Effect of tapping activity on the dynamics of radial growth of Hevea brasiliensis trees

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1 Tree Physiology 26, Heron Publishing Victoria, Canada Effect of tapping activity on the dynamics of radial growth of Hevea brasiliensis trees UNAKORN SILPI, 1 PHILIPPE THALER, 2 POONPIPOPE KASEMSAP, 3 ANDRE LACOINTE, 4 ARAK CHANTUMA, 5 BORIS ADAM, 4 ERIC GOHET, 2 SORNPRACH THANISAWANYANGKURA 6 and THIERRY AMÉGLIO 4,7 1 DORAS Center, Kasetsart University, Bangkok 10900, Thailand 2 Cirad-cp Rubber Program, Doras Center, Kasetsart University, Bangkok 10900, Thailand 3 Department of Agronomy, Faculty of Agriculture, Kasetsart University, Bangkok 10900, Thailand 4 UMR-PIAF, INRA, Site de Crouelle, Clermont-Ferrand, France 5 Rubber Research Institute, Department of Agriculture, Bangkok 10900, Thailand 6 Department of Botany, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand 7 Corresponding author (ameglio@clermont.inra.fr) Received October 11, 2005; accepted February 12, 2006; published online September 1, 2006 Summary Rubber tree (Hevea brasiliensis Müll. Arg.) radial growth dynamics were monitored with displacement sensors, together with latex production, to investigate three aspects of the dual production of latex and wood: (1) the usefulness of fine-scale dendrometric measurements as a physiological tool to detect water shortage through radial growth; (2) the dynamic aspects, both at the seasonal and at the multi-year scale, of the competition between latex and wood production; and (3) the spatial distribution of radial growth rates around the tapping cut. Radial growth of untapped control trees started with the onset of the rainy season and lasted until the onset of the dry season, ceasing completely during the driest period. Displacement sensors provided a sensitive means of detecting water shortage, with a clear correlation between diameter variations and changes in water availability (both daily evapotranspiration and monthly rainfall) over the whole annual cycle. However, the correlation was significantly disturbed in tapped trees. After resumption of tapping, the radial growth rate dropped sharply within two weeks and the effect persisted throughout the whole season, so that the cumulative growth of tapped trees was about half that of untapped trees, with the cumulative growth deficit reaching 80% for the period from mid-june to November. This long-known negative impact of tapping on growth was much stronger in the second year of tapping than in the first, whereas latex production increased significantly between the first and second year of tapping. The increased latex production, which could not be ascribed to climatic conditions, shows that the establishment of an artificial latex sink is a progressive, long-term process likely involving many aspects of metabolism. As expected, ethylene significantly increased latex production in both years; however, ethylene had no effect on the growth rates of tapped trees. Radial growth was differentially affected at different locations around the tapping cut, with growth rates significantly lower in the tapped panel than in the untapped panel, and higher above the cut than below the cut. Thus, caution is needed when deriving whole stem wood production from girth measurements at one location on the stem, especially from girth measurements made close to the tapping cut. This also provides new evidence for the location of the latex regeneration area in the tapped panel, below the cut. Keywords: ethylene, latex production, radius and diameter variations, rubber tree, tapping. Introduction The rubber tree (Hevea brasiliensis Müll. Arg.) is one of the major economic crops in south-west Asia, valued for the production of not only latex, but also of timber. Farmers must maintain a balance between latex production and tree growth; however, there is little information on the physiological processes underlying the effects of regular tapping or stimulation on the growth of the rubber tree. Moreover, because of the lack of available land and competition with other crops (mainly oil palm), rubber plantations are being extended to drought-prone areas that provide sub-optimal growing conditions for rubber tree cultivation, e.g., the Central Highlands of Vietnam, north-central Vietnam, northern India, south-west China, the southern plateau of Brazil and the north-eastern part of Thailand, where there is a long dry season. Growth reduction, an increased immaturity period (Manmuen et al. 1993) and up to 30% reduction in latex production (Wichitchonchai and Manmuen 1992, Chandrashekar et al. 1996, Chandrashekar 1997)

