Study of Five Clones with Combinations of Growth Regulators Based on Growth and Anatomical Characteristics of Rubber Plant (Hevea brasiliensis L.)
Try Koryati, T.1*, Wiwik Yunidawati2, Radite Tistama3, Nurhayati N4, Mazlina5, Rosmaiti6
1,2,5Faculty of Agriculture, Amir Hamzah University, Medan
3 Indonesian Rubber Research Institute, Sumatera Selatan
4 Faculty of Agriculture, Islamic University of North Sumatera, Medan
6Faculty of Agriculture,, Samudra University, Langsa
* Corresponding Author Email: trykoryati@unhamzah.ac.id
ABSTRACT
As an export commodity, rubber is a major contributor to Indonesia’s foreign exchange. Regarding the increase in rubber production, a study on combinations of growth regulators and rubber clones has been carried out based on plant growth and anatomical characteristics during the immature plant period. This research was conducted for eighteen months at the KSO Karang Inong plantations, PTPN-I, and PTPN- III, East Aceh District, using five clones. A nested design was adopted with three factors: clones, Growth Regulators (GRs), and paclobutrazol. The clones used were PB 260, IRR 104, IRR 112, IRR 39, and 105. The GRS combination factor between IAA and Kinetin consisted of seven levels, and three levels of paclobutrazol. The results showed that the rubber clones PB260 and IRR105 gave higher stem girth and plant height than the other clones when applied with GRs at six different levels. The widest leaf area was found in clone IRR 104 compared to other clones when GRs were applied. In addition, paclobutrazol affects plant height. Application of paclobutrazol could reduce plant height for all clones but increases stem girth, skin thickness, number and diameter of latex vessels, and total chlorophyll at the age of 46 months after treatment.
Keywords: Rubber clone; Plant Growth Regulatory Substance; Paclobutrazol; Hormones
INTRODUCTION
Rubber is an export commodity which contributes to Indonesia’s foreign exchange. The plantation area is around 3.4 million hectares, and 80% of this area is smallholder rubber, with a national production of 3.16 million tons (Ditjenbun, 2018). The National Rubber Development Policy was established by Government to put the long-term target for national rubber production about 3.80−4.00 million tons in 2025 (Boerhendhy & Amypalupy, 2011).
The interest of planters in replanting rubber is still an obstacle. The immature period of rubber plantations is longer than that of other plantation crops. The economic life of tapping rubber plants is 4.5 -5 years (Anwar, 2006). This condition requires high investment costs and a longer return on investment compared to oil palm plantations with an immature three-year period.
Assembling cultivation technology to speed up stem girth is carried out by combining conventional and unconventional technologies. One unconventional technology that has not been applied intensively is growing substances when the plants are immature. Substance or microorganism that applied to plants is called a plant biostimulant. It aims to increase nutritional efficiency, abiotic stress tolerance, and/or plant quality properties, regardless of their nutritional content (du Jardin, 2015), for example, growth regulators (GRs). Specifically, organic chemical compounds that modify or regulate physiological processes in plants when applied in small concentrations and in sufficient size is called plant growth
regulator (Desta & Amare, 2021; Tesfahun, 2018). These substances include plant hormones (natural or synthetic) and non-nutritional chemicals that do not occur naturally in plants (Pidlisnyuk et al., 2022).
As responsible factor for secondary growth in the plant stem, hypocotyl, and root tissue, the vascular cambium, proliferate and produce daughter cells. In the next step, it can differentiate into secondary xylem and phloem (Wang, 2020). Cytokinins and gibberellins play an important role in cambium activity (Wang, 2020).
In addition, Paclobutrazol is a retardant whose action is inhibiting gibberellin biosynthesis (ICI, 1986;
Sponsel, 1987; Davis et al., 1988). Its inhibition occurs in the caurenoate formation pathway, so the pathway changes its role to carry out abscisic acid biosynthesis. The impact of this process is to inhibit the vegetative growth of plants so that several factors such as climate, stadia, growth phase, and plant conditions must be considered to be more effective. This physiological effect is closely related to immature rubber plants’ growth, stimulating the stems, so that plant height growth is temporarily inhibited by paclobutrazol so that the impact on the plants becomes shortened and the effect of enlargement of the stem girdle occurs.
