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• Groundwater depth (GT) controlled transpiration dynamics of Populus tomentosa. • Diurnal (Q day) and nocturnal water use (Q night) had different seasonal trends. • Groundwater recharged Q night and reduced the water consumption from the stem. • Q night helped poplars refill the consumption of water stored in the stem (R e). • When GT < 170 cm of the surface, R e / Q night increased with increasing GT. For theories of forest water utilization over day-night cycles, it is important to have a reliable information on plant transpiration behaviors during daytime and nighttime and their environmental controls. With the help of thermal dissipation probes, our study investigated the seasonal variations in the diurnal (Q day) and nocturnal stand water use (Q night) of a Populus tomentosa plantation and explored their responses to groundwater table (GT) fluctuations and the meteorological factors. The results indicated that Q night occurred on most nights during the growing season in the P. tomentosa plantation, but with a different seasonal dynamic from Q day. A distinct increase in the ratio of nocturnal to daily water use appeared at the beginning and the end of the growing season. During the experimental period, stem refilling accounted for 61% of Q night , indicating that P. tomentosa was able to draw on stored water in the stem to support transpiration. The Q day and Q night were positively and negatively related to GT , respectively. When the GT varied within 170 cm of the surface, the proportion of stem refilling accounting for Q night increased with GT. The positive and negative correlations were found in meteorological factors with Q day and Q night , respectively. Among the single meteorological factors except for ET 0 , vapor pressure deficit (VPD) and photosynthetically active radiation (PAR) explained the highest variation in Q day in 2010 and 2011, respectively. In contrast, Q night was dominantly affected by VPD in 2011. Nocturnal stand water-use could help P. tomentosa to recharge the consumption of water stored in the stem and overcome the seasonal drought. The compensation effect of groundwater on the nocturnal stand water-use of P. tomentosa could reduce its consumption of water stored in the stem. As the stand age of P. tomentosa plantation varied, the key meteorological drivers of both Q day and Q night also changed. Our results enable better understanding of the water-use strategies of poplars in the North China Plain and can lead to refinement of current silvicultural techniques. [ABSTRACT FROM AUTHOR]

Highlights • Four different methods were applied to determine the optimal tensiometer placement. • The optimal tensiometer placement location should not change with stand age. • The tensiometer should best be placed 10 cm deep and 15 cm distant from the dripper. • This will optimize the schedule for drip irrigation in different aged Populus tomentosa forests. Abstract Determining the optimal tensiometer placement is crucial for scheduling irrigation based on soil water potential (SWP) in poplar plantations. However, such work has never been done. The stand-age effect on the tensiometer placement is still unknown, hindering the development of a dynamic irrigation regime. In a surface drip irrigated Populus tomentosa , short-rotation plantation on sandy loam soil, the variation of SWP at different positions [hereafter expressed in spatial coordinates (distance from dripper in cm, depth in cm)] within the soil wetting volume (WV), and the daily tree growth rates were continuously monitored in one growing season. Whereas, the transpiration and fine root distribution were measured in two growing seasons. The HYDRUS model was calibrated using soil water and transpiration data from one growing season and validated using data in another growing season. Then the model was applied to simulate sixty scenarios, which were combinations of different SWP measurement positions (ten scenarios), root distribution patterns (three scenarios; mainly using rooting depth as a proxy for trees at different ages), and irrigation thresholds (two scenarios). Based on the simulation results, we estimated the ratio of actual to potential transpiration (T a / T p) and irrigation efficiency (IE; calculated as (T a / T p)/irrigation amount) for each scenario. The absolute value of the mean relative difference of SWP was smaller at (15, 10) than at any other positions within the WV. The proportion of variation in growth (R2 = 0.294) and transpiration rates (R2 = 0.247) explained by root water-uptake was also highest at (15, 10). Fine roots concentrated on both sides of dripper as the trees grew bigger, with the maximal root density finally occurring at about (15, 10) and (−15, 10). Under the (−15, 10), (0, 10), (15, 10) and (0, 30) position scenarios, the T a / T p was at a high level relative to other scenarios, while the IE were at a middle level. As rooting depth increased, the ranks of different tensiometer positions in T a / T p and IE changed only a little, and T a / T p decreased slightly, implying the optimal placement should not vary with stand age. Therefore, tensiometer was recommended to place at 10 cm depth and about 15 cm distance from the dripper to schedule drip irrigation in different-aged P. tomentosa plantations on sandy loam soil. This will bring benefits to minimizing the use of irrigation water and maximizing production. Besides, the comprehensive approach used in this study can be adopted, when determining the optimal soil water measurement positions for scheduling irrigation for other plant species growing on other soil types. [ABSTRACT FROM AUTHOR]

