Ji

Ministry of Agriculture’s Key Laboratory of Biology and Genetic Resources of Rubber Tree/Rubber Research Institute,Chinese Academy of Tropical Agricultural Sciences,Danzhou 571737,China

Responsible editor:Tingting XU Responsible proofreader:Xiaoyan WU

Labor shortage and increased spending on tapping rubber has seriously hampered the development of China’s natural rubber industry in recent years.The tapping technology and productivity should be improved continuously so as to improve the latex yield and output value per plant,breaking the current im passe of natural rubber industry in a short term[1].Micro-tapping adopted ethylene gas stimulation,and it is characterized by short cut (only 5-10 cm),simple operation,high tapping speed and low bark consumption.Micro-tapping can significantly improve the tapping productivity,reducing labor input and improving production efficiency.It is one of the effective ways to solve the shortage of tapping work ers[2].The studies on micro-tapping were started in 1996.After 10-year research,some technical difficulties in micro-tapping have been basically solved[3].In recent years,micro-tapping has been applied and promoted in Hainan,Guangdong and Yunnan,main rubber-planting areas in China,which has become an important direction for further deepening the reform of tapping system[4-9].

Instead of conventional ethrel,micro-tapping adopts high-concentration ethylene gas for stimulating latex production.The gas concentration used in current production and experiments is all 100% of ethylene,of which the stimulus intensity is theoretically 3-10 times higher than that of conventional ethrel[10].Under high-intensity stimula tion,the yield improvement effect of micro-tapping is significant in short term; however,the long term use of micro-tapping may lead to unstable latex yield,severe late dripping,increased thickness of phellem layer,etc[11].To reduce the side effects of high-intensity ethylene stimulation,the amount of inflated gas for micro-tapping has been studied[12].However,the gas concentration suitable for microtapping has not been studied.Currently,more attention is paid to the application and promotion of micro-tapping technology in field production tests,but there are rare reports on effects of ethylene concentration for micro-tapping on physiological parameters and yield of latex.PR107 is a line of which the latex production and discharging characteristics are more suitable for ethylene stimulation.It is an ideal material for researching the physiological responses of rubber tree to ethylene stimulation[13].Therefore,PR107 was selected as test material,and the effects of ethylene concentration for micro-tapping on yield and physiological parameters of latex were studied so as to screen out the optimum ethylene concentration for micro-tapping in rubber tree clone PR107.This will contribute to the large-scale application and promotion of micro-tapping technology,and lay a foundation for the further development of high-efficiency and high-security gas stimulation-based new tapping system.

Material and Methods

Material

The healthy rubber tree clone PR107,which was planted in the test base of Chinese Academy of Tropical Agricultural Sciences in 1987 and started to be tapped in 1995,was selected as test material.

Methods

The randomized complete block design was adopted.A total of seven treatments were designed,including S/2 d4 ET3.5% (CK1),S/4U d4 ET3.5% (CK2),S/8U d4 ETG20%(T1),S/8U d4 ETG40% (T2),S/8U d4 ETG60% (T3),S/8U d4 ETG80% (T4)and S/8U d4 ETG100% (T5).There were three duplicates,with 5 plants per duplicate,for each treatment.In T1 and T2 groups,the compound ethrel was smeared once every two cuts; in the other five treatment groups,the corresponding-concentration ethylene was inflated once every two cuts.The gas chamber was mounted 20-30 cm above the cut line.When the chamber was empty,40 ml of gas was filled every time.The test was carried out during June to November of 2013.Before the beginning of the test,all the rubber trees were cut three knives by d4 but not smeared with ethrel,and then the latex was collected for base analysis.Since the test was started,the microtapping with ethylene gas stimulation was applied.In each of the 16 stimulation cycles in 2013,the latex was sampled twice at the middle of every month.The physiological parameters of the collected latex were determined.

The latex within 5-35 min after tapping was collected.The latex collected from the five rubber trees in a same plot was mixed together and treated as one sample.All the collected samples were bathed in ice and taken to the laboratory for analysis.

The sucrose content,thiol content,inorganic phosphorus content,dry rubber content and magnesium ion content were determined by conventional methods[14-15].The latex yield and dry rubber content for every cut in each of the plots were determined.

Dry rubber yield = Latex yield ×Dry rubber content.

Data statistics and analysis

The data statistics and analysis were performed using SPSS 13.0.The differences in latex yield and physiological parameters before and after the treatment were analyzed using T-test;the differences in latex yield and physiological parameters among treatment groups were analyzed using Duncan’s multiple comparison test of the Oneway ANOVA model; the correlations between ethylene concentration and dry rubber content and physiological parameters were analyzed using Pearson analysis.

