GENETICS OF PRODUCTION TRAITS IN HEVEA-BRASILIENSIS (RUBBER) .1. CHANGES IN GENETIC-CONTROL WITH AGE

To determine the nature and extent of genetical and environmental cont rol of production traits in rubber trees, nine clones were studied in replicated trials in Sri Lanka at seven sites. The clones represented a range of indigenous and imported germplasm whereas the sites were se lected to represent a range of rubber growing areas. Trees were studie d for two important production traits over 15 years including 9 years after the initiation of tapping for latex: girth, a measure of vigour, and rubber yield. The results of the analyses of girth data show a co nsistent increase in heritability over the 15 years from 12 per cent t o over 75 per cent with a corresponding decrease in the effect of the environment (70 per cent to 12 per cent). With respect to yield, there was no such change from environmental control to genetic control of t he character over the 5 years during which yield was monitored. Correl ations between final tree size and earlier measurements indicate that different genes are involved in pre- and post-tapping growth. This imp lies that post-tapping size measurements alone are useful as predictor s of final size. There are nonsignificant but negative correlations be tween vigour and rubber production. The implications of these findings to breeding and selection of rubber for these two production traits a re discussed.