will place this cultivar at an advantage when growing in soils low in nitrogen. On an overall basis taking into account tuber yield, phosphorus use efficiency, and nitrogen use efficiency, TIS-2 may be considered the better candidate for introduction into soils poor in resources particularly phosphorus. This study was conducted with a limited number of cultivars due to limitation in the availability of germplasm. In spite of this, the differences in their abilities for phosphorus and nitrogen uptake and use are clearly visible which justifies large scale screening experiments using a broader germplasm base, in the future. 1. INTRODUCTION In intensive agricultural production, the primary aim is to increase productivity by the high application of agricultural inputs. It is in relatively recent years that the emphasis is shifting towards the search of such plants which may give high yields with minimum fertilizer application. These plants should obviously combine the two traits: efficiency of absorption and efficiency of utilization to produce sustainable yields. Sweet potato (Ipomoea batata) has great potential as a high starch human food and as a raw material for ethanol production [1,2]. It is a major root crop produced and consumed in developing countries of which Africa represents a major region. However, their average yields are far below the potential that could be reached primarily due to the low fertility of the soils in which they are usually grown. Being a subsistence crop, application of chemical fertilizers is not common and in many cases fertilizers are either not easily available or are not within the reach of farmers for economic reasons. Of the nutrients, phosphorus is a critical element in early stages of growth and development of the sweet potato plant. Previous studies have shown that phosphorus limitation affects nitrogen assimilation processes, therefore phosphorus sources could affect it as well. One possible means of alleviating the problem of phosphorus deficiency would be the exploitation of genetic variation in response to phosphorus. Earlier reviews have indicated that within many species there is substantial intra-specific variation in response to phosphorus and that scope exist for selection of accessions with improved phosphorus efficiency [3,4]. Where small amounts of phosphate fertilizers 147

2 are needed for optimum yield, rock phosphates could provide a relatively cheap and an ideal substitute for the expensive soluble phosphate fertilizers particularly in acid soils [5,6,7]. Rock phosphate deposits occur in several countries of Asia, Africa and Latin America. Some of these rock phosphates are reactive and can be used directly without pretreatments. In order to develop better selection criteria for agronomists, preliminary investigations of basic principles at the species levels may be useful. At present there is very little information on phosphorus uptake and its utilization by this crop. As sweet potato tubers start to enlarge at the early growth stage (around 40 days after planting) they develop in parallel with the growth of shoot (leaf and steam) for a long period of time. As a result, it is considered that the shoot may compete with the tubers for photosynthates and nutrients such as nitrogen and phosphorus during most of their growth period. The objective of this experiment was to evaluate the phosphorus and nitrogen use efficiency of three cultivars of sweet potato in the presence and absence of phosphorus applied as triple super phosphate () and rock phosphate (PR). 