Major quantum of natural rubber is being recovered as an industrial raw material from the latex obtained on tapping the bark of Hevea brasiliensis, a deciduous tropical tree with an economic life span of about thirty years. Genetic improvement for achieving better attributes including high yield is one of the prime areas of research in this crop. It has been established that high latex yielding clones are vulnerable to a syndrome termed Tapping Panel Dryness (TPD), when the trees are subjected to intensive tapping. Cessation of latex flow from the tapping panel is the initial symptom, followed by the occurrence of a number of morphological, structural, biochemical and physiological changes resulting in complete drying of the bark. Even though different reasons are attributed for this century old problem, till now there is no remedy other than a few management practices. Sieve tubes, the prime channel for photoassimilate translocation in the bark of the tree that cater nourishments for latex biosynthesis exhibit an array of deformations following TPD. In this context, stress responsive plant deformations viz., permanently altered cambial activity, intense deposition of definitive callose and P-protein, chaotic dynamic system in the sieve tube, peroxidase activity, protein storing cells and phytoplasma with respect to TPD are reviewed.

Latex yield from Para rubber (Hevea brasiliensis Muell. Arg.) is highly sensitive to the prevailing ambient weather conditions and exhibits seasonal variations in the different agro-climatic regions where it is cultivated. Monthly share of latex yield of 260 clones from 24 field trials conducted by the Rubber Research Institute of India in four locations in the traditional region (Central and North Kerala, Kanyakumari district in Tamil Nadu and South West Karnataka), four in North and North East India (Assam, Meghalaya, Tripura and West Bengal) and two locations in Central India (Maharashtra and Odisha) was examined from published sources to identify the lean and high yielding months/seasons in each region. The traditional region experienced a productive period of 10 months which was reduced to eight months in Central and NE India due to varying stresses. The average yield during the three lean months from February to April in all three regions was to the tune of 15 per cent of the total annual yield. Price of rubber remained largely insensitive to monthly yield variations. The study throws light on how to regulate tapping by growers in each region by skipping of tapping in the extremely low yielding months and harvesting maximum yield in the high yielding months rather than abstaining from tapping altogether under low price situations.

The hilly regions of Kerala state witnessed two massive landslides causing many human casualties and serious damages to the environment and properties in the past couple of years. Natural rubber (NR) plantations are mostly located on the undulating and sloping terrains along the foothills of the Western Ghats of the state which are increasingly becoming vulnerable to landslides. Extremely intensive rainfall can destabilize hilly terrains where NR is a popular crop among others in Kerala. To estimate the spatial extent of rubber plantations according to proneness to landslide in Kerala, satellite-based area under rubber (age three years and above) was geospatially analyzed in landslide susceptible zones of the state. Further district-wise extent of rubber plantations susceptible to landslide was estimated. Landslide susceptibility map prepared by Kerala State Disaster Management Authority was utilized for characterizing area under rubber plantations into low, medium and high susceptible zones prone to landslide. Results showed that out of the total rubber plantation area in Kerala, 1.6 per cent (9,485 ha) was in the high susceptible zone, six per cent (32,398 ha) in the medium and two per cent (13,072 ha) in the low susceptible zone. All three zones together account for 9.7 per cent of the total rubber cultivation in Kerala whereas more than 90 per cent of rubber holdings in the state are outside landslideprone region. Different districts in eastern parts of Kerala (except Alappuzha) have landslide vulnerable regions. Plantation areas under rubber in the high susceptible zone of landslide were the highest in Kottayam district followed by Idukki, Kannur, Palakkad, Pathanamthitta and Kasaragod districts. Substantial area is under medium landslide susceptible zone spread over various districts. Policymakers and planners are devising strategies to minimize the occurrence of natural hazards and mitigate the risks on the background of increased incidences of landslides, flood etc. in the recent past. The present information is highly useful for planning appropriate conservation and management strategies for rubber plantations in the highly vulnerable areas. An open source WebGIS portal was developed by integrating landslide susceptibility of rubber plantations in different panchayats in Kerala for easy dissemination of this information to rubber growers. Good agricultural practices are recommended through this platform for adoption at a location according to its vulnerability to landslide as well as avoiding certain cultural operations to minimize the occurrence/impact of landslides.

