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

The physiological response of two species of grasses with C₃ and C₄ mechanisms syndromes, Napier grass (Pennisetum purpureum Schumach × Pennisetum glaucum (L.) R. Br) and hydric common reed grass (Phragmites australis (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 (E) and executing larger osmotic adjustment (OA) at an average of 0.85 MPa compared with 0.42 MPa for common reed; carbon assimilation (P_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 P_N than that of common reed. Recovery in LWP and P_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 E control in Napier grass resulting in superior physiological profile over the common reed subjected to water and heat stress.