Osmotic adjustment, stomata morphology and function show contrasting responses to water stress in mesic and hydric grasses under elevated CO2 concentration

Abstract

<p>The physiological response of two species of grasses with C₃ and C₄ mechanisms syndromes, Napier grass (<i>Pennisetum purpureum</i> Schumach × <i>Pennisetum glaucum</i> (L.) R. Br) and hydric common reed grass (<i>Phragmites australis</i> (Cav.) Trin. Ex Steud) was examined under ambient (aCO₂) and elevated CO₂ (eCO₂), in combination with water and temperature stress treatments. Under eCO₂ and subjected to water and temperature stress, the Napier grass maintained higher daytime leaf water potential (LWP) by reducing transpiration (<i>E</i>) and executing larger osmotic adjustment (OA) at an average of 0.85 MPa compared with 0.42 MPa for common reed; carbon assimilation (<i>P</i>_N) was thus higher for the Napier grass. Under aCO₂ and low temperature, water stress induced no significant differences in OA between the grasses, but Napier grass still had higher <i>P</i>_N than that of common reed. Recovery in LWP and <i>P</i>_N following re-watering of water-stressed plants was more rapid in Napier grass than that in the common reed; the former had also higher water-use efficiency due to its low specific water use (water use/leaf area) that was just a fraction (less than 6%) that of the common reed. Exposure of common reed to eCO₂ reduced stomata number, but increased it in the Napier grass, especially when subjected to water stress and high temperature. Exposure to eCO₂ enhanced OA capacity and <i>E</i> control in Napier grass resulting in superior physiological profile over the common reed subjected to water and heat stress.</p>