American Journal of Agriculture and Forestry
Volume 3, Issue 5, September 2015, Pages: 192-201

Comparative Effects of Foliar Application of Gibberellic Acid and Benzylaminopurine on Seed Potato Tuber Sprouting and Yield of Resultant Plants

Martin Kagiki Njogu1, Geofrey Kingori Gathungu1, *, Peter Muchiri Daniel2, 3

1Department of Plant Science, Chuka University, Chuka, Kenya

2Department of Plant Science and Crop Protection, University of Nairobi, Kangemi, Kenya

3MoA, Wambugu Agriculture Training Centre, Nyeri, Kenya

Email address:

(G. K. Gathungu)

To cite this article:

Martin Kagiki Njogu, Geofrey Kingori Gathungu, Peter Muchiri Daniel. Comparative Effects of Foliar Application of Gibberellic Acid and Benzylaminopurine on Seed Potato Tuber Sprouting and Yield of Resultant Plants.American Journal of Agriculture and Forestry.Vol.3, No. 5, 2015, pp.
192
-201.doi:10.11648/j.ajaf.20150305.14


Abstract: Seed potato tubers planted immediately after harvest is characterized by delayed plant emergence, poor establishment and low yields. Gibberellic acid (GA) and Benzylaminopurine (BA) or their combinations cause dormancy breakage though little information is available on their combined application to dormancy termination. The effects of foliar application of GA and BA on potato tuber sprouting and subsequent yield were studied. Three potato varieties with different tuber dormancy period; ‘Asante’ (short dormancy), Dutch Robyjn (medium dormancy) and ‘Kenya Sifa’ (long dormancy) were planted at National Potato Research Centre, Tigoni and sprayed with a factorial combinations of 0, 50, 100, 300 ppm GA and 0, 50, 75, 100 ppm BA separately and combined at the rate of 1000 lts/ha spray volume towards the end of maturation. The resulting tubers were put in diffuse light storage (DLS) and data on number, length and vigour of sprouts recorded. Sprouted seed tubers were subsequently planted and evaluated for both growth characteristics and yields. The data collected was subjected to analysis of variance and significantly different means were separated using Fishers protected least significant difference at p≤0.05. Higher rates of foliar application of GA+BA (300 ppm + 100 ppm) compared with the control (0 + 0) resulted in significant increase insprout length (cm), number of sprout/tuber, sprout vigour (score), and % sprouting from 3.24 to 7.02 and 3.84 to 9.03, 2.04 to 4.45 and 2.07 to 4.8, 1.7 to 3.06 and 1.63 to 3.23, 61.21to 86.67and 63.3 to 83.7 in Asante, 2.94 to 8.03 and 2.8 to 7.99, 1.84 to 5.24 and 1.87 to 4.76, 1.3 to 3.0 and 1.27 to 2.63, 50.61 to 92.7 and 52.7 to 85.7in Dutch Robyjn and 0.79 to 6.43 and 1.32 to 6.99, 0.61 to 3.49 and 0.79 to 3.33, 0.61 to 3.03 and 0.73 to 2.83, 22.12 to 85.76 and 28.3 to83.7 in Kenya Sifa after storage in 2008 and 2009 respectively. A combination of BA and GA resulted in significantly more growth than using only GA or BA alone at the same level. Similarly the subsequent tuber numbers per plant and yield (tons/ha) in resultant plants increased from 7.13 to 12.53 and 24.66 to 32.27, 6.93 to 10.47 and 16.73 to 23.37, and 5.63 to 9.6 and 17.53 to 30.13 in Asante, Dutch Robyjnand Kenya Sifa respectively. Combined application of GA + BA at varied rates can be used to improve sprouting characteristics of seed potato and yield of resultant plants.

Keywords: Potato, Giberrelic Acid, Benzylaminopurine, Seed Sprouting, Resultant Plants, Yield

1. Introduction

Potato tuber buds normally remain dormant throughout the growing season until several weeks after harvest [1] and the conditions that influence tuber formation and growth in potatoes may influence the duration of dormancy [2].Plant internal factors including plant hormones such as gibberellic acid, auxins, ethylene, cytokinins and abscissic acid have been known to affect potato sprouting [3]. There have been a number of studies of the effects of exogenous plant growth regulators (PGRs) on dormancy in potato where they have been applied to leaves shortly before harvest [4] or to whole tubers at harvest or during storage [5,6,7].The most consistent effects have been observed with gibberellins [8,9].

