Very high CO2 reduces photosynthesis, dark respiration and yield in wheat

Although terrestrial CO2 concentrations, [CO2], are not expected to reach 1000 mu mol mol(-1) for many decades, CO2 levels in closed systems such as growth chambers and glasshouses, can easily exceed this concentration. CO2 levels in life support systems in space can exceed 10 000 mu mol mol(-1) (1 %). Here we studied the effect of six CO2 concentrations, from ambient up to 10000 mu mol mol(-1), on seed yield, growth and gas exchange of two wheat cultivars (USU-Apogee and Veery-10). Elevating [CO2] from 350 to 1000 mu mol mol(-1) increased seed yield (by 33 %), vegetative biomass (by 25 %) and number of heads m(-2) (by 34 %) of wheal plants. Elevation of [CO2] from 1000 to 10000 mu mol mol(-1) decreased seed yield (by 37 %), harvest index (by 14%), mass per seed (by 9 %) and number of seeds per head (by 29 %). This very high [CO2] had a negligible, non-significant effect on vegetative biomass, number of heads m(-2) and seed mass per head. A sharp decrease in seed yield, harvest index and seeds per head occurred by elevating [CO2] from 1000 to 2600 mu mol mol(-1). Further elevation of [CO2] from 2600 to 10000 mu mol mol(-1) caused a further but smaller decrease. The effect of CO2 on both wheal cultivars was similar for all growth parameters. Similarly there were no differences in the response to high [CO2] between wheal grown hydroponically in growth chambers under fluorescent lights and those grown in soilless media in a glasshouse under sunlight and high pressure sodium lamps. There was no correlation between high [CO2] and ethylene production by flag leaves or by wheal heads. Therefore, the reduction in seed set in wheal plants is not mediated by ethylene. The photosynthetic rate of whole wheat plants was 8 % lower and dark respiration of the wheat heads 25 % lower when exposed to 2600 mu mol mol(-1) CO2 compared to ambient [CO2]. It is concluded that the reduction in the seed set can be mainly explained by the reduction in the dark respiration in wheat heads, when most of the respiration is functional and is needed for seed development. (C) 1997 Annals of Botany Company.