Abstract: An investigation was conducted in Kuru (Lat. 09044’N; Long. 08047’E and Alt. 1350 m asl.) on the Jos-Plateau, Nigeria to study the flowering and fruiting behavior of Livingstone potato (Plectranthus esculentus). The treatment consisted of three local cultivars namely Ryiom, Longat and Bebot laid out in a Randomized Complete Block Design (RCBD) with four replications. Results obtained indicated that Livingstone potato cultivars produced flowers from 158-215 Days after Planting (DAP). The number of branches, nodes, flowers and inflorescences per plant were significantly higher in Longat (p<0.05) than in Ryiom, while cultivar Bebot maintained an intermediary position in most parameters tested. Flowers dried up without fruiting or seed-setting in all cultivars probably because of the interplay of genetic and environmental factors. Determination of these factors and how to overcome them should form the focus of subsequent research.
INTRODUCTION
Living stone potato (Plectranthus esculentus) is a dicotyledonous perennial shrub belonging to the family Lamiaceae. It is known locally as Rizga, Mvat, Vu, Sima or Rungwabi and is popular among the middle belt region of Nigeria (Schippers, 2000; Olojede et al., 2005). The crop is extremely hardy and easily grown in regions without frost (Van Wyk, 2005). The Livingstone potato is indigenous to southern tropical Africa. It was also first cultivated in the upper Niger valley of the Hausa land in Nigeria and in the Central African Republic (Kyesmu, 1994). Although it is seen as a lost crop of Africa and it was cultivated since pre-historic times, it is a crop that is still relevant today (Kyesmu, 1994).
Tuber yield per hectare of locally available cultivars is low hence there is need for a genetic yield improvement. Some of the tubers that are cultivated can weigh up to1.8 kg (Nazmul, 2003). Yields of 2-6 t ha–1 have been seen before as well as yields of 70 t ha–1 were documented when the leaves could be harvested (Nazmul, 2003). Although some cultivars flower from 158-215 DAP, there are cultivars that can be harvested 120-200 DAP.
The ability of a cultivar to flower and set fruit determines the extent to which it can be improved upon by conventional breeding methods. Variations in the mean number of stems, branches, nodes and flowers depend on the length of time in which new stems, branches, nodes and flowers are produced as well as the length of life of individual stems, branches, nodes and flowers (Rieger and Sedgley, 1996).
Cultivars with prolonged vegetative phase tend to have a low root-top ratio since most of the assimilates produced will be used in stem, leaf and branch growth instead of tuber growth or flower production. This indicates a negative relationship between the vegetative phase and the reproductive phase (Siddique et al., 1988).
Roberts et al. (1996) noted that there is a critical photoperiod (Pc) that when it is exceeded, flowering is delayed. The delay increases with photoperiod until a ceiling period (Pce) is reached. In photoperiods longer than the ceiling, there is no response to either the number of days to onset of flowering or the rate of flowering. Flowering studies therefore provides the basic information needed by plant breeders for the improvement of the crop. Livingstone potato has been reported to flower and set fruit and viable seeds in South Africa (Schippers, 2000). Preliminary studies in Jos-Plateau showed that it produced flowers but failed to set seeds.
This experiment was set out, therefore, to investigate the flowering and fruiting behavior of the livingstone potato with a view to exploring and understanding the fertility problems in the crop in order to develop procedures to overcome the barriers.
MATERIALS AND METHODS
The experiment was carried out in the wet season of 2014 at the Kuru sub-station (Lat. 09°44’N; Long. 08°47’E and Alt. 1350 m asl.) of the National Root Crops Research Institute (NRCRI) Umudike, Nigeria. Three cultivars of Livingstone potato were used namely Ryiom, Longat and Bebot. These three cultivars were laid out in a Randomized Complete Block Design (RCBD) with four replications. The net and gross plot sizes were 6 and 12 m2, respectively. The plant density was 33,333 plants per hectare arrived at by spacing plants 30 cm within ridge and 1 m between ridges to allow for a full expression of the flowering and fruit setting potential of each plant. The plots were first weeded manually at 40 Days After Planting (40 DAP) and thereafter weeded regularly to keep them weed free. At 41DAP the plots received a blanket application of fertilizer NPK (15:15:15) at the rate of 60 kg ha–1 each of nitrogen, phosphorus and potassium, which was equivalent to 360 g plot–1. Readings were taken at 7 day interval.
Field observations: Field observations began at 30 DAP and continued until 215 DAP. The total number of stems from ten randomly sampled plants was divided by ten to obtain the mean number of stems per plant.
The total number of branches produced by ten randomly sampled plants was divided by ten to obtain the mean number of branches per plant.
Ten plants were sampled from each plot and the total number of nodes was divided by ten to get the mean number of nodes per plant.
The mean number of days to onset of flowering was recorded as the number of days after planting to when the first flower emerged from each plot.
The total number of flowers produced by ten randomly sampled plants was divided by ten to obtain the mean number of flowers per plant.
The total number of inflorescences from ten sampled plants was divided by ten to obtain the mean number of inflorescence per plant.
The total number of fruits produced by ten sampled plants divided by ten gave the mean number of fruits per plant.
Data analysis: Data collected were subjected to analysis of variance (ANOVA) test and the means were compared using the Duncan’s new Multiple-Range test.
