MATERIALS AND METHODS

The experiment took place in a research station located at the Agriculture Faculty of Padjadjaran University from March-September 2015. The altitude of 685 m.a.s.l., with D3 rainfall in inseptisol soil. Split-plot design was being used during the experiment. The main-plot factor applying paclobutrazol consisted of three different levels of applications (0, 50 and 100 ppm). The split-plot was the experiment design with the amount of water given in four different levels (100, 85, 70 and 55% of water field capacity). From those two treatments, 12 treatment per unit were obtained with three replications, so in total there were 36 plots treated. Each treatment was provided with 10 polybags of 15 kg and each polybag consisted of 1 plant, thus, 360 pots were obtained.

Growing media, which had been prepared in polybag, was given 20 t ha^–1 of fertilizer on the left and the right side of the plant. About 300 kg ha^–1 of N fertilizer was given twice on the 1st day planting and on the 30th day after planting. While 100 kg ha^–1 of K and 150 kg ha^–1 of P fertilizer were given at one time on the 1st day planting. After being fertilized, a diemeter of 10 cm hole was initially made to grow potato seed. The second generation of the seed of Atlantic cultivar of 30-45 g per piece was planted in 7.5 cm depth. To avoid soil insects and other pests, 3 g of furadan was spread around the seeds with a dose of 37.5 kg ha^–1. About 30% paranet shield was put up before growing the potatoes⁵. Paclobutrazol application was given on the 30th day after planting by spraying 15 mL per plant. Based on 100% water capacity for inseptisol soil, 1,950 L of water was needed. Plants were watered everyday with the appropriate amount of water accordingly. To mantain the amount of water on treatment, the provision of water was given as much water loss through evapotranspiration added with fresh weight of the plant as a correction factor that is maintained by weighing.

Weeds were removed and then soil was loosen and tiled and added with N fertilizer. Spraying dithane M-45 fungicide and decis 2,5 EC insecticide were aimed to control insects and pests based on the frequency of their attack. Harvesting could be started when the top of potato plants, the stems and the leaves, became yellow and began to fall and the skin of tubers were not peeled off.

Observation was done on the changing factors; leaves Relative Water Content (LRWC) calculated based on calculation:

Chlorophyl content was counted by using chlorophyl meter. The parameters of growth consisted of plants height, leaf area and dry weight, while the parameter of yield consisted of numbers of tubers per plant, the weight of tubers per plant and per hectare as well as the yield quality which consisted of starch and sugar content. The F test was performed to determine the differences in each treatment. If there’s significant effect of treatment, Duncan’s multiple range test at the 5% significance level will be applied.

RESULTS AND DISCUSSION

Content of leaves relative water: From Table 1, it was found that leaves relative water content with 100 ppm paclobutrazol concentration were much higher than with 50 and 0 ppm concentration. Reducing water had caused decline of the content of leaves relative water.

Plants growth-plant height, leaf area and chlorophyll content: Variance analysis results showed no interaction between the effects of packlobutrazol and the amount of the provision of water in the height of potato plant but the effect independently showed that paclobutrazol and the amount of water administration show the effect on plant height, leaf area and chlorophyll content. Treatment of 50 and 100 ppm paclobutrazol shown lower plants height and leaves area than 0 ppm paclobutrazol, which however, showed higher chlorophyll content. Plant height, leaves area and chlorophyll content became lower as a greater amount of water given (Table 2).

Number of tubers and its total weight per plant: Variance analysis result showed that there’s no interaction between paclobutrazol application and the water procentage of field capacity on the number of tubers per plant but it had independently showed influence. Application of 50 and 100 ppm of paclobutrazol could increase the number of tubers and its total weight per plant. The amount of water given had also affected them.The number of tubers watered by 85% of field capacity showed most tubers and differed from other treatments. However, they indicated different result when watered by 100% of field capacity. Then, the number and the weight of potatoes decreased as less water given (Table 3).

Sugar and starch content: The result of statistic analysis showed that the sugar content of a plant with 100 ppm paclobutrazol application was 0.45 as the lowest value, while the starch content showed the highest value, 12.50%. Less water of field capacity percentage had caused higher sugar content, yet lower starch content (Table 4).