A field experiment was conducted in 2010-2012 to investigate the effects of different N fertigation regimes on diameter growth (measured as individual tree basal area increment), N uptake and biomass allocation in a triploid Populus tomentosa plantation in the North China Plain. The experiment included six N fertigation treatments (combination of three N application rates [NR: 115 (N), 230 (N) or 345 (N) kg ha year] and two application frequencies [NF: two (F) or four times (F) per year] and a single irrigation control treatment. Diameter at breast height, biomass and N uptake were monitored. N fertigation significantly increased aboveground diameter and N uptake compared with the control. Use of NR above 115 kg N ha year did not result in growth and N uptake benefits. NF showed almost no effect on tree growth, but F resulted in significantly higher N uptake. N fertigation resulted in a shift in biomass allocation from roots to leaves, but had no effect on relative allocation to wood biomass, which was significantly influenced by ontogeny (size effect). Biomass allocation of triploid P. tomentosa can be altered by N fertigation to some extent, but this was also under ontogenic control. Higher NR (N or N) and NF (F) are not necessary to achieve optimum growth, but higher NF is required for optimal N uptake. NF is recommended as the N fertigation regime for triploid P. tomentosa as this results in higher N uptake and optimal growth. [ABSTRACT FROM AUTHOR]

A field experiment was conducted in 2017 and 2018 to investigate the responses of fine root growth and morphology plasticity to drip irrigation and nitrogen fertigation (DIF) in a young triploid P. tomentosa plantation in the North China Plain. We monitored four DIF treatments (I20F0, I20FH, I45F0, I45FH), which were the combinations of two irrigation thresholds (− 20 kPa and − 45 kPa, denoted as I20 and I45) and two N fertigation amounts (0, denoted as F0; 220 and 260 kg N ha−1 year−1 in 2017 and 2018, denoted as FH). Some soil properties and aboveground growth parameters were also monitored. I20 significantly improved fine root biomass density (FRBD) by 156.92% and induced a significant reduction of specific root length (SRL) by 28.79% in 0–30 cm soil layers than I45, and FH significantly decreased fine root length density (− 62.88%), surface area (− 54.01%), and SRL (− 39.01%) in 30–80 cm soil layers than F0 in 2017. Fine root traits differed little among DIF treatments in 0–30 cm soil layers in 2018, but I20 significantly improved FRBD by 259.98% than I45 and FH significantly improved fine root averaged diameter by 67.14% in 30–80 cm soil layers than F0. These suggested that fine root had a conservative foraging strategy in water- and nitrogen-rich conditions. All the trait plasticity might be due to the changeable soil properties (i.e. soil water content, organic matter, nitrate and ammonium nitrogen, and total phosphorus). Based on the little difference of fine root mass among DIF treatments in two years, we hold the view that fine root distribution pattern and some growth and morphological traits adapted to water-nitrogen heterogeneity more easily than the mass plasticity in young aged P. tomentosa plantations. Yet enhancing our understanding of fine root foraging strategy in response to DIF practices and how it affects tree growth requires more attention to some physiological and biochemical processes and the growth dynamics of the root systems. • Fine root had a conservative foraging strategy in resource-rich conditions. • Key indices affecting fine root trait plasticity were SWC, OM, NO 3- -N, NH 4+ -N, and TP. • Fine root showed higher trait plasticity compared with mass plasticity under DIF practices. [ABSTRACT FROM AUTHOR]