A foliar spray of gibberellic acid, 3–6 days before haulm killing shorten potato tuber dormancy period and induced sprouting [10,11]. When GA3 was applied to the foliage of potato plants grown from true seeds towards the end of the vegetative cycle (60 days after transplantation), it induced rapid breakage of tuber dormancy, a reduction in specific weight, a higher rate of respiration and increased weight loss during storage [4]. The magnitude of the GA effect depends on the cultivar and storage temperature regime [11]. Most studies of foliar application of cytokinins have been based on potato yields [12,13]. Foliar applications of 50 mg/L benzylaminopurine (BAP) and 50 mg/L gibberellic acid (GA) at early tuberization phase increased both tuber number and yields [12]. Dwelle[13] found that foliar application of a commercial sea weed extract "Cytex" containing cytokinins equivalent to 100ppm resulted to substantial increase in potato yields.

However, reports show that tuberization and sprouting of tubers are associated with high level of cytokinins[14]. Supplying cytokinins to tubers with innately dormant buds induce sprouting growth.It was found that cytokinins is the primary factor in the switch from innate dormancy to non-dormant state in the potato tuber buds but probably do not control the subsequent sprout growth [15].Early bioassay data suggested that increases in endogenous cytokinins accompanied dormancy break [3,16]. It is proposed that cytokinins may be responsible for dormancy breakdown [15] while the gibberellins promote the subsequent sprout growth [14,17]. Since potato tuber dormancy is thought to occur on or about the time of tuber initiation, it may be possible that application of gibberellic[4] acid and cytokinins[12] at late vegetative (tuber bulking) phase may shorten the dormancy period. However, there are no reports of the effect of combined use of GA and cytokinins on potato seed sprouting.

However, in Kenya there exists less information on the effects of exogenous application of gibberellic acid and benzylaminopurine on tuber sprouting characteristics and subsequent effect on yield of generated plants. This study was aimed at investigating the effects of foliar application of a factorial combination of Gibberellic acid and Benzylaminopurine in breaking dormancy and subsequent sprouting in three cultivars of different dormancy period.

2. Materials and Methods

2.1. Establishment of the Experiment

The experiment was conducted at Kenya Agricultural & Livestock Research Organisation (KALRO) formally Kenya Agricultural Research Institute (KARI), National Potato Centre, Tigoni from September 2008 to October 2009. Three potato varieties with different tuber dormancy period [18]; ‘Asante’ (short dormancy), Dutch Robyjn (medium dormancy) and ‘Kenya Sifa’ (long dormancy) were planted in staggered manner so that all varieties flowered at the same time. The experiment was laid out as a randomized complete block design (RCBD) with a split plot layout where the main blocks were the different rates of combinations of GA and cytokinins and the sub plots were the different genotypes. Each experimental plot was 4.5 m by 0.9 m. Plots and blocks were separated by 1 m and 1.5 m respectively. Di-ammonium phosphate (DAP) fertilizer was applied during planting at the rate of 500 kg/ha. The potatoes were sprayed with a factorial combinations of concentrations of 0, 50, 100, 300 ppm Gibberellic acid (GA) and 0, 50, 75, 100 ppm Benzylaminopurine (BA) at the rate of 1000 litres/ha spray volume towards the end of tuberization phase (75 days after emergence). Dehaulming was done 6 days after treatment application [11] and harvesting was done 14 days thereafter. Untreated varieties were used as control.

2.2. Data Collection

2.2.1. Tuber Dormancy and Sprouting Characteristics

At harvest, twenty uniform potato tubers from each treatment were randomly selected and put in paper trays, labeled and put in diffuse light store (DLS). Data on dormancy period, number, length and vigour of sprouts and subsequent yields were collected, recorded and analyzed. Tuber sprouting was defined as when a tuber had at least one visible sprout of at least 2mm long [3].For dormancy period, the buds of all seed tubers from each treatment were observed after every week. The number of sprouted tubers was counted and recorded. Sprouting was got as a percentage of the number of sprouted tubers in a sample. A sample was considered to have broken dormancy when its sprouting was 80% and dormancy period was given by the duration from when the sample was treated to time when sample tuber dormancy was broken [6]. For the number and length of sprouts, five tubers from each treatment were picked at random after every week and the number of sprouts per tuber and the length of the longest sprout per tuber were noted.

