Abstract: Three levels of nitrogen (0, 225 and 300 kg N ha-1) and 4 levels of potassium (0, 125, 175 and 225 kg K ha-1) fertilizers were applied to potato mother plants (MF-II) for the production of high quality True Potato Seed (TPS). The effect of N and K application on the quality of TPS was evaluated by nutritional analysis, germination tests in Petri dishes and growth performance in nursery beds. Increase in N application significantly increased N, P, Ca, Mg and Na concentrations in TPS but K did not increase. Increase in K application significantly increased N, P and K concentrations, while decreases in Ca, Mg and Na concentrations in TPS were recorded. TPS that were harvested under different NxK combinations were classified into large-(>1.18 mm) and small sizes-(1.18 to 1.00 mm). Among them, large TPS showed the highest germination rate (97.7%) 12 days after sowing when 300 kg N and 125 kg K ha-1 was applied. The coefficient of velocity of germination in large TPS was highest (10.4) when 300 kg N and 125 kg K ha-1 were applied. Large TPS also showed high emergence rate (94%), seedling vigor (4.8) and dry matter content (10.5%) in nursery beds when 300 kg N and 125 kg K ha-1 was applied. Large TPS always showed better performance than small TPS. In conclusion, the combination of 300 kg N and 125 kg K ha-1 was the best combination for application to potato mother plants for the production of high quality TPS.
INTRODUCTION
In a previous report (Roy et al., 2007b), we grew potato mother plants (MF-II) with different levels of Nitrogen (N) and Potassium (K) fertilizers (twelve combinations in total) to obtain hybridized True Potato Seed (TPS) with TPS-67 (♂) and investigated the flowering habits of the mother plants and yield components of harvested TPS. The results clearly showed that the combination of 225 kg N and 225 kg K ha-1 (hereafter described as N225 K225), N300 K125 and N225 K125 resulted in the highest yield of berries plant-1 (207.2 g plant-1), highest 100-TPS weight (83.8 mg) and highest TPS yield (145.3 kg ha-1), respectively. By considering all the above described results together, we concluded that N300 K125 was the best combination for the commercial production of TPS that were obtained from the hybrid of MF-IIxTPS-67 when the production was focused on the yield and size of TPS.
However, for the production of TPS, seed quality, which is commonly evaluated as high germination rate, uniform germination property and vigorous seedling growth after germination, is also a crucial element (Upadhya et al., 2003; Islam et al., 2000; Pallais, 1987; Dickson, 1980; McDonald, 1980).
Kanzikwera et al. (2000) demonstrated that the combination of N240 K265.6 resulted in low germination rate and less seedling vigor compared with N240 K132.8. Thus, the quality of TPS also varies significantly according to the different combinations of N and K fertilizers applied to the mother plants (Roy et al., 2007b; Pallais et al., 1984), mainly due to the different nutrient levels that are contained in the harvested TPS.
Wheat seeds containing high N germinate faster and develop into larger seedlings than those containing normal N (Lopez and Grabe, 1973). In lettuce, a linear relationship was found between N concentration in the seed and seedling vigor after germination (Soffer and Smith, 1974). On the other hand, a negative correlation was found between K concentration and germination rate in primrose (Primula vulgaris H.) seed (Zerche, 2005).
In addition to the importance of nutrient levels in TPS, the size of TPS also affects germination and subsequent seedling growth. Almekinders and Wiersema (1991) found that large TPS showed better germination and earlier emergence than small TPS. When small TPS take a long time to emerge after sowing, subsequent crop production will be greatly diminished by soil related problems such as fast-growing weeds and pathogens.
The objective of this study was to assess the effect of N and K application to potato mother plants (MF-II) on the growth performance of harvested TPS.
