Adoption of high yielding clones Low Frequency Tapping (LFT) are essential for sustainable rubber cultivation.  The present paper reports the response to yield stimulation in 31 newly developed high yielding pipeline clones. Yield under pre-stimulation and stimulation were recorded to understand the response to yield stimulation in each clone in comparison to that of the check clone RRII 105. As a result of stimulation, 23 clones showed higher yield than RRII 105 of which seven clones viz. HS PB 242/172, HS PB 242/117, HS PB 252/132, HS Ch 26/161, HS PB 5/51/82, HS RRII 105/112 and HS PB 217/180 had significantly higher yield. Three clones, despite being low yielders compared to RRII 105 in the unstimulated condition showed better yield on stimulation. When response to stimulation in clones under different yield categories was analyzed, all clones in the top yield category except clone HS RRII 105/112 and HS PB 252/132 showed low response to stimulation and hence not suitable for LFT.  Clone HS RRII 105/112 was found to be an ideal candidate for LFT with an increase in yield of 85 per cent by virtue of stimulation. In contrast to top yield category, clones in middle and bottom category generally showed very high relative response to stimulation. Response to stimulation in clones under each family indicated that yield category to which the clone belongs rather than the family played a much more important role in deciding the clone’s relative response to stimulation. The dry rubber yield pattern after the application of stimulation was identical in all months under study and was found to be the highest in the first tapping day immediately following the application of stimulation as expected. 

Effect of frequency of stimulation with ethephon (2-chloro ethyl phosphonic acid) on yield performance of five clones viz. RRII 417, RRII 422, RRII 429, RRII 430 and RRIM 600 were evaluated under S/2 d3 6d/7 tapping system with stimulation and S/2 d2 6d/7 tapping system without stimulation with two months tapping rest. A five year study showed increase in dry rubber yield due to application of three to four stimulations per year in d3 system of tapping in all the clones. Highest yield was observed in clone RRII 429 which also had the highest growth. Incidence of tapping panel dryness was non-significant. Four rounds of stimulation in a year gave a higher yield of 1866 kg 400 trees-1 year-1 in clone RRII 429 under d3 system which was comparable with yield under d2 system (without stimulation) in North Eastern region of India.

Eleven ortet selections from the states of Assam, Tripura and Meghalaya were evaluated for growth parameters and juvenile yield in a clonal nursery trial where RRIM 600 was used as a check clone. Observations on growth were recorded one year after planting onwards and continued up to five years of crop growth. Three years after planting, the young plants were subjected to test tapping and continued up to five years. The ortet selection RRST 37 attained the highest girth (30.3 cm) and juvenile yield (71.7 g t1 10t ap-1) followed by RRSG 9 (66.0 g t-110 tap-1) and RRSA 121 (57.2 g t-110 tap-1). The check clone RRIM 600 recorded a juvenile yield only of 44.4 g t-110 tap-1. The ortet selection RRST 24 attained the highest girth increment of 24 cm over five years followed by RRST 37 (22 cm) and RRST 39 (21.5 cm) whereas the check clone RRIM 600 recorded a girth increment of 21.2 cm indicating the superior performance of the new ortet selections. There is good prospect for preliminary selection of clones through clonal nursery evaluation of these orlets.

Polyclonal seedling trees raised from seeds collected from diverse environments viz. Kamrup (Assam), Jalpaiguri (West Bengal), Tura (Meghalaya) and Kanyakumari (Tamil Nadu) were evaluated under the agroclimate of Sub-Himalayan West Bengal. Girth and test tap yield of seedlings from various seed sources was not significantly different during the initial four years of tapping. Overall, 21 per cent seedling trees showed more than 50 g t-1 t-1  among all the tapping trees. Trees showing above average yield were more  in seedling trees raised from seeds of Kanyakumari followed by Tura. However, in case of seedlings showing yield above 50 g t-1 t-1, it was high in seedlings sourced from Guwahati. Five seedling trees showed high girth and yield which were selected for further studies to evolve new clones for North East India.

To find out whether potassium accumulation is more in drought tolerant accessions/clones of rubber (Hevea brasiliensis) and whether its concentration is high in accessions/clones during stress period, studies were conducted in selected clones and germplasm accessions. In the first experiment, leaf samples from mature trees were collected from a clone evaluation trial in the traditional rubber growing region of Kerala.  In the second experiment, leaf samples of young natural rubber plants from already identified drought tolerant and susceptible wild accessions and hybrid clones grown in a drought prone area were assessed  during pre-stress, stress and post-stress period. Significant differences in K content in leaves were recorded between genotypes in both experiments. RRII 430, a known drought tolerant clone recorded high K content in the leaves in both experiments.  There was significantly higher K content in leaves in drought tolerant germplasm accessions than susceptible accessions.  The leaf K concentration was high during stress compared to pre-stress and post-stress period in almost all accessions/clones. Five accessions/clones viz. MT 4788, MT 43, RO 2153, RRII 430, RRII 414 were identified as potential drought tolerant clones based on K content of leaves.  High leaf K concentration in proven drought tolerant wild accessions/clones and also during drought stress, indicates high leaf K as an indication of adaptation to drought stress in Hevea. Hence, high leaf K concentration can be used as a screening tool for drought tolerance along with other physiological/ biochemical/parameters molecular.

Ribbed Smoked Sheets (RSS) are generally made from fresh latex and graded by visual grading as per the “Green book” specifications. As sheets in India are mostly made in the small growers sector the uniformity in quality of sheets is a concern. Good grading can be achieved by adopting recommended processing practices through group processing centres (GPCs). To ensure a regular supply of latex to the GPCs it is necessary to store latex and prepare sheets from preserved field latex (PFL). Present study is an attempt to develop a method to prepare sheets from stored latex without compromising the quality. Both low ammonia latex and high ammonia latex were pre-treated before adding the preservative and stored for various periods. The lattices were coagulated at regular intervals and the raw rubber properties were measured. The study revealed that the PRI values can be retained as a satisfactory level by pretreating the latex before adding preservatives. This method can be used for making sheets from stored latex without adversely affecting the rubber properties.