Genetic transformation offers a viable approach in Hevea brasiliensis for crop improvement by adding valuable genes for specific characters in a relatively short period. 3-Hydroxy 3-methylglutaryl Coenzyme A reductase (HMGR) is considered as a key enzyme in the rubber biosynthetic pathway. With a view to increase the yield potential in Hevea, attempts were made to over-express this rate limiting enzyme by Agrobacterium tumefaciens mediated genetic transformation. Selection of an appropriate bacterial strain is one of the key parameters required for an efficient genetic transformation system. To identify a suitable bacterial strain giving maximum transformation efficiency with the hmgr1 gene in Hevea, three Agrobacterium strains (EHA 105, LBA4404 and pGV 1301) harbouring the transgene were experimented. The binary vector contained hygromycin phosphotransferase gene (hpt) as the plant selectable marker. The effect of different explants (primary callus, embryogenic callus and embryogenic suspension cultures) as well as co-cultivation temperature on transformation efficiency was evaluated. Highest transformation efficiency was observed with the strain EHA 105, irrespective of the target tissues tried for transformation. Low temperature incubation (20 oC) of the infected tissues during co-cultivation period improved the frequency of transformation. Among the different target tissues tried, embryogenic suspension cultures gave the maximum number of transgenic cell lines (32%). The presence of the transgene was confirmed in the transgenic cell lines by PCR using gene specific primers.

Low temperature during winter season is the major constraint for the extension of rubber cultivation in subtropical environments that are prevailing in north-eastern India. Cold stress strongly affects the growth and productivity of rubber plants. In the present study, four Hevea brasiliensis clones viz., RRII 105, RRIC 100, SCATC 88/13 and Haiken 1 were exposed to cold stress under controlled environmental conditions. Morphological symptoms such as yellowing and drying of leaves were more prominent in RRIC 100 followed by RRII 105 and less prominent in SCATC 88/13 and Haiken 1. Photobleaching of the photosynthetic pigments, chlorophyll and carotenoids were comparatively lesser in Haiken 1 and SCATC 88/13. Maximum photochemical efficiency of PS II (Fv/Fm) and effective quantum yield of PS II (Φ PS II) were stable in SCATC 88/13 followed by Haiken 1 under low temperature conditions. The rate of lipid peroxidation was severe in RRII 105 and RRIC 100, indicating that these two clones succumbed to cold mediated oxidative stress. Two stress proteins were found in the chloroplast protein profile of Haiken 1 and SCATC 88/13 that may probably be involved in conferring cold tolerance to these two clones.

Breeding and selection of fast growing clones of rubber with high latex yield and timber with good wood quality traits is a priority area of research at the Rubber Research Institute of India. A study was under taken on the clonal variability among certain newly released high yielding RRII 400 series clones and their parents with the objective of identifying clones with high timber yield and quality. Timber volume of clones was determined from tree stands at the age of 20 years from five clones viz., RRII 414, RRII 417, RRII 422, RRII 429 and RRII 430 along with their parents RRII 105 and RRIC 100. Six trees each were selected for estimating quality traits. Wood samples were prepared for physical, mechanical and anatomical properties and clonal variability was analyzed according to standard procedures. Timber volume of all the new clones in general, was higher than that of the parental clones. RRII 430 and RRII 417 recorded the highest wood density 664 kg m-3. These two clones showed comparable strength properties with RRII 105, which is known for quality timber traits. Volumetric shrinkage values from air dry to oven dry condition of all the clones were on par with that of parental clones. The higher timber output of the RRII 400 series and quality traits comparable with those of RRII 105, especially for the clones RRII 430 and RRII 417, make them good candidates as latex timber clones.

The growth, girth increment, early dry rubber yield and timber yield of RRII 300 series, IRCA and a few other promising clones in the dry sub humid climate of Odisha in eastern region of India is reported. Prolonged high temperature, low rainfall and soil moisture are the major environmental constrains affecting the growth, yield and adaptability of clones in the region. Clone RRII 351 recorded the initial highest mean rubber yield (31.03 g t-1t-1) followed by IRCA 109 (30.69 g t-1t-1 ), whereas popular clones RRIM 600 and RRII 105 recorded 26.59 and 26.75 g t-1t-1, respectively. Highest and lowest yield was recorded during winter and summer months, respectively. RRII 300 attained highest girth (53.03 cm) and girth increment but yield was relatively low. Highest bark thickness was recorded in PB 28/59 (6.97 mm) followed by 6.75 mm in RRII 357 and RRII 51. Highest bole volume was recorded in RRII 300 (0.07 m3 tree-1) followed by IRCA 111 (0.06 m3 tree-1). IRCA 111 recorded medium yield of 26.03 g t-1t-1, on par with that of the popular clones. In general, the clones RRII 351, RRII 352 and IRCA 109 were found to be more promising than the popular clones RRII 105 and RRIM 600 for early yield. The results of the present study will aid in the identification of potential clones for possible commercial cultivation in the region.

The effect of panel changing on long term yield response of Hevea brasiliensis (clone RRII 105) under different frequencies of tapping and stimulation was studied over a period of eleven years. There were eight treatments comprising of d2, d3 and d4 frequencies of tapping of half spiral cuts with and without panel change at different levels of stimulation. Considerable yield variation was observed among various treatments over the years. Effect of panel change on yield increase was more prominent in the initial five years. No significant beneficial impact of panel change on yield increase was observed under different systems of tapping. Higher yield could be obtained under d2 and d3 frequency of tapping with upper panel change (CUT). Comparable yield could be obtained under various frequencies of tapping. Cumulative yields observed within similar systems of tapping with or without panel change were also comparable. In general, similar trend was also noticed in kg per tree, g t-1t-1 and kg tap-1. Significant increase in yield per tap and g t-1t-1 was noticed under d4 frequency of tapping. However, highest cumulative yield was observed under d2 frequency of tapping which was observed to be at par with d3 frequency of tapping with or without panel change. Panel change resulted in higher TPD under d2 frequency of tapping compared to lower frequency of tapping. Moreover, benefit of panel change was reflected only in the initial five years but panel management after first five years of tapping is difficult. Hence, continuous panel change cannot be considered for managing TPD or to get sustainable high yield over long period.

A specific stain for the identification of sieve tubes in the bark of Hevea particularly in transverse plane is lacking, hence a new staining procedure has been developed. Fresh sections of bark preserved in 4 per cent glutaraldehyde were treated with a mixture containing O-dianisidine 1mg mL-1 and 1 per cent hydrogen peroxide in 0.1 M phosphate buffer (pH 7), for about 0.5-1 minute. Observations and photomicrographs were taken by using Leica QWin V3 image analysis system attached to Leica DM 1000 microscope. Transverse sections of the soft bark when stained with O-dianisidine gave deep brown coloration for the cell walls of sieve tubes present in the inner soft bark, as well as the recently differentiated ones from the cambium. The cell wall of companion cells, axial and radial parenchyma, and laticifers remained unstained. The study revealed that the staining method using O-dianisidine is suitable for identifying sieve tubes in cross sectional plane of Hevea bark.