Results and Analysis

Effect of different ethylene concentration for micro-tapping on latex yield

As shown in Fig.1,the average dry rubber yields of upward tapping(CK2 and the other five treatments)were all lower than that of conventional tapping with ethrel stimulation (CK1).Among all the upward tapping treatment groups,the yields of T1 and T2 were significantly higher than that of CK2,and they were also higher than those of the other micro-tapping treatment groups with different concentrations of ethylene stimulation.The yields (per plant,per time) of T1 and T2 reached 111.78 and 109.62 g,respectively.The average yields of T3,T4 and T5 were all higher than that of CK2(P ＞0.05).

Effect of different ethylene concentration for micro-tapping on dry rubber content

As shown in Fig.2,the average dry rubber contents of upward tapping(CK2 and the other five treatments)were lower than that of conventional tapping with ethrel stimulation (CK1).The dry rubber contents of micro-tapping treatments were lower than that of upward tapping with ethrel stimulation (CK2).Among all the micro-tapping treatments,the dry rubber content was negatively related to ethylene concentration(R=-0.955,P=0.012).With the increase of ethylene concentration,the dry rubber content in latex was reduced.Among all the micro-tapping treatments with ethylene stimulation,the dry rubber content of T1 was highest (30.78%),which showed no significant differences with those of CK1 and CK2; while the dry rubber content of T5 was lowest (27.5%),which was significantly lower than those of T1 and T2.

Effect of different ethylene concentration for micro-tapping on physiological parameters of latex

Table1 showed that the sucrose contents of upward tapping treatments(CK2 and the other five treatments)before the test were higher than that of downward tapping treatment (CK1) after the test.After the test,the sucrose content of downward tapping treatment was significantly increased,but it was still much lower than those of upward tapping treatments.Among all the upward tapping treatments,great differences were shown in variation amplitudes of sucrose content before and after the test.The declining amplitudes of CK2 (-3.62),T4 (-1.93) and T5 (-2.03) were largest,while the variation amplitudes of T1,T2 and T3were relatively small.After the test,the sucrose content of T2 was slightly increased.Similar to sucrose content,the inorganic phosphorus contents of upward tapping treatments before the test were significantly higher than that of downward tapping treatment; the inorganic phosphorus content of downward tapping treatment after the test was significantly increased,but it was still lower than those of micro-tapping treatments and higher than that of CK2.Among all the upward tapping treatments,the inorganic phosphorus content of CK2 was significantly reduced after the test(-5.14); among all the micro-tapping treatments,the inorganic phosphorus contents of all the treatments were significantly increased after the test except that of T3,and the increasing amplitude showed certain positive relationships with ethylene concentration (R =0.962,P=0.009).

Table1 Comparison of sucrose content and inorganic phosphorus content among different treatment groups

Table2 Comparison of thiol content and magnesium ion content among different treatment groups

Table2 showed that there were no significant differences in thiol content among all the treatments before the test; the thiol contents of upward tapping treatments were significantly increased after the test,but the thiol content of CK1 did not change significantly after the test.So the thiol contents of upward tapping treatments were significantly higher than that of CK1 after the test.Among all the upward tapping treatments,the increasing amplitudes of thiol content of T1 and T2 were larger than those of CK2,T3,T4 and T5.After the test,the thiol contents of T1 (1.159 mmol/L)and T2(1.135 mmol/L)were highest.

Before the test,there were great significances in magnesium ion content among all the treatments.After the test,the magnesium contents of upward tapping treatments were lower than that of CK1 except that of T1.In terms of variation amplitude,the declining amplitudes of magnesium content of CK1,T4 and T5 were relatively large,while the declining amplitudes of magnesium content of CK1,T1,T2 and T3 were relatively small(Table2).

Conclusions and Discussion

Effect of different concentration of ethylene on latex yield

The test results showed that,compared with that of conventional tapping with ethrel stimulation,the yield improvement effect of micro-tapping with ethylene stimulation was not obvious.This might be caused by lower tapping height,larger tree spiral and longer tapping length in control groups.In addition,the tapping with ethylene stimulation led to the occurrence of long-time latex flow.Due to the effects of weather,security and other factors,part of the latex was not calculated in yield,and this was also a reason for the non-ideal treatment effect of tapping with ethylene stimulation.Among all the upward tapping treatments,the yields of micro-tapping treatments,especially the micro-tapping treatments stimulated by lower concentrations of ethylene,were higher than that of CK2.It was suggested that at the same tapping height,in the premise of upward tapping,micro-tapping with ethylene stimulation,especially with low-concentration ethylene stimulation,still had good yield improvement effect.Among all the tapping treatments stimulated by different concentrations of ethylene gas,the yield of PR107 was not linearly related to ethylene concentration.Under conditions of tapping stimulated by high concentration(60%-80%)of ethylene,the dry rubber yield was not improved significantly,and the risk of long-time latex flow was high; instead,the increase of dry rubber yield was relatively large for tapping stimulated by lower concentration of ethylene.These study results were consistent with the previous study results of micro-tapping with different concentration of ethylene in RRIM600 (unpublished).However,20% ethylene showed good stimulation effect in PR107,but not in RRIM600,indicating that lower concentration of ethylene was more applicable to PR107 than to RRIM600.

