Introduction

Tapping in the rubber tree (Hevea brasiliensis Müell. Arg.) consists of making a cut in the cortex of the tree with a given frequency during the year (^{Compagnon, 1998}) in order for the tree to produce latex. The stem is divided in two symmetrical longitudinal zones, in east-west direction, each one called “tapping panel”. The low panel is tapped from 1 m above the ground level and the high panel starts at 1.5 m above ground level upwards (^{Rojo et al., 2011}). The rubber tree has the particular quality of regenerating the cortex of the cut made in the tap. The intensive tapping systems in rubber trees can lead to higher production, but also to high rates of depletion of the tapping panel, short life cycles of the plantations and low labor productivity (^{Chantuma et al., 2011}).

The variation of phloem turgor pressure with the hour of the day, height, age of the tree, circumference, age of the cortex, and stimulation with ethrel or ethephon (2- chloroethyl phosphonic acid) has a positive relationship with the latex yield of the tree. Phloem turgor pressure is an indicator of the phloem development of the tree and the potential latex yield (^{An et al., 2014}).

The rubber plantations established in Mexico in the middle of the 1980’s were habilitated with genetic material whose maximum yield, under optimum conditions, was estimated at 900 kg of dry latex ha^-1 year^-1. Due to poor management of the plantations in their pre-productive period, their development has not been satisfactory, therefore they present low yield (500 kg of dry latex ha^-1 year^-1) (^{SAGARPA, 2012}).

Of the rubber plantations, 30 % were exploited for more than 25 years, and currently present very deteriorated lower panels and high panels in good conditions. In this percentage there are also young plantations, which were poorly managed and consequently have panels in bad condition (^{Fundación Produce Oaxaca, 2007}).

Stimulation with ethrel is essential to increase latex production in rubber trees of small producers and agro-industrial plantations (^{Lacote et al., 2010}). This treatment of rubber trees is to increase latex productivity by decreasing tapping frequency. An effective system of management and utility is achieved by adequately combining the systems of tapping and stimulation (^{Rojo et al., 2011}).

The systems with low tapping frequency and stimulation with ethrel in the system in half spiral every four and five days (½ S d/4 5d/7 ET 2.5 % and ½ S d/7 5d/7 ET 2.5%), produce more latex per plant and per tap in the lower panel, with respect to the system of tapping every two days (½ S d/2 5d/7 ET 2.5 %) in the same time, with lower extraction of nutrients (^{Silva et al., 2012}).

Stimulation with ethrel should not be applied during the renovation of leaves in rubber trees (^{Compagnon, 1998}). In Mexico defoliation is from February to May, which coincides with the dry season, thus neither the application of stimulants nor tapping is recommended during this period (^{Hernández, 2005}).

Latex yields in polyclonal plantations of regenerated cortex increment 28 % with the application of ethrel and 38 % in those of virgin cortex, in half spiral with downward cut in lower panels; the lower increments could reach 20 % (^{SAGARPA-INIFAP, 2011}). However, the excessive use of ethrel can have a negative effect on the life of the rubber trees and reduce yield (^{Jetro and Simon, 2007}; ^{She et al., 2013}).

The surface of rubber planted in the state of Tabasco, Mexico, is distributed in six municipalities: Huimanguillo, with 84 %, is the most important in the Chontalpa Region; Macuspana, Jalapa, Teapa and Tacotalpa, 15.93 % in the Sierra Region; and Tenosique, with 1.86 % in the Rivers Region (^{SAGARPA-INIFAP, 2011}).

The plantations that are used have an age of 7 to 46 years. 24 % is in the “young” stage, 7 to 12 years; 65 % is in the “adult stage, with 13 to 32 years; and the rest are considered plantations in the “old age” stage, with over 32 years. The plantations with the greatest planted surface, according to their physiological state, are those in the “adult” stage (^{Izquierdo et al., 2011}). Latex yield of rubber trees decreases from 25 to 30 years of productive age, and are later abandoned, burned or cut (^{Monroy et al., 2006}).

The diseases which are frequent in the lower panel, with downward cut in half spiral (1/2 S) in the tree stem, are mouldy rot in recent wounds, during tree tapping, produced by Ceratocystis fimbriata Elliot, black stripe in the stem produced by Phytopthora palmivora Bult and gangrenous spot or stem cancer that affects the tapping panel with the appearance of parallel vertical lines and exudates propagated to the woody zone of the tree (Picón et al., 1997).

