CORRELATION BETWEEN CO2 ASSIMILATION RATES AND OTHER GAS EXCHANGE PARAMETERS OF Hevea WITH REFERENCE TO COMMENCEMENT OF TAPPING

The present study was supplement to a longer experiment to determine the feasibility of early commencement of tapping of two Hevea genotypes i.e. RRIC121 and RRISL 211. The main objective of this study was to develop a correlations between CO2 assimilation rates and other leaf gas exchange parameters of Hevea with reference to the commencement of tapping. Tapped and untapped trees of two Hevea genotypes, i.e. RRIC121 and RRISL 211 at the same age were selected for the study. All cultural practices were performed according to the Rubber Research Institute of Sri Lanka. To evaluate the significance of differences between all possible pairs of treatment means, mean separation of treatments was performed with Duncan’s Multiple Range Test (DMRT). The strength of the relationships between yield and photosynthetic gas exchange parameters was estimated by linear correlation analysis. The leaf gas exchange parameters were measured under optimal environmental conditions clearly showed 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. 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. This was primarily because of the greater photosynthetic rates of RRISL 211, rather than lower transpiration rates.When considering the correlation analysis of gas exchange parameters of Hevea, CO2 assimilation rate (A) had highly significant correlations with gs, Tr and intensity of PAR. However, the absence of a correlation between PAR and WUE showed that, when PAR increases both A and Tr increase in similar magnitudes. Therefore, WUE remains approximately constant. The presence or absence of correlations in gas exchange parameters of tapped and untapped treatments were broadly similar to the overall correlation analysis. In the tapped treatments, 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. Hence WUE and ivpd increase latex flow within the tree and finally increase the latex yield with increasing latex volume. On the other hand, latex yield has a negative correlation with gs. The study findings clearly revealed that, in top leaves reduced Ci increases the CO2 gradient and uptake of CO2 for more photosynthesis, which increases the latex yield. Faster utilization of absorbed Ci may enhance the yield potential.