The period of unproductive age (the immature plant period) is closely related to the size of the girth of the stem. Stem girth is closely related to production (Gonçalves et al., 2011). In rubber, production parameters such as initial latex flow, total latex volume, and plugging index (IP) are indications of the importance of stem circumference criteria in tapping rubber plants (Gonçalves et al., 2011). Koryati (2016) stated that rubber clones with GRs positively responded to plant growth parameters, anatomical characters, and latex production. To study rubber clones that respond to GRs, it is necessary to carry out further research on other clones based on latex-producing clones and latex and wood-producing clones. Considering that the type of clone involves critical production aspects, namely the productivity level per unit area, the length of the TBM period that must be passed, the stability of production during the TM period, the maintenance costs that must be incurred for each maintenance, and the quality of the rubber produced, the selection of clone types carefully should be a critical consideration for every plantation (Azwar and Ginting, 1990; Anwar, 2006)). This study aimed to examine five rubber clones with a combination of plant growth regulators based on plant growth and anatomical characteristics during the immature plant period.
MATERIALS AND METHODS
The research was conducted from March 2019 to August 2020 for 18 months at the KSO Karang Inong, PTPN-I, and PTPN-III plantations, East Aceh District using clones PB-260, IRR 104, IRR 112, IRR 39, and IRR 105 (Plant age ±28 months). This study was based on a nested design of three factors:
clones, GRs, and paclobutrazol. The clones used for testing were PB 260, IRR 104 and IRR 112, IRR 39, and IRR 105. The GRs combination factor between IAA combined with kinetin consisted of seven levels: 1) Control (H0, without IAA and Kinetin), 2) 400 ppm IAA combined with 50 ppm kinetin (H1), 3) 400 ppm IAA combined with 60 ppm kinetin (H2), 4) 500 ppm IAA combined with 50 ppm kinetin (H3), 5) 500 ppm IAA combined with 60 ppm kinetin (H4), 6) 600 ppm IAA combined with 50 ppm kinetin (H5), and 7) 600 ppm IAA combined with 60 ppm kinetin (H6). The third factor was application of paclobutrazol (P), which consists of three levels, namely 1) Control (P0, without paclobutrazol), 2) application of 2 ml/l water (500 ppm) through the soil (P1), and 3) application of 2 ml/l water (500 ppm) through the leaves (P2). The experiment was nested in the clone-type treatment factor and repeated twice, finally the number of experimental units was 7 x 5 x 3 x 2 = 210 experimental units.
The number of plants used was 840 because of there were four plants in each experimental unit.
Before applying the treatment in this study, it was necessary to make trial plots in selected planting areas where the plant spacing was relatively close between the clones used. For each clone, 168 plants with relatively high homogeneity were selected. Criteria for plant selection included plant height, stem
circumference, crown condition, and root and leaf disease was free. The number of plants was divided into two groups replication and 84 trees respectively.
IAA and Kinetin were mixed with lanolin and applied to a circle of skin that has been rubbed with sandpaper. The stem was lubricated 2.5 cm wide for each plant into two position. The two positions were located on 70 cm and 100 cm above the ground. Once a month for eight applications was applied.
Paclobutrazol was applied according to the treatment. Applications through the soil were only given twice during the study and six months after treatment, while applications by spraying the leaf surface were given eight times starting from the implementation of the research with an interval of once a month.