2 1580 SILPI ET AL. have been reported in such areas. To improve rubber cultivation under suboptimal growth conditions, more drought hardy trees and different cultivation systems are needed. Because rubber tree breeding is a slow process, it is critical to identify reliable and relevant markers of high latex and wood production during the early stage of tree development under suboptimal conditions. The first aim of this study was to evaluate the capacity of displacement sensors to detect water shortage on radial growth. Specifically, we continuously monitored radial growth with displacement sensors (Linear Variable Differential Transformer (LVDT) or Linear Motion Position Sensor (LMPS)) at a resolution of 1 10 µm, which has proved effective in monitoring water deficiency in several woody perennial plants (Kozlowski 1971, Huguet 1985, Garnier and Berger 1986, Cohen et al. 1997, Luque et al. 1999, Goldhamer and Fereres 2001, Améglio et al. 2001). Rubber tree growth and latex production are determined to a great extent by the carbon budget (Chandrashekar 1997). It has been known for decades that a negative relationship exists between latex production and wood biomass production (Karling 1934, Gooding 1952), and tapping is known to activate latex metabolism (Annamalainathan et al. 2001). Because little is known about the dynamic aspects of the competition for carbon between growth and latex production in either the short- or the long-term, our second aim was to determine: (1) on a seasonal scale, how long it takes to alter the radial growth after tapping; (2) whether the partitioning ratio between sinks is stable between years, such that value of the ratio in the first year after tapping would be a reliable predictor for the future; (3) alternatively, whether the competitive status change over time, denoting longer-term shifts in carbon metabolism; and (4) whether these dynamics of competition status are affected by the widely used latex production enhancer ethrel (an ethylene-releasing compound) and, if so, how (Amalou et al. 1992)? Tapping is performed on a very limited area of the stem, which raises the question of the spatial extent of the affected area with respect to its impact on radial growth. Rubber tree growers generally estimate total wood production or production deficit due to tapping on the basis of girth measurements at a particular height, which is assumed representative of the whole stem growth (Gohet 1996). The third objective of our study was to test the validity of this assumption, by comparing growth rates in both the tapped and untapped sides of stems, and both above and below the tapping cut. Materials and methods Plant material The experiment was performed with rubber tree (H. brasiliensis) Clone RRIM600, growing in a monoclonal plot in a 2.5 m 7.0 m planting design (571 trees ha 1 ) in Chachoengsao Rubber Research Center (CRRC-DOA: N; E), which is located in a region of Thailand receiving an annual rainfall of about 1300 mm, with a marked dry season Figure 1. Time courses of (a) cumulative rainfall (mm month 1 ) and monthly mean of daily reference evapotranspiration (ET o ); (b) stem diameter variation of the two untapped control trees from December 2001 December 2002 (vertical arrows indicate leaf fall and budbreak); and (c) monthly average of daily diametral growth rate of untapped control trees. In (a) and (c), vertical bars indicate SE for each month (n = number of days in month). (Figure 1a). The trees were planted in 1993 and were opened for tapping in July 2001 at 1.30 m above ground level (a.g.l). Tapping involves periodically cutting bark on the trunk, and hence severing latex vessels. Cuts are made at a 30 angle from the horizontal, from high on the left of the tree to low on the TREE PHYSIOLOGY VOLUME 26, 2006

3 EFFECT OF TAPPING ON RADIAL GROWTH IN RUBBER TREES 1581 right, exposing the maximum number of latex vessels per length of incision. The same cut is regularly reopened by excising at each tapping a new, thin shaving of bark from the sloping cut. As a result, latex flows immediately along the cut into a cup attached to the trunk. The flow progressively diminishes, and eventually stops after 1 3 h as severed vessel ends become plugged by caps of coagulum. In this experiment, tapping was performed twice weekly according to the most widespread system in Thailand, i.e., with the tapping cut spiralling over half of the stem circumference, thus delimiting two sides on the stem referred to as the tapped panel and untapped panel, respectively. Treatments included 12 untapped trees (control), 12 tapped trees without stimulation (tapped, nil-stim) and 12 tapped trees where latex production was enhanced by 2-chloroethylphosphonic acid (Ethephon) application eight times per year (tapped, stimulated), which is the usual way to increase latex yield in commercial crops (Silpi et al. 2005). Tapping started on July 4, 2001 and stopped