MATERIALS AND METHODS
Planting materials and cultivation method: Seed tubers of two desired parental lines (MF-II and TPS-67 as female and male lines, respectively) were grown during 2005 to 2006 in the field of the Tuber Crops Research Center, Bangladesh to make hybridized TPS. Farm yard manure which contains approximately 1.0 to 1.2% N was applied (10 t ha-1) in the experimental plots one week before (14 October 2005) the planting of the seed tubers. On 18 October, chemical fertilizers which contain P-Gypsum-Zinc sulphate-Borax at 120-120-12-6 kg ha-1 were applied in the experimental plots. Effect of NxK fertilizers on the quality of TPS was investigated by applying 12 different NxK combinations of 3 levels of nitrogen (N0, N225 and N300 kg ha-1 as total amount of application) and 4 levels of potassium (K0, K125, K175 and K225 kg ha-1 as total amount of application) for the production of the female plants. The entire quantity of K (K0-225) was applied 3 days before planting of the seed tubers (18 October) because it is slowly absorbed into the plants, while only one-third of entire quantity of N (0, 75 and 100 kg ha-1 for N0, N225 and N300) was applied on 18 October as a basal dressing, irrespective of total amount of NxK application. On 21 October 2005, seed tubers were planted in a split-plot design with 3 replicates. The rest amount of N fertilizer was equally applied in 4 installments (0, 37.5 and 50 kg ha-1 for N0, N225 and N300) at 10 day intervals starting from 30 Days after Planting (DAP) (21 November, 1, 11 and 21 December). Splitting of N application in 4 equal amounts helped in reducing the loss of excessive N fertilizer applied at planting and better utilization of the fertilizer particularly during rapid growth of plants, flowering, berry setting and development of seeds (Phillips et al., 2004; Pallais et al., 1987). The procedure of fertilizer’s application was the same as described in our previous reports (Roy et al., 2007a, b). For the production of male plants, seed tubers were planted in separate plots on 14 November. A standard commercial amount of NxK combination (N150 K125) was applied as basal dressing (Roy et al., 2005). On 30 November (39 DAP), flowering of the female plants was started and they were hand pollinated with the previously collected pollen grains of the male plants. The crossing between the two parental lines and the subsequent production of TPS were the same as described previously (Roy et al., 2007a). Berries were harvested during 12 to 17 January 2006 (42 to 47 days after pollination). TPS that were produced from the 12 different fertilizer combinations were stored in a desiccator for approximately 5 months at room temperature.
Determination of N level in TPS: Dried TPS samples were homogenized in a mortar and approximately 0.5 g powder was transferred to a 100 mL flask. Approximately 1.1 g digestion mixture (1.0 g Na2SO4 + 100 mg mercuric oxide) and 5 mL concentrated H2SO4 were added to the sample and then heated to 160°C for 3 to 4 h in a digestion chamber until the sample was digested completely. The flask was then cooled in tap water and brought up to 100 mL with distilled water. A 10 mL sample of the solution was put into a Micro-Kjeldahl distillation tube with 5 mL 50% NaOH and 2.5 mL 15% Na2S2O3 and the solution was distilled with steam for 10 min. A 15 mL sample of the distillate was decanted into a test tube with 25 mL of 2% boric acid solution, 4 to 6 drops of 0.2% methyl red and methylene blue as an indicator. The solution was then titrated against 0.02% N HCl. In each analysis, water was also digested and distilled as a blank. The concentration of N in TPS was determined by the method of Ma and Zuazaga (1942).
Determination of P, K, Ca, Mg and Na: TPS samples (approximately 50 mg) from each treatment were digested with 2 mL 16 N HNO3 overnight at room temperature. Two milliliters of 60% HClO4 was added and then the solution was successively heated to 80°C for 30 min, 120°C for 30 min and 160°C for 90 min. The digested solutions were then cooled to room temperature, filtered through a glass fiber filter (25 μm in diameter) and brought up to 25 mL with distilled water. Phosphorus, K, Ca, Mg and Na concentrations in each sample were determined using an Inductively Coupled Plasma (ICP) spectrophotometer (model Liberty 220, Varian, Victoria, Australia).
