A study of litter fall from rubber trees, litter in standing crop and their decomposition was carried out for a period of two years at Agartala, Tripura, North East India. Annual litter fall was estimated in a 14 year old rubber plantation of clone RRIM 600 which had a density of 317 mature rubber trees per ha. Litter samples were collected at monthly intervals from 12 traps randomly placed in an area of one ha. The total annual litter fall ranged from 6.8 to 7.8 t per ha. Leaf litter accounted for 66 per cent of the total litter and the major portion of the leaf fall occurred from January to March. There were no significant differences in litter production between two consecutive years. Maximum litter in standing crop occurred in March and it decreased progressively as the rates of decomposition increased.  Nutrients ranging from 94 to 130 kg of N, 5 to 6 kg of P, 22 to 25 kg of K, 106 to 168 kg of Ca and 17 to 33 kg of Mg were returned to the soil through total litter fall. Decomposition of leaf and petiole litter was studied for a period of 360 days.  After 120 days a weight loss of 16-21 per cent was observed for leaf litter, whereas a higher weight loss of 28-30 per cent  was  observed for  petiole litter indicating faster decomposition of petioles.  As the decomposition progressed, the concentrations of K, Mg and Ca decreased markedly. Decomposition quotient (K_L) for leaves ranged from 1.97 to 2.92, for twigs from 3.24 to 4.06 and from 2.28 to 3.20 for petioles.  The total quantity of leaf fall in the mature plantation in Tripura and the nutrient return through litter was comparable to that in the traditional rubber growing tracts.

The performance of a seedling population of polyclonal origin was evaluated under the agro climatic conditions of Tripura. Comparison of the characters of the polyclonal seedling population with a multiclonal population of the same age revealed that the seedling population is highly heterogeneous with respect to growth, yield and other useful secondary characters. Yield evaluation of the seedling population over the initial three years revealed that some genotypes are outstanding performers which recorded two to three times higher yield than the mean of the multiclonal population.  In addition to growth and yield, the population was assessed for incidence of powdery mildew, wind damage and wintering pattern. Ten genotypes with high yield and useful secondary attributes were selected for further evaluation.   This report, which is the first of its kind from North East India, reveals the scope for selection of promising genotypes from polycross progeny.

Rootstock-scion interaction effects on certain physiological and biochemical parameters  were studied in five clones of Hevea brasiliensis. Photosynthetic C0₂   assimilation rate (A) and stomatal conductance(gs) were measured in 18-month-old polyclonal rootstock seedlings before budding and 18 months after budgrafting them with the scions. The photosynthetic C0₂ assimilation rate was found to be a clonal character.  Existence of a strong positive correlation between A of the stock plants and scion indicated that A is also being influenced by the rootstock. No significant relationship existed between gs and instantaneous water use efficiency (A/gs) of the stock plants and scion. Very high CV was observed among plants within the scion clones in gs and A/ gs, may be due to stock-scion interaction.  The considerable CV observed between the individual plants within a_clone in total soluble sugars, reducing sugars, phenol and amino acid contents, indicated the existence of stock-scion interaction.

Widespread incidence of Corynespora leaf fall (CLF) disease in mature rubber plantations was reported in 1996 from Nettana  in South Karnataka, India.  Surveys carried out in coastal Karnataka and  North  Malabar region of Kerala during 1998-2000 disease  seasons indicated that the disease incidence and severity of infection varied between light to severe in all the locations.  The intensity of infection was found to increase gradually and the incidence to widen year after year. Subramanya region in Karnataka exhibited maximum intensity of the disease.  Fresh incidence was noticed in Hosdurg and Nileshwar in Kerala during 2000. Variation in intensity of disease among different clones was observed with the clone RRII 105 showing high and RRIM 600 and GT 1showing low infection  in all the locations surveyed.

A nursery experiment was conducted for two consecutive years with a view to study the effects of agroclimate on sprouting success and growth of two popular Hevea clones viz. RRIM 600 and RRII 105. Budded stumps of these clones were planted in the second week of every month consecutively for two years.  The sprouting success and growth parameters recorded showed wide variations. Maximum air temperature and sunshine hours had a positive effect on sprouting and growth. The more favourable period for planting budded stumps in polybags was from March to May under the agroclimatic conditions of Mizoram.

A trial composed of ten Hevea clones was established at 600 m altitude in 1986 at Tura in Meghalaya, North East India, to evaluate their growth and yield performance. Results showed that maximum growth (44%) and yield (36%) occurred during  the monsoon and the post-monsoon seasons respectively.  The lowest growth of 5 per cent was noted during the winter season succeeded by 11 per cent in the summer while the lowest yield of 19 per cent was accounted for in the summer followed by 22 per cent in the winter and 23 per cent during monsoon.  The growth was vigorous in the clones RRIC 105, PB 311, PB 310 and RRII 118 but low in PR 255, RRII 105 and RRII 5. While the highest yield was recorded in PB 311 followed by RRII 208 and RRII 118 the lowest was recorded for RRIC 105 and RRIC 102.  Growth and initial yield performance of the ten clones under the agroclimatic conditions of the region are discussed.