Plant height was measured at the start of the experiment (28 months), 31 months, and 46 months using bamboo or wood with the set height/length. Measured from the top of the bottom to the top of the tallest straight plants. Stem circumference measurements were made at the start of the experiment (28 months), aged 31 months, and 46 months at the height of 100 cm from the bottom. At the start of the experiment (28 months), aged 31 months, and 46 months, measured leaf area by taking samples of newly formed mature leaves. Determination of the area per leaf was calculated using the following formula: Ld LA =
€ a PL, where Ld LA = Leaf area (cm2), a = constant leaf area = 0.654; P = leaf length (cm), and L = leaf width (cm). Measurements of skin thickness, number of latex vessels, and diameter of latex vessels were carried out at the beginning of the experiment, aged 31 months, and 46 months, while the amount of chlorophyll was carried out at the last observation.
Analysis of variance (ANOVA) was performed to evaluate differences between treatments. Differences between treatments are observed with Tukey HSD. All analyzes were run by R software.
RESULTS AND DISCUSSION
Single Factor of Plant Growth and Anatomical Characters
The single-factor statistical analysis results showed that each hormone, paclobutrazol, and clone tested had a significant effect, as shown in Table 1. Table 1 showed that each clone tested had differences in the parameters observed. The largest plant height, stem girth, and skin thickness were found in clone IRR 104, the largest diameter of latex vessels was found in clone PB 260, and the largest number of latex vessels was found in clone IRR 105. Hormone treatment at various levels significantly increased plant height, stem girth, skin thickness, number of latex vessels, the diameter of latex vessels, leaf area, and amount of chlorophyll. While the paclobutrazol treatment markedly reduced plant height and leaf area, and the application of paclobutrazol through the soil increased stem girth, skin thickness, number of latex vessels and diameter of latex vessels, and the amount of chlorophyll.
Table 1. Average plant height, stem girth, skin thickness, number of latex vessels (JPL), the diameter of latex vessels (DPL), leaf area, and total chlorophyll at 46 months of age in the Clone, GRs, and paclobutrazol treatments
Treatment Growth variables at 46 months
Plant height (m)
Girth (cm)
Skin thickness (mm)
Number of latex vessels
Diameter of latex vessel (mµ)
Leaf width (cm2)
Number of chlorophylls (/mm2) Clone
PB260 10.408 b 40.365ab 6.130 a 5.048 c 24.890 a 56.858 bc 51.560 c IRR 104 10.448 a 40.515 a 6.180 a 5.548 b 23.950 b 65.198 a 51.060 c IRR 112 11.018 a 40.265 b 5.880 b 5.548 b 25.200 a 52.648 c 55.890 ab IRR 37 11.198 a 40.315 b 6.150 a 5.548 b 25.180 a 52.828 c 53.410 bc
IRR105 11.368 a 40.415 a 6.060 ab 6.548 a 24.890 a 60.088 ab 57.290 a Hormone (GRs)
H0 10.345 c 43.429 c 6.380 b 5.621 c 24.909 c 53.240 bc 55.033ab H1 10.465 c 48.819 b 7.44 a 6.821 b 25.799 a 59.270 ab 52.953abc H2 10.765 ab 48.869 ab 7.520 a 6.921 b 26.049 a 55.060 abc 53.603abc H3 10.745 ab 48.699 b 7.620 a 6.421 a 26.339 ab 53.160 bc 53.903abc H4 10.845 a 49.429 a 7,250 ab 5.721 b 25.089 b 54.270 abc 51.163bc H5 10.785 ab 48.449 b 7.500 a 6.221 ab 26.009 b 59.850 a 50.143c H6 10.635 bc 48.819 b 7.280 ab 6.321 ab 27.009 a 52.290 c 55.503a Paclobutrazol
P0 11.190 a 43.456 b 5.788 b 4.823 b 24.711 c 56.832 a 50.120 b P1 9.750 c 48.966 a 7.348 a 6.123 a 25.751 a 53.032 b 51.770 a P2 10.260 b 48.656 a 7.098 a 6.023 a 25.191 b 51.802 b 52.640 a Note: Control (H0, without IAA + kinetin), IAA 400 ppm + kinetin 50 ppm (H1), IAA 400 ppm + kinetin 60 ppm
(H2), IAA 500 ppm + kinetin 50 ppm (H3), IAA 500 ppm + kinetin 60 ppm (H4), IAA 600 ppm + kinetin 50 ppm (H5), IAA 600 ppm + kinetin 60 ppm (H6); Control (P0, without paclobutrazol), 2 ml/l water 500 ppm through the soil (P1) and 2 ml/l water 500 ppm through leaf (P2).Numbers followed by the same letter in the same column group show no significant difference at the 5% level based on the Tukeys’ Honest Significant Difference (HSD) Test
The observation results showed a correlation between plant height, stem girth, and skin thickness. It is well known that one of the most obvious effects of paclobutrazol is growth modification. This change in development manifested phenomena through the inhibition of gibberellin biosynthesis. Its inhibition occurred in the pathway to form caurenoate from oxidation of caurene, so this pathway changed its role to carry out abscisic acid biosynthesis (ICI, 1986; Sponsel, 1987; Davis et al., 1988, Wang, 2020). The impact of this process was to suppress the growth of vegetative cell elongation so that the height growth of rubber clone plants during TBM was inhibited and the effect was on stem enlargement and skin thickness due to the shift in the use of photosynthates.
