An experiment with the objective of evaluating the performance of 15 clones of rubber (Hevea brasiliensis) was conducted in the Konkan region of Western India. The clones under evaluation were RRII 5, RRII 6, RRII 105, RRII 208, RRII 308, RRIC 52, RRIC 100, RRIC 102, RRIC 105, RRIM 605, PB 260, PB 310, PB 311, PR 255, and PR 261. The region is a trial environment for the crop and experiences more than seven rainless months and severe drought in the summer months. Data on monthly girth growth w ere collected for six years from a trial with randomized block design. The growth of clones in terms of monthly girth increment growth (GIN, cm month−1) seasonal mean girth increments (MGIN, cm season−1) and mean relative increment rates (MRIR, mm cm−1 season−1) as well as annual MGIN (cm a−1) and MRIR (mm cm−1 a−1) was studied. Correlation analysis was performed to understand the effect of seasonal growth on the final growth. At the beginning of the study, the largest girth noted was for the clone RRII 6 (22. 5 cm) followed by RRII 208 (22.0 cm). PR 261 with a girth of only 14.2 cm was the least vigorous among the clones. A large portion of the growth occurred in the wet season only. During the dry season the growth rates of the clones declined substantially and decreases in girth ranging from 0.2 mm to 0.5 mm were noticed in most of the clones. At the end of the study period the largest girth observed was for clone RRII 208 (49.3 cm) and the lowest for PR 261 (39.3 cm). The highest proportion of tappable trees noted was for clone RRII 208 (52.4%) and the lowest for PR 261 (2.7%). The pooled average of tappable trees was on ly 17.5%. The data revealed that the immaturity period for Hevea in the region will not be less than 9 years under rainfed conditions. From the analysis based on the final girth it was concluded that clones RRII 208, RRIC 52, RRII 6, RRIC 100 and RRIC 102 are more tolerant to drought while RRII 105, RRIC 105, RRII 5, RRIM 605, PB 310, PB 260, PB 311, PR 255, RRII 308 and PR 261 are less tolerant. The results of correlation indicated that by analysing the growth, potentially drought-toleran t clones can be identified.

No abstract

Seven Hevea brasiliensis clones were evaluated on two types of rootstocks, assorted seedling rootstocks (AR) and monoclonal rootstock (MR) over 19 years. Influence of rootstock on scion growth and rubber yield was assessed based on juvenile height, circumference of the main trunk (cm), number of branches, branching height and cumulative dry rubber yield (g per tree per tapping, conventionally abbreviated gt-1t-1). Highest cumulative yield (g per tree per tapping) over the 12 years for which the trees were tapped was obtained from clone RRII 105 (MR: 1076 g per tree per tapping and AR: 497 g per tree per tapping), followed by RRII 203 (MR: 661; AR: 538), RRII 208 (MR: 477; AR: 486), RRII 118 (MR: 497; AR: 452). Gl 1 yielded the least, 219 g per tree per tapping (MR) and 378 g per tree per tapping (AR); GT1 produced 335 g per tree per tapping (MR) and 375 g per tree per tapping (AR). RRII 118 had the greatest circumference at age 19 (91.4 cm on MR) 88.8 on AR, followed by RRII 105 (MR: 87.4 cm AR: 89.2 cm) and GT 1 (MR: 88.5 cm; AR: 84.4cm). Effect of scion clone was significant (p<0.01) only for trunk circumference at opening, but not for cumulative rubber yield at age 11 (4 years after opening) or cumulative rubber yield at age 19 (12 years after opening). Most importantly, rootstock and clone × rootstock interaction did not significantly affect rubber yield or tree circumference at any evaluation time. There was no evidence to suggest that growth and yield of clones was influenced significantly by rootstock type.

With the objective to explore the possibility of optimising selection for reducing the number of years of yield recording that could be sufficient enough to establish the lowest risk tapping-age for selecting the potential clones from field-level clone trials, long-term yield data (13-16 years) from eight clone evaluation trials were subjected to formal graphical/statistical analytical techniques. Non-linear curves were fitted to yield data and the year of start of upper asymptotic phase was worked out. Correlations were worked out between mean yield of all the years' data and cumulative mean yield over 3-10 years of tapping. Success rate achieved in the selection of high yielding clones from the third year of tapping was worked out. The mean maximum dy/dx (first derivative of the curve) across all trials ranged from 3 to 7 years of tapping with mean maximum dy/dx at 4 years. Correlations observed from the sixth year of tapping were highly significant in most of the trials. Considering the results from all the three analytical techniques, it was concluded that six years of yield recording would be required for optimising selection of top yielding clones from large-scale clone trials. The possible use of these results is discussed in the framework of a global selection scheme with a view to shorten the breeding cycle of Hevea brasiliensis.