on April 6, 2002 for two months; it was resumed on June 1, 2002 and continued until March 29, Dry rubber production for each tree (n = 36) was recorded monthly. Radial growth measurements For each tree (n = 36), stem circumference was measured with tape once monthly from May 2001 to May 2003 at 1.7 m a.g.l. For two control trees, diameter variations at 1.7 m a.g.l. were measured from December 2001 to March 2003 with LVDT sensors (Model D.F.2.5, Solartron Metrology, Massy, France). The LVDTs were unguided miniature displacement inductive transducers, where the needle (the moving part of the measurement sensor) was applied with glue to the stem and separated from the body (bobbin). There was no physical contact between the sensing element providing position measurement and the needle (friction-free movement). The LVDT sensors were mounted on a specially designed Invar (an alloy comprising 65% Fe and 35% Ni that has minimal thermal expansion) frame (Huguet 1985) and attached to the stem. For stem radius measurement, four LMPS sensors (LM10, AB Electronic, Essex, England) were installed in December, 2001 onto each of two tapped and two stimulated trees, i.e., two on the tapped panel and two on the opposite panel at the same levels: one above the cut at about 1.5 m a.g.l., and one below the cut at about 1.0 m a.g.l. For each of two control untapped trees, one LMPS was also installed at 1.5 m a.g.l. To attach the LMPS devices (18 in total) to the trunk, an Invar rod was inserted horizontally deep inside the heartwood and the sensor was attached to the rod with only a small needle left free to move inwards or outwards according to trunk movement. All LVDT and LMPS sensors were connected to a data logger (Model CR 10X, Campbell Scientific LTD, Logan, UT), with a resolution of 1.6 and 10 µm, respectively. Average measurements were recorded every 15 min. The voltage signal received from both kinds of devices was monitored continuously. The correct position of the sensor needle was adjusted regularly depending on tree growth rate. Because no significant differences were detected between the cambial growth of tapped unstimulated versus stimulated trees for the two tapping years (6.1 ± 0.4 versus 6.4 ± 0.3), unless otherwise stated the presentation of growth analyses is simplified by grouping those two treatments into a single tapped treatment. Climatic data In parallel with radial growth measurements, trunk temperatures were measured by thermocouples and recorded with data loggers as 15-min means. The daily reference evapotranspiration (ET o ) and rainfall (mm) were measured in the weather station of Chachoengsao Rubber Research Center (CRRC- DOA), Thailand. Statistical analyses Means and standard errors were computed for all measurements, and the significance of a treatment effect was evaluated at the P = 0.05 threshold by the non-parametric tests of Mann and Whitney, or Kruskal and Wallis (Sprent 1992) when more than two treatments were compared. All statistical analyses were performed with XLSTAT 6.1 (Addinsoft, Paris, France). Results Seasonal dynamics of radial growth of untapped control trees Radial growth dynamics of untapped control trees, measured by two LVDTs during one year, showed seasonal changes (Figure 1b). The cambium of untapped control trees appeared inactive from mid-december to mid-april. In early February, concomitant with the expansion of new leaves (the leaves were fully expanded in the second week of February), the trees showed progressive trunk shrinkage which lasted until the end of March, when limited rainfall (Figure 1a) partly reversed the shrinkage. True growth started in late April with the onset of the rainy season. Rapid growth occurred during May and early June (about µm day 1 ), followed by a slight but steady decrease in stem growth for the remainder of the season until November (Figures 1b and 1c), leading eventually to a complete cessation in stem diameter growth in December. Annual trunk diameter increment was 8 11 mm. Stem diameter dynamics for the period between refoliation and true growth resumption, i.e., the late dry season (February and March), are shown in detail in Figure 2a. Both control trees exhibited reversible, daily variations in diameter, with amplitude increasing progressively to a maximum value of µm, reflecting elastic reversible shrinkage. The reversible daily shrinkage was clearly superimposed on a continuous decrease in diameter. This general decreasing tendency was temporarily reversed within one day following a significant rainfall (see February 26 and March 28). By contrast, fine-scale dynamics in the rainy season (Figure 2b) exhibited a generally increasing tendency, superimposed on the daily nighttime expansion and daytime shrinkage. After a few days without rain, we observed an increased amplitude of daily variations, together with a flattening of the general increasing TREE PHYSIOLOGY ONLINE at

