Physiological and biomass partitioning shifts to water stress under distinct soil types in Populus deltoides saplings

Abstract

Aims

Although soil environments exist extensive heterogeneity for many plants with a wide range of distribution, researches about effects of soil conditions on plants’ tolerance and adaptation are particularly inadequate. In our study, the aims are to reveal physiological strategies of Populus deltoides against drought stress under different soil conditions and to select the most suitable soil type for P. deltoides plantation.

Methods

Under controlled conditions, we used P. deltoides as a model species to detect differences in gas exchange rate, antioxidative capacity, nitrogen metabolism and biomass accumulation and partitioning in response to drought stress under three mineral soil types with distinct physicochemical characters, i.e. red soil (RS), yellow soil (YS) and yellow-brown soil (BS).

Important Findings

Exposure to 25% of field water holding capacity in soil for 3 months had significantly decreased biomass of all organs, photosynthetic rate, enzyme activities related to N assimilation, but increased H₂O₂, malondialdehyde and content of both NO₃⁻ and NH₄⁺, when P. deltoides was planted in both RS and YS. In contrast, under BS, there are slightly negative effects exerted by water deficit on total biomass, gas exchange rate, activities of enzymes related to nitrogen metabolism and membrane damage caused by reactive oxygen species, which can be associated with a consistent increase in superoxide dismutase, peroxidase and catalase, and a higher ratio of root mass to shoot mass. It is concluded that, such higher capacity in tolerance and adaptation against drought stress under BS relative to both RS and YS could be accounted for more sufficient nutrient provision in soil parental materials and better soil aeration conditions which play a vital role in plant acclimation to water shortage. Our study also revealed that, distribution areas of BS might be preferable for cultivation of P. deltoides, when compared with those of RS and YS.

摘要

土壤类型是影响植物分布和生产力的重要环境因素，但有关土壤环境异质性对植物抗逆性效应的研究非常缺乏。本研究以美洲黑杨 (Populus deltoids)为对象，以3种典型土壤类型(红壤、黄壤和黄棕壤)为栽培基质，在控制实验条件下，经过三个月的干旱胁迫(25%田间持水量)处理，测定了不同处理条件下美洲黑杨的气体交换速率、抗氧化能力、氮代谢特征、生物量积累与分配特征。研究结果表明，在红壤和黄壤条件下，与对照(75%田间持水量)相比，干旱胁迫显著降低了美洲黑杨各器官的生物量、光合速率、叶片氮同化酶的活性，显著增 加了叶片中过氧化氢、丙二醛和无机氮的含量。在黄棕壤条件下，干旱对美洲黑杨总生物量、光合速率、氮同化酶以及质膜完整性的负面 影响较小，这与其维持较高的超氧化物歧化酶，过氧化物酶和过氧化氢酶的活性相关，也与其生物量分配模式(如提高根冠比)密切相关。 由此可见，生长在黄棕壤条件下的美洲黑杨表现出较强的抗旱能力，这可能与其土壤母质中较高的土壤养分和良好的通气状况相关。因此，就土壤类型而言，与红壤和黄壤相比，黄棕壤提供的土壤环境条件有利于美洲黑杨的抗逆表现和栽培利用。

INTRODUCTION

Drought is one of the most common abiotic stresses limiting plant production worldwide. At present, up to one-third of agricultural lands are adversely affected by water shortage globally (Massacci et al. 2008). In China, arid and semi-arid regions account for 45% of land areas (Wang et al. 2004). Meanwhile, the continuous changes in global warming have been influencing the degree and area of drought lands (Dai 2013). Forest plantation species, including poplar, are often established on marginal lands where water deficit in soil often happens without regular irrigation (Amichev et al. 2012). Therefore, drought is become one of most widespread factors i.e. affecting growth, photosynthesis and physiology of tree species.

Populus spp. is an ideal model tree species and is easy to be genetically modified with a large variety of gene resources, and as a short-rotation plantation species are usually cultivated intensively for timber production. In China, plantation area of poplar is approximately 7 million hm², ranking the first in the world (Fang 2008). After the accomplishment of genome sequencing of P. trichocarpa (Tuskan et al. 2006), which noticeably speeds up exploitation of excellent allelic genes, genetic recombination and improvement, and nowadays Populus has been widely used as a model species for research of plant biology, physiology and ecology. Up to now, there have been a large number of researches involving in morphological, anatomical, physiological and molecular responses of poplar to adverse conditions. For example, omics technologies flourishing in recent years, such as transcriptomics and metabolomics, have been used to study the mechanisms of poplars related to photosynthesis physiology, signal transduction, osmoregulation, antioxidation, redox homeostasis, metabolic processes of carbon and secondary metabolites and so forth, when exposed to water or nutrient deficiency (Engel et al. 2004; Jia et al. 2017; Tschaplinski et al. 2019; Xia et al. 2020; Zhang et al. 2019a, 2019b). However, it should be noted that, effects induced by drought are also regulated by soil characteristics, such as physical properties, nutrient supplies and microbial activities and such studies are very scarce to date.

