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Transpiration is a significant part of water cycle in forest ecosystems, influenced by meteorological factors and potentially constrained by soil moisture. We used Granier-type thermal dissipation probes to monitor xylem sap flow dynamics of three tree species ( Quercus liaotungensis , Platycladus orientalis , and Robinia pseudoacacia ) in a semi-arid loess hilly region, and to continuously monitor the key meteorological factors and soil water content (SWC). We established the SWC thresholds delineating soil moisture-limited and -unlimited sap flow responses to transpiration drivers. The results showed that mean sap flux density ( J s ) of Q. liaotungensis and R. pseudoacacia was significantly higher during period with higher soil moisture compared to lower soil moisture, while the difference in J s for P. orientalis between the two periods was not significant. We used an exponential saturation function to fit the relationship between the J s of each tree species and the integrated transpiration variable (VT) which reflected solar radiation and vapor pressure deficit. The difference in the fitting curve parameters indicated that there were distinct response patterns between J s and VT under different soil moisture conditions. There was a threshold in soil moisture limitation on sap flow for each species, which was identified as 0.129 m 3 ·m -3 for Q. liaotungensis , 0.116 m 3 ·m -3 for P. orientalis , and 0.108 m 3 ·m -3 for R. pseudoacacia . Below the thresholds, J s was limited by soil moisture. Above these points, the normalized sensitivity index (NSI) for Q. liaotungensis and P. orientalis reached saturation, while that of R. pseudoacacia did not reach saturation but exhibited a significant reduction in moisture limitation. Among the three species, P. orientalis was the most capable of overcoming soil moisture constraints.

Significant improvements to the centrifuge water-extraction method of measuring the percentage loss volume of water (PLV) and corresponding vulnerability curves (VCs) are reported. Cochard and Sperry rotors are both incapable of measuring the VCs of species with long vessels because of premature embolism induced by hypothetical nanoparticles that can be drawn into segments during flow measurement. In contrast, water extraction pushes nanoparticles out of the sample. This study focuses on a long-vessel species, Robinia pseudoacacia, for which many VCs have been constructed by different methods, and the daily water relations have been quantified. PLV extraction curves have dual Weibull curves, and this paper focuses on the second Weibull curve because it involves the extraction of water from vessels, as proven by staining methods. We demonstrate an improved water extraction method after evaporation correction that has accuracy to within 0.5%, shows good agreement with two traditional methods that are slower and less accurate, and is immune to nanoparticle artefacts. Using Poiseuille's Law and the geometry of vessels, we argue that the percentage loss of conductivity (PLC) equals 2PLV-PLV2 in a special case where all vessels, regardless of size, have the same vulnerability curve. In this special case, this equation predicts the data reasonably well., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Water entry at germination for black locust (Robinia pseudoacacia L.) seeds which are known as hard seeds with impermeable seed coat to water, was examined using micro-magnetic resonance imaging (MRI). The MRI apparatus equipped with a low-field (1 T; Tesla) permanent magnet was used, which is open access, easy maintenance, operable and transportable. The excellent point of the apparatus is that T 1-enhancement of water signals absorbed in dry seeds against steeping free water is stronger than the apparatuses with high-field superconducting magnets, which enabled clear detection of water entry. Water hardly penetrated into the seeds for more than 8 h but approximately 60 % of seeds germinated by incubating on wet filter papers for several days. Hot water treatments above 75 °C for 3 min effectively induced water gap; scarification was 70 % at 100 °C and 75 °C, declined to 15 % at 50 °C and decreased further at room temperature. Water entered into the scarified seeds exclusively through the lens, spread along the dorsal side of the seeds and reached the hypocotyl, whereas water migrated slowly through hilum side to radicle within 3 h.

