Plant water relations influence carbon gain in a grass occurring along sharp gradients of soil temperature.

• The effects of supraoptimal soil temperatures on plants are not well known, despite their importance with fluctuating temperatures in many habitats. The effects of warm soils on carbon gain and water relations were evaluated for Dichanthelium lanuginosum , a grass that can persist in soils sustained at > 45°C by geothermal heating. • Microclimate, root and shoot biomass, photosynthetic gas exchange, and soil/plant water relations were measured under different root-zone temperatures in the field and glasshouse. • Carbon gain at midsummer decreased markedly for plants in 35-50°C vs c. 30°C soils, due to foliar necrosis and stomatal constraints to photosynthesis. Daily rates of dehydration, estimated from changes in leaf water potential, were also 30% greater for plants in the warmer soils. Soil water, evapotranspiration, and ratios of root : shoot biomass and area could not explain greater plant dehydration in warmer soils. • Reduced uptake of soil water may therefore lead to water stress and stomatal limitations to photosynthesis in warm soils, and may thereby influence the local-scale distribution of D. lanuginosum along steep gradients of soil-temperature within geothermal basins.