Techniques for applying  pesticides to rubber trees are reviewed. Projection of fungicide into the canopy  of tall trees has required  the use of thermal fogs applying  aerosol particles generally smaller than 50 µm in diameter. But applications need to be repeated to achieve adequate control. Maintenance of pathways  by application of herbicides can be improved with knapsack sprayers by fitting a spray  management valve to provide a constant  1 bar pressure  at the nozzle. There has also been  limited  use of hand-held  spinning  disc sprayers  to apply  250µm droplets of herbicides.

Basic studies were carried out to improve somatic embryogenesis in Hevea. These investigations called for histological and biochemical analyses of plant material and culture media in relation to water, atmospheric gases in culture vessels, minerals, polyamines, phenols and growth regulators. Mechanisms of somatic embryogenesis are partially outlined with the help of histological and  biochemical  evidences  at  each  step  of culture.  In primary  culture,  water parameters  and mineral analyses indicated  that initiation of exchanges between medium  and explant has to be enhanced. During callus development metabolism of ethylene and polyamines is very active and must be controlled by adjustment of growth regulators by the type of medium support and/or by inhibitor  supply.  Disorgnisation  of the oxidative  metabolism  has  been studied in relation to browning of the callus. Adjustment of culture medium to enhance somatic embryogenesis   has  been  correlated  with  the  setting  up  of a  specific  water  status  in  the embryogenic callus.  Embryogenesis can  be expressed in  two different  forms  depending on cultural conditions and genotype. Enhancement of the basic knowledge led to systematic embryogenesis  from the clones PB 260, PB 235, PR 107 and  RRIM 600.

The dynamic mechanical properties of natural rubber(NR) filled with untreated and  chemically treated short sisal  fibre  have  been investigated.  By incorporating short sisal fibre  the storage modulus (E') of NR vulcanizate was improved. The effect of fibre-matrix interface adhesion on the viscoelastic properties of the composite has been evaluated and  it was found  that interface adhesion increased the dynamic modulus and  mechanical  loss. The effects of fibre orientation, fibre   loading and   temperature  on   the   dynamic  mechanical    properties  have   also   been investigated.  With  increase in  temperature, mechanical loss and  modulus decreased sharply indicating a possible  deterioration of the polymer at higher  temperatures. The storage modulus (E'), loss  modulus (E") and  loss  factor  (tan  d)  increased   continuously with  increase  in  fibre loading.

Polyisoprene  synthesis  in   ethylene stimulated laticiferous cells of Hevea brasiliensis  requires an exogenous supply of sugar.  Plasmodesmata are absent from  these non-chlorophyllian cells. The  transplasmalemmal electrical  gradient  recorded at  steady   state  was  about-115   mV  for laticiferous cells  and  -36 mV for  laticiferous protoplasts. Sucrose  and  glucose  depolarize the plasmalemma of laticiferous cells by about  15 to 25 mV and  by about  1 to 7 mV of laticiferous protoplast. Results  show  that  with  depolarization due  to sucrose  (1 mM) or glucose  (1 mM), a slight alkalinization (0.1 to 0.2 pH units) can be detected on the outer surface of the cell. Fructose and  3-0-methyl-glucose have  no such  effect. The extent of depolarization due  to the addition of sugars is lower  than  the electrogenic component of the membrane potential produced by  functioning of the   H⁺-excretion pump (vanadate sensitive-ATPase). Furthermore, in presence of vanadate or DNP,  no shift in pH value  was observed with  glucose  or sucrose. The effect of phlorizin on the shift of the membrane potential due  to sugar  uptake across  the plasmalemma has been  tested.  Neither sucrose nor glucose demonstrate any depolarization and  alkalinization in  the  presence of phlorizin. Stimulation of the H⁺-pump by ethylene hyperpolarizes cells by approximately -40 mV and increases the extent of the depolarization induced by sugar transport. Even  without the cellwall, with  laticiferous protoplast, glucose and  sucrose  induce depolarization  of  plasmalemma. On  these  protoplasts, sugars do  not  induce any  depolarization in  the presence of DNP.  These  results  suggest an active  transport of the sugars from  the  apoplasm towards the cytosol.  Evidence  for  the existence  of H⁺ cotransport with  sucrose and/ or glucose at  the  plasmalemma is discussed.

A method  and apparatus for using a gaseous unsaturated hydrocarbon, or oxide thereof, as a yield  stimulant   by  its  direct  absorption  into  the  live  bark  tissues  of  Hevea brasiliensis are described.*  A polythene or rubber jacket sealed to the tree bark is used for holding the stimulant. This prolongs latex flow, when a hypodermic puncture  is made in the bark, for upto 40 h. or more at a maximum  rate of one ml per minute.   The latex is collected by means of a hollow tube or needle (of about one mm internal diameter)  that leads into a covered latex collection container, from  the hypodermic  puncture.   The total outflow of latex from one aperture was between 1000 ml and 2000 ml per extraction, compared  to only about 150 ml obtainable  from conventional  half spiral  tapping systems.   The higher yield realized by this method promises a  bright future  for the natural  rubber industry.   Physiological parameters  relevant  to sucrose translocation  and  its subsequent  synthesis  into isoprene  in  situ  were also monitored  over  a period.  The physiological dynamics of this convertion of sucrose into isoprene is also discussed briefly.  Anatomical examination  of stimulated  bark samples taken from  trees under  the new method  of latex extraction  did  not indicate any undesirable  effects.  Microscopic observation of the stimulated bark samples indicated no decrease in the number of healthy latex vessel rings.

The nature and  magnitude of genetic  variability, heritability and  heterosis were  assessed in 23 F1  hybrid clones, derived from  the cross between RRII 105  and  RRIC 100 of Hevea brasiliensis, at  the  premature phase  of 4 ½ years  after  planting. The hybrid clones  displayed substantial differences and  heterotic responses for  various characters. Genetic variability was  highest for total  volume of latex  (GCV 42.7%) and  lowest  for  bark  thickness (GCV 4.24%). Mean annual yield exhibited a heritability of 68% with a genetic advance of 59 per cent. Yield following ethrel stimulation, initial flow rate per  unit  length  of tapping cut  and  plugging index  recorded high heritability coupled with high genetic advance. The high heritability and genetic advance suggest preponderance of additive gene action.  Among the  23 hybrids, 15  outyielded their  superior parent (RRII 105), of which  six displayed significant heterosis over  RRII 105.