Sprout vigour was determined as a 5 point rating score based on sprout base thickness and sprout length where;1= Very low vigour (where half or more of the tubers in a treatment sample had produced sprouts of at least 1mm base diameter and 2mm long), 2=low vigour (where half or more of the tubers in a treatment sample had produced sprouts of at least 2mm base diameter and 3mm long), 3= medium vigour (where half or more of the tubers in a treatment sample had produced sprouts of at least 3mm base diameter and 4mm long), 4=high vigour (where half or more of the tubers in a treatment sample had produced sprouts of at least 4mm base diameter and 4mm long) and 5=very high vigour (as described in score 4 but had green colouration, firm and had no defects).

2.2.2. Field Evaluation of Sprouted Tubers

After data collection, the sprouted tubers were conventionally planted at the same station for field yield evaluation. Pest and disease management was done using karate® and ridomil®. Supplementary irrigation was also done. The experiment was laid in a complete randomized block design with three replications. In the field, data on %germination (score), number of stems per plant and number of leaflets per plant were taken after every two weeks and recorded.

Germination score was determined as a 4 points rating whereby; 1 means ≤ 25% germination, 2 means > 25% ≤ 50% germination, 3 means > 50% ≤ 75% germination and 4 means > 75% germination. After germination, 5 plants per treatment per genotype were selected at random, labeled and the number of stems per plant and leaflets per plant were counted and recorded. At harvest, the tuber were graded into three grades (Chatts, Seed, and Ware) based tuber size where; Chatts = tubers < 25mm diameter, Seed = tubers ≥ 25 mm ≤ 55 mm and Ware = tuber > 55mm.

2.3. Data Analysis

The data collected was subjected to Analysis of variance (ANOVA) using the PROC ANOVA procedure of Genstat (Lawes Agricultural Trust Rothamsted Experimental station 2006, Version 9). Where the treatment was significant, difference among the treatment means were compared using the Fishers protected LSD test at p≤0.05 probability level.

3. Results

3.1. Sprouting

Potato tuber sprouting exhibited significant differences among treatments and genotypes in both seasons compared with the control (Tables 1 and 2). Duration to commencement of sprouting of tubers from plants treated with GA alone or its combination with BA decreased with increased rate of GA concentration in all genotypes in both seasons except for 50ppm GA which was not significantly different with the control.GA at 50ppm level had no significant difference in all genotypes in both seasons compared with the control. However, at 100ppm level duration to sprouting was reduced by 1 week in Asante, 3 weeks in Dutch Robyjn and Kenya Sifa with respect to control. Visible sprouts were observed during the first week after harvest in Dutch Robyjn and Kenya Sifa at 300ppm GA while all the three genotypes sprouted in the second week at the same rate in the second season. When GA and BA were sprayed in combination, duration to sprouting was the same as that of GA alone at the same rate of concentration in all genotypes in both seasons. Benzylaminopurinealone exhibited no significant difference in sprouting at all rates in all genotypes in both seasons compared with the control. Kenya Sifa took the longest time (7weeks) to sprout at low rates of GA (0-50ppm) while Asante sprouted during the 4th week at the same rate. At higher rates (300ppm) all varieties took the same time (2 weeks) except in 1st season where visible sprouts were observed during the 1st week in Kenya Sifa.

Table 1: Effects of foliar application of rates of combination of gibberellic acid and benzylaminopurine on sprouts length, number of sprouts per tuber, sprouts vigour and % sprouting of potato genotypes Asante, Dutch Robyjn and Kenya Sifa (Year 2008).

*Means followed by the same letters along the column are not significantly different (p≤0.05). LSD 0.05 =least significant difference at 5% probability level; CV% =percent coefficient of variation and xG.yC means combination of x parts per million (ppm) gibberellic acid and y ppm Cytokinins (benzylaminopurine).

