Evaluation of Some Potato Hybrids and Their Parents in a Location with Supra- Optimal Temperatures

Transcription

1 PAT 2008; 4 (2): ISSN: Online copy available at Evaluation of Some Potato Hybrids and Their Parents in a Location with Supra- Optimal Temperatures 1 Amadi, C. O., 2 Ene Obong, E. E, 3 Okonkwo, J. C. and 3 Danbaba, A. K. 1. Corresponding author. National Root Crops Research Institute Umudike, Potato Programme Kuru, P. M. B. 04 Vom, Jos, Plateau State. okeyamadi2003@yahoo.com 2. Michael Okpara University of Agriculture Umudike Abia State 3. National Root Crops Research Institute Umudike, Potato Programme Kuru, P. M. B. 04 Vom, Jos, Plateau State. ABSTRACT One hundred and forty nine potato hybrids (from 32 families), ten parents and a check variety Desiree known for heat tolerance were evaluated during the rainy season of 2002 and 2003 at Toro in the northern guinea savanna of Nigeria to determine the extent and nature of variability in some economically important traits within this population. It was also intended to compare their performance in order to identify and select genotypes with enhanced potential for growth and yield in climates with high ambient temperatures. Highly significant variability was observed for most traits of the genotypes evaluated suggesting the existence of sufficient variability upon which selection for improvement in these attributes can be based. Partitioning of the observed variation into their genotypic and phenotypic components showed that variation in plant emergence, number of stems, number of wilted stands, average tuber weight and tuber yield were mostly due to environmental influences while the reverse was the case for other attributes. Traits with high environmental influence are expected to respond slowly to selection. Six hybrid genotypes had tuber yield which were significantly higher than the highest yielding parent B Five of these out yielded the check variety Desiree significantly. Twelve others genotypes had tuber yields which were higher than but not statistically different from the check variety. Seventeen hybrid genotypes are recommended for further evaluation as heat tolerant genotypes suitable for the northern guinea savanna. Key words: Variability, genotypes, high temperature, selection, yield INTRODUCTION The potato Solanum tuberosum L originated in the tropical highlands of the Andes of South America (Horton and Anderson, 1992) and are generally best adapted to cool temperate zones (Hawkes, 1978). Over the last two decades, potato production has rapidly expanded to tropical and subtropical areas of the world, becoming one of the most valuable staple foods due to its high nutritional value (Van der Zaag and Horton, 1983;

2 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids CIP, 1984). The optimum temperature for tuber formation and growth for most potato cultivars is about o C (Borah and Milthorpe, 1962). In tropical lowland parts of Africa, potatoes are exposed to day and night temperatures far above the optimum. Almost all Solanum tuberosum L cultivars breed in northern latitudes respond to such conditions with delayed emergence (Midmore, et al., 1988), excessive haulm growth at the expense of the tubers (Ewing, 1981), increased rate of respiration (Sale, 1974), reduced dry matter accumulation, delayed tuber initiation and maturity (Amadi, 2005) and a significant loss of tuber yield and quality (Levy, 1983, 1984). The Adverse effect of high temperature on tuber yield and quality of potato is a major obstacle for potato production in hot regions, and local breeding of heat tolerant cultivar has been necessary to improve potato crops in hot environment (Levy, 1984). Evaluation of potato genotypes at warm locations enables not only the study of the response of important plant characters to such locations but also the analysis of nature and extent genetic variability and inter-relationship among these characters. In this study, some potato hybrids and their parents were evaluated to determine the nature and extent of variability in some economically important traits and compare their performance in order to identify and select high yielding genotypes suitable for cultivation in climates with high ambient temperatures like we have in the northern guinea savanna of Nigeria. MATERIALS AND METHOD This experiment was carried out during the rainy season of 2002 and 2003 at Toro - a warm location situated at lat N, and Long 9 0 E in the northern guinea savanna agricultural zone of Bauchi State Nigeria. Weather data of Toro for the relevant months in 2002 and 2003 are presented in Fig 1.One hundred and sixty potato genotypes consisting of one hundred and forty nine hybrids (from 32 families), ten parents and Desiree, a cultivar well known for some degree of heat tolerance (Midmore, 1984) were used. The experimental design was randomized complete block design (RCBD) replicated 2 times because of shortage of planting materials. Each replication consisted of six blocks. Gross plot size was 9m 2 while net plot size was 3m 2. The seed tubers were stored under diffused light storage and were sprouted before planting. They were planted at the rate of 1 tuber per stand, with inter and intra row spacing of 1m and 30cm respectively giving a plant density of 33,333 plants per hectare. Weeding was carried out manually at the 4th and 8th weeks after planting (WAP). Fertilizer was applied at the rate of 100kg N, 100kg P and 40kg K 2 0 per hectare by banding two weeks after planting. No fungicide was applied. The plants were harvested when the leaves began to senesce. Data were collected on the following attributes; seedling emergence at 4 WAP, number of stems per plant, plant height (cm), number of leaves per plant, days to tuber initiation, days to maturity, severity of early blight, number of wilted stands, scab