Ascending tap is an alternative for lengthening the productive cycle and that the lower panels regenerate with time. The objective of the present study was to know the response in latex production of the high panel in plantations of productive age, stimulated with ethrel at 5 %, and three tapping frequencies. It is expected that this will prolong the harvesting time of the plantations; considering that in Tabasco 11 % of the surface planted with rubber is in the state of “old age”; the high panel of these trees can be exploited with upward tapping, and 65 % of the adult plantations which have had poor management of the panel or deficient control of diseases, generating bad regeneration of the lower panels. Therefore, it is necessary to identify the diseases that affect the high panel in upward tap. The hypothesis was that the application of upward tapping in the high panel along with stimulation with ethrel can prolong the useful life of the plantations, given that they present good production.

Materials and Methods

The study was conducted in four rubber plantations in Chicoacán and Pedregal Moctezuma first section of the municipality of Huimanguillo, Tabasco, Mexico (17° 19’ N and 93° 23’ W) (Figure 1), with climate Am(f), warm humid with rains in summer, maximum temperatures 31.7, minimum 21.9 and mean annual 24.1 °C. The soils are of the Acrisol group (^{Zavala-Cruz et al., 2014}).

Figure 1 Location of rubber plantations of the study in the municipality of Huimanguillo, Tabasco. 

To evaluate dry latex production, with ethrel stimulant at 5 %, in the high panel, work was carried out in two plantations with 20 years of age and two with 28 years.

In each plantation six treatments were established, with three tapping frequencies stimulated with ethrel (5 %), and three controls that correspond to the same tapping frequencies without stimulation (0 %). The treatments with stimulation were three replications and the controls with one. Each treatment included 10 trees and 129 trees per plantation. The taps were: (¼ S ↑ d6 6d/7) upward tap in quarter spiral, every six work days of seven days per week (one tap per week), (¼ S ↑ d3 6d/7) upward tap in quarter spiral every three work days (two taps per week) and (¼ S ↑ d2 6d/7) upward tap in quarter spiral every two work days (three taps per week). In the three controls the same tapping frequencies were applied without stimulant (Table 1).

Table 1 Distribution of the upward tapping frequencies during the week in rubber plantations 

¼ S ↑ d6 6d/7 = upward tap in quarter spiral every six work days; ¼ S ↑ d3 6d/7 = upward tap in quarter spiral every three work days; ¼ S ↑ d3 6d/7 = upward tap in quarter spiral every two work days. Six days a week are considered work days.

The perimeter of the rubber trees was measured at 1.30 m height with a tape measure, to analyze the correlation of production with the stem thickness of the tree. In the installation of the study, the stem was divided in two symmetrical longitudinal parts, in east-west direction, each part is referred to as “tapping panel”. The high panel began with a cut in quarter spiral of the laticiferous vesicles, at 1.5 m from the base of the tree. The cut was made with a knife for upward tap at 45° inclination, dividing each tapping panel in two halves and using ¼ S of the panel in upward tap.

The rubber trees were stimulated with 2- chloroethyl phosphonic acid (ethrel) (21.70 g of active ingredient) diluted to 5 % with water base. The ethrel mixture was applied with a one inch brush, scraping the cortex of the high panel in a width of 2 cm, above the cut. The first application of ethrel was made on June 13, the second on August 13 and the third on October 13.

The experimental design was split plots, modification of the number of replicates of the control, using the available material, randomly assigning the treatments in each one of the participating plots. In the four rubber plantations six treatments were distributed, three of which were treatments of tapping frequency with application of stimulant and three without application of stimulant (controls) (Table 2).

Table 2 Treatments for the application of upward tapping in quarter spiral in plantations of 20 and 28 years of age. 

Upward tap in quarter spiral every six work days: ¼ S ↑ d6 6d/7; upward tap in quarter spiral every three work days: ¼ S ↑ d3 6d/7; upward tap in quarter spiral every two work days: ¼ S ↑ d2 6d/7.

The experiment was carried out from June 16 to December 14 with the purpose of evaluating latex production in the period of highest production and to determine whether the upward tapping is a production alternative.

Latex production (g) was measured weekly. Coagulated latex was obtained in the form of “cheese” and was dried at ambient temperature, then weighed (precision scale of 0.1 mg, OHAUS) and the data was used to calculate monthly dry latex production per tree. The high panel of upward tapping was also visually inspected each week to detect fungal diseases.

The data of latex production were analyzed as a mixed linear model in R with Infostat. Age of the trees, tapping frequencies and their interaction were considered fixed effects. The effect of unit (each tree) within the treatment, that of the treatment within the plantation and that of the plantation (production as a function of the site where it is found) are considered random effects. The comparison of means was carried out with the test of significant minimum differences of Fisher with correction of Bonferroni (LSD Fisher). Tree perimeter was used as covariable, but it was not significant; therefore it was not included in the analysis.

