Effect of the use of Potassium Fertilizer on the Resistance and Growth of Tomato to Bacterial Wilt caused by Ralstonia solanacearum

The research aims to study the effect of sources and doses of potassium fertilizer on the resistance and growth of tomato to bacterial wilt caused by Ralstoniasolanacearum. We conduct experiment in a screen house in Faculty of Agriculture, Islamic University Malang. The research is conducted experimentally using completely randomized block design (RAK) arranged in factorial with three repetition. There are six combinations of treatment. Factor I: source of potassium fertilizer, consists of two levels: KCl and K2SO4. Factor II:dose of K2O, consists of three levels: 50 kg ha-1, 100 kg ha-1 and 200 kg ha-1. The inoculation of Ralstonia solanacearum is conducted a week after transplanting. There is significant effect on the use of different sources and doses of potassium fertilizer. As whole, the use of potassium fertilizer originated from K2SO4 is better than that of KCl and the magnitude of the increase depends on dose applied. The best result indicates by treatment of the use of K2SO4 with dose of 200 kg ha-1 K2O that able to extend the incubation period of 6,27 days, decrease the attack level of 73,15%, increase the uptake of potassium and leaf chlorophyll of 4,58% and 7,17%, respectively,and increase root lignin of 3%, whereas total phenol is decreased of 27,27% compare to the use of KCl in the same dose.


INTRODUCTION
Tomato is one of superior horticultural commodities in Indonesia and has a promising economic prospect. Therefore, it needs serious handling, especially, in terms of increasing its yield and fruit quality. The projection of national tomato demand for 2014-2019 is around 970.499 -1,107,168 ton, whereas tomato product by 2013 was only 922,780 ton with average productivity of 16.61 t.ha -1 (Bureau of Statistics and Directorate General of Horticulture, 2014). One of obstacles in the low production of tomato is the occurrence of wilt disease caused by Ralstonia solanacearum bacteria, which is a threat for hot climate areas or areas with warm rainy season. Result of field observation shows that the disease has caused the loss of fresh fruit in approximately 7.1 -63.7% (Rosyidah et al., 2014). It is unfavorable for farmers since the investment for production cost is high. Various efforts of controlling the disease have been conducted, such as the use of organic material from chicken manure (Rosyidah, A., 2012), the use of cabbage family as bio-fumigant, and the use of resistance variety (Rosyidah et al., 2014). Another effort is the use of potassium fertilizer with appropriate source and dose. This effort is another alternative to increase plant resistance against environmentally friendly disease. Tomato plant absorbs large amount of potassium element in approximately 1-5% of plant's dry weight (Chen and Gabelman, 2000). Potassium plays important role in plant metabolism (Farhad et al., 2010), helps in the formation of protein, carbohydrate, enzyme activity, regulation of osmotic, water use efficiency, translocation of photosynthate (McKenzie, 2001), stimulate the development of root and increase the size of fruit (Marsono and Sigit, 2001), and increase the transportation of sugar and acid to storage organ (Bernardi et al., 2013). The application of potassium could increase the formation of thick lignin compound; therefore, wall cell will be stronger and able to protect plant from pathogen interference (Fageria et al., 2009). Potassium fertilizer mostly used in Indonesia is muriate of potash (KCl) containing about 60% of K2O. Recently, however, there is a development in the use of potassium sulphate (K2SO4). Some research found that the incident of disease is higher when potassium fertilizer used is originated from KCl, whereas potassium sulphate is proven to improve some characteristics of quality of various vegetable products (Gunadi, 2009). At present, the role of potassium in increasing plant resistance, especially in tomato, has not been studied. The aim of the research is to study the effect of combination of source and dose of potassium fertilizer on the resistance and growth of tomato toward bacteria wilt caused by Ralstonia solanacearum.

