IRRIGATION REQUIREMENT OF RUBBER TREES (HEVEA-BRASILIENSIS) IN THE SUBHUMID TROPICS

North Konkan region in the west coast of India is only marginally suit able for rubber (Hevea brasiliensis) cultivation due to subhumid clima te. The region has a rainless period with severe soil moisture deficit s in summer. Heavy leaf injury and shedding is experienced in summer m onths. The immaturity period of the crop, attainment of 50 cm girth by main trunk, in the region under rainfed conditions is very long exten ding to more than 10 yrs. An irrigation experiment was laid out to qua ntify the requirement of irrigation water and for comparative evaluati on of basin and drip irrigation systems. In the experimental site, the drip irrigation was inferior to basin irrigation. Growth rates achiev ed under irrigation regimes of 1.00 crop evapotranspiration (ETc) and 0.50 ETc were comparable suggesting that only 50% of the estimated cro p water requirement for the summer season is the irrigation requiremen t. The immaturity period could be reduced to 6 yrs by irrigation. Suff icient irrigation was found to eliminate foliar injury and to result i n a high photosynthetic rate. Path coefficient analysis revealed that photosynthetic rate, leaf area index, turgor pressure and leaf tempera ture have direct and positive relationship with the fortnightly relati ve growth rate. The results of path coefficient analysis of photosynth etic rate with physiological factors revealed that latex solute potent ial is the major parameter affecting the photosynthetic rate under str ess conditions. Other parameters namely, leaf epicuticular wax, turgor pressure, stomatal resistance and chlorophyll indirectly affect photo synthesis. (C) 1998 Elsevier Science B.V.