Early evaluation techniques to shorten the breeding cycle of Hevea have been investigated at large, but few have systematically studied the juvenile-mature correlations which are of paramount significance in deciding early selection criteria. A population of 150 polycross seedlings belonging to ten half-sib progenies was subjected to investigations on correlations among five juvenile traits and 18 mature traits. Juvenilemature correlations among these traits recorded over three years in the juvenile phase in a nursery and over 11 years in the subsequent mature phase of the 150 resultant clones planted in a compact family block design in the main field were studied. Juvenile girth, bark thickness and the number of laticifers emerged as important components showing positive association with yield determined by test tapping. Rubber yield in the first four years of tapping at maturity was significantly correlated with summer yield, girth, number of latex vessel rows in the soft bast and clear bole volume. Clear bole volume showed a higher positive correlation with girth than forking height, an indication that high girthing clones can be assumed to give high timber yields. The juvenile-mature correlations indicated test tap yield of two year old seedlings in the nursery to be the only juvenile parameter with a significant positive association with yield of the resultant clones at maturity in the main field. Girth, bark thickness and the number of latex vessel rows in the juvenile seedlings had no association with yield per se in the mature stage and hence, resorting to selection of seedlings based on juvenile vigour alone would lead to erroneous conclusions and loss of potential high yielders. Among the 29 clones with high yield at maturity 90% were from the high and moderately high test tap yield category while only 72% belonged to the moderate to high girth category. Only 7% of the low juvenile yielders gave high yield at maturity while 25% of the seedlings with low juvenile vigour gave high yield at maturity. The results suggest that nursery screening based on juvenile yield by test tapping remains the only reliable criterion in breeding for yield improvement. However, the relatively low strength of juvenile – mature correlation indicates that a combined and less stringent selection for traits such as juvenile yield, girth and the number of latex vessel rows in 2- 3 year old seedlings could give better recovery of high yielding clones at maturity.

Changes in the status of soil organic carbon (SOC), available phosphorus (av. P), available potassium (av. K), pH and bulk density (BD) were studied in rubber plantations at an interval of two decades in Tripura. Composite soil samples collected at two depths viz. 0-30 cm (surface soil) and 30-60 cm (subsurface soil) during 1986 and 2006 from growers’ fields, were analyzed for the physico-chemical properties. The data showed that SOC content increased appreciably under rubber, particularly in the surface layer. It was observed that in a span of twenty years, SOC status in surface and sub-surface soil increased from 0.81 to 0.96 and 0.66 to 0.73 per cent, respectively. The frequency distribution curve for SOC showed a positive shift indicating an increase of SOC under rubber soils over time, up to a depth of 60 cm. A total amount of 10.2 mt C/ha was stored under rubber soils during twenty years of rubber cultivation and the rate of carbon assimilation in rubber soils was found to be 0.24 mt/ha/yr .The frequency distribution curve for av. K showed a building up at both the depths, whereas a decline in av. P status of the soil was observed during that period. No appreciable change in soil pH was recorded during this time and variation in BD of soils was inconclusive. The study revealed that cultivation of rubber in Jhum-lands of Tripura will be beneficial for restoring soil health.

Multilocus amplified fragment length polymorphism (AFLP) markers are used in genetic mapping, diversity analysis and genotyping. Success of the technique mainly depends on the identification of the primer combinations for selective amplification. In this study a simple method was demonstrated to identify the potential primer combinations using a mini polyacrylamide gel electrophoresis (PAGE) followed by silver staining. A modified silver staining protocol was developed to save time without compromising on the sensitivity. Further, the utility of the protocol in developing AFLP markers in rubber (Hevea brasiliensis) for assessment of genetic diversity among cultivated clones from different South East Asian rubber growing countries was demonstrated.

A mature phase evaluation of selected wild accessions of Hevea brasiliensis from the 1981 IRRDB Brazilian wild germplasm collection established at the Rubber Research Institute of India was done on the basis of various agro-morphological traits such as growth, yield, bark anatomical characters and timber potential. These wild accessions represent three provenances of Brazil, viz. Acre, Rondonia, and Mato Grosso. Field experiments were conducted at the Regional Research Station of Rubber Research Institute of India at Padiyoor, in Northern Kerala. The popular clone, RRII 105 was used as control. The accessions showed significant variability for all the characters. In the year of opening, six out of eighty accessions were found superior to RRII 105 for girth and this trend continued for the subsequent five years under regular tapping. The accession MT 2233 was significantly superior to RRII 105 in bark thickness. Two Acre accessions (AC 635 and AC 166) had significantly higher number of latex vessel rows than that of RRII 105 and the number of latex vessel rows was on par with the control in 44 accessions. One Rondonian accession (RO 368) showed significant superiority for density of laticifers over RRII 105 and 11 accessions comprising two from Acre and nine from Mato Grosso provenances were on par with that of the check clone. Thirty three accessions had latex vessel diameter on par with that of RRII 105, indicating the scope for selection in the wild accessions for yield-related anatomical traits. Increased number of laticifer rows found in an accession might have contributed its high yield potential. Six accessions had significantly higher bole volume than that of RRII 105. Among these, two accessions with low dry rubber yield can be considered as potential timber yielding accessions and the remaining four accessions with high timber yield and comparatively medium latex yield can be considered for developing timber-latex clones. The wild accession AC 166, gave good yield and this was reconfirmed in further on-farm evaluation. The possibility of using the inherent genetic variability available in this wild population for future crop improvement programmes is discussed.

The influence of intercropping on growth of rubber and soil physico-chemical properties was studied in a field experiment involving three intercropping systems viz. rubber/tapioca, rubber/banana and rubber/ pineapple in comparison with the standard rubber-cover crop system. Pineapple and cover crop were retained for 3-5 years and three crops of tapioca and banana were raised during this period. The girth of rubber was recorded at periodic intervals at a height of 125 cm from bud union. Soil samples were collected before and 6.5 years after planting from each system and analysed for physical and chemical properties. Bulk density and porosity of soils were not affected by planting different intercrops while an improvement in cation exchange capacity (CEC) and organic carbon status was observed in banana and cover crop established areas. An increase in available phosphorus was noticed in all the systems and the availability of potassium increased significantly in banana area compared to other systems. The tappability of rubber in 6.5 years was 43.75% in banana-intercropped areas. Growth of rubber was superior together with banana compared to other systems while in tapioca, pineapple and cover crop established areas, tappability was 37.8, 17.02 and 37.8 % respectively. The study showed that among the three intercrops and cover crop, banana intercropping is good for improving growth of rubber followed by cover crop and tapioca.

Studies were undertaken in the polybag nursery for two successive years to investigate whether indole butric acid (IBA) and water enhanced establishment of green budded stumps of Hevea in polybags. Three different concentrations viz. 250 ppm, 500ppm and 1000 ppm IBA were used for the study. A simple technique of keeping tap root of green budded stumps dipped in fresh water for 4-5 hours before planting in polybags was also attempted. Examination of the stumps after four months from application showed that IBA at 500 ppm and water gave 73.16 % and 66.27% establishment, respectively whereas it was 53.50% in the control. IBA at the concentration of 500 ppm and also fresh water treated stumps recorded better height, diameter, and number of whorls than the untreated stumps. Dry weight of lateral roots showed better response of 138% (500 ppm IBA) and 125% (fresh water) over the control. The results of this study indicated higher production of lateral roots after the IBA and water treatments and thereby better establishment of green budded stumps in polybags.