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

Abstract

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.