Drought stress is a limiting factor for a wide range of physiological processes in plants, including photosynthesis. Under drought stress, plants can use different mechanisms to control water loss to some extent. One of these mechanisms is to close stomata to avoid further water loss via transpiration. This decrease in gas exchange restricts water loss, but it also simultaneously reduces daily carbon assimilation at the leaves. The drought-tolerant species control the stomatal function to either allow some carbon fixation under stress, thus improving their water use efficiency, or to open stomata rapidly when water deficit is relieved.

Populus euphratica, a dominate species in the lower reaches of the Tarim River, is very important in maintaining the ecosystem function in arid and semi-arid regions due to its tolerance to severe drought and high salinity and alkalinity in soils. As a riparian tree, P. euphratica is directly influenced by the change of groundwater. Moreover, disruption of water flow in the lower reaches of the Tarim River imposes a threat on the stability of local vegetation.

Diurnal courses in gas exchange are one of the best indicators of plants’ ability to maintain their photosynthetic apparatus in response to the environment. Therefore, researchers from XIEG and Xinjiang University measured the diurnal photosynthetic rate (P_N), stomatal conductance (g_s), transpiration rate (E) and water use efficiency (WUE) of P. euphratica at different groundwater depths. This study aimed to examine whether the variation in groundwater depth (GD) would change the photosynthetic capacity and WUE in P. euphratica, and to characterize the response of photosynthetic parameters of P. euphratica growing in the presence of groundwater at different depths to environmental variables.

It was found that the P_N was not sensitive to the change of GD within the range of 4.2–6.8 m. Compared to g_s and E of P. euphratica grown at GD 4.2, 5.6 and 5.8 m, g_s and E at GD 6.8 m both markedly declined in June, suggesting that P. euphratica at deeper GD can avoid overall water loss by stomatal adjustment. The intrinsic WUE of P. euphratica first decreases with the increasing GD, but when GD increased to 6.8 m, intrinsic WUE increased by 1.2–2.2 fold, compared with the WUE of P. euphratica at GD 4.2–5.8 m, indicating that intrinsic WUE of P. euphratica will increase when the plant suffers from moderate drought stress. This study will help researchers improve the understanding of P. euphratica’s adaption to drought stress in arid areas.

The result has been published on Environmental Earth Sciences, 2012, 66: 433-440. The paper is also archived at http://www.springerlink.com/content/f04850m0263tj1p6/.