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Natural rubber produced by I1evea brasiliensis Muell. Arg. is an industrially important rawmaterial for which demand continues both locally and internationally. It is produced fromthe latex extracted/tapped from the rubber trec. In this study, variation in yield and some yield determining factors, i.e. girth, depth oftapping cut, tapping height and tapping angle and their relationships were studied on threegenotypes (clones) of I1evea PB 217, RRIC 117 and RRIC 102. Clones have been plantedin 1988 in a commercial estate in Sri Lanka and tapping began in 1994. From each clones,an area having ca. 100 trees was selected for the study. Yield measured as latex volume was found to be highly significant clonal character.Among the three clones tested, the highest mean latex volume per tree per tapping wasrecorded in RRI C 102 whilst the lowest in PB 217. Further, a significant tree-to-tree variation in yield within a clone was observed in all threeclones. The maximum co-efficient of variation was found in PB 2 I7 while minimum wasin RRIC 102. Yield varied through different ranges in each of the three clones and PB 217had more low yielding trees whilst RRIC 102 and RRIC 117 had more medium yieldingtrees. Yield determining factors i.e. girth, depth of tapping cut found to vary within a clonein all three clones. Further, they were significantly and positively correlated to the yield.Tapping height and tapping angle also varied within a clone and they too contributed tothe yield variation within a clone. Out of the yield determining factors considered above, girth was the most important factorwhich governed the yield and it was further significantly and positively correlated to thedepth of tapping cut. It was clearly evident that yield determining factors used in the present study playa majorrole in determining rubber yield. Those arc mainly governed by the rootstock of the plant,environment conditions and agrornanagcmcnt practices. Therefore by the adaptation of thecorrect agromanagement practices and technically correct tapping, such variations withinclones can be minimized and it will enhance the existing productivity levels in Sri Lankarubber plantations.

THE LEVEL OF VARIATION IN YIELD AND YIELD DETERMINING FACTORS AND THEIR RELATIONSHIP IN DIFFERENT HEVEA GENOTYPES.