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Many factors have been influencing the natural rubber (NR) industry in the past quarter of a century. Growth in population and the development of world economy have enhanced the demand for goods and services, including rubber products. But the supply of rubber has not kept pace with the rise in demand.

There are also other factors like impact of climate change on NR output, role of hedge funds, futures market activities, exchange rate variations in currencies, rise in oil prices and stock position that have influenced the NR market. These have brought about fluctuations in NR prices, which have ultimately affected the growth prospects of the rubber producing sector.

The fluctuating NR price may also promote the tendency of NR consuming countries to go for more and more synthetic rubber (SR) in place of the high-cost NR component in their products and the growing demand for reclaimed rubber and the hunt for alternatives to NR are also matters of great concern for NR producers.

The NR industry therefore needs a dynamic production improvement policy to ensure its long-term viability and to meet the growing demand of the rubber products sector, which is currently reeling under insecurity of NR supply.



Clones of future
We should aim at developing ‘smart clones’ which should have high rubber and timber yields, faster growth and shorter gestation period, improved tolerance to diseases/pests, better adaptability to climate stress and are location-specific.

We must not worship any particular breeding strategy but select the one which can help to achieve the desired end-goal speedily and surely. Research should continue its clone-centric approach but it should make use of molecular breeding techniques such as marker-assisted selection to shorten the breeding cycle and get more number of agronomically important genes into an elite clone.





We should aim at developing ‘smart clones’ which should have high rubber and timber yields




Further, genetic improvement in Hevea (e.g. breaking the yield ceiling or pyramiding of multiple traits in one single clone, etc.) may be difficult through conventional breeding/selection route. The latest technologies such as molecular technology, nano technology, information technology, GIS and remote sensing, etc. should be profitably made use of for advancing scientific research in NR.



Developments
Substantial progress has been made during the last century in improving the production and productivity of natural rubber. While the average yield of tree raised from open-pollinated seeds derived from the original” Wickham gene pool” was only about 320 kgjha, plantations which can yield over 3000 kg/ha are now in existence. Nevertheless, we have a long way to go before reaching the potential yield plateau.



Limits to yield of rubber
Rubber is a vegetative plant product and superficially it may appear important to improve the capacity to accumulate dry matter. If all the solar energy that falls in the tropics is utilised for dry matter production, it could produce approximately 1300 tons ha-1 yr-1 but this is just not possible because 60 percent of the solar radiations cannot be absorbed by plants.

Of the remaining 40 percent in a non-limiting environment, there are still physical losses of radiations besides respiratory losses. This works out to 71 gjm2jday dry matter if only carbohydrates are synthesized. If there is uniform radiation throughout the year, about 260 tons of dry matter can be produced per year. At present the best ratio between dry matter production and rubber yield is 11.1 percent.

Disregarding possible negative correlations, a maximum of 28.6 tonnes of rubber per year may be the upper limit of yield. This calculation assumes that throughout the year there is radiation approximately 500 cal em 2 day-> without limitation of nutrients, water and carbon dioxide and that the interception is complete.

In rainy months the amount of radiations tends to be low, whereas during the sunny days water usually becomes a limiting factor. If we give due regard to these factors, it is unlikely that presently more than 124 tonnes dry matter ha-1 yr-1 could be produced.

Assuming that 11 percent of this could be rubber, we can expect about 13.6 tonnes rubber ha-1 yr-1. Indeed it would be reasonable to fix this as an attainable aim. At present, the published reports suggest that yields of the order of 3.5 tonnes/ha/yr of rubber have been obtained without Yield stimulation.

However, In High Yielding Estates only 1200 kg ha-! yr-! of rubber has been obtained. The average yield in most countries including Sri Lanka continues to be only around 1000 to 1400 kilogram ha per year. Thus, we have a gap of nearly 10 tonnes between the realisable upper yield limit and the record yield so far obtained. The gap between record yields and the average yield of good estates is about two tonnes. The national averages tend to be only half of that of good estate.



Factors controlling yield in rubber
The yield of rubber depends upon the ability of plants to accumulate dry weight and convert a proportion of it into latex and rubber. The proportion of rubber to dry matter accumulation capacity is a measure of harvest index (HI). Variability in this component was observed to range from 3.0 to 11.1 percent.