4 1582 SILPI ET AL. Figure 4. Simulated radial growth rates (RGR) versus measured RGR from a multiple regression for untapped control trees against daily reference evapotranspiration (ET o ) and monthly rainfall (R): RGR = (ET o ) R. The model explained a good percentage of the variance (r 2 = 0.60, P < 0.003) with a significant effect of daily ETo (P < ) and of monthly rain (P < ). Closed symbol, control treatment; and open symbol, tapped treatment. Figure 2. Fine-scale dynamics of stem diameter variation of untapped control trees, in parallel with daily rainfall: (a) in the dry season (February April) and (b) in the rainy season (June). tendency. Effect of tapping on radial growth activity Although tapping in Thailand generally starts in May, tapping Figure 3. Time course of radial growth increment of untapped and tapped trees from December 2001 to April Filled horizontal bars show the tapping period. Vertical bars indicate the maximum SE for each month (n = 2 for control and n = 4 for tapped trees). resumption in this experiment was delayed by one month (began June 1) to compare the growth behavior of tapped versus untapped control trees during the period of rapid growth in May. Until tapping was resumed, the growth dynamics of tapped and untapped trees were similar (dry season and early rainy season; Figure 3). However, just a few weeks after tapping resumed, a significant tapping effect was observed as a sharp decrease in radial growth rate (RGR) in all locations. This strong effect lasted for the whole season, so that the cumulative growth of tapped trees was about half that of untapped trees, with the cumulative growth deficit reaching 80% for the period from mid-june to November. Furthermore, there was some interaction between tapping and water availability (Figure 4). As expected, there was a significant correlation between radial growth rate and climatic data (daily ET o and monthly rainfall) for control untapped trees. In contrast, for tapped trees, no relationship was observed during the tapping months, whereas the trees followed the same relationship as untapped control trees when tapping was stopped and during the dry season. Tapping spatially affects radial growth rate Radial growth was differentially affected at different positions on the stem, both in the vertical (above versus below the cut) and in the horizontal direction (tapped versus untapped panel). There was a significant difference in radial growth rates above and below the tapping cut from July to September (Figure 5a), with a higher radial growth rate above the cut than below cut. On the contrary, in December, radial growth rate was higher below the cut than above the cut. Horizontal growth rate dropped quickly in the tapped panel (Figure 5b), the growth rate dropped quickly after tapping resumption and remained significantly lower than that in the untapped panel for the next TREE PHYSIOLOGY VOLUME 26, 2006

5 EFFECT OF TAPPING ON RADIAL GROWTH IN RUBBER TREES 1583 Figure 5. Time course of radial growth for tapped trees relative to the maximum value (May) reached by untapped control trees: (a) below and above the tapping cut; and (b) untapped and tapped panels. Values are means ± SE (n = 8). An asterisk (*) indicates a significant difference at P < 0.05 between locations (below versus above the tapping cut, or untapped versus tapped panel). Abbreviation: NS = nonsignificant. Figure 6. Summary of data for two tapping seasons and Timecourses of (a) monthly girth at 1.7 m above ground level for the untapped control, the nil-stimulated tapped trees and the ethylene-stimulated tapped trees; and (b) cumulative latex yield for the trees in the two tapped treatments. Values are means ± SE (n = 12). Abbreviation: a.g.l. = above ground level. six months (June November). Trunk girth and latex yield production The two tapping treatments affected the seasonal dynamics of trunk girth and latex yield production. In the first year of tapping ( ), the trunk girth increment was slightly (P = 0.001) lower in tapped trees than in control trees, and this negative effect was much stronger in the second year ( ; Figure 6a; Table 1). Latex production increased by a factor of about 2.5 between the first and second years of tapping (Figure 6b; Table 2). This shift between the two tapping years could not be ascribed to climatic conditions, which were similar in both years (Table 3). When all data for the three treatments and both tapping years were pooled, the annual girth increment at 1.7 m a.g.l. was negatively correlated with annual latex yield production (Figure 7). The negative correlation was altered by ethylene stimulation. Although the ethylene-stimulated trees had a significantly higher latex production than the non-stimu- Table 1. Girth increment (cm) at 1.7 m above ground level of control and tapped trees during the first two tapping seasons (mean ± SE). Abbreviation: NS = not significant. Treatment Year 1 Year 2 P value Control (n = 12) 4.5 ± ± 0.4 NS Tapped (n = 24) 3.5 ± ± 0.2 < 0.01 P value < < Table 2. Latex yield (kg) of control and ethylene-stimulated tapped trees during the first two tapping seasons (mean ± SE; n = 12). Treatment Year 1 Year 2 P value Control 1.07 ± ± 0.16 < Ethylene stimulated 1.39 ± ± 0.14 < P value < < 0.01 TREE PHYSIOLOGY ONLINE at