Seed quality tests in vitro: TPS that were produced from each combination of NxK were classified into large- (>1.18 mm) or small- (1.00 to 1.18 mm) seed. TPS were then soaked in 0.15% Gibberellic Acid (GA3) solution at room temperature for 24 h, one hundred seeds per each size class were sown in Petri dishes (15 cm diameter) equipped with wet filter paper (Whatman No. 1, Middlesex, UK) and then incubated at 20°C for 12 days. The seeds were watered as necessary. The germination rate of TPS was recorded at 6, 9 and 12 Days After Sowing (DAS). Coefficient of velocity (CoV) of germination was also calculated as follows (Scott and Williams, 1984):
CoV = 100[ΣNi/ΣNi
Ti] |
| N | = | No. of seeds germinated on day |
| i and T | = | No. of days after sowing |
The experiment was conducted in a completely randomized design with 3 replicates.
Seed quality tests in vivo: Seed quality in vivo was determined according to the growth performance of the seedling. The experiment was conducted following a split-plot design with 3 replicates in a nursery bed of Sher-e-Bangla Agricultural University during 2006-2007. Soil substrates of the nursery bed were soil, sand and farmyard manure (1:1:1 v/v). The unit plot size was 1 m2. TPS that were harvested under different NxK combinations were sown and grown as described previously (Roy et al., 2005) and the percentage of emerged shoots at 10, 15 and 20 DAS and CoV were recorded.
Seedling vigor was visually evaluated at 21 DAS using five staged scales i.e., 1 = small seedlings with stunted growth, 2 = moderate growth but visually stunted, 3 = good growth, 4 = vigorous, tall plants with green foliage and 5 = transplantable seeding with vigorous, green, strong stem. At 30 DAS, the seedlings were cut at the soil surface, weighed, oven-dried at 70°C for 72 h and re-weighed.
Statistical analysis: Analysis of variance was performed according to MSTAT-C statistical software (MSTAT-C, 1991). Means were compared using the Least Significant Differences (LSDs) test at a 5% probability level.
RESULTS
Nutrient concentration in TPS as affected by different combination of N and K fertilizers: Nitrogen, P, K, Ca, Mg and Na concentrations were significantly (p≤0.01) influenced by different levels of N application. All ions except K showed the highest level when N300 was applied (Table 1). On the other hand, N, P and Mg concentrations decreased with increasing K application.
The combined effect of NxK on the nutrient concentration was also significant, except for Ca and Mg (Table 1, 2). The highest values of N, P, K and Na concentrations were obtained when N300 K125, N225 K125, N225 K175 and N225 K0 (45.4, 11.4, 8.0 and 2.1 mg g-1 dry weight) were applied, respectively.
Effect of NxK and TPS size on germination rate and CoV of germination: When N was not applied (N0), germination of both large and small TPS was strongly inhibited, irrespective of K application level throughout the experimental period (Table 3). However, germination rate of TPS changed largely by different K applications even if N was applied together with K; i.e., large TPS showed the highest germination rate (43.0-45.3%) at 6 DAS when N225-300 K125 was applied, but were largely inhibited (8.6-9.0%) when K225 was applied (Table 3). Among the small TPS samples the highest germination rate (23.7-18.3%) at 6 DAS was when N225-300 K125 was applied, but the rate was less than half of the rate of large TPS and this lower germination rate of small TPS did not recover even at 12 DAS (Table 3).
The combination of N225-300 K125 also resulted in the highest CoV of germination (10.3-10.4), irrespective of TPS size (Table 3).