A long term field trial was conducted under sub-humid climatic condition at Agartala to study the effect of planting density on growth and yield of Hevea brasiliensis, clone RRII 429. Performance under six planting densities viz. 408, 445, 489, 544, 613 and 699 trees ha-1 was evaluated. Growth of the plants was significantly influenced by different densities only after the sixth year. Growth and yield per tree decreased with increasing planting density. Plants at the lower density showed high girth increment in all the years and produced thick virgin bark. Higher density resulted in lower girth, bark thickness and higher crotch height. However, it was vice-versa in the case of yield per hectare due to increase in number of trees under tapping with increasing planting density. From the study it appeared that the highest seven year average yield could be obtained at the density of around 613 trees ha-1. However, planting density of 544 trees ha-1 gave the highest Benefit-Cost Ratio (BCR) and Internal Rate of Return (IRR) indicating that it is the economically viable planting option for North-East India.

This paper discusses the role of parental selection in classical Hevea breeding programmes based on results from clone evaluation trials. Among the major clonal types examined (W x W, W x A and ortets), W x W origin clones (360 clones) showed an average yield potential of 5.4 kg tree-1 yr-1. Yield potential of the W x A hybrids (42 clones) and Wickham origin ortets (165 clones) were 3.7 and 4.5 kg tree-1 yr-1 respectively.The average yield was higher in clonal families with RRII 105 as the female parent than as the male parent. The same trend was observed in reciprocal crosses also indicating the significance of RRII 105 as a female parent in cross combinations. Among the various cross combinations involving RRII 105 as the female parent, the highest average family yield (7.9 kg tree-1 yr-1) and the maximum yield (15.9 kg tree-1 yr-1) were recorded in the family of RRII 105 x RRII 118. In the 1982 HP programs, clones in the family of RRII 105 x RRIC 100 recorded 63 per cent average yield improvement over the family of GT 1 x RRII 105 and 90 per cent improvement over the family of GT 1 x RRIC 100. RRII 118 and RRIC 100 are clones known for high growth vigour and yield potential which strongly suggests the critical role of both the parents in Hevea breeding programmes. 

The average yield potential of the important introduced clones, popular primary clones and secondary clones were 6, 4 and 6 kg tree-1 yr-1 respectively. The average yield improvement of the tertiary clones (RRII 400 series) over the secondary clones was 17 per cent while that over the primary clones was 75 per cent. Recombination breeding in Hevea is rewarding although a yield ceiling seemed to be reaching as evidenced from the decreasing per cent of yield improvement. Further yield improvement and widening of the gene pool is envisaged through new introductions. The recently imported clones, through the international Multilateral Clone Exchange (MCE) programme from the IRRDB member countries, having proven traits for yield, growth and other important secondary attributes are expected to contribute more in the Hevea breeding programmes and in the plantation industry.

Secondary gene pool available in the wild relatives of a cultivated variety can be employed when the variation within the primary gene pool is exhausted to develop improved varieties through introgression hybridization. The time-consuming breeding cycle makes introgressive hybridization through repeated backcrossing a highly challenging strategy in Hevea. Three promising Wickham (W) x Amazonian (A) hybrids identified earlier viz. 90/10 and 90/34 (RRII 105 x RO 142) and 90/274 (RRII 105 x MT 196) were selected as the paternal parents. The maternal parents used in the back cross included RRII 105, along with RRII 414, RRII 430 and RRII 429 which are the hybrids of RRII 105 x RRIC 100. A total of 353 hybrid seedlings derived from 13,859 hand pollinations were planted in the nursery for evaluation. The hybrids were screened for yield and girth under seedling nursery evaluation. Sixty three promising hybrids were identified and the per cent recovery of high yielding hybrids from a cross combination varied between 0 to 33 per cent. Cross combinations involving RRII 430 as maternal parent resulted in maximum number of selections. Number of selections from crosses involving 90/10 as paternal parent was the highest with an average of 26.2 per cent. The average test tap yield of hybrids of RRII 430 as maternal parent was the highest. The promising hybrids identified in the present study were forwarded to the next level of evaluation to develop improved clones.