3.2. Sprout Length

Sprout length showed significant difference following treatment with different rates of GA, BA or their combinations and genotypes in both seasons compared with the control. When GA was applied alone, it exhibited significant ascending difference of sprouts length with increasing doses of GA in all varieties. However, when BA was applied alone, variation amongst different rate was genotype dependent with significance observation from 50 ppm for Asante, 75ppm for Dutch Robyjn and 100ppm for Kenya Sifa with respect to control(Tables 1 and 2).At lower doses of GA (0-50ppm), Asante genotype produced longer sprouts than Dutch Robyjn but at higher doses (100-300ppm) GA, it was the vice versa. Kenya Sifa recorded the shortest sprout in all treatments. There were higher sprouts length observed when a combination of GA and BA was employed than when each hormone was applied alone at the same rates. It was noted that BA exhibited significant difference after each increase rate at highest rate of GA (300ppm) unlike at lower rates of GA. There was a linear increase of sprout length with duration of storage.

3.3. Sprouts per Tuber

The number of sprouts per tuber was significantly different amongst treatments and genotypes in both seasons (Tables 1 and 2). The number of sprouts per tuber was significantly higher with increased rate of GA in all genotypes while the same occurred in BA treated tubers except in Kenya Sifa where increasing the rate of BA caused no significant difference when applied alone. However, the number of sprouts per tuber increasingly and significantly varied with increase in rate of combination of GA and BA in all genotypes. At lower doses of GA (0-50ppm), Asante recorded higher number of sprouts per tuber than Dutch Robyjn but this was the vice versa at higher doses of GA (100-300ppm) (Tables 1 and 2).

Table 2: Effects of foliar application of rates of combination of gibberellic acid and benzylaminopurine on sprouts length, number of sprouts per tuber, sprouts vigour and % sprouting of potato genotypes Asante, Dutch Robyjn and Kenya Sifa (Year 2009).

*Means followed by the same letters along the column are not significantly different (p≤0.05).LSD 0.05 =least significant difference at 5% probability level; CV% =percent coefficient of variation and xG.yC means combination of x parts per million (ppm) gibberellic acid and y ppm Cytokinins (benzylaminopurine).

Kenya Sifa had the least number of sprouts per tuber in all treatments in both seasons.

3.4. Sprout Vigour

Sprouts vigour score varied significantly among treatments and genotypes in both seasons (Tables 3 and 4). Vigour score increased with duration of storage and with rates of application of GA, BA or their combination. Asante genotype had the highest vigour score in all treatments while Dutch Robyjn recorded higher vigour score than Kenya Sifa at lower dose of GA (0-100ppm) alone or in combination with BA but at 300ppm GA, the Kenya Sifa scored higher than Dutch Robyjn. At higher doses of GA (300ppm), Dutch Robyjn produced longer but slender sprouts.

3.5. Germination Score

Onset of germination was evident in all treatments in all genotypes by the second week after planting. However, there was significant difference in % germination score among treatments and genotypes (Table 5). At lower rates of GA applications, there was no significant difference in % germination score for Asante (0-50ppm) and Dutch Robyjn (0ppm) even with increased rates of concentration of BA. Kenya Sifa exhibited significant variation in the same rate bracket of GA with increase of BA concentration. Tubers treated with 300ppm GA recorded the highest % germination score in all genotypes while at the same rate no significant difference was observed with increase in BA concentration rates in all genotypes. Asante had the highest %germination score in all treatments. Dutch Robyjn recorded higher germination than Kenya Sifa at lower rates of GA (0-50ppm) while the opposite occurred at higher rates (100-300ppm) of GA applications.

3.6. Stems per Plant

Variation in the number of stems per plant differed significantly between treatments and genotypes (Table 5). Gibberellic acid treated tubers produced significantly more stems per plant with increase in the rate of application in all genotypes. Plants treated with BA alone showed no significant variation with increased rates of BA for Asante and Dutch Robyjn genotypes. However, increase in BA concentration exhibited significant difference in stems per plant when applied in combination with GA. A combination of BA and GA gave significantly more stems than when each hormone was applied alone. Asante genotype had the highest number of stems per tuber in all treatments while Kenya Sifa had the lowest.

3.7. Leaflets per Plant

The number of leaflets per plant differed significantly among treatments and genotypes.Increase in concentration of GA or BA when applied alone or in combination resulted in significant increase in the number of leaflets per plant. However, results for the combination were significantly higher than when each hormone was applied alone at the same rate. The number of leaflets per plant increased with time in each treatment in Asante and Dutch Robyjnupto the 8th week after which no increase was observed (Table 5).