During periods of water deficit, growing roots may shrink, retaining only partial contact with the soil. In this study, known mathematical models were used to calculate the root-soil air gap and water flow resistance at the soil-root interface, respectively, of Robinia pseudoacacia L. under different water conditions. Using a digital camera, the root-soil air gap of R. pseudoacacia was investigated in a root growth chamber; this root-soil air gap and the model-inferred water flow resistance at the soil-root interface were compared with predictions based on a separate outdoor experiment. The results indicated progressively greater root shrinkage and loss of root-soil contact with decreasing soil water potential. The average widths of the root-soil air gap for R. pseudoacacia in open fields and in the root growth chamber were 0.24 and 0.39 mm, respectively. The resistance to water flow at the soil-root interface in both environments increased with decreasing soil water potential. Stepwise regression analysis demonstrated that soil water potential and soil temperature were the best predictors of variation in the root-soil air gap. A combination of soil water potential, soil temperature, root-air water potential difference and soil-root water potential difference best predicted the resistance to water flow at the soil-root interface., (© The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

From April 14 to October 10, 2009, the dynamic changes of Robinia pseudoacacia sap flow in the hilly-gully region of Loess Plateau were measured with thermal dissipation probe. Meantime, the photosynthetic active radiation (PAR), air temperature, relative air humidity, water vapor pressure deficit (VPD), wind speed, and rainfall were monitored. The diurnal variation of the sap flow velocity (SFV) displayed a single-peak curve, but the pattern varied among different months. In April, the sap flow started to increase at about 12:00, and reached the peak at about 18:00. From May to August, the sap flow started to increase at 5:30-7:30, and reached the peak at about 15:00. In September and October, the sap flow started to increase at about 8:00, and reached the peak at 11:30-13:00. The monthly average SFV was the highest in August and the lowest in April, with the value being 0.002610 cm x s(-1) and 0.000549 cm x s(-1), respectively. During the monitoring period, the sap flow velocity was significantly correlated with PAR, air temperature, VPD, wind speed, and air relative humidity, and the correlation coefficients declined in the order of air temperature > VPD > PAR > relative humidity > wind speed. The sap flow velocity could be estimated by the linear equation with variables PAR and VPD, and the regression coefficients were highly significant.

The study on the root distribution characters of Robinia pseudoacacia and soil moisture status in the Loess Plateau showed that on shady slope, there was no obvious difference in the horizontal distribution of R. pseudoacacia fine roots within 2.0 m from the stem, and the maximal distribution depth of the roots could reach 2.0 m. In sunny slope, the vertical distribution of the roots within 0.5 m from the stem could also reach that depth. The root amount in shady slope was larger than that in sunny slope, and the affected depth of R. pseudoacacia roots on soil moisture status reached 3.3 m in shady slope, while 2.7 m in sunny slope.

With locust (Robinia pseudoacacia), the main tree species in afforestation, as test material, this paper studied the characteristics of its water physiological ecology and productivity under four habitats, i. e. , sunny, shady, semi-sunny and semi-shady hillsides, in the North Shaanxi gully areas of Loess Plateau. The mean water content in 0 - 500 cm soil layer was 8.87% (shady), 8.06% (semi-shady), 7.62% (semi-sunny), and 6.96% (sunny), respectively. There was a significant discrepancy between shady, semi-shady, semi-sunny and sunnyhilisides (a 0.01), and the difference between shady, semi-shady and semi-sunny, as well as between semi-sunny and sunny hillsides was also significant (a 0.05) . Remarkable relationships were found between leaf RWC, locus WSD and soil water content (SWC) . The leaf RWC and locust water potential under sunny hillside were lower, but those under shady hillside were higher. The daily mean transpiration was in order of shady (4.07 microg x cm(-2) x s(-1)) > semi-shady (3.89 microg x cm(-2) x s(-1)) > semi-sunny (3.05 microg x cm(-2) x s(-1)) > sunny (2.70 microg x cm(-2)s(-1)) hillside. The remarkable difference of transpiration appeared at 11:00 and 13:00, and there existed a remarkable relationship between transpiration and light intensity, RH and soil water content. All of these resulted in a diversity of locust biomass under different habitats, the highest under shady hillside, and the lowest under sunny hillside, and the differences between different habitats were all significant. It could be concluded that soil water content was the main factor affecting locust growth.