6 1584 SILPI ET AL. Table 3. Cumulative rainfall and reference evapotranspiration (ET o ) for 3 years at the Chachoengsao Rubber Research Center (CRRC- DOA), Thailand. Year Cumulative rainfall (mm) Cumulative ET o (mm) lated trees in both years, they did not exhibit lower radial growth rates in either year. Discussion Water as a major factor for radial growth: evidence from fine-scale dendrometric measurements Stem radial variations reflect the action of four components: (1) irreversible radial growth; reversible shrinking and swelling in relation to changing amounts of (2) hydration and (3) thermal expansion (Klepper et al. 1971, Améglio and Cruiziat 1992, Simonneau et al. 1993, Zweifel et al. 2000, Cochard et al. 2001); and (4) contraction and expansion of dead conducting elements in response to increases in and relaxation of internal tensions (Irvine and Grace 1997, Offenthaler et al. 2001, Sevanto et al. 2002). Of these components, the first two are usually the most significant; hence, radial growth measurements are commonly used as a basis for scheduling irrigation of orchards (Huguet 1985, Garnier and Berger 1986, Schoch et al. 1988, Li and Huguet 1990, Huguet et al. 1992, Goldhamer and Fereres 2001). Although we did not measure tree water status directly, we found a clear correlation between diameter variations and water availability over the whole annual cycle. Moreover, radial growth rate of control untapped trees (Figure 4) was correlated Figure 7. Relationships between annual latex yield and annual girth increment at 1.7 m above ground level. Closed symbols, tapping season; open symbols, tapping season. The coefficients of regression (r) and determination (r 2 ) of the linear regression (n = 72) are presented. with daily ET o (P < ) and with monthly rainfall (P < ), and growth ceased completely during the driest period (December early April). At the time of refoliation, which occurred in the middle of the dry season, the trunk exhibited net shrinkage (cf. Figures 1b and 2a), indicating that the trees were drought stressed because soil water uptake did not compensate for the daily water loss by transpiration. Trunk shrinkage has commonly been observed in rubber trees in the dry season when soil water content is low (Chandrashekar et al. 1996, 1998, 2002, Devakumar et al. 1999). Under our experimental conditions, leaf fall in late December allowed more light to penetrate to the ground surface, which in turn affected the soil temperature and vapor pressure deficit. After bud break in late January, new leaves that were initiated in this extreme environment enhanced the rate of water loss through transpiration and, as a result, the trunk showed no radial growth until soil water content was replenished by rainfall in April (Figure 2a). However, rapid and significant trunk expansion was observed the day after a rainfall. This radial expansion within one day of a rainfall reversed the trunk shrinkage that occurred during the previous 2 weeks, indicating that the rate of expansion following abundant rainfall was much faster than the rate shrinkage caused by dehydration, as found by Kozlowski and Peterson (1962) in red pine trees. During the dry season, radial expansion following occasional rainfalls did not reflect growth, just the recovery of turgor pressure, or cell water content, which had declined during the preceeding severe drought (Kozlowski 1971, Garnier and Berger 1986, Améglio and Cruiziat 1992, Simmoneau et al. 1993, Chandrashekar et al. 1996, Cochard et al. 2001). True radial growth started with the onset of the rainy season and lasted until the onset of the next dry season, with periods of temporary limitation that can be ascribed to water shortage. Thus, rubber trees exhibit the well known drought-induced reduction in radial growth (Gooding 1952, Lustinec et al. 1969, Kozlowski 1971). One of our objectives was to determine if this growth limitation was a direct consequence of the altered water status, through alteration of the functions that are involved in tissue enlargement (cell division and cell expansion), or an indirect effect mediated through an effect on carbon