2. MATERIALS AND METHODS A greenhouse experiment was conducted at the FAO/IAEA Agriculture and Biotechnology Laboratory in Seibersdorf, Austria using three sweet potato cultivars, TIS-2, TIS-3053 and TIS The plants were brought from the International Institute of Tropical Agriculture (IFFA) in Ibadan, Nigeria and multiplied by stem cutting. Uniform apical stem cuttings with about three fully expanded leaves were obtained from the same source. A single-stem cutting was transplanted in the month of August in each pot. Experimental units were 10 kg plastic pots. At transplanting a homogeneous sample of 6 plants per variety were taken to measure dry weight and initial phosphorus content. The three sweet potato cultivars were grown at two levels of phosphorus (0 kg P/kg soil and 60 kg P/kg soil). Two sources of phosphorus were used, Gafza rock phosphate (GRP) with 14.19% total phosphorus and triple super phosphate with 19.76% total phosphorus. Phosphate fertilizers were applied to each pot by prior mixing with the soil. 13 N labelled urea of 10% atom excess was added at planting to each pot in quantities equivalent to 37 mg N/kg soil. Potassium is especially important to promote swelling of the tubers. Potassium and others nutrients were applied as nitrogen/phosphorus free Hoagland nutrient solution at the rate of 30 ml/pot, each week. The soil used was non-sterilized soil and each pot was filled with 10 kg of 1:2 mixture of sand and soil classified as Brown earth, having a ph (KC1) of 4.38 and low available P. Pots were maintained at a moisture content near field capacity throughout the experiment. Plants were maintained in the greenhouse at a mean day/night temperature of 28/20 C and 50 to 70% humidity. Sequential harvests were performed at 35, 60 and 90 days after planting. At each harvest, the fibrous roots and tubers were carefully picked by hand from the soil and the roots were washed with tap water and fibrous roots and tubers were separated. Dry weight of shoots, tubers and fibrous roots were obtained after chopping and drying in an oven at 70 C for 48 h. Total phosphorus was determined using the phospho-vanadomolybdate yellow method [8] and the 1S N was used as a tracer [9] to measure nitrogen uptake and use by different cultivars. Plant samples were analysed for %N on a Kjeldahl digest [10] and the 15N isotope ratio analysis was performed on a VG-isogas mass spectrometer using Dumas Combustion method [11] Vesicular-arbuscular mycorrhizal (VAM) root infection was also determined. Fibrous roots were cut into 1 cm long pieces, cleared with 10% KOH and stained with Trypan Blue according to procedures as described by Philips and Hayman [12]. The phosphorus uptake was calculated taking into account the total phosphorus accumulated in the plant. Nitrogen uptake was calculated directly through the use of 15 N as a tracer [9]. Physiological nutrient use efficiency of phosphorus and nitrogen was determined as the biomass per unit of tissue nutrient concentration, as described by Siddiqi and Glass [13]. According to this: E = 148

3 W/C and C = N/W thus E = 2W/N where E is efficiency of utilization, W is biomass, C is tissue concentration and nitrogen is amount of nutrient in biomass. The percent of fertilizer nutrient utilization (FNU) or recovery has been calculated as: FNU = Nitrogen uptake/nitrogen applied x 100 Analysis of variance was performed to assess the effects of phosphorus sources and cultivars on nitrogen and phosphorus uptake and utilization. A split-split plot design with randomized complete block was used, with phosphorus sources as main plot and cultivars and harvest as sub-plot and subsub plot respectively. Treatments were replicated three times. Comparisons between means of treatments for various measured parameters were made by the least significance difference (LSD) test (P<0.05). 3. RESULTS AND DISCUSSION 3.1. Dry matter yield Dry matter yield of different varieties of sweet potatoes at three growth stages are presented in Table I. There were significant differences in dry matter yield between phosphorus sources and cultivars at harvest 1, 2 and 3. Therefore the cultivars differed among themselves in their response to availability of phosphorus. Application of phosphorus either in the form of triple super phosphate or Gafza rock phosphate, resulted in significant increase in dry matter yield of TIS-1487, TIS-2 and TIS-3053 over control (). Dry weight of fibrous roots, shoots and tubers increased with time and phosphorus application. The phosphorus source x time x cultivar interaction was not significant. Among the sweet potato cultivars tested, TIS-1487 had a high capacity to grow in the absence of added phosphorus at harvest 1 and harvest 2 but this cultivar was low in