There was an inverse relationship between dry matter accumulation and rubber yield. We can compare this situation with that of other crop plants, such as wheat, wherein the number of tillers is often negatively correlated to the number of grains per ear or grain weight.

A further analysis of components is essential for deciding the methodology for improvement.

When there is more dry matter accumulation but a low percentage of rubber, it could be due to fewer laticiferous cells per unit area or a high plugging index. Conversely, the high proportion may be related to more laticiferous cells per unit area and low plugging index. If one attempts to combine these apparently contrasting characters, two points have to be kept in mind.

• Whether a higher proportion of rubber would automatically reduce the dry matter accumulation rate?
   
• Whether the combination of high proportion of rubber and high dry weight can be sustained by enhancing the capacity of source i.e. the photosynthetic potential.

In fact, there are studies on the first aspect. Increased production of rubber results in reduced dry matter accumulation and growth. An increase of one kilogram of rubber is equivalent to at least 2.25 kilogram of carbohydrate and a factor of 2.5 kilogram would be more appropriate.
Some clones have been identified which show minimal change in growth even after tapping. Such clones give medium yield but seem to have the compensatory mechanism. Would such clones retain their present characteristics if the proportion of rubber to dry matter is doubled? To answer this question integrated studies in plant morphology and physiology would be useful.



Photosynthesis and productivity
In the improvement of a plantation crop such as rubber the effort has to be to bring light interception to 100 percent but without making the lower leaves parasitic. A plant having large leaves at the top could intercept almost all of the light but would result in so much shading that the efficiency of lower leaves would be reduced. Therefore, selection for smaller leave may provide a better plant canopy for higher photosynthetic efficiency.
In rubber, where a vegetative growth is important, it is necessary to know the relationship between leaf area, leaf photosynthesis and structure and photosynthesis rate. Methods need to be developed to assess it with simpler indices such as protein or nitrogen content so that the determination of photosynthesis on the plant top need not be essential.



Water and nutrients as constraints
Available data indicate that the yield of rubber increases with increasing water availability. This can affect rubber yield in more than one way. Firstly, water shortage can result in reduced water potential and consequently increased viscosity leading to poor flow rate in laticiferous cells. Limitation in water availability can also cause reduction in photosynthesis and, therefore, reduced yield.

Therefore, efforts have to be made to reduce transpiration and improve water use efficiency (WUE). One way to do this is to obtain isolines with pubescent character as has been done in soyabean. These isolines were similar in photosynthesis and yield but had improved water use efficiency.
From the point of view of nutrition, it is clear that the latex contains considerable proteins and minerals. It also contains DNA and RNA. When these substances are lost, the roots must be deprived of synthetic materials. This in turn can influence root growth adversely. Although there are extensive studies on latex metabolism and composition, much less attention has been given to the root system. This gap in knowledge will have to be filled.



Stimulants
The properties of various stimulants have been assessed in relation to plugging and latex flow rates. Earlier, substances such as 2, 4-ST and related compounds were used. Now it is known that acetylene and ethylene can serve as good stimulants and many innovative technologies have been developed, but the industry is not willing to accept. The problem seems to be one of reduction in yield in subsequent years. The flow of latex is not only the function of plugging but also of the synthetic activity in the plant.

Most of the stimulants have the property of inducing senescence. There is a need for searching for a stimulant which would enhance either photosynthesis rate or stimulate the emergence of new leaves and growth. In other words, the studies on stimulants should be associated with growth analysis which can provide a better insight for the development of new stimulants.



Conclusion
A gap analysis in rubber reveals that in contrast to the theoretical potential yield of over 10 tonnes of rubber per hectare, less than four tonnes per hectare per year has been obtained so far in experimental plots. With Ethrel stimulation the yield goes up.

The task of the research scientists is the development of material and techniques which can enhance the ceiling to rubber yield. Only an inter-disciplinary thrust aimed to increase the total biological yield as well as the diversion of photosynthates to latex production can help to achieve the goal. The rubber industry’s support is also crucial.


(N. Yogaratnam can be contacted at treecrops @gmail.com)