balance via stomatal limitation and reduced photosynthesis. The daily pattern of diameter variations provides indirect evidence for the former possibility because the pattern was remarkably similar to that found by Daudet et al. (2004) in an experiment on walnut trees involving continuous measurement of gas exchange. From the dynamics of stem respiration, Daudet et al. (2004) suggested that structural radial growth activity, i.e., carbon incorporation into new structures, occurred mainly during nighttime when water status was improved and turgor pressure was higher, as predicted by Lockhart s (1965) model. Thus, the daily variations in stem diameter would be a consequence of structural radial growth through variations in stem water content. However, water was not the only factor that limited radial growth, as evidenced by two observations. First, the long-term declining tendency in radial growth rate (Figure 1c) cannot be ascribed to a similar pattern in rainfall. For example, radial TREE PHYSIOLOGY VOLUME 26, 2006

7 EFFECT OF TAPPING ON RADIAL GROWTH IN RUBBER TREES 1585 growth rates in August and September were significantly lower than in May and June, whereas the rainfall pattern was the opposite. This is in agreement with previous results of Chandrashekar et al. (1998, 2002) and de Fay (1999). Second, although the climatic conditions of both tapping years were similar and resulted in a similar degree (and seasonal pattern) of water limitation, the tapped trees had significantly lower radial growth in the second year than in the first. These observations led us to consider the role of carbon, which is another major determinant of growth (Daudet et al. 2004), as a second limiting factor of rubber tree radial growth. Carbon as a limiting factor for radial growth Carbon serves as a structural material and as the source for metabolic energy. In our study, carbon limitation was manifested most clearly as a severe growth limitation of tapped trees compared with untapped controls, which can be explained on the basis of strong competition for carbon between two sink functions. This competitive effect between sinks has been qualitatively known for many years (Karling 1934, Pyke 1941, Gooding 1952, Templeton 1969, Wycherley 1976, Sethuraj 1981, 1985, Gohet 1996, Gohet et al. 1996); however, by continuously monitoring growth for the first time, our data provide new insight into the dynamics of the competition. The most striking finding was the sharpness of the decline in radial growth rate which occurred almost immediately (within 2 weeks) after tapping was resumed demonstrating the strength of the artificially induced and maintained sink compared with growth, which is the naturally dominant sink. The shift in the balance between the two sinks from the first to the second tapping year reflects the significantly lower radial growth of the tapped trees in the second year accompanied by a significantly higher latex yield (by contrast, the untapped control trees, which produced no latex in either year, exhibited similar growth rates in both years). This shows that the establishment of artificial latex sink is a progressive, long-term process likely involving many aspects of metabolism, with a memory effect that can survive a 2-month-long interruption. As expected (Audley et al. 1976, Silpi et al and references therein), the ethylene-stimulated trees had a significantly higher latex production than the non-stimulated trees; however, the radial growth rates of the ethylene-stimulated tapped trees were no lower than those of the unstimulated tapped trees. In other words, the total amount of carbon used by both sinks was significantly higher in ethylene-stimulated trees than in unstimulated trees. This might reflect a stimulating effect of ethylene on local import from circulating assimilates, whether direct or indirect (Lacrotte 1991, Gohet 1996, Silpi et al. 2005) by using carbohydrate reserves from trunk parenchyma xylem cells. Furthermore, this stimulation of sink activity might explain the ethephon-induced net increase in total carbon use, through stimulation of net assimilation at the tree level, that has been demonstrated in several species (Chalmers et al. 1975, DeJong 1986). Spatial extent of the tapping cut effect on local radial growth There were significant differences in radial growth rates among different locations around the tapping cut, indicating that caution is needed when deriving whole-stem wood production from girth measurements at one location on the stem, especially at a location close to the tapping cut. Additionally, the differential impact of tapping on radial growth at different locations around the cut provides some insight into the location of the latex regeneration area, or drained area, which has concerned