Irrigation requirement of rubber trees (Hevea brasiliensis) in the subhumid tropics

Abstract North Konkan region in the west coast of India is only marginally suitable for rubber ( Hevea brasiliensis ) cultivation due to subhumid climate. The region has a rainless period with severe soil moisture deficits in summer. Heavy leaf injury and shedding is experienced in summer months. The immaturity period of the crop, attainment of 50 cm girth by main trunk, in the region under rainfed conditions is very long extending to more than 10 yrs. An irrigation experiment was laid out to quantify the requirement of irrigation water and for comparative evaluation of basin and drip irrigation systems. In the experimental site, the drip irrigation was inferior to basin irrigation. Growth rates achieved under irrigation regimes of 1.00 crop evapotranspiration (ET c ) and 0.50 ET c were comparable suggesting that only 50% of the estimated crop water requirement for the summer season is the irrigation requirement. The immaturity period could be reduced to 6 yrs by irrigation. Sufficient irrigation was found to eliminate foliar injury and to result in a high photosynthetic rate. Path coefficient analysis revealed that photosynthetic rate, leaf area index, turgor pressure and leaf temperature have direct and positive relationship with the fortnightly relative growth rate. The results of path coefficient analysis of photosynthetic rate with physiological factors revealed that latex solute potential is the major parameter affecting the photosynthetic rate under stress conditions. Other parameters namely, leaf epicuticular wax, turgor pressure, stomatal resistance and chlorophyll indirectly affect photosynthesis.