II. MATERIALS AND METHODS
The research was conducted experimentally in a screen house in Faculty of Agriculture, Islamic University Malang in April -July, 2016. The altitude of the location was 460 above sea level. Type of soil is loam. Air temperature is around 22.5 0 C -25.5 0 C with air humidity of 80% -86%. The research was conducted experimentally using completely randomized block design (RAK) arranged in factorial with three repetition. There were six combinations of treatment. Factor I: source of potassium fertilizer, consisted of two levels: KCl and K2SO4. Factor II was dose of K2O, consisted of three levels: 50 kg ha -1 , 100 kg ha -1 and 200 kg ha -1 . Each treatment consisted of ten sample plants. Tomato seeds from Lentana variety were planted in seeding basin with media of soil + sand + compost that previously sterilized with hot steam for 3 hours with ratio of 1:1:1. At the age of 10 days, plant's seedlings were transferred to seedling glass, one seedling per glass. Growing media used weresoil:sand:organic material of chicken manure (C/N = 12) (ratio of 2:1:1) that previously sterilized with hot steam for 3 hours. Growing media of 8 kg was put into a polybag. The transplanting of tomato seedling was conducted when the seedling has height of 10 cm and 4 leaves. Inorganic fertilizers applied were SP-36 and it was applied on 3 days after transplanting with dose of 150 kg/Ha and Urea on 7 days after transplanting with dose of 150 kg Ha -1 . The application of KCl and K2SO4 fertilizers was conducted at the age of 7 days after transplanting with dose in accordance with the treatment. The isolate of R. solanacearum used was the result of isolation of tomato plant attacked by R. solanacearum in Donowarih Village, Karangploso, Malang. The purification isolation and propagation of R. solanacearum was conducted using media of TZC (2,3,5-triphenyl tetrazolium chloride). Population density for inoculation was 2.78x10 8 cfu.mL -1 measured with spectrophotometer in OD 600. Plants at the age of 1 week after transplanting were inoculated with R. solanacearum with concentration of 10 8 cfu/ml of 20 ml by wounding the plant roots using scalpel. Observation was conducted on: incubation period of the disease, level of disease attack (Sinaga, 2003), potassium uptake in leaves (through extraction using NH4Oac), root lignin (Acid detergent fiber method), total phenol (Folin-Denis method), level of leaf chlorophyll (SPAD) and plant height. In order to see the influence of treatments on observation conducted, data of observation result was statistically analyzed based on analysis of variance (ANOVA) and followed by Least Significance Difference test in confidence level of 95% to see their significances.

III.
RESULT ANDDISCUSSION The status of soil fertility used in the experiment is presented in Table 1. The content of carbon (C), nitrogen (N), and C/N ratio is classified as low. Phosphor (P) and potassium (K) are classified as medium and low, respectively, whereas, cation exchange capacity (KTK) is classified as medium. Incubation period and level of attack of R. solanacearum disease Based on research result (Table 2), it can be seen that there was a significant interaction (p < 0.05) in the treatment of the use of potassium fertilizer sources and doses on incubation period of the disease and the attack level of bacterial wilt disease. Based on Table 2, it is known that the use of different potassium fertilizer sources and doses resulted in different disease incubation period and level of attack. The incubation period of the disease was ranged from 16.21 to 25.74 days after inoculation. Level of attack of the disease was ranged from 6.22% to 12.21% at the age of 35 days after transplanting. The application of potassium fertilizer of K2SO4 is better than those of KCl. It is estimated that it is due to the content of SO4 in potassium sulphate fertilizer since one of the functions of sulfur (S) is to reduce the attack of the disease (Tisdale et al., 1990). The higher the dose of potassium fertilizer (200 kg ha -1 K2O) applied, the longer the incubation period and the lower the attack level caused by R. solanacearum pathogen. It happens because the initial content of K element in soil used for the experiment was low. With the addition of potassium of 200 kg ha -1 K2O in the treatment gives sufficient nutrient and good plant resistance. One of the function of K element is to improve plant resistance by strengthening plant tissues and thickening epidermic wall. Nurhayati (2008) stated that potassium in plant plays role in the formation of protein and carbohydrate as well as in the increasing of resistance against pathogen.