The results of the study on five clones tested for 18 months (from 28 months to 46 months in the field) show that the clones growing faster in stem girth were clones IRR 104, followed by IRR 105, IRR 112, IRR 39, and PB 260 in 46 months old in the field. It is presumed that the clones tested were commercially recommended clones which were grouped into two groups, namely clones IRR 104, PB 260, and IRR 112, belonging to latex-producing clones, while clones IRR105 and IRR 39 were latex and wood-producing clones (Daslin, 2014; Daslin et al., 2009; Anwar, 2006).
The number of latex vessels strongly affects production, where the more the number of latex vessels, the higher the latex production. Aidi-Daslin et al. (1987) stated that the number of latex vessels and the diameter of the latex vessels were important criteria in clone selection to obtain high-yielding clones.
This is because a large number of vessels and large-diameter vessels will provide high production.
Factors of Interaction between Plant Growth and Anatomical Characters
The analysis of variance shows an interaction between the treatment of rubber clones and GRs (auxin + kinetin) aged 31 and 46 months on the parameters of stem girth and plant height. IRR 104 differed from PB260 and IRR 39 in all GRs treatments. Figures 1 and 2 show a consistent increase in trunk circumference. The combination of clones and PGR gave stem girth growth at IRR 104, and IRR105 compared to PB260 positively. However, there was no difference between IRR 104 and IRR105, and IRR 39 for rubber trees aged 31 months (Figure 1). In a rubber tree aged 46 months, IRR 104 differed from IRR 105 and IRR 112 (Figure 2).
In rubber clones, each clone gave a different growth. Based on these conditions, in a rubber tree breeding program, there were several rubber clone selection parameters, such as rubber yield, precocity
(initial attainment of the poultice circumference), and wood yield potential (Gonçalves et al., 2011).
Several clones were reported to provide differences in plant height and stem diameter under drought stress such as IRR434 (Pasaribu et al., 2022). Under environmental stress, several clones showed different responses in leaf and flowering phenology, for example, the response of rubber clones to drought stress (Gutiérrez-Vanegas et al., 2020).
