Genotypic variability in potential yield was analysed using the published yield data of as many as 622 Hevea clones derived from various field trials conducted by the Rubber Research Institute of India at different points of time in the past 70 years.  These clones were experimental materials with promising yields or cultivated clones popular in Asia and were mostly of Wickham origin.  They included ortets and hybrids between Wickham clones and a few hybrids between Wickham clones and wild Amazonian accessions in more recent times.

Since the trials were conducted with standard management practices in the agro-climatically better suitable traditional rubber growing regions of the country, the realised yields represent the potential yield of each clone. The frequency distribution of the 622 clones according to their potential yields followed a normal distribution with an extended distribution to the extreme right and with just one or two clones each falling in the highest yield classes. While there were eight clones appearing in the lowest yield class of <2 kg tree-1 year-1, there were 12 clones falling in the very high yield classes between 11 and 16 kg tree-1 year-1. The population mean was 5.3 kg tree-1 year-1 and the mode and median were 3.3 and 4.8 kg tree-1 year-1, respectively. 

From unselected trees with hardly an yield potential of one kg tree-1 year-1 planted at the beginning of rubber plantation industry in the country, popular clones of the later years such as Tjir 1 and RRIM 600 (until the 1960s/70s) showed better potential yields of 4.5 and 5 kg tree-1 year-1, respectively.  The first Indian high yielding clone, RRII 105 (1980) had an yield potential of 6.2 kg tree-1 year-1, while the subsequently released five clones in the RRII 400 series (2005 to 2020) had a yield potential of 7.2 kg tree-1 year-1. The present analysis indicates that yield potential of future clones, if any, to emerge from the present genetic resource, may stagnate at about 8-9 kg tree-1 year-1. This clearly indicates that the available genomic resources are getting exhausted and the law of diminishing marginal returns will apply making future clone development with still higher yields increasingly difficult from the existing genetice stock.  We discuss science-based strategies such as widening the genetic base of the breeding population (e.g. incorporating wild accessions collected from the Amazon and high yielders recently imported from other countries), applying latest developments in population genetics and genetic recombination, marker assisted molecular breeding and selection, ploidy manipulation and creation of pure-lines, gene editing etc. for breaking the yield ceiling in this species.

Regional Trade Agreements (RTAs) are proliferating in the world. They affect different sectors of the economy differently. The present study analysed tariff policies of raw materials of rubber under India’s RTAs and their impacts on the growth in imports into the country. All the tariff lines of natural rubber (NR), synthetic rubber (SR) and reclaimed rubber (RR) at the six-digit level of the Harmonised System (HS) were analysed. Along with the gravity modelling, breakpoint analysis and descriptive statistics were used for the analysis. The results of the study show that in the case of NR, no tariff lines exhibited growth in import due to the tariff policies under the RTAs of India. In the case of SR, the tariff policies of India under the RTAs led to growth in import of five tariff lines. In the case of RR also the tariff concessions given under the RTAs were a major factor for the growth of import into India.

Corynespora leaf fall disease, caused by the fungus Corynespora cassiicola, is a highly devastating disease adversely affecting the growth and productivity of Hevea brasiliensis. Early screening of the newly developed, improved clones towards this disease is highly desirable. Cultivating tolerant clones with improved yield and other secondary characters will be a better strategy for disease management rather than adopting chemical control measures for susceptible clones. Towards this objective, we explored the possibility of employing in vitro assay for early screening, making use of the pathotoxin, cassiicolin produced by the fungus. Eight different clones of Hevea, four each belonging to the susceptible and tolerant groups as established from previous field observations were selected for this study. Both detached leaves and calli derived from these clones were employed in the bioassay experiments wherein the disease sensitivity was tested using crude culture filtrate generated from a highly virulent strain of C. cassiicola. Two experiments were performed using detached leaves, one being leaf wilt bioassay using whole detached leaf and the second one employing leaf segments subjected to vacuum infiltration with crude culture filtrate. In the whole leaf assay, sensitivity to crude culture filtrate was evaluated by visual identification of the degree of wilting of different clones at specific time intervals. In the case of leaf segments subjected to vacuum infiltration, visual observation of the necrotic lesions as well as electrolyte leakage measurement were carried out. In the in vitro screening experiment of callus using fungal exudates, changes in the colour and texture of the callus were taken into account for evaluating the extent of cellular damage. Results of all these experiments conform earlier field observations of sensitivity ranking of different clones towards Corynespora leaf fall disease. These methods, being quite simple and fast, can be employed in the early screening of new hybrids and pipeline clones to determine the degree of tolerance towards this disease.