However, Kenya Sifa exhibited linear increase in the number of leaflets per plant throughout the 12 weeks. The plants in the control had the lowest leaflets while those treated with a combination of 300ppm GA and 100ppm BA gave the highest in all genotypes. Asante gave the most number of leaflets per plant while Kenya Sifa produced the lowest in all treatments (Table 5).

3.8. Tubers per Plant

Number of tubers per plant varied significantly between treatments and genotypes (Table 5). When GA was applied alone, increase in concentration caused significant upward variation in all the genotypes compared with the control and previous rate except Kenya Sifa where concentration of 50ppm caused no significant difference with respect to control. Increase in the rate of concentration of BA resulted in significant increase in number of tuber per plant in all genotypes except in Kenya Sifa. When GA and BA were used in combination, variation of BA concentration rate had no significant difference at high level of concentration of GA (100-300ppm) in all genotypes. At low concentration rare of GA (0-50ppm), the number of tuber per plant in Asante and Dutch Robyjn were not significantly different except at when BA was 100ppm at 50ppm GA. When GA concentration was 100ppm and 300ppm, Asante genotype significantly outweighed Dutch Robyjn in the number of tubers per plant. Kenya Sifa had the lowest number of tubers in all treatments.

Table 3: Effects of sequence of application of gibberellic acid and benzylaminopurine on sprouts vigour of potato tubers stored under diffuse light conditions (2008).

Treatment Genotypes
Asante Dutch Robyjn Kenya Sifa
Sprouts vigour score Sprouts vigour score Sprouts vigour score
2Wks 6 Wks 10 Wks 2 Wks 6 Wks 10 Wks 2 Wks 6 Wks 10 Wks
Control 0a* 2a 3a 0a 1a 3a 0a 0a 2a
0G.50C 0a 2a 3.33b 0a 1a 3a 0a 0a 2.33b
0G.75C 0a 2a 3.33b 0a 1.33a 3a 0a 0a 2.33b
0G.100C 0a 2a 4c 0a 1a 3a 0a 0a 2.67c
50G.0C 0a 2a 4c 0a 1.33a 3a 0a 0a 2.33b
50G.50C 0a 1.67a 4c 0a 2b 3a 0a 0a 2.33b
50G.75C 0a 2ab 4c 0a 1.33a 3.33b 0a 0a 3d
50G.100C 0a 2.33b 4c 0a 1.33a 3.67c 0a 0a 3d
100G.0C 0a 2.33bc 4c 1b 2b 4d 0a 1b 3d
100G.50C 0a 2.67cd 4.33d 1b 2.33bc 4d 0a 1.67c 3d
100G.75C 0.33b 3de 4c 1b 2.33bc 4d 0a 2cd 3d
100G.100C 0a 3de 4.33d 1b 2b 4d 0a 2.33d 3.33e
300G.0C 1c 3de 5e 1b 2.67cd 3.67c 1b 3e 3.33e
300G.50C 1c 3de 5e 1.33c 3d 4d 1b 3e 3.67f
300G.75C 1c 3de 5e 2d 2.67cd 4d 1b 2.67de 3.67f
300G.100C 1c 3.33e 5e 2d 3d 4d 1b 3e 3.67f
LSD0.05 Treat.(T) LSD0.05 Genotype (G) CV% 0.12 0.05 17.8 0.37 0.13 12.1 0.28 0.13 4.7 0.12 0.05 17.8 0.37 0.13 12.1 0.28 0.13 4.7 0.12 0.05 17.8 0.37 0.13 12.1 0.28 0.13 4.7

*Means followed by the same letters along the column are not significantly different (p≤0.05). LSD 0.05 =least significant difference at 5% probability level; CV% =percent coefficient of variation and xG.yC means combination of x parts per million (ppm) gibberellic acid and y ppm Cytokinins (benzylaminopurine).

Table 4: Effects of sequence of application of gibberellic acid and benzylaminopurine on sprouts vigour of potato tubers stored under diffuse light conditions (2009).