total yield with Gafza rock phosphate fertilizer at harvest 1. However all the cultivars responded strongly to the triple super phosphate fertilizer. Tuberization started first in cultivar TIS-1487 at 35 days after planting and the highest tuber dry weight was observed in cultivars TIS-1487 and TIS-2 at harvest 3. On the basis of tuber yield response the susceptibility of sweet potato to phosphorus stress at harvest 3 was in the order TIS > TIS-2 > TIS From a practical point of view the cultivars which produced well under a low level of phosphorus and responded well to added phosphorus are the most desirable. Cultivars TIS-1487 and TIS-2 fall into this category. TABLE I. EFFECT OF PHOSPHORUS SOURCE ON DRY MATTER YIELD OF SHOOT, ROOT AND TUBERS OF THREE SWEET POTATO CULTTVARS p sources Cultivars TIS Harvest 1 Root Harvest 2 Dry matter yield (g/plant) Root Tubers Harvest 3 Root Tubers * LSD at the 0.05 level of probability to compare P sources (P), cultivars (C), harvest (H) and any two means, respectively. LSD (p<0.05) shoot; P. 1.25; C:1.06; H: 0.82; PxH: 1.42; CxH; LSD (p<0.05) root;c:0.39; H: 0.61; PxH: 1.05; CxH; LSD (p<0 05) tubers; P: 59; C:

4 3.2. Phosphorus P uptake The results showing the total uptake of phosphorus by different cultivars are presented in Table II. These data indicate that phosphorus uptake from the two sources used differed widely among cultivars, with triple super phosphate providing the marginally better values. Accumulation of phosphorus varied significantly within sweet potato cultivars at each harvest showing that cultivars may differ in their ability to utilize the phosphorus applied as fertilizer. At earlier growth stages with triple super phosphate application, the total uptake of phosphorus was highest in TIS-1487 cultivar. However at harvest 2 and 3 greater amount of phosphorus was absorbed by TIS-2 and TIS-3053 showing a better performance to triple super phosphate fertilization. The cultivar which accumulated less phosphorus at early stages (TIS-3053) showed high susceptibility to phosphorus stress. However the best performance at level was TIS-1487 but at 90 days after planting with T-1487 and TIS-2 showed high phosphorus uptake at all phosphorus sources. However, high shoot and tuber growth and high phosphorus uptake of these cultivars was not associated with high fibrous root growth. This indicates that phosphorus uptake efficiency may not be directly related to growth of the fibrous roots at least in plants where roots develop into storage organs as in sweet potatoes. With the rising cost of phosphorus fertilizer, the potential of using cultivars efficient in use of nutrients in order to increase and/or stabilize crop production is becoming increasingly attractive. Results of this study indicate that sweet potato cultivars differ significantly in their phosphorus requirements and phosphorus utilization from different sources. TABLE II. EFFECT OF PHOSPHORUS SOURCE ON PHOSPHORUS ACCUMULATION IN SHOOT, ROOT AND TUBERS OF THREE SWEET POTATO CULTIVARS p sources Cultivars TIS1487 TIS2 TIS3053 US 1487 TIS2 TTS 3053 ITS 1487 TES Harvest 1 Root Harvest 2 P uptake (mgp/plant) Root Tubers Harvest 3 Root Tubers * LSD at the 0.05 level of probability to compare P-sources(P), cultivars (C), harvest (H) and any two means, respectively. LSD (p<0.05) shoot; P: 3.3; C2.5; H: l.fr, PxH: 3.1; CxH: 3.2. LSD 0X0.03) root;: C: 1.1; H: 1.2; CxH: 2.2. LSD(p<0.05) tuber, P: ll.s;c:12.3. At low phosphorus and nitrogen levels in the soil, sweet potatoes are usually colonized by VAM fungi [14]. The magnitude of the growth response is affected by the soil phosphorus and nitrogen contents, inherent effectiveness of the VAM fungi, and inherent differences among sweet potato cultivars [8]. Recent experiments have shown increases in plant growth, P/N incorporation, and tuber yields when the sweet potato plants were inoculated with VAM fungi. The potential inoculum in the soil used in this experiment was low and as a result the plants even at level were not infected by the native mycorrhizas. The results are therefore not reported here. 3.3 Phosphorus use efficiency The results on phosphorus use efficiency of the three sweet potato cultivars in the presence of the different phosphorus sources are shown in Fig. la and Appendix Table XXXIII. The phosphorus use efficiency was highest in the absence of applied phosphorus followed by triple super phosphate and the rock phosphate indicating that there are pronounced differences in phosphorus 150

