Carbon assimilation, 𝛅13C and water relations of Elaeagnus angustifolia grown at two groundwater depths in the Minqin desert, China

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We investigated the physiological responses of Elaeagnus angustifolia to variation in groundwater depth. Elaeagnus angustifolia seedlings were grown in the Minqin desert in lysimeters supplied with underground water at the soil depth of 1.40 m and 3.40 m. Results showed that constant access to groundwater allowed plants supplied with water at the lower soil layer to meet their water requirement and, consequently, they were not affected by water stress. There were no differences in A max (the net CO2 assimilation rate under conditions of photosynthetically photon flux density and CO2 saturation), J max (maximum rate of electron transport) and stomatal conductance between the two underground water treatments. However, plants with deeper groundwater had a significantly higher V cmax (i.e. a higher carboxylation efficiency of Rubisco) and mesophyll conductance resulting in increased photosynthesis measured at the CO2 growth condition (A) and, consequently higher intrinsic water-use efficiency (WUE). However, respiration was also increased in plants grown with deeper groundwater. This may have offset the increased A and led to a similar long-term WUE, as expressed by carbon isotope discrimination (δ13C), between the two ground water treatments. In the present study, we also found a high foliage nitrogen concentration in the E. angustifolia plants (3.75% on average), that may be very significant ecologically in improving soil properties. The physiological traits of E. angustifolia found in this study confirm that the use of perennial phreatophytic, nitrogen fixing species has significant potential to positively impact soil fertility and carbon sequestration under environmental conditions found in the Minqin desert.