Treatment Genotypes
Asante Dutch Robyjn Kenya Sifa
Sprouts vigour scores after n weeks Sprouts vigour scores after n weeks Sprouts vigour scores after n weeks
n=2 n=6 n=10 N=2 n=6 n=10 n=2 N=6 N=10
Control 0a 3.47a 3.73a 0a 3a 3.33a 0a 2a 2.33a
0G.50C 0a 3.33a 3.73a 0a 3a 3.47a 0a 2.33b 2.33a
0G.75C 0a 3.33a 3.67a 0a 3a 3.33a 0a 2.33b 2.47ab
0G.100C 0a 3.47a 4b 0a 3.33b 3.27a 0a 2.67cd 2.67bc
50G.0C 0a 4b 4.13b 0a 3a 3.33a 0a 2.33b 2.47ab
50G.50C 0a 3.93b 4b 0a 3.33bc 3.53ab 0a 2.47bc 2.47ab
50G.75C 0a 4b 4b 0a 3.47cd 3.47ab 0a 2.93d 2.73cd
50G.100C 0a 4b 4b 0a 3.67d 3.67b 0a 3d 2.933d
100G.0C 0.67c 4b 4.14bc 1.33c 4e 4c 0a 3d 3d
100G.50C 0.67c 4b 4.33c 1b 4e 4c 0a 3d 3d
100G.75C 0.69c 4.33c 4.27bc 1b 4e 4c 0a 3.33e 3.33e
100G.100C 0.67c 4.33c 4.33c 1b 4e 4c 0a 4f 4f
300G.0C 1.63c 4.67d 4.73d 1b 4e 4c 1b 4f 4f
300G.50C 1.33b 4.67d 5d 1b 4.13e 4.13cd 1b 4f 4f
300G.75C 1.33b 4.73d 5d 1b 4e 4.33d 1b 4f 4f
300G.100C 1.63c 4.57cd 5d 1b 4.33 4.33d 1b 4.33 4.13f
LSD0.05 Treat.(T) LSD0.05 Genotype (G) CV% 0.210 0.99 28.0 0.298 0.136 5.0 0.282 0.138 4.7 0.210 0.99 28.0 0.298 0.136 5.0 0.282 0.138 4.7 0.210 0.99 28.0 0.298 0.136 5.0 0.282 0.138 4.7

*Means followed by the same letters along the column are not significantly different (p≤0.05).LSD 0.05 =least significant difference at 5% probability level; CV% =percent coefficient of variation and xG.yC means combination of x parts per million (ppm) gibberellic acid and y ppm Cytokinins (benzylaminopurine).

Table 5: Effects of foliar application of rates of combination of gibberellic acid and benzylaminopurine on subsequent germination, number of stems per plant and number of leaflets per plant of potato genotypes Asante, Dutch Robyjn and Kenya Sifa (2009).

*Means followed by the same letters along the column are not significantly different (p≤0.05).LSD 0.05 =least significant difference at 5% probability level; CV% =percent coefficient of variation and xG.yC means combination of x parts per million (ppm) gibberellic acid and y ppm Cytokinins (benzylaminopurine).

Table 6: Effects of foliar application of rates of combination of gibberellic acid and benzylaminopurine on subsequent yields of potato genotypes Asante, Dutch Robyjn and Kenya Sifa (Year 2009).

*Means followed by the same letters along the column are not significantly different (p≤0.05).LSD 0.05 =least significant difference at 5% probability level; CV% =percent coefficient of variation andxG.yC means combination of x parts per million (ppm) gibberellic acid and y ppm Cytokinins (benzylaminopurine). Chatts= tubers< 25mm diameter, Seed= tubers ≥25mm≤ 55mm and Ware=tuber >55mm

3.9. Yields of Generated Plants

Both treatments and genotypes showed significant variation in total yields compared with the control (Table 6). When GA singly applied, there was a significantly difference in total yields in all genotypes with increased hormone application rates. However for BA treated plants, effects were only significant at higher application rates (75-100ppm) for Asante but total yields showed no significant difference in Dutch Robyjn and Kenya Sifa.

When a combination of GA and BA was employed, significant total yields was only observed at higher dose (100ppm) of BA at each rate of GA except at 300ppm where no significant difference was observed compared to that of GA treated plant at that rate in all genotypes. Asante gave the highest total yields while Dutch Robyjn gave the lowest in all treatments.