The data did not comply with the assumptions that should be considered in the model, given that they presented heterocedasticity and autocorrelation among the periods (Figure 2 and 3); therefore a model to correct them was employed.

Figure 2 Studentized residuals of the predicted values (Res cond stand) that show heterogeneity of variances. 

Figure 3 Autocorrelation of the residuals of the model of the observations of dry rubber production (g). 

The residuals obtained with the model, with distinct variances in each population, did not show heterocedasticity (Figure 4). Furthermore, the model that considers the correlation among the times eliminated the autocorrelation (Figure 5).

Figure 4 Distribution of predicted values as a function of the standardized residuals of Pearson with a model that contemplates distinct variances for each population. 

Figure 5 Function of autocorrelation of the residuals of the model, that included the modeling of the serial autocorrelation. 

The graph Q-Q in the residuals of the last model showed its approximation to the normal distribution (Figure 6).

Figure 6 Standardized residuals with a normal distribution. 

Results and Discussion

Of the number of taps programmed in the treatments, 45 % were applied in the tapping frequency of once a week in the plantations of 20 years, and 48 % in those of 28 years; in the tapping frequency of twice a week 47 % in plantations of 20 years and 52 % in those of 28 years. In the tapping frequency of three times per week, 47 % was applied in those of 20 years and 49 % in those of 28 years. These differences were due to the frequent rainfall in September, November and December. When it rains, the water runs down the stem of the tree and during tapping, the latex spills or does not flow through the channel to the collection cup (Table 3).

Table 3 Number of tapping of the three frequencies in each rubber plantation stimulated with ethrel for six months. 

The rains decreased the number of tappings programmed for the study, increased the time without harvest of the tree and caused the cortex to harden and scar over. The frequency of three tappings per week was the most affected by the rains, with respect to that of one and two tappings per week, given that the rains occurred once or twice a week and the tappings were made three times a week.

In hypothesis tests of the fixed effects, statistical differences were observed (p>0.05) in the production of latex generated in the tapping frequencies applied (one, two and three tappings per week) and in the effect of interaction of the ages of 20 and 28 years of the plantations with the tapping frequency (Table 4).

Table 4 Hypothesis test of the age, tapping frequencies and their interaction (fixed effects) that determines whether there is some effect between them 

Rubber production in the tapping frequencies of two and three times per week, without ethrel stimulation, did not present significant differences between each other. The trees with tapping frequencies of once a week, stimulated with ethrel and without stimulation, presented the lowest yields of dry latex (Table 5).

Table 5 Multiple comparison of means of LSD treatments of Fisher with correction of Bonferroni, in dry latex production (g) per tree stimulated with ethrel, for six months, in plantations of 20 years 

Means with different letter are statistically different (p≤0.05)

The comparison of means showed that the highest mean production of dry latex per tree occurred in the plantation with 28 years of age, in the frequency of two tappings per week stimulated with ethrel; however, in the tapping system of three tappings per week stimulated with ethrel there were no statistical differences. The tapping frequencies of one, two and three times per week without stimulation did not present significant differences among each other, nor with the frequency of once per week stimulated with ethrel. The lowest yields of rubber in the plantations without stimulation occurred when applying the tapping frequencies of once and twice per week, with no differences between them (p>0.05); however, there were differences with respect to the stimulated plantations. Thus, for the producer it would be convenient to apply ethrel with two tappings per week (Table 6).

Table 6 Multiple comparison of means of LSD treatments of Fisher with correction of Bonferroni, in dry latex production (g) per tree stimulated with ethrel, for six months, in plantations of 28 years 

Means with different letter are statistically different (p≤0.05)

Production of dry latex was higher in the plantations of 28 years of age compared with those of 20 years stimulated with ethrel (Tables 5 and 6). The tapping frequencies of two and three times per week showed higher yield in the plantations of 28 years and in the plantations of 20 years the difference was minimal. In this case the ethrel did not increment production with respect to the tapping system without stimulation. In July, August and September, highest production was obtained and contrasted with that of September and November. These months presented more rainfall, and less favorable conditions for tapping. In June and December the evaluations were incomplete, because the study began on June 16 and concluded on December 14, and therefore production of these months was lower (Figure 7).

Figure 7 Monthly production of dry rubber in plantations of 20 years and 28 years of age, stimulated with ethrel, with upward tapping in quarter spiral in one hectare. 

