Recognizing the significance of maintaining genetic diversity in rubber plantations, systematic efforts on polycross breeding in rubber were initiated by the Rubber Research Institute of India in 1989. The study also addressed the challenge faced by plant breeders to provide rubber growers with diverse planting materials without compromising on yield.  Open pollinated progenies comprising 1680 half sib seedlings from 20 promising parent clones were evaluated in a nursery. The superior seedling progenies were identified and 15 genotypes per progeny from 10 parents were cloned and evaluated in a field trial adopting a compact family block design for a period of 20 years which included 12 years of tapping. The progeny clones numbering 150 were evaluated for rubber yield in the two virgin panels, timber yield and secondary attributes.  Thirty five selections from these progenies were classified as dual purpose latex-timber clones and timber–latex clones as well as latex clones and timber clones.  In terms of yield in the long term, 25 promising half-sib clones were selected from the progeny of nine genetically diverse parents.  Among these, 15 clones exhibited stability in performance over 12 years of tapping. The most promising 12 selections in the pipeline out-yielded the check clone by over 20 per cent with estimated rubber yields of 2820 to 4429 kg ha-1 year-1. Clones PB 215, PB 28/83 and Ch 26 were confirmed to be prepotent for rubber yield with more than 25 per cent of their progeny being high yielding. Estimation of stability across years proved clones HS PB 252/132 the highest yielding clone to be inconsistent in performance and HS Ch 26/161 with more than 97 g t-1 t-1 along with 10 other high yielding clones to be consistent in yield in the long term. The reaction of these clones to TPD, pink disease, abnormal leaf fall and powdery mildew is also discussed.

The concept of harvest index (HI) requires redefining in a perennial crop like Hevea brasilienis where the economic yield is a secondary metabolite that is produced in small quantities by large trees. Partitioning of total biomass into shoot biomass and rubber yield was accounted in tapped trees of Hevea brasiliensis on an annual basis for 12 consecutive years in five clones viz. RRII 105, RRII 300, PB 235, PB 260 and PB 311. Ratio of rubber yield per year to annual increment in shoot biomass per year was defined as HI in this tree crop. Among the five clones studied, RRII 105 had the highest HI (0.32) indicating that this clone is an efficient clone capable of partitioning more photosynthates in to rubber.  But high yield and HI in this clone came with the cost of reduced shoot biomass compared to the PB clones. Rubber yield in PB 235 and PB 260 was on par with RRII 105, but the PB clones had much less HI than RRII 105. Improving HI in high biomass genotypes will result in better rubber yields and break the yield ceiling. Biomass production in untapped rubber trees was estimated and their CO2 sequestration potential is also discussed.

A trial on planting density was conducted in clone RRII 429 with six densities viz. 408, 445, 489, 544, 613 and 699 in rectangular planting. Growth of the plants was significantly influenced by the different densities sixth years after planting. The density up to 544 trees ha-1 attained required tappable girth by the end of  seventh year. The percentage of tappable trees was low in the denser plantings. Plants at the lower density showed high girth increment in all the years. Seasonal variation in growth was observed. Highest girth increment was observed in monsoon season followed by post monsoon and lowest in winter season. Lower planting density produced higher tree girth and thick virgin bark.

The lengthy unproductive immature phase of natural rubber is a matter of concern among rubber farmers, especially the smallholders. Majority of the rubber plantations in India takes about seven to eight years to attain tappable girth. This paper focuses on the results of a field experiment initiated to evolve an agronomic package to shorten the immaturity period of rubber in which the treatments included combinations of two types of planting material viz. green-budded stumps raised in polybags and plants raised by direct-seeding in polybags and two management options viz. current recommended package of practices, and an integrated management comprising enhanced nutrient application, selective manuring and conservation-oriented tillage. The results showed significant difference in the performance of two types of planting material. Direct- seeded green- budded plants were significantly superior to plants raised from green - budded stumps throughout the period under study. The effect of agromanagement practices was reflected in growth of rubber. The girth of the plants under integrated management was superior to that of respective type of planting material under current recommended practice. The soil nutrient status viz. the organic carbon and available K contents was significantly improved under integrated management. A higher canopy width was maintained by the direct seeded green-budded plants under integrated management. The soil moisture storage was also positively influenced by integrated management. Thus direct-seeded green-budded plants under integrated management was significantly superior to all others and 68 per cent of the plants reached tappable girth in five years and nine months as against 19 per cent in the control (budded stumps raised in polybags under the current recommended practices). The study revealed that the gestation period of natural rubber could be brought down to less than six years through the use of direct-seeded green-budded plants and adoption of integrated agricultural practices.

Latex production in Hevea brasiliensis can be optimized by appropriate tapping frequency and yield stimulation using ethephon. High frequency tapping and non-judicious use of yield stimulant may cause oxidative stress to the laticiferous system and leads to severe metabolic disorders like tapping panel dryness (TPD). In the present study, low frequency tapping systems with different levels of yield stimulation were tested in the clone RRII 105 for rubber yield. Biochemical parameters related to latex production and oxidative stress indicators were also studied. After four years of tapping, the cumulative yield under low frequency tapping systems was at par with S/2 d2 tapping frequency except S/2 d3 frequency without stimulation. TPD incidence was more than 21 per cent under high frequency (S/2 d2) tapping without yield stimulation. Under weekly tapping (S/2 d7), TPD incidence was only six per cent with the highest rounds of annual ethephon application. No significant stress effects were observed subsequent to various stimulation schedules under S/2 d4, S/2 d6 and weekly tapping as indicated by higher levels of invertase, ATP, protein synthesis, optimum thiol content, high utilization of sucrose and non-accumulation of stress indicators like proline and phenol. Sustainable high rubber yield obtained under weekly tapping without any stress effects due to frequent ethephon stimulation and low incidence of TPD suggests that S/2 d7 tapping system as the best option for rubber growers for harvesting latex from rubber trees to solve the current problems of low rubber price, scarcity of labour and high cost of production. 

Seven promising clones, screened on the basis of their performance over 12 to 15 years under the cold prone agro-climate of sub-Himalayan West Bengal, were evaluated as parents in polycross breeding by evaluation of their open pollinated half-sib progenies. Weight of seeds of RRII 429 was significantly higher than that of RRIM 600 while that of SCATC 88/13 was at par with RRIM 600. Viability of seeds from RRIM 600 and RRII 429 was above 95 per cent, while viability of SCATC 88/13 seeds was low. Among the half-sib progenies of the seven promising clones, performance of progenies of SCATC 88/13 was the best in terms of mean juvenile yield and yield efficiency, despite its low seed weight and viability. Moreover, supremacy of this clone was also evident while selecting half-sib progenies showing above average yield as well in the final screening process. Out of the 28 progenies from different mother clones showing yield efficiency >0.3 g t-1 t-1 cm-1 girth, 21 progenies belonged to SCATC 88/13 indicating prepotency of this parent clone of Chinese origin. This was the first time a wild Amazonian accession (RO 5363) was used in polycross breeding and the half-sibs produced by this clone had the second largest number of juvenile high yielders after SCATC 88/13. These clones generated more number of superior progenies than the Class 1 clones RRIM 600 and RRII 208. For further cloning and evaluation of potential genotypes for the agro-climate of sub-Himalayan India, 34 selected progenies (40% of the top yielders) would be conserved in source-bush nursery. The results also suggest that clones SCATC 88/13 and RO 5363 may be good female parents for further hybridization programs in cold prone regions.