Tuber grades (tuber sizes) differed significantly between treatments and genotypes compared with the control (Table 6). The potato seed and the quantity of chatts increased with increase in rates of rate of concentration of GA in all genotypes. A decrease in yields of ware was registered with increased concentration of GA when applied alone or in combination with BA. However, application of BA alone only reflected significant difference at 100ppm in Asante and Dutch Robyjn while variation of ware yield in Kenya Sifa was not sequential and did not vary greatly with concentration of both GA and BA. Yields of chatts were highest in Dutch Robyjn while Kenya Sifa had the lowest grades of both chats and seed in all treatments. Asante yielded most seed grade in all treatments. Dutch Robyjn gave the lowest ware yields while at lower levels of GA concentration (0-50ppm), Asante outweighed Kenya Sifa. However, at higher GA concentration (100-300ppm), Asante recorded lower ware yields (less than 10tons/ha) while Kenya Sifa steadily maintained yields above 10 tons/ha (Table 6).

4. Discussions

4.1. Effects of Foliar Application of Gibberellic Acid and Cytokinins on Subsequent Potato Tuber Sprouting

In this study, foliar application of GA alone or GA+BA resulted in significant decrease in duration of subsequent tuber dormancy period and sprouting and this varied among the genotypes. However, BA alone had no effect on duration of tuber dormancy and sprouting. These results are in agreement with [11] findings that a foliar spray of 300ppm- 375ppm gibberellic acid, 3–6 days before haulm killing shorten potato tuber dormancy period and induced sprouting. The results were also in agreement with [4] findings that when GA was applied to the foliage of potato plants grown from true seeds towards the end of the vegetative cycle, it induced rapid breakage of tuber dormancy. Alexopoulos [5] also observed that exogenous application of GA on potato plant foliage drastically increased the concentration of endogenous gibberellins and simple reducing sugars of the resulting tubers leading to visible sprouting. These results also agreed with[3] findings that application of low doses of cytokinins had no effect on potato dormancy.

This study revealed that foliar spray of GA, BA or GA + BA increased tuber sprout length, number of sprouts per tuber and sprout vigour in a dose dependent manner and also varied with genotype. The number of sprouts per tuber, sprout length and vigour were significantly higher with increased rate of GA in all genotypes. However, when BA was applied alone, variation of tuber sprout length and number for different rate was genotype dependent with significant observation from 50ppm for Asante, 75ppm for Dutch Robyjn and 100ppm for Kenya Sifa with respect to control. Results from various researchers while working on effects of foliar spray of individual hormones on different variable found similar findings. The growth length and vigour of potato tubers sprouts was greatly advanced by a foliar spray with gibberellic acid [10], the magnitude of the GA effects depended on the cultivar [11] and increased level of GA was found to exhibit premature sprouting [14]. The number and length of sprouts of tubers treated with a combination of GA and BA were higher than those of tubers treated with each hormone at the same rate and varied with increase in rate of combination of GA and BA in all genotypes. At each rate of BA, both sprouts length and number of sprouts per tuber increased with increased rate of GA concentration. However, it was noted that BA exhibited significant difference after each increase rate at highest rate of GA (300 ppm) unlike at lower rates of GA. Vigour score increased with duration of storage and with rates of application of GA, BA or their combination. Asante tubers produced the strongest sprouts while Kenya Sifa gave the weakest. However, at higher doses of GA (300ppm), Dutch Robyjn produced longer but slender sprouts.

Despite the involvement of BA in cell division, the number of cell may not necessarily results to visible growth [19,20]. The results from this study indicate that GA alone effected duration of dormancy termination and sprouts growth. This may indicate that GA is involved in both cell division and elongation. The above results may also be physiologically explained by several research findings that GA is involved in synthesis of α-amylase enzyme [19,21] involved in breakdown of starch to glucose and fructose and facilitation of the movement of cytokinins to the buds enhancing cell division [22].