Table 1:	Effect of paclobutrazol and the amount of water on the Leaves Relative Water Content (LRWC)

Table 2:	Effect of paclobutrazol and the amount of water on plants height, leaf area and the chlorophyll content of 56 days old potato
Average numbers followed by same letters are the same based on Duncan’s multiple range test on 5% level

Table 3:	Effect of paclobutrazol and percentage of water content of field capacity on the numbers of tubers
Average numbers followed by same letters are the same based on Duncan’s multiple range test on 5% level

Table 4:	Effect of paclobutrazol and the amount of water on starch and sugar content
Average numbers followed by same letters are the same based on Duncan’s multiple range test on 5% level

As water of field capacity was less given, content of leaf relative water was smaller. This was consistent with the research¹⁰ which had found that leaf area, dry weight of leaf, its relative water content and its index were decreasing when less water was given. It happened because the cell had became smaller and the leaf hadn’t developed well during the water stress. It also decreased the ability for photosynthesis due to smaller leaf area¹¹. The relative water content of leaves of the plants which had been given paclobutrazol were higher than one without paclobutrazol. One of the retardant substances of paclobutrazol was reducing leaf area on transpirational surface, which in turn reduced transpiration ability³. It could also induce stomatal closure, which then reduced transpiration as well. Therefore, the relative water content of the leaves could be maintained.

About 50 and 100 ppm paclobutrazol tretments showed that the plant was shorter and the leaves were smaller compare to 0 ppm paclobutrazol. Paclobutrazol application on plants would cause inhibition of the synthesis of gibberellins which implicated on shorter stems of the plants. The plants response on paclobutrazol application was there would be a reduction in the growth of the shoots^6,7,8,11. Plants applying paclobutrazol would be shorter and compact, that was related to shortened stems. It happened because applying paclobutrazol could reduce plants height through inhibition of gibberellins biosynthesis, an important hormone for cell elongation and enlargment^6,12. The content of photoshyntesis of potato plants which had been given paclobutrazol were higher than ones without paclobutrazol. Paclobutrazol application could cause the content of leaf photosynthesis went through the pathway of certain substances formation on gibberellin synthetic pathway, such as pytol which was chlorophyll formation precursor^4,13.

The values of plants height, leaf area and chlorophyll contents were lower as less water was given (Table 2). Plants growth are very much influenced by water provision, because water is one of the most improtant components needed for photosynthesis process besides CO₂ and sunlight. The height of stems, numbers of green leaves and the length of leaf were sensitive parameters for drought¹⁴. While, on less water or dry condition chlorophyll formation could be inhibited¹⁵. Drought is an inhibitory factor on growth and leaf development of potato. Early drought caused leaf limitation and branch development¹⁶. It happened because the cells had became smaller and under developed during the water stress, which resulted in reducing leaf area for photoshynthesis¹⁵. The numbers of tuber per plant with 85% water of field capacity showed higher number of tubers, different from other treatment. However, it didnt showed any difference when watered with 100%. Then, the number of tubers were decreasing as less water was given (Tabel 3). There was a significant difference between the high amount of water with the low amount of water given, yet giving 85-100% of water could still support plants growth and produce more tubers due to higher numbers of stolon forming the tubers. Number of tubers per plant tends to decrease as the response of water insufficiency¹⁷.

Packlobutrazol of 50 and 100 ppm application showed the numbers of tubers and its weight were much higher than without paclobutrazol. Paclobutrazol application could increase clear assimilation phase¹⁸. It’s related to the high content of chlorophyll and the postponment of senescence on potato leaf and it’s also related to the effect caused by paclobutrazol stimulating cytokinins hormone. The increased cytokinins hormone would trigger chlorophlast diffrentiation and biosynthesis, it would prevent chlorophyll degradation as well. Finally, those conditions affected tuberization on potatoes because the phase of photosynthate translocation into the tubers were increased due to paclobutrazol application^11,18.

The sugar and starch content of potato with paclobutrazol application were close to its standard value accepted in potato processing industry. Based on industrial standard, to make potato chips the value of starch content should be above 11.90% and sugar content^2,19 is below 0.5%. One of photosynthesis product is starch. Starch is a polysaccharide that is the end product of carbon fixation during photosynthesis. In addition to starch, sucrose is another product of photosynthesis^20,21.

Paclobutrazol application had lessened gibberellins accumulation in the tuber tissue, therefore, it increased the strength of sink storage to accommodate more assimilates and starch synthesis¹⁸. Furthermore, photoshyntate translocation occurs because the plants avoided high accumulation of sugar in order to maintain osmotic gradient. One of organ sink mechanisms in reducing the accumulation of sugar was to synthesize starch or other macromolecules substance which have a lower osmotic activity²².

CONCLUSION

ACKNOWLEDGMENT

We would like to thank the Directorate General of Higher Education Ministry of Research and Technology of Indonesia that has funded this research through the National Strategic Research Programme.