Root and shoot factors contribute to the effect of drought on photosynthesis and growth of the C4 grass Panicum coloratum at elevated CO2 partial pressures

We examined the hypothesis that root and shoot factors influence growth responses to elevated CO2 of the C4 grass Panicum coloratum var. makarikiense cv. Bambatsi (NAD-ME malic enzyme subtype) when well watered and droughted. Plants were grown at CO2 partial pressures (pCO2) of 36 (ambient) and 100 Pa (elevated) in pot ed soil in growth chambers for 3 weeks with adequate water (day 0) before being subjected to 15 d of drought. At day 15, enhancement of shoot growth by elevated pCO2 was 70% under drought, and 44% when well watered. During the drought period, leaf CO2 assimilation rates (A) and stomatal conductance (g) (measured at 36 Pa CO2) declined after day 2, but the decline was faster at 36 Pa CO2, and by day 9, A was negligible and intercellular pCO2 had sharply increased compared with 100 Pa CO2. Changes in carbon metabolism and water relations occurred during drought and elevated CO2 generally delayed these changes. Leaf growth rates were higher at elevated CO2 at day 0 and during drought. Importantly, the decline in soil water content was slower at elevated pCO2 due to lower transpiration rates. This explained the slower decline in A, gand shoot water relations at elevated CO2 and indicates that root factors were responsible for their decline. In contrast, leaf growth rates were higher at elevated CO2, irrespective of soil water content. We conclude that both soil and leaf factors contribute to the greater growth response of P. coloratum to high CO2 under drought, and that reduced transpiration rates explains their enhanced growth.