In order to meet the surging requirements for natural rubber, its production must be increased by way of expanding rubber cultivation to marginal areas which experience uncongenial climatic conditions.  Intrinsic growth and yield potential of Hevea brasiliensis is negatively regulated by abiotic stresses.  Hence identification of climate-resilient clones that can withstand such adverse conditions is imperative to maximize the productivity of Hevea.  With an objective to screen about 35 Hevea genotype for their inherent potential for drought tolerance, leaf characteristics and gas exchange parameters were measured in young plants experiencing soil moisture deficit stress. In terms of membrane stability, genotype P 173 exhibited better drought tolerance. Lesser stomatal density that plays a role in minimizing water loss was observed in genotype P 96 and P 47.  Under drought conditions, genotype P 96 maintained higher levels of chlorophyll content while genotypes P 96, P 65 and P 68 exhibited least chlorophyll degradation. Epicuticular wax content was the highest in genotypes P 65 and in check clones RRIM 600, RRII 430, RRII 422 under drought stress while rate of wax build up during the stress was the highest in genotype P 173.  The genotype P 200 excelled in gas exchange parameters along with clones RRII 430 and RRII 208. Drought injury symptoms in leaves viz. leaf yellowing, drying and leaf fall were comparatively lesser in pipeline clones P 200, P 201 and P 65 and in check clones RRII 430 and RRIM 600 which also corroborated with the cumulative ranking results that these are relatively drought tolerant clones.  These pipeline clones need further field evaluation to ascertain their field tolerance to drought stress and their suitability for cultivation in drought stress prone regions. 

An experiment was conducted to study the effect of farm yard manure (FYM) integrated with recommended dose of chemical fertilizers (RDF) on growth of rubber (Hevea brasiliensis) and soil nutrient status in north eastern India. Nine treatments were laid out in randomized block design with three replications. The treatments were control (without fertilizer and organic manure), 50 per cent of RDF, 100 per cent RDF alone, 10 and 20 kg FYM per plant alone and in combination with 50 per cent and 100 per cent RDF.  Soil nutrient status increased on application of FYM with chemical fertilizers compared to the application of chemical fertilizer alone. The soil microbial population was also higher in the plots where the FYM was applied.  After seven years girth of rubber plants was 3.6 per cent higher with application of 10 kg of FYM with RDF and 12.6 per cent higher on application of 20 kg of FYM in combination with RDF compared to RDF alone. Girth was 24.8 per cent lower in control without application of any chemical fertilizer compared to RDF alone and the control plants recorded the lowest leaf area index. Tappable girth of rubber was achieved in six and half years with integrated application of 20 kg FYM with RDF in North-East India. 

Changes in the soil chemical properties and nutrient availability with liming were studied in extremely acidic (pH 4.4) and strongly acidic (pH 5.5) soils by conducting an incubation experiment. Response of young rubber plants to liming and the changes in rhizosphere soil chemical properties were studied by growing rubber plants in polythene bags. Incubating the soil with lime for two months improved pH, cation exchange capacity (CEC) and availability of phosphorus (P) in extremely acidic soil. Similarly, incorporation of lime improved exchangeable potassium (K) and reduced exchangeable magnesium (Mg) and aluminium (Al) in both the soils. Lime treatment improved the diameter and height of young rubber plants in the extremely acidic soil. With the incorporation of lime, apart from the significant improvement in pH and organic carbon (OC), the availability of P and Ca including fractions of P and exchangeable Ca were increased significantly in the rhizosphere. At the same time, significant reduction in the availability of K and Mg including fractions of K and exchangeable Mg were recorded in the rhizosphere of rubber plants with liming. Liming improved the pH from 4.4 to 5.5 in the rhizosphere and improved the growth of plants indicating that a pH of around 5.5 is congenial for growth and establishment of rubber.