researchers for a long time. Several authors, using radio-labeled isotopes (Lustinec and Resing 1965, Lustinec et al. 1969) or turgor pressure measurements (Buttery and Boatman 1964, 1966, Pakianathan et al. 1975), concluded that the latex regenerating bark area was located mostly on the tapped panel. Furthermore, the latex regeneration process is thought to be most active below the tapping cut (Tupý 1973, Silpi et al. 2001, 2004). In the frame of competition for carbon between growth and latex regeneration, our results provide a new kind of evidence supporting latex regeneration taking place preferentially in the tapped panel below the cut. Besides competition, however, a girdling effect cannot be overlooked as part of the explanation for the stronger inhibition of growth below than above the tapping cut. Every time the tree is tapped, bark is wounded and partly removed. The part that remains on the tapped panel consists of cambium and the young phloem; as a consequence, the translocation of current assimilates from the canopy is partly obstructed by tapping. In a girdling experiment, Daudet et al. (2004) observed an accelerated radial growth rate of walnut stem above the girdle, whereas almost no radial growth was evident in the area that was isolated by double girdling, and concluded that current photoassimilate flux was important for active radial growth. Wound responses and bark regeneration are carbon consuming processes, and hence should be considered as sink functions. Tapping is thought to activate the vascular cambium (Gomez et al. 1972, de Fay and Jacob 1989). Thomas et al. (1995) reported that, after two weeks of tapping, the cambial derivatives were produced in large numbers and the differentiation of these was initiated after about one week. Newly differentiated sieve tubes in the regeneration bark had a larger diameter than those produced from the cambium in the uncut area. Thus, a qualitative reorientation in the functioning of cambium (phloem side versus xylem side) would contribute to further decreases in wood production. To conclude, the displacement sensors proved an efficient tool for detecting water shortage, for following the annual pattern of radial growth and for investigating the production of both latex and wood. The dynamics of the competition between latex and wood production investigated at the seasonal and multi-annual scales provides new evidence for the location of latex regeneration in the tapped panel below the cut. Acknowledgments This study was funded by the French Ministry of Foreign Affairs (MAE), the French Ministry of National Education (MENESR) and the Thai Ministry of University Affairs (Commission on Higher Education: CHE) in the Thai-French Cooperation on Rubber Tree Agronomy and Physiology. We thank the French Embassy in Bangkok for TREE PHYSIOLOGY ONLINE at

8 1586 SILPI ET AL. supporting bilateral fellowships. The discussions with André Clement-Demange, CIRAD-CP, Rubber Program, Montpellier, were very stimulating and constructive. References Amalou, Z., J. Bangratz and H. Chrestin Ethrel (ethylene releaser)-induced increases in the adenylate pool and transtonoplast ApH within Hevea latex cells. Plant Physiol. 98: Améglio, T. and P. Cruiziat Daily variations of stem and branch diameter: short overview from a developed example. In Mechanics of Swelling. NATO ASI Series Vol. H.64. Ed. T.K. Karalis. Springer-Verlag, Berlin, pp Améglio, T., H. Cochard and F.W. Ewers Stem diameter variations and cold hardiness in walnut tree. J. Exp. Bot. 52: Annamalainathan, K., R. Krishnakumar and J. Jacob Tappinginduced changes in respiratory metabolism, ATP production and reactive oxygen species scavenging in Hevea. J. Rubber Res. Inst. Malay. 4: Audley, B.G., B.L. Archer and I.B. Carruthers Metabolism of ethephon (2-chloroethylphosphonic acid) and related compounds in Hevea brasiliensis. Arch. Environ. Contam. Toxicol. 4: Buttery, B.R. and S.G. Boatman Turgor pressure in phloem: measurements in Hevea latex. Science 145: Buttery, B.R. and S.G. Boatman Manometric measurement of turgor pressures in laticiferous phloem tissues. J. Exp. Bot. 17: Chalmers, D.J., R.L. Canterford, P.H. Jerie, T.R. Jones and T.D. Ugalde Photosynthesis in relation to growth and distribution of fruit in peach trees. Aust. J. Plant Physiol. 2: Chandrashekar, T.R Stomatal response of Hevea to atmospheric and soil moisture stress under dry subhumid climatic conditions. J. Plant Crops 25: Chandrashekar, T.R., J.G Marattukalam and M.A. Nazeer Growth reduction of Hevea brasiliensis to heat and drought stress under dry subhumid climatics conditions. Indian J. Nat. Rubber Res. 9:1 5. Chandrashekar, T.R., M.A. Nazeer, J.G. Marattukalam, G.P. Prakash, K. Annamalainathan and J. 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