Performance of TPS in nursery beds
Percentage of emerged seedlings: The percentage of emerged seedlings in
the nursery beds was significantly influenced by the NxK combination and TPS
size (Table 4).
| Table 1: | Single effect of different levels of nitrogen (N) or potassium (K) fertilizers supplied to the potato mother plant on nutrient concentrations in the harvested TPS |
| N0 to N300 and K0 to K225 indicate that nitrogen and potassium were applied at 0 to 300 kg ha-1 and 0 to 225 kg ha-1, respectively. *,**: Significant at p≤0.05 and 0.01, respectively. Different letter(s) within each column indicate significant differences by LSD test at p≤0.05. z ns: Non-significant | |
| Table 2: | Combined effect of different levels of NxK fertilizer supplied to the potato mother plants on N, P, K, Ca, Mg and Na concentrations in the harvested TPS |
| z: LSD (p≤0.05) for comparing means in columns, y: ns = Non-significant | |
The rate decreased with increasing K application levels, irrespective of N level or TPS size. Large TPS emerged at rates higher than 90% when N225 K125 or N300 K125 was applied (Table 4).
CoV of emergence: Coefficient of velocity of emergence was significantly influenced by the NxK combination and TPS size. High CoV of emergence was found in large TPS when N300 K125, N300 K175, or N225 K125 was applied (6.73, 6.65 and 6.64, respectively) (Table 4).
Seedling vigor: Seedling vigor was also significantly influenced by the NxK combination and TPS size. Among the 12 treatment combinations, large seeds showed the best performance (4.8) compared with all other treatments (less than 4.3) when N300 K125 was applied (Table 4).
Seedling weight: Fresh and dry weight of the seedlings was also significantly influenced by the NxK combination and TPS size. Large seeds resulted in the maximum fresh and dry seedling weight when N300 K125, N300 K175, or N225 K125 were applied (6.4 and 0.68 g, 6.23 and 0.64 g and 6.13 and 0.63 g, respectively) (Table 4).
The highest percentage of dry matter was found in seedlings grown from large
seeds when N300 K125 was applied; whereas, the lowest
was found in seedlings grown from small seeds when N0 K225
was applied (10.5 and 7.8%, respectively) (Table 4).
| Table 3: | Combined effect of different levels of NxK fertilizer supplied to the potato mother plants on the percentage of germination at 6, 9 and 12 DAS and CoV of germination of harvested TPS sown in Petri dishes |
| z: TPS were classified into large and small sizes (>1.18 mm and 1.00-1.18 mm, respectively). y: LSD (p≤0.05) for comparing means in columns | |
| Table 4: | Combined effect of different levels of NxK fertilizer supplied to the potato mother plants on the percentage of emerged seedlings at 10, 15 and 20 DAS, CoV of emergence, seedling vigor, seedling fresh wt., seedling dry wt. and percentage of dry matter of harvested TPS grown in nursery beds |
| z: TPS were classified into large and small sizes (>1.18 mm and 1.00-1.18 mm, respectively), y: LSD (p = 0.05) for comparing means in columns | |
DISCUSSION
The concentration of nutrients in TPS was influenced by different levels of N and K fertilizers supplied to the mother plants. Nitrogen level in TPS increased as the N fertilizer level increased and just reverse when K fertilizer level increased. Such a negative interaction between N and K application has also been reported in other seed potato plants (Bester and Maree, 1990).
Different levels of N and K fertilizers supplied to the mother plants also affected the accumulation of other cations in TPS. Potassium application at a high (225 kg ha-1) level resulted in decreases in P, Mg and Na concentrations in TPS, while N application resulted in increases the same (Table 1). Similar negative interactions between N and K fertilizers on cation concentrations in the harvested seeds were also reported in corn (Karien et al., 1987) and wheat (Karien and Whitney, 1980). The negative influence of K application on the accumulation of other cations such as Mg and Na (Table 1) also suggests K-Mg and K-Na antagonism as reported by Marschner (1990). A positive correlation between N application and P and Ca concentrations in TPS was found (Table 1) as reported by Karien and Whitney (1980) in wheat. In our experiment, TPS with high N, P and Ca concentrations also showed the highest emergence rate and seedling vigor (Table 4) as reported in tobacco (Thomas and Raper, 1979).
