THE EFFECT OF ELEVATED ATMOSPHERIC CO2 AND DROUGHT ON STOMATAL FREQUENCY IN GROUNDNUT (ARACHIS-HYPOGAEA (L))

The effects of elevated atmospheric CO2, alone or in combination with water stress, on stomatal frequency in groundnut (Arachis hypogaea (L.) cv. Kadiri-3) were investigated. CO2 exerted significant effects on stomatal frequency only in irrigated plants. The effects of drought on leaf development outweighed the smaller effects of CO2 concentration, although reductions in stomatal frequency induced by elevated atmospheric CO2 were still observed. When stands of groundnut were grown under irrigated conditions with unrestricted root systems, an increase in atmospheric CO2 from 375 to 700 ppmv decreased stomatal frequency on both leaf surfaces by up to 16%; in droughted plants, stomatal frequency was reduced by 8% on the adaxial leaf surface only, Elevated atmospheric CO2 promoted larger reductions in leaf conductance than the changes in stomatal frequency, indicating partial stomatal closure. As a result, the groundnut stands grown at elevated CO2 utilized the available soil moisture more slowly than those grown under ambient CO2, thereby extending the growing period. Despite the large variations in cell frequencies induced by drought, there was no treatment effect on either stomatal index or the adaxial/abaxial stomatal frequency ratio. The data suggest that the effects of future increases in atmospheric CO2 concentration on stomatal frequency in groundnut are likely to be small, especially under conditions of water stress, but that the combination of associated reductions in leaf conductance and enhanced assimilation at elevated CO2 will be important in semi-arid regions.