3 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids severity, number of tubers per plant, average tuber weight/plant, and tuber yield per plant. These attributes were measured as follows Seedling emergence: Emergence per plot was recorded at 4 weeks after planting by counting the number of stands whose shoots have broken through the soil surface. Number of stems per plant: Only the main stems i.e. those originating from the mother tubers were counted. The record was taken at full flowering Plant height (cm): Plant height was obtained by measuring from the base of the plant to the apical bud. The measurement was taken at the 8th week after planting Number of leaves per plant: The compound leaves from the base to the tip of the plants counted at 8 weeks after planting. Days to tuber initiation: This was counted from planting to the time when the first tuber is initiated by any plant in the plot. A plant was considered to have formed tubers when the swelling at the end of the stolon was twice by visual estimates the diameter of its stolon. To observe this, beginning from the 3 WAP, two plants from the boarder rows were carefully removed each week to expose the stolon for observation. Days to maturity: This was obtained by counting the number of days from planting to the time when less than 50% of the canopy remain green. Early blight severity: Early blight severity was recorded at 10 weeks after planting based on a scale developed by Martin and Thurston, (1989). The scale ranges from 1 to 9 with a mid point of 4. 1 = No blight spots seen on the foliage 4 = 25 50% of the foliage infected by blight 9 = foliage completely covered/killed by blight. Incidence of bacterial wilt: The numbers of stands with bacterial wilt symptoms per plot. Number of tubers per plant: This was obtained by counting the whole tubers produced by all plants in the plot and dividing by the number of plants in the plot Average tuber weight per plant: This was obtained by dividing the total weight of the tubers produced in a plot by the number of tubers Tuber yield: The total weight of tubers/plant (g) Data from both years were subjected to combined analysis of variance (ANOVA), simple correlation, partial correlation and regression using the STATVIEW for windows software version 4.5. The genotypes variance was obtained as the mean difference between cultivar and error mean squares. The genetic coefficient of variation and phenotypic coefficient of variation were obtained using the formula of Burton, (1952); GCV = σ 2 g x 100 and σ 2 ph x 100 x x

4 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids Where σ 2 g is the genetic variance, σ 2 ph is the phenotypic variance and x the sample mean. Heritability was obtained by the variance component analysis (Hanson et. al, 1956) and the expected genetic advance (%) was calculated from the formula by Johnson et. al, (1955) as: Gs = K (100 σ g) x (σ g) (X) (σ 2 ph) Where K estimate as 2.06 (for 5% selection in large samples from a normally distributed population) is the selection differential measured in terms of phenotypic standard deviations. RESULTS Plant emergence Plant emergence varied significantly (P<0.05) between the genotypes and their parents ranging from %. Genotypes 1-19, 5-12, 9-26, 17-28, 31-8, and had 100% emergence while genotypes and 20-6 had the least emergence of 55%. The percentage emergence of some of the genotypes is shown in Table 1. Number of stems/plant The mean number of stems per plant of some potato hybrids and their parents are shown in Table 1. The observed differences in stem number where however, not significant between the genotypes (P<0.05, Table 1). The stem number of the parental genotypes ranged from 1.2 stems per plant in BR to 1.95 stems per plant in Plant height The mean plant heights for some hybrid genotypes and their parents are shown in table 1. There were significant variations in height (P<0.05) between the genotypes (Table 1). Number of leaves/plant The mean number of leaves per plant varied significantly (p<0.01, Table 1) between the potato genotypes varying from 105 in genotype to 34 in genotype 5-13 for the hybrids; and from 35 in B to 134 in for the parental genotypes. Days to tuber initiation The mean number of days to tuber initiation varied significantly with the potato genotype (P<0.001, Table 1). Amongst the hybrids, tuber initiation was earliest in genotype 25-2 and 31-8 which initiated tuber at 41 days after planting but did not however, differ significantly from 16 other genotypes. Genotype 26-5 was the last to initiate tubers. Amongst the parental genotypes, days to tuber initiation varied from 39 in BR to 54 in the check variety Desire (Table 1).