A plant regulatory substance, auxin is essential for growth of plant and regulates some developmental processes (Bozsó & Barna, 2021; Celik & Tuluce, 2006; Novak et al., 2014; Okazawa et al., 1967;
Pidlisnyuk et al., 2022; Su et al., 2011; Woodward & Bartel, 2005). Circumference and chlorophyll count increase with GRs, such as IAA, in combination with Kinetin (Koryati et al., 2015; Okazawa et al., 1967). In addition, kinetin functions to maintain callus development. It suggests that callus stimulation due to exogenous auxin is thought to be mediated by the kinetin addition (Okazawa et al., 1967). Most of the research conducted has been concerned with the auxin-like activity of melatonin which, like IAA, is able to stimulate shoot and root growth and induce root formation, increase the new lateral and adventitious roots (Arnao & Hernández -Ruiz, 2018; Su et al., 2011). Even though many compounds (both natural and synthetic) demonstrate auxin-like activity in bioassays, indole-3-acetic acid is known as a crucial auxin in almost all plants. The indole-3-acetic acid (or IAA) is resulted from tryptophan using a Trp-dependent pathway and from indolic Trp precursors via a Trp-independent pathway (Woodward & Bartel, 2005). There is a relationship between auxin and cambium, but the mechanism is not clearly explained. Some evidence is described, such as the removal of the shoot apex stops cambium activity and secondary growth, whereas exogenous application of synthetic auxin reactivates cambium cell proliferation (Wang, 2020). The role of cambium as an internal meristematic tissue is a niche for stem cells and is arranged in tube-like domains spanning the growth axis (Sehr et al., 2010). The vascular cambium provides to lateral growth in dicotils and gymnosperms (Wang, 2020).
Figure 1. Differences in stem girth (cm) based on the combination of GRs and rubber clones aged 31 months. Control (H0, without IAA + kinetin), IAA 400 ppm + kinetin 50 ppm (H1), IAA 400 ppm + kinetin 60 ppm (H2), IAA 500 ppm + kinetin 50 ppm (H3), IAA 500 ppm + kinetin 60 ppm (H4), IAA 600 ppm + kinetin 50 ppm (H5), IAA 600 ppm + kinetin 60 ppm (H6). The same letter followed numbers in the same column describe no significant difference at the 5% level based on the Tukeys’ Honest Significant Difference (HSD) Test
Figure 2. Differences in stem girth (cm) based on the combination of GRs and rubber clones aged 46 months. Control (H0, without IAA + kinetin), IAA 400 ppm + kinetin 50 ppm (H1), IAA 400 ppm + kinetin 60 ppm (H2), IAA 500 ppm + kinetin 50 ppm (H3), IAA 500 ppm + kinetin 60 ppm (H4), IAA 600 ppm + kinetin 50 ppm (H5), IAA 600 ppm + kinetin 60 ppm (H6). The same letter followed numbers in the same group describe no significant difference at the 5% level based on the Tukeys’ Honest Significant Difference (HSD) Test
Figure 3. Differences in plant height (m) based on the combination of GRs and rubber clones at 31 months of age. Control (H0, without IAA + kinetin), IAA 400 ppm + kinetin 50 ppm (H1), IAA 400 ppm + kinetin 60 ppm (H2), IAA 500 ppm + kinetin 50 ppm (H3), IAA 500 ppm + kinetin 60 ppm (H4), IAA 600 ppm + kinetin 50 ppm (H5), IAA 600 ppm + kinetin 60 ppm (H6). The same letter followed numbers in the same group describe no significant difference at the 5% level based on the Tukeys’ Honest Significant Difference (HSD) Test.
Figure 4. Differences in plant height (m) combination of GRs and rubber clones at 46 months of age.
Control (H0, without IAA + kinetin), IAA 400 ppm + kinetin 50 ppm (H1), IAA 400 ppm + kinetin 60 ppm (H2), IAA 500 ppm + kinetin 50 ppm (H3), IAA 500 ppm + kinetin 60 ppm (H4), IAA 600 ppm + kinetin 50 ppm (H5), IAA 600 ppm + kinetin 60 ppm (H6). The same letter followed numbers in the same group describe no significant difference at the 5% level based on the Tukeys’ Honest Significant Difference (HSD) Test
Based on the analysis of variance, the combination of GRs and rubber clones gave a difference in leaf area, while paclobutrazol showed no difference. Figure 5 consistently shows that the combination of GRs and clones gave different parameters of leaf area at 46 months of age. The largest leaf area was obtained from clone IRR104 and given GRs IAA 600 ppm + Kinetin 50 ppm, and different from other treatment combinations and leaf area. Meanwhile, the smallest leaf area was found in clone IRR112 and given GRs IAA 500 ppm + kinetin 60 ppm, and there was no difference with other clones. This is because GRs was applied in stems, and the transport of each hormone was different. IAA has a rather complicated distribution, but it is clear that IAA is synthesized in meristematic areas, from leaf shoots and side shoots. IAA dispersal in plants is mainly regulated by IAA polar transport (Davies, 1997;
Salisbury and Ross, 1992; Pidlisnyuk et al., 2022). Besides, the plant age and rubber clones affect the auxin content in rubber leaves (Koryati, 2011).