RESULTS
The highest mean number of stems per plant was observed in the cultivar Ryiom while cultivar Bebot recorded the least and the difference is significant (p<0.05) but there was no significant difference (p>0.05) between Longat and Bebot (Table 1). The highest mean number of branches and nodes was observed in cultivar Longat while Ryiom had the least in that order (Table 2 and 3), with Bebot occupying the middle position. The mean number of days to the onset of flowering showed significant difference (p<0.05) and was earliest and latest in Longat and Ryiom respectively, with Bebot taking the middle position (Table 4). Table 5 shows the mean number of flowers per plant where Longat had the highest (34), followed by Bebot (24) and lastly Ryiom (4) with a significant difference (p<0.05) between the cultivars. The mean number of inflorescences is shown in Table 6 where Longat had the highest while Ryiom had the lowest and the difference is significant (p<0.05). None of the cultivars produced fruits (Table 6).
| Table 1: | Mean number of stems per plant in livingstone potato in Jos-Plateau |
| Means followed by the same letter (s) are not significantly different at 5% level of probability (Duncan’s new multiple-range test) | |
| Table 2: | Mean number of branches per plant in livingstone potato in Jos-Plateau |
| Means followed by the same letter (s) are not significantly different at 5% level of probability (Duncan’s new multiple-range test) | |
| Table 3: | Mean number of nodes per plant in livingstone potato in Jos-Plateau |
| Means followed by the same letter (s) are not significantly different at 5% level of probability (Duncan’s new multiple-range test) | |
| Table 4: | Mean number of days to onset of flowering in livingstone potato in Jos-Plateau |
| Means followed by the same letter (s) are not significantly different at 5% level of probability (Duncan’s new multiple-range test) | |
| Table 5: | Mean number of flowers in livingstone potato in Jos-Plateau |
| Means followed by the same letter (s) are not significantly different at 5% level of probability (Duncan’s new multiple-range test) | |
| Table 6: | Mean number of inflorescence and fruits per plant in livingstone potato in Jos-Plateau |
| Means followed by the same letter (s) are not significantly different at 5% level of probability (Duncan’s new multiple-range test) | |
In this study, cultivar Bebot maintained an intermediary position in almost all the parameters tested, if this is a vantage position indicating genetic and environmental stability remains to be seen when it is subjected to further investigation and research.
In this study, the cultivar Longat had the highest mean number of branches, nodes, flowers and inflorescences per plant while cultivar Ryiom had the least in those parameters, with cultivar Bebot occupying the middle position in that regard. None of the cultivars produced fruits (Table 6).
DISCUSSION
The effect of genotype on the number of stems, branches and nodes of Livingstone potato has shown that cultivar Ryiom which had the highest number of stems also had the lowest number of branches and nodes. Conversely, Longat which had the lowest number of stems turned out with the highest number of branches and nodes. A similar observation was reported in the cut flower species Banksia coccinea and Banksia hookeriana (Rieger and Sedgley, 1996). This could imply that there is a negative correlation between the number of stems and number of branches per plant on the one hand and between stems and nodes on the other hand between cultivars Ryiom and Longat. Negative correlation had been reported by Siddique et al. (1988) on the relationship between vegetative phase and reproductive phase. The effect of genotype on days to onset of flowering indicated that cultivar Ryiom took the longest time to produce the first flower possibly because its vegetative phase is dominant over its reproductive phase when compared with the other two cultivars (Van Wyk, 2005). The delay in the production of flowers as observed in the cultivar Ryiom could be attributed to poor sensitivity to photoperiod in which the cultivar took a longer time to transit from the vegetative phase to reproductive phase (Chung and Myeong, 1996; Roberts et al., 1996). At the time it produced its first flower (213DAP) it was already two months into the dry season during which time most of the leaves have senesced and dropped off. The cultivar Longat had the highest number of flowers and inflorescences while cultivar Ryiom maintained the least in that regard and the difference was significant (p<0.05). This had been noted earlier by the report of Rieger and Sedgley (1996). Schippers (2000) had reported that in South Africa, the crop produces its yellow flowers before the rain starts after which the leaves appear, whereas in Cameroun it does not produce flowers until the end of the rainy season. Allemann and Hammes (2006) had reported on the crop’s ideal pH of 6.5-7, annual rainfall of 700-1100 mm and photoperiod of 12.5 and 13 h which also agrees with the work of Van Wyk (2005). Flowers dried up without fruiting or setting seed in all the genotypes probably because of interplay of genetic and environmental factors which contradicts the findings of Schippers (2000), who had reported that the crop flowered and set fruit and viable seeds in South Africa. The ability of flowers to set fruit and viable seeds depends on the length of life of the flowers (Rieger and Sedgley, 1996).
CONCLUSION
In this study, the cultivar Longat had the highest mean number of branches, nodes, flowers and inflorescences making it the most preferred while cultivar Ryiom had the least in these parameters mentioned above making it the least preferred.
The livingstone potato flowered but failed to produce fruit and seeds under the normal growing season in Kuru. The determination of these genetic and environmental factors and how to overcome them should be the focus of subsequent research.
ACKNOWLEDGMENT
The authors are grateful to the management and staff of the National Root Crops Research Institute (NRCRI), Kuru sub-station for the logistic and technical assistance during the field work.