5 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids Days to maturity The mean numbers of days to maturity for some genotypes are shown in table 1. Amongst the hybrid genotypes, variation in the mean number of days to maturity, ranged from 83 in four genotypes (4-8, 4-27, 7-2, and 9-16) to 94 days in 16-3 and five other genotypes. Amongst the parental genotypes, days to maturity ranged from 84 in BR to 95 days in Kondor (Table 1). Early blight severity The genotypes varied significantly (P<0.05) in their resistance to early blight (Tables 1). The variation ranged from score 2 to 4. Fifty three genotypes showed very high resistance (score 2) to the early blight pathogen while 23 others had moderate resistance (score 4). Number of wilted plants/m 2 The genotypes did not vary significantly in their number of wilted stands. Most of the genotypes were susceptible to bacterial wilt. Scab There was a significant variability (P<0.01, Table 1) in the severity of infection of the tubers of potato genotypes by scab inciting pathogen. The incidences of scab amongst the genotypes were generally low ranging from none (score 1) to less than 25% infection (score 3). Number of tubers per Plant Potato genotypes differed significantly in their mean number of tubers, (P<0.05, Table 2). Genotype 1-28 produced the highest number of tubers (10) but did not differ significantly from 8 other genotypes in this trait (Table 2). Genotypes 5-2 and 9-15 produced the least number of tubers. Average tuber weight Average tuber weight varied significantly with both the genotypes (P<0.05 Table 2). The mean average tuber weights of some genotypes are shown in tables 2. The average tuber weight of the hybrids ranged from to 15.35g compared with to 32.40g amongst the parental genotypes. Tuber yield Table 2 shows the mean tuber yield of top yielding hybrid and parental genotypes. There were significant differences (P<0.05 Table 2) between the genotypes in mean tuber yield. Five hybrid genotypes namely 30-3, 18-4, and 27-1 gave the highest tuber yields which were significantly higher than the check variety and all the parents (Table 2). The lowest tuber yield /plant of 99.85g were produced by genotype Variance component Estimates of the genotypic, phenotypic and environmental variances for the different attributes of the potato genotypes are shown in table 3. Environmental variances were

6 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids higher than genotypic variances for plant emergence, tuber number and tuber weight. The reverse was the case for plant height, number of leaves, maturity, blight and scab severity, and average tuber weight. Estimates of genotypic and phenotypic coefficients of variability, broadsence heritabiltiy and genetic advance are shown in table 4. Heritabilities for early blight severity and scab were and respectively. Plant emergence, tuber number and weight had very low heritabilities. Inter-relationship between attributes The correlation coefficients for the relationship between different attributes are shown in table 5. Days to maturity, number of tubers, average tuber weight and early blight severity were positively and significantly correlated with yield. Plant height was significantly correlated with leaf number (P<0.01, r=0.172). Number of stems were positively and significantly associated with tuber number (P<0.01, r=0.165). Tuber number and average tuber weight were negatively correlated (P<0.01, r=-0.318). Simple regression and partial correlation coefficients for the relationship between tuber yield and other attributes are shown in Table 6. These coefficients were significant for both tuber number and average tuber weight. DISCUSSIONS Variation amongst potato genotypes for different traits has been reported by various authors (Sidhu and Pandita, 1979; Levy, 1986; and Birhman and Kang, 1993). Highly significant variability in plant attributes within a population suggests the existence of sufficient variability upon which selection for improvement in these attributes can be based. Partitioning the observed variation in metric characters into their genotypic and environmental components showed that variation in plant emergence, number of tubers per plant, and tuber yield were mostly attributable to environmental influences and consequently had low broad sense heritability while the reverse was the case for other attributes. Low heritability values for yield have been reported in African Yam bean by Okoye and Ene-Obong, (1992). This is attributable to the polygenic control of this character hence slow progress is expected when selecting for improvement in this character. However a more rapid improvement by selection is expected for characters such as leafiness, and days to maturity, plant height, number of tubers per plant and resistance to early blight which had high broad sense heritability. Successful emergence and establishment of a crop are of paramount importance to achieve maximum potential yield (Midmore, 1992). Plant emergence was high in majority of the genotypes. This is inspite of the reported negative effect of high soil temperatures on rate of crop emergence and percentage germination (Kishore 1983; Midmore et al 1988; Timm et al 1980). Plant height and number of leaves per plant observed in this study were higher than what was reported for the potato grown under cool mid altitude conditions of the Jos