The increment of dry latex production with the tapping frequencies of twice a week in ¼ S, stimulated with ethrel, in plantations of 20 years, was unexpected, given that in the frequencies of twice a week, stimulated with ethrel, 26 % less was produced than with the same frequency without stimulation (Figure 7).

Production of dry rubber with the tapping frequencies of once, twice and three times per week in quarter spiral, stimulated with ethrel, in plantations of 28 years surpassed the same treatments without stimulation with ethrel. The tapping frequency of twice a week incremented by 49 % with respect to the frequency of twice a week without stimulation. The tapping frequency of three times a week stimulated with ethrel increased by 32 % with respect to the frequency of three times a week without stimulation (Figure 8). The yields of our study are comparable to those obtained by ^{Izquierdo et al. (2011)} of 1,034 kg ha^-1 year^-1 of dry latex, with the downward tapping system of the lower panel ½ S ↓ d3 6d/7 twice a week without stimulation with ethrel. Production with the upward tapping system in ¼ S ↑ d3 6d/7 twice a week and stimulated with ethrel at 5 % in plantations of 28 years was 630 kg ha^-1 in six months. This indicated that in a year a production would be obtained similar to that of the tapping system of half spiral without stimulation with ethrel.

Figure 8 Average production of dry latex per hectare in six months, in plantations of 20 and 28 years. 

In the normal or downward tapping system, most of rubber plantations showed white mycelia in the lower panels or evidence of disease in the recent cut. In the trees harvested with this downward tapping system, it was found that as much as 80 % of the lower panels presented disease, such as black stripe and moldy rot in plantations of 20 years, and in the plantations of 28 years 72 % of trees were found to have the same diseases.

In the upward tapping system or inverse tapping in the high panel evaluated, in the period of 6 months, none of the 480 trees, of 20 and 28 years of age, showed some diseases caused by fungus.

The plantations of 20 years of age did not respond the same as those of 28 years. ^{Lacote et al. (2010}) observed that among the high yield clones, little stimulation with ethrel is needed. The GT1 clone showed the maximum yield (1.52 g per tree per tapping) with four applications of ethrel at 2.5 % in one year, PB 217 (1.78 g per tree per tapping) with 39 applications of ethrel per year, IRCA 130 (1.70 g per tree per tapping) without ethrel application, and IRCA 230 showed the maximum production with eight applications of ethrel per year. The slow start clones require stimulation in short periods of time, to produce more and in the time there will be no negative effect of the ethrel on the laticiferous cells.

Dry latex production in the tapping frequency of once a week (¼ S ↑ d6 6d/7), stimulated with ethrel, was 31 % higher compared with the same tapping frequency without stimulation; however, the production in the tapping frequency of twice a week (¼ S ↑ d3 6d/7) was 26% lower with the same frequency without stimulation. Production with the tapping frequency of three times per week (¼ S ↑ d2 6d/7) and with stimulation was 22 % higher than the control.

In plantations of 20 years with an average perimeter of 0.97 m, stimulation with ethrel produced 256 g per tree in six months, in one tapping per week, 499 g per tree in six months, in two tappings per week, and 551 g per tree in six months, with three tappings per week. The plantations of 28 years with average perimeter if 1.15 m produced 593 g per tree in six months, with one tapping per week, 1 146 g per tree in six months, in two tappings per week, and 1 013 g per tree in six months, with three tappings per week. However, in our study no correlation was found between latex production and the perimeter of the trees; it appears that latex production depends on the physiological age of the tree and little or nothing on the diameter. ^{Rojo et al (2002)} quantified latex production in downward tapping in half spiral without stimulation, in the region of Papaloapan (Oaxaca); they detected a relationship between the perimeter of the trees and production (1.561 kg per tree per year) of dry latex, and increment to a maximum (2.446 kg per tree per year) in trees with 78.5 cm perimeter; larger diameters produce less, up to 1.065 kg per tree per year in trees with 154 cm perimeter, with plantation useful life of 30 years.

The fungal diseases that affect the lower panel did not cause symptoms or damage in the high panel. The four plantations evaluated had 100 % of their high panels in productive conditions to be exploited with the upward tapping system. Therefore, the upward tapping system can be applied in those panels.

Conclusions

The production of dry rubber with the tapping system of quarter spiral in plantations stimulated with ethrel and evaluated with three tapping frequencies in the high panel is higher in the plantations of 28 years, with respect to those of 20 years.

Plantations of 20 years have lower panels in better conditions for a second tapping, compared with those of 28 years of age. Therefore, in the upward tapping system in high panels in quarter spiral it is better to apply ethrel to the plantations of more advanced age. In plantations of 28 years, production is higher with ethrel stimulation than in the same tapping frequencies without stimulation