Potassium uptake, root lignin and total phenol
Research result shows that there was significant interaction (p < 0.05) in the treatment of the use of potassium fertilizer sources and doses on potassium uptake, root lignin and total phenol (Tabel 3). Based on Table 3, it is known that the use of different potassium fertilizer sources and doses resulted in different potassium uptake, root lignin level and total phenol level of plant. The application of potassium fertilizer K2SO4 was better than that of KCl. In addition, the higher the dose of potassium fertilizer (200 kg ha -1 K2O) applied, the higher the leaf potassium intake and root lignin level and the lower the level of total phenol. Result of observation on the level of potassium uptake in leaves shows that bigger potassium uptake in leaves will increase availability status of potassium in plant organs. The sufficiency of potassium has function in increasing the status of plant defense to improve damage caused by pathogen since plant is able to increase the strength of its cell wall. Hardter, R (2003) and Pervez, H et al., (2007) add that the sufficient level of potassium in plant could increase the strength of paddy's stem and stalk due to the increase of its resistance. It is also explained that plant stomata and lenticel work well if sufficient potassium is exist. When pathogen invaded the plant, stomata and lenticel have the ability to close quickly. Potassium is also able to improve the work of enzyme for plant metabolism. The sufficiency of potassium in plant will increase the synthesis of molecular compounds with high molecular weight (protein, starch, cellulose) thus decreasing the synthesis of molecular compounds with low molecular weight, such as: organic acid, amino acid, and amide in plant tissues. It is the decrease of the synthesis of compounds with low molecular weight that able to increase plant resistance against pathogen infection (Marschner, P., 2012; Mengel, K., 2001). Potassium element also plays role in lignification of sclerenchyma tissue (Fageria et al., 2009). Therefore, the sufficiency of potassium could increase the formation of thicker lignin compound; thus, cell wall is stronger and able to protect plant from external disturbance. Observation on total phenol shows that the increasing of potassium dose applied will decrease the level of total phenol. In other words, the lower the doses of potassium fertilizer applied, the bigger the level of attack; therefore, a tendency of the increase in phenol compound level. The increase in phenol compound is the reaction of plant toward infection of R. Solanacearum pathogen and root wounding before pathogen inoculation. Pieterse et al., (2009) stated that the increase of plant resistance through SAR (Systemic Acquired Resistance) occurs after local pathogen infection in plant; the infected plant, then, activates genes that play role in the resistance to produce chemical compounds for plant resistance, such as salicylate. When the plant has the resistance, it will be able to protect itself if another pathogen exists; thus, pathogen infection will not be developed. According to Goodman et al., (1986), plant tissue infected by pathogen indicates a change in metabolic pattern, including, activating peroxide and other phenoloxidase enzymes. It is in line with Matern et al. (1995) and De Ascensao et al. (2003) stated that great phenol synthesis will occur if plant is attacked by pathogen. Agrios (2005) stated that pathogen microorganism causing mechanical and chemical damages will stimulate plant to produce toxin compound against pathogen (phytoalexin). Plants need peroxide enzyme to produce resistance compounds, such as lignin, chitin, and various compounds that build cell wall (Hallman, 2001). Further, Bruce et al. (1989) stated that peroxide is another component in the initial response of plant to pathogen attack and plays key role in the biosynthesis of lignin that limit the area of pathogen distribution.

Leaf chlorophyll and plant height
Research result shows that there was significant interaction (p<0.05) on the treatment of the use of potassium fertilizer sources and doses on the content of leaf chlorophyll. Regarding observation on final plant height, it shows no interaction between potassium fertilizer sources and doses tested ( Figure 1A and 1B). Treatment of the application of K2SO4shows more chlorophyll content than that of KCl. It is likely due to the content of sulfur in K2SO4 fertilizer. Sulfur is the main element in the formation of leaf chlorophyll that closely related to photosynthesis process and takes part in various metabolism reactions, such as carbohydrate, fat and protein (Tisdale et al., 1990). The increase in potassium fertilizer dose applied will increase chlorophyll content. It is due to the sufficiency of potassium in the plant that will increase the work of enzymes thus increasing the activation of plastid in leaf, synthesis of protein, photosynthesis and stomata movement. It results in the increase in the production of leaf chlorophyll. The optimum availability of potassium in leaf will make the leaf to be more efficient in the photosynthesis and plant will be more resistance and tolerance.

IV.
CONCLUSIONS In the research, we report the effect of the use of potassium fertilizer on the resistance and growth of tomato to bacterial wilt caused by R. solanacearum. The result can be summarized as follow: the effect of the interaction is significant in the component of resistance but not in the observation on plant height. Treatment of K2SO4 and doses of potassium fertilizer (200 kg ha -1 K2O) is the best as indicated by longer incubation period of 6.27 days, decrease in the level of attack of 73.15%, increase in potassium uptake and chlorophyll in leaf of 4.58% and 7.17%, respectively and the increase in root lignin of 3% and decrease in phenol of 27.27% compare to the use of KCl in the same dose.