Seed quality, which is shown as high germination rate and high seedling vigor, often correlates with the protein concentration in the seeds (Bhatt et al., 1989). These authors further observed that the TPS with high protein concentration germinated faster and developed into larger seedlings. Although the protein concentration in TPS was not measured in our experiment, the protein concentration in seeds correlates well with nitrogen concentration and it can be roughly calculated by multiplying the percentage of N by 6.25 (Ma and Zuazaga, 1942). In this aspect, TPS with a high N concentration in our experiment also contained high amounts of protein and showed better seedling performance (Table 4). On the contrary, high rates of K application reduced N concentration in TPS (Table 1) suggesting that excess K application may inhibit protein synthesis in TPS through interference with N uptake and reduced the growth performance after germination as Karien et al. (1987) reported in corn.
Although the germination rate of TPS in vitro was always higher than the emergence rate of the seedling in vivo as Gallagher and Nabi (1984) reported previously in TPS, the combination effect of NxK on the germination and emergence rates and their CoV were apparent both in vitro and in vivo. The highest emergence rate of TPS was always obtained when N300 K125 or N225 K125 were applied (data not presented), while the rate became lower as K application increased. Kanzikwera et al. (2001, 2000) also found a similar effect of N and K combination on the emergence rate of TPS.
A high CoV indicates that more seeds germinated or more seedlings emerged over a shorter time (Scott and Williams, 1984). In our experiment, both CoV of germination and emergence were always higher as N application increased, while they became lower as K application increased, indicating that N is a key factor for obtaining high CoV. In brinjal and soybean seeds, nitrogen application higher than those required levels for optimal crop production has been recognized to improve the seedling vigor (Naik et al., 1996; Van et al., 1982; Gray and Thomas, 1982; Delouche, 1980). In tomato and tobacco, increased N application to the mother plants increased the seed germination rate and enhanced germination uniformity (Seno et al., 1987; Thomas and Raper, 1979).
In this study, large TPS always performed better in all aspects than small TPS (Table 3, 4). Bhatt et al. (1989) reported that TPS size was associated with the quality because large seeds contained higher level of protein than small ones and they germinated faster and had the highest percentage of germination. In our experiment, large TPS that were produced under N300 K125 showed faster emergence and better seedling growth (Table 4). Lopez and Grabe (1973) also reported that the application of high N to wheat plants increased N concentration in the seeds. In addition, seeds with high N concentration germinated faster and developed into larger seedlings with higher dry matter content.
Although the percentage of emergence of TPS at 20 DAS differed little in both large and small TPS especially when N225 K125, N225 K175 or N300 K125 were applied, plant size at 30 DAS was significantly greater in large TPS than in small TPS (Table 4), because small TPS with low N concentration emerged slower and also performed less well in terms of seedling vigor than large TPS as reported by Malagamba (1988) in TPS. Baki (1980) also showed that N in soybean seed was important during seedling growth.
As a conclusion, present results show that application of N225-300 K125-175 to potato mother plants could improve TPS harvest quality when the quality was evaluated as growth performance both in vitro and in vivo. The range of this NxK combination was within the range of the best combination (N225 K125) for obtaining the highest TPS yield from the crossing of MF-II and TPS-67 (Roy et al., 2007b). However, less seedling vigor under N225 K125 as compared with N300 K125 (Table 4) suggests that the application of N at 225 kg ha-1 will be too low for obtaining high quality TPS. This is also confirmed from the view point of harvesting large TPS, where the application of N at 300 kg ha-1 was required (Roy et al., 2007b). Therefore, we conclude that 300 kg N and 125 kg K ha-1 is the best combination for obtaining not only large TPS (Roy et al., 2007b), but also preferable growth performance of the seedlings.
ACKNOWLEDGMENT
This study was supported by Ronpaku program of JSPS (Japan Society for the Promotion of Science).