Rubber plantation in the non-traditional area (Northeastern part of Thailand) exhibits growth reduction, longer immaturity period and lower latex production compared as traditional area (Southern part) (Wichitchonchai and Manmeun, 1992; Manmeun et. al., 1993). In order to improve rubber cultivation in these area, more drought-efficient trees and adapted cultivation systems have to be developed. The understanding of rubber tree stomatal function is a key step in the under-standing of drought performance. Plant hydraulic characteristics, such as vulnerability to embolism might provide a sound basis for an early selection for drought-efficiency rubber trees. The objectives of this study were to test the hypothesis of a correlation between stomatal closure and cavitation avoidance 

Materials and Methods

              Experiment were conducted on 1 1/2 year-old rubber clones RRIM 600 and RRIT 251 during June-July 2002 at Department of Agronomy, Kasesart University, Bangkok, Thailand. The irrigation of one plant per clone was successively stopped at 1-week interval for induced soil dehydrati Minimum xylem pressure potentials were determined with a pressure chamber. Leaf stomatal conductance (gs) was measured with an AP4 Porometer.The percentage of loss of hydraulic conductivity (PLC) was measured at the end of the drought treatment on branches, petioles and midribs with the technique described by Sperry et. al. (1998) with a new XYL’EM apparatus.Vulnerability curves (VC) were constructed by dehydrating branches or leaves to different xylem pressure and measuring the corresponding PLC values.and to evaluate a potential variability of rubber tree hydraulic among different clones.

Figure 1 Xylem embolism meter (a), Pressure chamber (b) and AP4 Porometer (c).

Table 1  Parameters of the logistic function fitted to the vulnerability curves. PPLC50 correspond to the xylem pressure (MPa) provoking 50% embolism and s is a slope parameter. For each clone, organs having a latter in common are not statistically different at p=0.05. Between clones, significant differences at p=0.1 are indicate by a *.

Results and Discussion

            Drought treatments provoked a strong stomatal closure and a decrease in xylem pressure (Figure 1). Less negative xylem pressure values were measured in the branches than in petioles and midribs. The dependence of gs on Ppetiole and Pbranch show no substantial differences between the two clones (Figure 2). The VCs for different organs of both clones were shown in Figure 4 and the parameters of the logistic fitting are given in Table 1. Petioles of the RRIT 251 clone were distinctly more vulnerable than that of RRIM 600. VCs of rubber tree in this study is in accordance with the results of Ransinghe and Milburn (1995). The high vulnerability of rubber seem a feature common to other tropical species adapted to rainforest-type environments vulnerabilities (Tyree and Zimmermann, 2002). The hydraulic system of rubber tree is its vulnerability segmentation like in Juglans regia (Tyree et. al., 1993). A basipetal gradient was found in this study.For xylem embolism and stomatal function, the results suggests that the stomata were nearly closed at the onset of xylem cavitation.

     Figure 2 Time course of leaf stomatal conductance (left) and xylem pressure potential (right) for RRIM 600 (close symbols) and RRIT 251 (open symbols) after pot watering was stopped at time 0.

Figure 3  Leaf stomatal conductance versus xylem pressure in the petiole and the branch of  RRIM 600 (close symbols, plain lines) and RRIT 251 (open symbols, dotted lines).The lines are logistic fits of the vulnerability curves from figure 4.

Figure 4  Midribs, petioles and branches vulnerability curves for RRIM 600 (close  symbols, plain lines) and RRIT 251 (open symbols, dotted lines) Circles correspond to air pressure-dehydrated sample and squares to measures on soil-dehydrated plants.  

Conclusions

        The xylem conduits in rubber trees are relatively vulnerable to cavitation. Substantial variations in vulnerability to cavitation was found in a comparison of two clones. A correlation was found between the stomatal behavior and the development of cavitation. The xylem embolism, especially in the petioles, may provide a sound criterion for the evolution of rubber tree clone behavior under drought condition in a long term breeding program.