Figure 5. Differences in leaf area (cm2) combined GRs (IAA+Kinetin) and rubber clones at 46 months of age
There was an interaction between the GRs combination (auxin + kinetin) and paclobutrazol on parameters of skin thickness, number of latex vessels, and diameter of latex vessels at 46 months of age in the field (Table 2).
0 10 20 30 40 50 60 70 80
PB 269 IRR 104 IRR 112 IRR 39 IRR 105 Leaf area (cm2)
Clone
H0 H1 H2 H3 H4 H5 H6
Table 2. Test for Difference in Mean Skin Thickness, Number of Latex Vessels, and Diameter of Latex Vessels at 46 months of age on Hormone (GRs) and Paclobutrazol Treatment Interactions Paclobutrazol
Hormones (IAA+Kinetin)
Growth variables at 46 months Skin
thickness (mm)
Number of latex vessels
Diameter of latex vessels (mµ)
H0 5.165 d 4.613 d 24.417 c
P0 H1 6.225 bc 5.813 bc 25.307 bc
H2 6.305 bc 6.113 b 25.557 bc
H3 6.405 b 6,413 ab 25.847 b
H4 6.035 c 5.713 c 24.597 c
H5 6.285 bc 6.213 b 25.517 bc
H6 6,065 c 6.213 b 26.327 a
H0 6.415 ab 5.913 bc 25.457 bc
H1 6.576 ab 5.513 c 25.257 bc
P1 H2 6.745 a 6.613 ab 25.327 bc
H3 6.625 a 6.913 a 25.237 bc
H4 6,035 c 6.113 b 25.173 bc
H5 6.285 bc 6.113 b 25.357 b
H6 6.065 c 5.833 bc 26.567 a
H0 6.465 b 5.813 bc 25.047 bc
H1 6.405 b 5.813 bc 25.047 bc
P2 H2 6.385 bc 6.113 b 24.557 c
H3 6.355 bc 5.713 bc 25.607 b
H4 6.475 b 6.013 b 25.487 b
H5 6.075 c 6.713 a 25.167 bc
H6 6.295 bc 5.533 c 26.077 b
Note: Control (H0, without IAA + kinetin), IAA 400 ppm + kinetin 50 ppm (H1), IAA 400 ppm + kinetin 60 ppm (H2), IAA 500 ppm + kinetin 50 ppm (H3), IAA 500 ppm + kinetin 60 ppm (H4), IAA 600 ppm + kinetin 50 ppm (H5), IAA 600 ppm + kinetin 60 ppm (H6); Control (P0, without paclobutrazol), 2 ml/l water 500 ppm through the soil (P1) and 2 ml/l water 500 ppm through leaf (P2). The same letter followed numbers in the same group coloumn describe no significant difference at the 5% level based on the Tukeys’ Honest Significant Difference (HSD) Test
Table 2 shows that the interaction of GRs and paclobutrazol combination treatment at 46 months of observation showed that skin thickness and the largest number of vessels were in the P1H2 treatment, followed by P1 H3, which was significantly different from the other treatments. Meanwhile, the diameter of the largest latex vessels was in the P1H6 treatment combination. It is well known that one of the most obvious effects of paclobutrazol is growth modification. This change in development manifested phenomena through the inhibition of gibberellin biosynthesis. Its inhibition occurred in the pathway to form caurenoate from oxidation of caurene, so this pathway changed its role to carry out abscisic acid biosynthesis (ICI, 1986; Sponsel, 1987; Davis et al., 1988; Desta and Amare, 2021). The impact of this process was to suppress the growth of vegetative cell elongation so that the height growth of rubber clone plants during TBM was inhibited, and the effect was on stem enlargement and skin thickness due to the shift in the use of photosynthates. The ability of GRs was related to the growth and development of plants; thus, the growth was directed towards the development of girth (Pahiihan and Wain, 1975). In addition, applying GRs (auxin + kinetin) can cause rapid cell differentiation so that vessels develop (Figure 6). It is suspected that exogenously administered auxin will activates with proteins from the plasma membrane, thus the protein's shape will be changed. It provides the changes in permeability properties of the membrane. Water together with organic ions and molecules (dissolved) left or entered the cell and changed cells on the osmotic pressure aspect. This changes affected on cell biochemical processes and a series of cellular reactions that produced a response on growth, i.e. the
plant organs formation, vessel development, and chemical composition (Wattimena, 1988; Pidlisnyuk et al., 2022).