7 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids Plateau Nigeria (Amadi, et al., 2008). This may be due to the relatively higher ambient temperature of the study location. High temperature favour shoot growth by decreasing the partitioning of assimilates to tubers (Ewing, 1981). Days to tuber initiation and foliage maturity were about one week longer than what was reported for potato grown under cooler conditions by Amadi, (2005). The higher mean temperatures at the study location delayed whole plant maturity by favouring the continued production of new leaves over a longer period. Khedner and Ewing (1985) also reported similar result. Since leaf production and tuber growth draw upon the same raw materials, continued foliage growth after tuber initiation clearly reduces the supply of carbohydrates available for tuber growth and may thus adversely affect yield. The disease reaction of the genotypes showed that they had low disease scores for early blight and scab infections. This suggests either that the genotypes had a moderate to high field resistance to early blight and scab infections or a low population of the innoculum at the study area. The symptoms of bacteria wilt found on most of the genotypes must have been aggravated by supra-optimal ambient temperature which is reported to favour the development of the pathogen Rhalstonia solanocearum (Tung, et al., 1992). Tuber yield were generally low due perhaps to high temperature induced increase in respiration and partitioning of assimilates in favour of the shoot. Six hybrid genotypes had tuber yield which were significantly higher than the highest yielding parent B Five of these out yielded the check variety Desiree significantly. Twelve others genotypes had tuber yields which were higher than but not statistically different from the check variety. The performance of these genotypes based on their superior tuber yield suggest that they have some tolerance to heat stress and may be suitable for cultivation in Toro and perhaps other high temperature locations in the Northern guinea savanna of Nigeria. Recommendation The 17 hybrid genotypes namely 30-3, 18-4, 25-9, 32-4, 27-1, 3-14, 1-28, 22-7, 32-3, 31-8, 5-25, 17-8, 30-15, 18-19, 27-4, 19-3, 23-11, which had tuber yields that compared favourably to the check variety Desiree are recommended for further evaluation as heat tolerance genotypes suitable for cultivation in Toro and perhaps other high temperature locations in the Northern guinea savanna of Nigeria. ACKNOWLEDGEMENT The authors are grateful to the Executive Director National Root Crops Research Institute Umudike for the permission to publish this work and to the technical and field staff of the Potato Programme for their assistance.

