Rhizosphere adaptations of Natural Rubber (Hevea brasiliensis) plants

Rhizosphere which is an important zone of nutrient changes and nutrient activity of plants which governs the nutrient release and uptake for plant growth. Rhizosphere chemical activity is species specific and the changes are in the immediate vicinity of roots. Rubber ( Hevea brasiliensis ), the important source of natural rubber latex generally growing in acidic conditions is a major crop for economic returns in the major rubber growing countries viz . Thailand, Indonesia, Malaysia, China, Vietnam and India. Repeated cultivation, heavy rainfall and topographic conditions contribute acidification of soils that limits the crop growth and productivity. The present attempt was to know the rhizosphere adaptations of rubber plants in three soil pH viz. extremely acidic (pH 4.4), strongly acidic (pH 5.5) and neutral soil (pH 7.4) with varying base status for pH and nutrient availability. Seedlings were raised in polybags and nutrient changes in the rhizosphere at the beginning of the experiment and periodic intervals at three months growth and eight months growth were monitored. It is observed that the extremely acidic pH (pH 4.4) was modified to a pH around 5.1 in the strongly acidic range and the neutral pH (pH 7.4) was reduced to acidic soil of pH 6.2. Soil organic carbon, the major soil health index, was maintained in the medium range as at the initial status in all three soils. The calcium and magnesium content of the extremely acidic pH and strongly acidic pH was improved whereas the very high status of neutral soil was decreased significantly. The Ca and Mg mobilized from the weathering of minerals by the root secretions according to plant demand and also by microbes association like mycorrhiza. Modifications in the availability of other nutrients were also observed. The observed alterations in the rhizosphere is an evidence of specific rhizosphere adaptations/modifications in the rubber plants for extremely varying soil conditions. In extremely acidic soil alkalization and in neutral soil, acidification was found when rubber seedlings were grown for eight months. However the fact that the strongly acidic soil (pH 5.5) does not change, indicates that the pH near 5.5 is favorable for the growth of rubber seedlings. Per cent change in rhizosphere pH and nutrient availability was different in three soils. The relative change in nutrient flux with pH change showed an increase in pH 4.4 soil. The study indicated that specific adaptations are operating in the rhizosphere in natural rubber plants to modify the pH and nutrient availability for initial establishment and growth.