ACCELERATED EARLY GROWTH OF RICE AT ELEVATED CO2 - IS IT RELATED TO DEVELOPMENTAL-CHANGES IN THE SHOOT APEX

The influence of elevated CO2 on the development of the shoot apex and on subsequent vegetative growth and grain yield was investigated usin g rice (Oryza sativa L. cv Jarrah) grown in flooded soil at either 350 or 700 mu L CO2 L-1. At 8 d after planting (DAP), elevated CO2 increa sed the height and diameter of the apical dome and lengths of leaf pri mordia and tiller buds but had no effect on their numbers. By 16 DAP, there were five tiller buds in the apex at 700 mu L CO2 L-1 compared w ith only three tiller buds at 350 mu L CO2 L-1. These changes in devel opment of the shoot apex at high CO2 were forerunners to faster develo pment of the vegetative shoot at elevated CO2 between 11 and 26 DAP as evidenced by increases in the relative growth rates of the shoot and tillers. Accelerated development at high CO2 was responsible for the 4 2% increase in tiller number at the maximum tillering stage and the 57 % enhancement of grain yield at the final harvest. The link between hi gh CO2 effects on development during the first 15 DAP and final tiller number and grain yield was demonstrated by delaying exposure of plant s to high CO2 for 15 d. The delay totally inhibited the tillering resp onse to high CO2, and the increase in grain yield of 20% arose from a greater number of grains per panicle. Consequently, it can be conclude d that accelerated development in the shoot apex early in development is crucial for obtaining maximum increases in grain yield at elevated atmospheric CO2 concentrations.