Populus tomentosa is a dominant poplar species in North China. Clarifying the short-term response of fine roots to soil water gradient in the P. tomentosa plantation is helpful to have a better understanding on the ecological adaption strategy of plant roots to soil moisture. Three irrigation treatments were set in a two-year-old P. tomentosa stand planted on the sandy loam, i.e. full irrigation (DIFI), controlled irrigation (DICI), and control (CK). After 2 months of irrigation, root sampling down to 150 cm depth was conducted in each treatment using the soil core method. Through the measurement of root samples, the root growth, distribution and morphology data in each treatment, soil depth and horizontal distance were obtained. Vertically, in each soil layer, there is no significant difference in fine root biomass density (FRBD) among treatments (p > 0.05). In the whole root zone and the area with a large moisture difference in shallow soil layer among treatments, the distribution depth of fine root showed a characteristic of CK > DICI > DIFI. However, this characteristic was not observed in the area with less difference in soil water content. Horizontally, FRBD in CK decreased gradually with the increase of the distance from tree, but this trend was very weak in DIFI and DICI treatment. There was no significant difference of FRBD among treatments at each horizontal distance (p > 0.05), except for the distance of 30 cm from tree, where FRBD of CK was obviously higher than DIFI and DICI treatment (p < 0.05). On a two-dimensional scale, fine roots distribution tended to be shallower with the increasing distance from tree. Under irrigation, fine roots mainly concentrated in the shallow soil layers at both sides of the dripper, while a majority of fine roots in CK distributed in the relatively deeper soil layers close to the tree. For each soil layer within 0–50 cm, no significant difference in fine root morphology was found among treatments (p > 0.05). Both the variation of the total fine root biomass and length among the treatments followed an order of CK > DIFI > DICI. In conclusion, when a short-term moisture difference occurred in the shallow soil layers, 1) the fine roots of P. tomentosa distributed deeper and tended to concentrate closer to the tree with the aggravation of drought stress, 2) P. tomentosa preferentially adjusted the fine root distribution rather than the fine root morphology in the shallow soil layer to adapt to soil water stress, 3) as to the total amount of fine roots, P. tomentosa adopted a regulation strategy of slight decrease at first and then distinct increase. [ABSTRACT FROM AUTHOR]

Late embryogenesis abundant (LEA) genes were confirmed to confer resistance to drought and water deficiency. An LEA gene from Tamarix androssowii (named TaLEA) was transformed into Xiaohei poplar (Populus simonii × P. nigra) via Agrobacterium. Twenty-five independent transgenic lines were obtained that were resistant to kanamycin, and 11 transgenic lines were randomly selected for further analysis. The polymerase chain reaction (PCR) and ribonucleic acid (RNA) gel blot indicated that the TaLEA gene had been integrated into the poplar genome. The height growth rate, malondialdehyde (MDA) content, relative electrolyte leakage and damages due to salt or drought to transgenic and non-transgenic plants were compared under salt and drought stress conditions. The results showed that the constitutive expression of the TaLEA gene in transgenic poplars could induce an increase in height growth rate and a decrease in number and severity of wilted leaves under the salt and drought stresses. The MDA content and relative electrolyte leakage in transgenic lines under salt and drought stresses were significantly lower compared to those in non-transgenic plants, indicating that the TaLEA gene may enhance salt and drought tolerance by protecting cell membranes from damage. Moreover, amongst the lines analyzed for stress tolerance, the transgenic line 11 (T11) showed the highest tolerance levels under both salinity and drought stress conditions. These results indicated that the TaLEA gene could be a salt and drought tolerance candidate gene and could confer a broad spectrum of tolerance under abiotic stresses in poplars. [ABSTRACT FROM AUTHOR]

Late embryogenesis abundant (LEA) genes were confirmed to confer resistance to drought and water deficiency. An LEA gene from Tamarix androssowii (named TaLEA) was transformed into Xiaohei poplar (Populus simonii × P. nigra) via Agrobacterium. Twenty-five independent transgenic lines were obtained that were resistant to kanamycin, and 11 transgenic lines were randomly selected for further analysis. The polymerase chain reaction (PCR) and ribonucleic acid (RNA) gel blot indicated that the TaLEA gene had been integrated into the poplar genome. The height growth rate, malondialdehyde (MDA) content, relative electrolyte leakage and damages due to salt or drought to transgenic and non-transgenic plants were compared under salt and drought stress conditions. The results showed that the constitutive expression of the TaLEA gene in transgenic poplars could induce an increase in height growth rate and a decrease in number and severity of wilted leaves under the salt and drought stresses. The MDA content and relative electrolyte leakage in transgenic lines under salt and drought stresses were significantly lower compared to those in non-transgenic plants, indicating that the TaLEA gene may enhance salt and drought tolerance by protecting cell membranes from damage. Moreover, amongst the lines analyzed for stress tolerance, the transgenic line 11 (T11) showed the highest tolerance levels under both salinity and drought stress conditions. These results indicated that the TaLEA gene could be a salt and drought tolerance candidate gene and could confer a broad spectrum of tolerance under abiotic stresses in poplars. [ABSTRACT FROM AUTHOR]