8 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids REFERENCES Amadi, C. O. (2005). Evaluation of potato genotypes for adaptation to heat stress. PhD Thesis, Michael Okpara University of Agriculture Umudike. 285pp Amadi, C. O., Ene-obong, E. E., Okonkwo, J. C. and Lenka, D. M. (2008) Preliminary studies on yield and some yield attributes of potato grown under high and low ambient temperature The Nigerian Agricultural Journal 39(1) 65:75 Birhman, R.K. and Kang G. S. (1993). Analysis of variation and inter -relationships in potato germplasm. Euphytica 68:17-26 Borah, M.N. and Milthorpe F.L. (1962) Growth of potato as influenced by temperature. Indian Journal. Plant Physiology, 5:53-72 Burton, W.G. (1952). Studies on the dormancy and sprouting of potatoes 2: The carbon dioxide content of the potato tuber, New phytolologist. 50: Burton, W.G. (1989) The Potato 3rd ed. Longman scientific and technical. England 742pp CIP (1984) Potatoes for the Developing World: A Collaborative Experience. International Potato Center, Lima, 150pp Ewing, E.E. (1981). Heat stress and tuberization stimulus. American Potato Journal, 58:31-49 Hanson, C.H., Robinson, H.F. and Comstock, R.E (1956) Biometrical studies of yield in segregating population of Korean lespediza. Agronomy Journal. 48: Hawkes, J.G. (1978) History of potato. In: P. M. Harris ed. The Potato Crop. The Scientific Basis for Improvement. Chapman and Hall, London, pp Horton, D.E. and Anderson, J.L. (1992) Potato production in the context of the world and farm economy. In: P.M. Harris ed. The Potato Crop: The Scientific Basis for Improvement. Chapman and Hall. London. 909pp Johnson, H. W., Robinson, H.F. and Comstock, R.E. (1955) Genotypic and phenotypic correlations in Soybeans and their Implications in Selection. Agronomy Journal 47: Kheder, M. B. and Ewing, E.E. (1985) Growth analysis of eleven potato cultivars grown in the screen house under long photoperiods with and without heat stress. American Potato Journal, 62: Kishore, H. (1983) Potato for hot dry tropics. In: Research For The Potato In The Year 2000 International Potato Centre, Lima pp Levy, D. (1983) Varietal differences in the response of potato to repeated short periods of water stress in hot climates. 2. Tuber yield and dry matter accumulation and other tuber properties. Potato Research, 26: Levy, D. (1984) Cultivated Solanum tuberosum L. as a source for the selection of

9 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids cultivars adapted to hot climates. Tropical Agriculture (Trinidad), 61: Levy, D. (1986) Genotypic variation in the response of potatoes (Solanum tuberosum L.) to high ambient temperature and water deficit. Field Crop Research. 15:85-96 Lopez, D.F., Boe, A.A., Johansen, R.H. and Jansky, S.H. (1987). Genotype x Environment interactions, correlations and combining ability of six traits in potato (abstract) American Potato Journal, 64:447 Martin, C. and Thurston, H. D. (1989). Factors affecting resistance to Alterneria solani and progress in early blight research at CIP. In: Fungal diseases of the potato. Report of the planning conference on fungal diseases of potato held at CIP, Lima Midmore, D. J. (1984). The Potato (Solanum spp) in the hot tropics 1: soil temperature effects on emergence, plant development and yield. Field crops Research. 8: Midmore, D. J. (1988) Potato (Solanum spp) in the hot tropics VI: plant population effects on soil temperature, plant development and tuber yield. Field Crops Research., 19, Okoye, Florence and Ene-obong, E. E. (1992) Genetic variability and correlation studies in the African Yam Bean -Sphenstylis stenocarpa. Nigerian journal of Botany 5:75-83 Sale, P.J.M. (1974) Productivity of vegetable crops in a region f high solar input III. Carbon balance of potato crops, Australian Journal Plant Physiology 1: Sidhu, A. S. and Pandita, M. L., (1979). Genetic variability and correlation studies in potato (Solanum tuberosum L). Journal Indian Potato Association, 6: Timm, H., Timm, G. H. and Bishop, J. C. (1980) Ethylene production and growth of potato sprouts under simulated high temperature soil condition (abstract) Horticultural Science, 15:383. Tung, P. X., Hermsen, J.G.T., Zaag, P. V, Schmiediche, P. (1992) Effects of heat tolerance on expression of resistance to Pseudomonas solanacearum (E. F Smith) in potato Potato Research (Netherlands).35 (3) : Van der zaag, D. E. and Horton, D. (1983) Potato production and utilization in world perspectives with special reference to the tropics and sub-tropics. Potato Research., 26:

10 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids Table 1: Potato genotypic response to environment with supra-optimal temperature in Toro Nigeria Genotype %Plant Emergenc e 4WAP Number of Stems/ plant Plant height (cm) Number of leaves /plant Days to Tuber Initiation Days to maturity Early blight severity Number of wilted stands/m 2 Hybrids Scab score

11 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids Parents B BR Desiree Famosa Kondor RC VC WC CV% SED

12 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids Table 2: Tuber yield characteristics of some high yielding potato hybrids and their parents in Toro Nigeria Genotype Pedigree Number Tuber /plant Average tuber weight/plant Tuber yield / plant(g) 30-3 VC785-2 x WC VC785-2 x BR63-18 x Kondor x VC Famosa x Kondor Famosa x BR Kondor x BR RC x x VC BR63-18 x WC B x BR x VC785-2 x WC VC785-2 x / x Kondor Famosa x BR63-18 x Desiree Urgenta x Depesche B X927-3 x ACC VC785-2 B x A RC A-1 x B BR63-18 Atzimba x A-1USDA Monalisa x XY (BR63.74 x.xy.9) WC732-1 Greta x P Kondor Famosa (BR63.74 x.xy.9).45 x CV% SED

13 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids Table 3: Potato genetic variability estimates in Toro Nigeria Attributes Genotypic variance Phenotypic variance Environmental variance Plant emergence(4wap) *Number of stems/plant Plant height(cm) Number of leaves/plant Days to tuber initiation Days to maturity Early blight severity *Number of wilted plants/m Scab score Number of tubers/plant Average tuber wt/plant Tuber yield/plant * Variation between the potato genotypes in number of stems/plant and number of wilted stands/m 2 were not significant. Table 4: Genetic coefficients of variability and heritability estimates in potato hybrids evaluated in Toro Nigeria Attributes Genotypic Coefficient of variability (%) Phenotypic Coefficient variability (%) PCV - GCV Broadsence Heritability (%) Expected Genetic Advance (%x) Plant emergence(4wap) *Number of stems/plant Plant height(cm) Number of leaves/plant Days to tuber initiation Days to maturity Early blight severity *Number of wilted plants/m Scab score Number of tubers/plant Average tuber wt/plant Tuber yield/plant * Variation between the potato genotypes in number of stems/plant and number of wilted stands/m 2 were not significant.

14 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids Table 5: Simple correlation between attributes of potato hybrids and their parents evaluated in Toro Nigeria Attributes Plant emergence (4wap) Number of stems/plant Number of stems /plant Plant height (cm) Number of leaves /plant Days to tuber initiation Days to maturity Early blight severity Number of wilted stands/m 2 Scab score Number of tubers / plant Average tuber wt/plant plant height (cm) Number of leaves/plant ** Days to tuber initiation ** Days to Maturity ** 0.637** Early blight severity * 0.121* Number of wilted stands/ m Scab Score * Number of tubers / plant ** Average tuber weight/plant * 0.202** 0.129* ** Tuber yield /plant * 0.147** 0.123* ** 0.632**, ** = Correlation significant at P< 0.01, * = Correlation significant at P< 0.05

15 PAT 2008; 4 (2): ISSN: ; Amadi, C. O et al.,; Evaluation Of Some Potato Hybrids Table 6: Simple regression and partial correlation coefficients between tuber yield and some attributes of potato hybrids Attributes Simple regression Coefficients Partial correlation Coefficients Plant emergence(4wap) ns Number of stems/plant ns.0094 plant height(cm) ns.0318 Number of leaves/plant ns.0540 Days to maturity 2.257*.0791 Early blight severity 8.268*.0786 Number of wilted stands/ m ns Scab score 5.165* Number of tubers / plant ***.9536*** Average tuber weight/plant 3.592***.9484*** *** = Coefficient significant at P<0.001 ** = Coefficient significant at P<0.01 * = Coefficient Significant at P<0.05 ns = Not significant Fig1: Temperature and rainfall data of Toro from June to Sept and 2003 Temp and rainfall values Jun- 02 Jul- 02 Aug- 02 Sep- 02 Jun- 03 Jul- 03 Aug- 03 Sep- 03 max. temp. min. temp. rainfall Months of the year Fig. 1. Temperature and rainfall data of Toro from June to Sept and 2003