VARIATION OF LEAF GAS EXCHANGE PARAMETERS WITHIN THE CANOPY OF RUBBER (Hevea brasiliensis)

The main objective of this study was to investigate the variation of leaf gas exchange parameters of Rubber (Hevea brasiliensis Muell. Arg.) within the canopy of tapped trees of two Hevea genotypes, I.e. RRIC121 and RRISL 211. The leaf gas exchange parameters, I.e.Stomatal conductance (gs),Intercellular CO2 concentration (Ci), Leaf temperature (Tleaf),Transpiration rate ( E ),Water use efficiency (WUE) and Leaf-air vapour pressure deficit (lvpd) were measured under optimal environmental conditions. Study findings clearly revealed that the intercellular CO2 concentration (Ci) gradually decrease with increasing light intensity in both clones. The same trend was observed in the different canopy layers under both tapping treatments.Clone RRISL 211 showed a higher leaf temperature than RRIC 121 under tapping. The highest leaf-air vapour pressure deficit (lvpd) was shown in the leaves of the upper canopy and lowest in the bottom canopy layer. Furthermore, the water use efficiency (WUE) of clone RRISL 211 was higher than that of clone RRIC 121 under tapping. In the tapped treatment, because of tapping, the photosynthetic rate is stimulated relative to the transpiration rate. The positive latex yield response to WUE and lvpd of top leaves indicates that, in top leaves when lvpd increases transpiration also increases accordingly. Clone RRIC 121 showed a greater contribution from the middle layer to overall canopy photosynthesis while RRISL 211 showed a lower contribution. This could have been due to the more open canopy architecture in clone RRIC 121 or maintenance of a higher A in low light levels.