5 f o GRP Phosphorus source TIS-1487 TIS-2 i Phosphorus source GRP FIG. 1. Genotypic differences in phosphorus use efficiency (a) and nitrogen use efficiency of three cultivars of sweet potato in the presence of triple super phosphate (), Gafia rock phosphate (GRP), and in the absence of applied phosphorus (). 151

6 nutrition between sweet potato cultivars. TIS-1487 and TIS-2 appear to have access to a source of phosphorus in Gafza rock phosphate that was less available to the other cultivars. In the earlier harvest there was little interaction between cultivars performance and phosphorus sources. Cultivars which grow well with phosphorus application also performed well at level. This indicates that the differences in performance between cultivars at and phosphorus application were related more to differences in the efficiency with which the absorbed phosphorus is utilized for growth than to inherent differences in ability to absorb phosphorus. Growth of TIS-3053 rapidly increases at later stages producing high amount of dry weight of shoot compared with dry weight of tuber at harvest 3. TIS-3053 is also much slower to establish than TIS-2 and TIS-1487, and this may be related to its poor ability for phosphorus uptake and use. The magnitude of the differences in phosphorus nutrition between cultivars observed in this study has important implications, as scarcity of phosphorus fertilizers is likely to become a major problem in the future. The fact that TIS-2 and TIS-1487 are able to absorb phosphorus better from Gafza rock phosphate and produce higher tuber yields indicate their greater potential for using alternative sources of natural phosphate fertilizers more effectively. 3.4 N uptake and nitrogen use efficiency The results (Appendix Table XXXIV) show that nitrogen uptake by shoots although significantly different among cultivars and between phosphorus treatments, these differences became less obvious at harvest 3 when plants were more developed. On the other hand, nitrogen accumulation in tubers showed marked differences with TIS-1487 being superior to other cultivars and in all phosphorus treatments. Application of Gafza rock phosphate clearly increased accumulation of nitrogen in TIS-1487, a characteristic which will have great advantage for cultivation in areas low in soil nitrogen. The %Ndff in tubers (Fig. lb and Appendix Table XXXV) did not show much of a difference between harvests 2 and 3. In the 3rd harvest, the values ranged from 57 to 66. The %Ndff was in general similar in shoots and in tubers. However, the recovery of nitrogen showed significant differences between shoots and tubers, the recovery in tubers being lower than in the shoots. Much of the nitrogen applied as fertilizer accumulated more in the shoots and less in tubers probably because tubers accumulate more carbohydrates than nitrogenous compounds. This is also evidenced by the fact that the total nitrogen in tubers is usually about half that of the shoots. The nitrogen use efficiency of the tubers (Table III) varied significantly between cultivars and within cultivars depending on the phosphorus source. TIS-2 showed the best nitrogen use efficiency in the presence of Gafza rock phosphate followed by TIS-1487 in the presence of. The nitrogen use efficiency of TIS-3053 was the lowest (1.07) in the absence of applied phosphorus but improved in the presence of triple super phosphate and GRP. On an overall basis taking into account tuber yield, phosphorus use efficiency, and nitrogen use efficiency, TIS-2 may be considered a good candidate for introduction into soils poor in resources particularly phosphorus. 