Figure 6. Diameter of latex vessels (A) and number of latex vessels (B) treated by GRs and Paclobutrazol.
There was no interaction between the three treatment factors (clones, GRs and paclobutrazol).
It can be concluded that the combination of rubber clones and PGR affects stem girth and plant height.
PB260 and IRR 105 showed differences from other clones when applied by GRs. Genetic aspects (Sant’Anna et al., 2020) combined with PGR affect rubber growth, such as stem girth and plant height (Gutiérrez-Vanegas et al., 2020; Novak et al., 2014; Woodward & Bartel, 2005). Clone IRR 104 showed the widest leaf area compared to other clones when applied with GRs, and clone IRR 112 was the smallest. In addition, paclobutrazol showed differences in plant height; application of paclobutrazol through soil suppressed plant height for all clones but increased stem girth. The largest plant height was found in clone IRR 105 with the application of paclobutrazol through the soil. The combination application of GRs and paclobutrazol positively affected skin thickness, the number of latex vessels, and the diameter of latex vessels. Thus, using GRs (IAA+Kinetin) and paclobutrazol on rubber during TBM had a very positive effect. The five clones tested all gave different responses to GRs and paclobutrazol treatment; this is in line with the results of a previous study by Koryati (2016). Therefore, it can be seen that studies of the PB 260, IRR 104, IRR 112, IRR 39, and IRR 105 clones that were tried gave a positive response to the application of GRs and paclobutrazol in various combinations.
The use of superior rubber clones with a high production is the main requirement for determining the success of rubber plantation agribusiness. Thus, rubber plant breeders continue striving to obtain new superior clones with high yield potential and the desired agronomic characteristics (Woelan et al., 2007). The five clones tested were superior latex and latex and wood-producing clones and responded to GRs at various levels. The results of the GRs application research on five clones can accelerate the increase in stem girth and the maturity of rubber clone tapping because stem girth is a criterion for rubber plants that can be tapped. In rubber plantations, the standard provision for early tapping (exploitation) is that 70% in the area of the location has reached tapping maturity or has been able to produce with the criterion that the stem girth measured at the 100 cm height from the grafting link has reached 45 cm with at least 7 mm of a skin thickness of (Anwar, 2006).
CONCLUSIONS
There were significant differences between the clones tested on plant height, stem girth, skin thickness, leaf area, total chlorophyll, number of latex vessels, and diameter of latex vessels. PB 260 and IRR 105 rubber clones gave higher stem girth and plant height than other clones when applied with GRs under six different levels. Applying Paclobutrazol through the soil in each clone reduced plant height and leaf area but increased stem girth, skin thickness, number of latex vessels, and diameter of latex vessels; in this case, clone IRR 105 gave greater stem circumference compared to other clones. The single and combination application of GRs and paclobutrazol effect positively growth variables of each clone especially stem girth, skin thickness, number and diameter of latex vessels.
ACKNOWLEDGEMENT
The authors thank the Faculty of Agriculture, Amir Hamzah University, Rubber Research Center, and Islamic University of North Sumatera, Medan, Indonesia, for facilitating this research. In addition, the authors thank all partners for contributing to this research
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