4.2. Effect of Foliar Application of Gibberellic Acid and Cytokinins on Subsequent Potato tuber Yields

Foliar application of GA alone caused significant increase in subsequent germination, stems per plant, leaflets per plant and number of tubers per plant and yields at harvest with increased rate of application in all genotypes. Asante had the highest germination score in all treatments. Dutch Robyjn recorded higher germination than Kenya Sifa at lower rates of GA (0-50ppm) while the opposite occurred at higher rates (100-300ppm) of GA applications. At low concentration rate of GA (0-50ppm), the number of tuber per plant in Asante and Dutch Robyjn were not significantly different. These results are in agreement with Abd[23] findings who observed thatfoliar application of GA enhances vegetative growth, length of plant, average number of shoots, leaves number, fresh and dry weight of shoots and gives more yields. They also agree with Alexopoulos[22] findings that foliar application of GA in plants derived from TPS caused an increase in the number of tubers per plant and increased the yields.

Spraying of BA alone caused significant increase in germination in Dutch Robyjn and Kenya Sifa genotypes but not in Asante, whereas application of 100ppm BA resulted in increased stems/tuber, leaflets/plant and tubers/plant except in Kenya Sifa for tubers/plant. This was in tandem with Dwelle[24] and Blunden [25] who while working separately found thatfoliar application of ‘Cytex’ product, a commercial aqueous seaweed extract equivalent to 100ppm cytokinins activity and synthetic cytokinin ‘kinetin’ produced a significant increase in the yield of potatoes. However, at lower rates of GA applications, there was no significant difference in germination score for Asante (0-50ppm) and Dutch Robyjn even with increased rates of concentration of BA.

Plants treated with a combination of BA and GA gave significantly more stems per plant, leaflets per plant than when each hormone was applied alone. Asante genotype gave the highest stems per plant and leaflets per plant and total yields while Kenya Sifa gave the lowest except the total yield which was given by Dutch Robyjn. These findings were in agreement with Caldiz[12] findings who also observed that foliar applications of a combination of benzylaminopurine and gibberellic acid under both field and glasshouse conditions increased both tuber number and tuber yields. However,yields of ware decreased with increased concentration of GA when applied alone or in combination with BA but in BA treated plants, yields of ware tuber differed only at 100ppm in Asante and Dutch Robyjn. Production of ware yield did not vary with concentration of GA, BA or their combination in Kenya Sifa. Both the number and weight of seed and chatts tubers increased with increase in rates of concentration of GA alone or its combination with BA in all genotypes.These observations agree with Stuik[26] results who similarly observed that increase of GA application on potato plant increased the number of tubers and total yields but production shifted the tuber size distribution towards the smaller grades. It was observed that not all sprouts gave rise to stems. Increase in the numbers of sprouts per tuber may have resulted in increased stems per plant, leaflets per plant and faster rate of canopy cover. This may have resulted to higher amount of intercepted radiation increasing number of tubers per plant and total yields [27].

5. Conclusions and Recommendations

This study revealed that foliar application of GA alone or a combination of GA and BA late in the growth cycle results in significant decrease in duration of subsequent tuber dormancy period and sprouting and this vary among the genotypes. Foliar spray of GA, BA or GA + BA increase tuber sprout length, number of sprouts per tuber and sprouts vigour in a dose dependent manner but varies with genotype. A combination of BA and GA results in significantly more growth than using only GA or BA alone at the same level.It seems that BA and GA has dose dependent synergistic effect on potato tuber sprouting and sprouts growth.

Foliar application of GA alone or GA+BA causes significant increase in subsequent germination, stems per plant, leaflets per plant and number of tubers per plant at harvest with increased rate of application in all genotypes. GA enhances vegetative growth; average number of stems, leaflets number and more yields. More sprouts per tuber may result in more stems per plant. Consequently, more stems per plant results more leaves, and the ground cover taking place at a faster rate of ground cover, higher amount of intercepted radiation and assimilation and hence higher total yields. Foliar application of gibberellic acid or a combination of GA and BA late in the growth cycle may be of practical value in cases where tubers are required for planting soon after harvest. However, higher doses of hormones are recommended for foliar application than when the hormones were directly sprayed on tubers

Acknowledgements

The authors appreciate assistance by International Potato Centre (CIP), Nairobi in importation of True Potato Seed (TPS) from Lima, Peru; Centre Director, KALRO, National Potato Research Centre, Tigoni and Principal, Njabini Agricultural Training Centre, Nyandarua, for providing land resource and other facilities essential for the field work.


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