4. CONCLUSIONS The results from these preliminary studies indicate that there is much genotypic differences in the efficiency with which phosphorus and nitrogen are taken up and used by different cultivars of sweet potatoes. Their response to different sources of phosphorus are also variable. TIS-2 and TIS have a greater ability to absorb phosphorus from Gafza rock phosphate and produce higher tuber yields indicating their greater potential for using alternative sources of natural phosphate fertilizers more effectively. Gafza rock phosphate also increased accumulation of nitrogen in TIS-1487, a characteristic which will place this cultivar at an advantage when growing in soils low in nitrogen. On an overall basis taking into account tuber yield, phosphorus use efficiency, and nitrogen use efficiency, TIS-2 may be considered the better candidate for introduction into soils poor in resources particularly phosphorus. This study was conducted with a limited number of cultivars due to limitation in the availability of germplasm. In spite of this, the differences in their abilities for phosphorus and nitrogen uptake and use are clearly visible which justifies large scale screening experiments using a broader germplasm base, in the future. 152

7 TABLE HI. EFFECT OF PHOSPHORUS SOURCE ON TOTAL N, %NDFF AND %N RECOVERY (%FNU) IN SHOOT, ROOT AND TUBERS OF THREE SWEET POTATO CULTIVARS p Cultivars Harvest 1 Harvest 2 Total N (mg N/plant) Harvest 3 sources Tubers Tubers TIS %Ndff %FNU

8 ACKNOWLEDGEMENTS This work was carried out at the FAO/IAEA Agriculture and Biotechnology Laboratory in Seibersdorf, Austria as part of back-up research in support of the FAO/IAEA/SIDA Co-ordinated Research Programme on The Use of Isotope Studies on Increasing and Stabilizing Plant Productivity in Low Phosphate and Semi-arid and Sub-humid Soils of the Tropics and Sub-tropics (D ). The authors thank Ms. Helga Axmann for her assistance in 13 N analytical services. REFERENCES [I] DANGLER J. M., LOCASCIO S. ]., HALSEY L. N., Sweet potato, for biomass. Biomasg 4 (1984) 2S [2] LOCASCIO S. J., DANGLER J. M., Starch and mineral nutrient accumulation by sweet potato cultivars. In Smith V. H. (ed.). Biomass energy development. Plenum Press, New York, (1986) [3] AHLOOWALIA, B. S., KUMARASINGHE, K. S..S1GURBJOERNSSON, B., MALUSZYNSKI, M., Genotypic selection for improved phosphorus utilization in crop plants, Genetic manupulation of crop plants to enhance inegrated nutrient management in cropping systems-1, Phosphorus: Proc. of an FAO/ICR1SAT Expert Consultancy Workshop, March 1994, ICRISAT Asia Centre, India (Johansen, C, Lee, K.K. Sharma, K.K., Subbarao, G.V., Kueneman, E.A., Eds) (1995) [4] FOHSE, D., CLAASEN, N., JUNGK, A., Phosphorus efficiency of plants, Plant and Soil 110 (1988) [5] Mahotra C. L., Efficiency of rock phosphates compared to super phosphates. Pert. News. 13 (9) (1986) [6] LEON L. A., PENSTER W. E., HAMMOND L. L., Agronomic potential of eleven phosphate rocks from Brazil, Colombia, Peru and Venezuela. Soil. Sci. Am. J. 50 (1986) [7] HAMMOND D. T., CHIEN S.H., MOKWUNYE A. U., Agronomic values of unacidulated and partially acidulated phosphate rocks indigenous to the tropics. Adv. Agron. 40 (1986) [8] HUNG L. L., SYLVIA D. M., O'KEEFE D.M., Isolate selection and phosphorus interaction of vesicular arbuscular mycorrhizal fungi in biomass crops. Soil Sci. Am. J. 54 (1990) [9] ZAPATA F., Isotope techniques in soil fertility and plant nutrition studies of soil-plant relationships. International Atomic Energy Agency (1990) Vienna. [10] EASTIN, F. P., Total nitrogen determination for plant materials, Anal. Biochem., 85 (1978) [II] FIEDLER, R., PROKSCH, G., The determination of N-15 by emission spectrometry in biochemical analysis, A Review, Anal. Chim. Acta 78 (1975) [12] PHILLIPS J. M., HAYMANN D. S., Improved procedures for clearing roots and staining parasitic and vesicularmycorrhizal fungi for rapid assessment of infection, trans. British. My col. Soc. 55 (1970) [13] SIDDIQIM. Y., GLASS A. D. M., Utilization index: a modified approach to the estimation and comparison of nutrient utilization efficiency in plants. Journal of Plant Nutrition. 4(3) (1981) [14] NGEVE J. M., RONCADORI R. W., The interactions of vesicu'lar-arbuscular mycorrhizae under aseptic conditions. J. Gen. Microbiol. 27 (1985)