Variation of phloem turgor pressure in Hevea brasiliensis: An implication for latex yield and tapping system optimization

Highlights

• The latex tapping system could be optimized according to the variation of phloem turgor pressure.

• Phloem turgor pressure is positively related to the yield potential of rubber clones.

• Not only the initial phloem turgor pressure before tapping but also its kinetic change after tapping are linked with rubber tree yield.

Abstract

Phloem turgor pressure (PTP) is the initial driving force for latex flow after a rubber tree is tapped and therefore plays an important role in rubber tree latex production. Variation in PTP with rubber tree clone, age, yield potential and commonly used Ethrel (an ethylene releaser) stimulation have, however, not been comprehensively studied to date. The aim of this study was to investigate these relations and examine whether PTP can be used as an index for rubber tree clone assessment and tapping system optimization. The results showed that: (1) the daily change of PTP in the foliation season suggests that a high PTP can ensure a high latex yield and tapping could be moved forward to midnight or earlier in the night; (2) the decrease of PTP from the basal to distal stem indicates the benefit of a controlled upward tapping system; (3) the logarithmic increase in PTP with rubber tree planting age and age-based mean girth suggests that the preferred age for the commencement of rubber tree tapping is eight years; (4) the change of PTP with regenerated bark age suggests that the regenerated bark could be exploited again after the second year; (5) PTP is positively related to the yield potential of rubber tree clones; (6) although Ethrel stimulation could not significantly increase the initial PTP of a rubber tree, it delays the recovery of PTP after tapping. Therefore, PTP is an indicator of rubber tree latex yield and can be used for tapping system optimization.

Introduction

Natural rubber is an indispensable and irreplaceable industrial raw material widely used in a variety of industrial applications. It derives from the milky latex flowing out from rubber plants, mainly the rubber tree (Hevea brasiliensis) once the tree is tapped by regularly removing a slice of bark. The latex yield of each tapping varies with several factors including the tapping system, the exploitation hour, the season, the rubber tree clone and the tree age. The phloem laticifer turgor pressure is directly responsible for latex flow. Therefore, there might be a close relationship between phloem laticifer turgor pressure and these factors.

Turgor pressure is the force exerted on elastic plant cell walls by the contents of the cell. This pressure is caused by the presence of osmotica, which set a negative osmotic potential allowing water to pass into the cell (D’Auzac et al., 1989). According to the widely accepted “mass flow theory”, photosynthetic assimilates are transported in the phloem from leaves to the consuming tissues under osmotically generated turgor pressure gradients (De Schepper et al., 2013, Dinant and Lemoine, 2010). Due to limitations of the measurement methods, the measured turgor pressures in rubber trees are actually derived from a group of laticifer vessels and the conducting phloem. It is more precise to say that the phloem turgor pressure (PTP) or phloem hydrostatic pressure rather than laticifer turgor pressure is what is measured.

The PTP is directly responsible for latex flow at each tapping (Priyadarshan, 2011, Yeang, 2005). During tapping, the laticifer vessels are breached and the turgor pressure of the laticifer system nearest the cut is reduced to atmospheric pressure. Immediately after cutting, a turgor pressure difference of 0.7–1.4 MPa between the nearby laticifer vessels and the cut itself will be established (Buttery and Boatman, 1966, Buttery and Boatman, 1967). The sudden release of turgor pressure will result in a laticifer wall contraction and the cytoplasm fluid, the latex, being expelled. Latex then flows out from the anastomosed laticifer system until the arrest of latex flow by the plugging of laticifer vessel extremities. Consequently, Yeang (2005) suggested that the production of latex from each tapping could be expressed as a function of PTP and time, without the involvement of other parameters such as fluid dynamics, latex vessel contraction and latex dilution during the process of exudation.

Although it is commonly believed that PTP is a crucial determinant for rubber tree latex production, only a few papers have studied this topic. Buttery and Boatman, 1964, Buttery and Boatman, 1966, Buttery and Boatman, 1967 developed a manometric technique, examined the daily and seasonal variation of PTP, and the changes of PTP subjected to tapping and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) stimulation. Milburn and Ranasinghe (1996) compared several methods of measuring the PTP of rubber trees. Yeang (2005) investigated the relationship between latex flow kinetics and PTP. Until now, the variation of PTP with rubber tree clone, age, yield potential and also the currently commonly used Ethrel (an ethylene releaser) stimulation, especially during the tapping flow course, has not been extensively studied.

The change of PTP with time of day, rubber tree height, clone, tree age and girth, regenerated bark age and Ethrel stimulation was investigated in this study. These results provided valuable insights into rubber tree yield prediction and the optimization of tapping systems.