Effect of different concentration of ethylene on dry rubber content

The dry rubber content in latex reflects the hydrocarbons and latex synthesis and regeneration ability of lacticifer vessel system.It affects the viscosity of latex.Too high dry rubber content may lead to blocked latex flow,affecting latex yield;too low dry rubber content indicates incomplete latex regeneration,reflecting excessive tapping or impaired functions of lacticifer vessel system[16].The test results showed that the average dry rubber content of micro-tapping with ethylene stimulation was significantly lower than that of conventional tapping with ethrel stimulation,which was consistent with the study results of Sivakumaran and Xiao et al[12,17].The comparison of dry rubber content among micro-tapping treatments stimulated by different concentrations of ethylene showed that the average dry rubber content was negatively related to ethylene concentration.The higher the ethylene concentration is,the lower the dry rubber content in latex is.It was suggested that too high concentration of ethylene may lead to excessive latex flow,incomplete latex regeneration and impaired functions of lacticifer vessel system,as well as increased risk of long-time latex flow,resulting in the loss of effective yield.

Effects of different concentration of ethylene on physiological parameters of latex

Sucrose is the original material for synthesis of polyisoprene,and it reflects the coordination between rubber-producing capacity of rubber trees and rubber tapping intensity to some extent.Inorganic phosphorus is closely related to energy metabolism.High inorganic phosphorus content helps to enhance the activity of metabolism.Thiol is the sum of several reductants in latex,such as glutathione,and it reflects the protection ability of latex on cell membranes.High content of thiol can extend the latex flow time,increasing latex yield;while too low content of thiol may weaken the ability of non-enzymatic protection system,leading to tapping panel dryness.In the metabolism in lacticifer vessel system,magnesium ions have contradictory effects:it can neutralize the negative charge carried by rubber particles,promoting latex coagulum;it is an essential activator for some enzymes in latex,including ATP enzymes,transferases and PEP carboxylase,but it is also an inhibitor for some enzymes in latex,including invertase and acid phosphatase for hydrolyzing nucleotides[16].

The test results of this study showed that the thiol and inorganic phosphorus contents in latex obtained by micro-tapping with ethylene stimulation were increased in varying degrees,but the sucrose and magnesium ion contents in latex obtained by micro-tapping with ethylene stimulation,especially high-concentration ethylene stimulation,were greatly decreased,which were consistent with the study results of Xiao and Yang et al[18].It was indicated that the ethylene stimulation improved the metabolic activity of latex,enhanced the stability of latex and extended the latex flow time,expanding the latex drainage area of rubber trees,thereby improving the yield.Under stimulation of high-concentration ethylene,the latex yield was not significantly increased;however,the inorganic phosphorus content in latex was significantly increased,sucrose and magnesium ion contents were significantly decreased,and the thiol content in latex was slightly increased.It was suggested that high-concentration ethylene produced high-intensity physiological response to stimulation,resulting in hypermetabolism in rubber trees,thereby consuming large amounts of sucrose,slowing the latex coagulation and extending the latex flow.However,longterm and high-intensity stimulation caused physiological deficiencies in rubber trees,represented by reduced yield and reduced sucrose content.These were similar to the effects of micro-tapping with high-concentration ethylene stimulation on latex yield and physiological parameters of rubber tree RRIM600[16].

Based on the analysis of latex yield and physiological parameters above,it can be concluded that the ethylene concentration applied by micro-tapping is not linearly related to latex yield of rubber tree clone PR107,and too high-concentration ethylene cannot improve latex yield.Under stimulation of high-concentration (＞60%)ethylene,the latex metabolism of rubber trees will be enhanced,the sucrose consumption will be increased,the magnesium ion content will be reduced,the rubber discharging time will be extended and the risk of late dripping will be increased,resulting in reduced dry rubber content and unstable latex yield.However,when low-concentration(＜40%)ethylene is adopted,the physiological features of rubber trees will be relatively steady,and the dry rubber content in latex also be stable,thereby improving latex yield.

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