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There are considerable variations in the percentage loss of hydraulic conductivity (PLC) at mid-day minimum water potential among and within species, but the underpinning mechanism(s) are poorly understood. This study tested the hypothesis that plants can regulate leaf specific hydraulic conductance (K l) via precise control over PLC under variable Δ Ψ (water potential differential between soil and leaf) conditions to maintain the −m/b constant (−m : the sensitivity of stomatal conductance to VPD ; b : reference stomatal conductance at 1.0 kPa VPD), where VPD is vapor pressure deficit. We used Populus euphratica , a phreatophyte species distributed in the desert of Northwestern China, to test the hypothesis. Field measurements of VPD , stomatal conductance (g s), g s responses to VPD , mid-day minimum leaf water potential (Ψ lmin), and branch hydraulic architecture were taken in late June at four sites along the downstream of Tarim River at the north edge of the Taklamakan desert. We have found that: 1) the −m/b ratio was almost constant (=0.6) across all the sites; 2) the average Ψ 50 (the xylem water potential with 50% loss of hydraulic conductivity) was − 1.63 MPa, and mid-day PLC ranged from 62 to 83%; 3) there were tight correlations between Ψ 50 and wood density/leaf specific hydraulic conductivity (k l) and between specific hydraulic conductance sensitivity to water potential [d(k s)/dln(−Ψ)] and specific hydraulic conductivity (k s). A modified hydraulic model was applied to investigate the relationship between g s and VPD under variable Δ Ψ and K l conditions. It was concluded that P. euphratica was able to control PLC in order to maintain a relatively constant −m/b under different site conditions. This study demonstrated that branchlet hydraulic architecture and stomatal response to VPD were well coordinated in order to maintain relatively water homeostasis of P. euphratica in the desert. Model simulations could explain the wide variations of PLC across and within woody species that are often observed in the field. [ABSTRACT FROM AUTHOR]

Under the climatic and ecological conditions of Xingjiang, Northwest China, different degrees of drought stress were installed during the growth stages of cotton, and the drip irrigation with mulch was adopted, aimed to study the effects of drought stress and re-watering on the endogenous hormones (abscisic acid, ABA; and zeatins, ZRs) contents of cotton roots and leaves and the stomatal conductance (gs) of cotton leaves. With the increase of drought stress at different growth stages, the ABA contents of cotton roots and leaves increased, while the ZRs contents of cotton roots and leaves and the gs and photosynthetic rate (Pn) of cotton leaves decreased, with greater decrements in the treatment of soil moisture content being 40% -45% of field capacity at early floweringfull flowering stage. After re-watering, the ABA contents of cotton roots and leaves did not have a decrease with the improvement of soil moisture regime, while the ZRs contents of cotton roots recovered rapidly or exceeded the control after 1-3 days of re-watering. There was a positive correlation between the ZRs contents of cotton roots and the gs of cotton leaves. In the treatment of soil moisture content being 50% -55% of field capacity at full budding-early flowering stage, the ZRs contents and gs of cotton leaves recovered more quickly and with greater increments. It was suggested that the higher ZRs contents of cotton roots after re-watering could be the main cause for the higher stomatal conductance and photosynthetic rate of cotton leaves. [ABSTRACT FROM AUTHOR]

Drought slows net photosynthetic rate ( A N ) but increases water use efficiency (WUE). Farmers give an artificial drought pretreatment to some crops in the early growth stage and find that yield increases accompanying with the improvement of WUE. We conducted well-watered, non-drought, mild drought and moderate drought pretreatments of potted cotton cultivars. The aims of the present study were to analyse the importance of mesophyll conductance ( g m ) as a factor that may simultaneously improve A N and WUE under drought pretreatment conditions, and to analyse the role of anatomical structure and biochemical mechanism in the variability of g m . Our results showed that significant variability of g m estimated by gas exchange and chlorophyll fluorescence was observed between non-drought pretreatment and drought pretreatment associated with change in A N and WUE. There was great difference in anatomical structure and expression of aquaporins ( GhAQP1 ) among all the treatments. In addition, expression of carbonic anhydrase ( CA ) may not be important in the regulation of g m under drought pretreatment conditions. We concluded that the variability of g m offers a potential target for improving leaf A N and WUE simultaneously by the regulation of anatomical structure and GhAQP1 . [ABSTRACT FROM AUTHOR]