There is a wide range of Populus deltoides plantation in China which respond differentially under various soil types and cultivation areas. Among them, red soil (RS), yellow soil (YS) and yellow-brown soil (BS) are the most typical soil types. RS is one of most common zonal soil types in tropical and subtropical regions of China, accounting for 6.4% of land area in China (He et al. 1983). It is characterized by a red, acid and highly unsaturated iron-alumina-enriched type as a result of desiliconization and aluminum enrichment. YS, accounting for 3.6% of land area of China (He et al. 1983), is a typical acidic soil formed in the subtropical warm and wet climate areas (Zhang et al. 2014b), which is widely distributed in mountainous region and plateau in Yunnan, Hunan, Sichuan and Fujian in China. BS, accounting for 1.9% of land area of China (Chen et al. 2013), is the main zonal soil type in the northern subtropics, which is a transitional soil type between brown earth and yellow earth, showing weak acidity and high soil organic carbon (Li et al. 2001). Above soil types are adjacent to each other and are important agricultural soils in China. Although rainfall is relatively abundant, seasonal drought occurs frequently due to the uneven distribution of rainfall that ultimately exerts negative effects on growth and yield of poplars.

Populus deltoides was first introduced to china in 1950s, and has become an important afforestation species in subtropical climate area, especially in the middle and lower reaches of the Yangtze River and Huai River basin, where RS, YS and BS are the most common soil types. In present study, we conducted a comparative study focusing the effects of soil types on growth, photosynthesis rate, quenching of reactive oxygen species (ROS) and N metabolism under drought stress. We tested the following hypotheses: (i) there are distinct performances in growth and physiology when poplars are planted under different soil types with different physical–chemical properties. (ii) Nutrient-rich soil type is beneficial to the growth and resistance of poplar against water deficit. Our aim is to reveal the physiological strategies of P. deltoides against drought under different soil conditions, to select the most suitable soil type for P. deltoides plantation, and to lay theoretical grounding for water management of P. deltoides.

MATERIALS AND METHODS

Plant growth conditions and treatments

In April 2017, annual healthy shoots of P. deltoides cv. ‘35/66’, a major variety in Jiangsu Province in China, were collected from the germplasm nursery at the Communist Youth League farm in Jingkou district of Zhenjiang in Jiangsu province. The cuttings of poplar were firstly soaked in aminobenzothiazole rooting powder solution (50 mg·kg⁻¹) for 6 h. Then, the cuttings were preliminarily cultivated in a seedbed containing mixture of alluvial soil and fine sand (3:1, v/v) as a substrate. After 1 month, cuttings with similar height and root length (about 10 and 15 cm, respectively) were selected for the present study. RS, YS and BS soil samples were collected from Fengle, a village of Shimian county (102°54′ E, 29°33′ N), Baolin, a village in Qionglai city (103°30′ E, 30°22′ N), and Xutang, a village of Pingwu county (104°53′ E, 32°32′ N), respectively, all of which are located in Sichuan province China. For each soil type, mineral soil was collected from five points with a distance of more than 2 km. The soils of the same soil type were fully homogenized to represent for a certain soil type. Soil properties, including soil organic matter, total nitrogen, total phosphorus, bulk density, total porosity, were measured according to methods of Jiao et al. (2011). Soil physical–chemical properties are shown in Table 1.

Our experiment was a completely randomized design with six factorial combinations of three types of soil and two levels of water content. In our preliminary experiment in 2016, it was found that 75% water content was optimum for height growth of P. deltoides when compared with other conditions supplied with more water, such as 100% of the field water holding capacity in soils (FC). This phenomenon suggests excessive water in soils is unfavorable for growth of P. deltoides, and similar results have been observed in Puelia sinense (Zhang et al. 2015) and Zea mays (Ali et al. 2011). Nevertheless, 25% FC is considered as severe deficit irrigation conditions for Populus (Zhang et al. 2012). Therefore, 75% and 25% FC were set as the control conditions (CK) and drought treatment, respectively. Each pot was filled with 13 kg soils and was transplanted with one cutting. The diameter and height of pot were 30 and 25 cm, respectively. For each soil type and treatment, a total of 25 cuttings was contained, i.e. 5 replicates with 5 cuttings in each replicate. Our experimental treatment began on 1 June and ended on 31 August 2017. During this period, soil water content in the pots was controlled by daily supplementary irrigation. The relationship (Y = 0.975 + 0.112X, R² = 0.968, P < 0.001) between fresh weight (Y, g) and height (X, cm) of the plant was used to correct the water volume error caused by changes in biomass of poplar (Li et al. 2004). The pot-culture experiment was conducted at the Forestry Ecological Engineering in the Upper Reaches of Yangtze River Key Laboratory of Sichuan Province (102°59′E, 29°58′N, a.s.l. 620 m). It belongs to the subtropical zone with a warm and moist climate, 16°C average annual temperature, 1732 mm average annual precipitation and 294 days frostless duration per year. The average annual pan evaporation is 1412 mm, and range of potential evapotranspiration per year is 651–900 mm in Sichuan basin (Chen et al. 2017).

Measurement of gas exchange rate

At the end of experiment, for each treatment, five cuttings were randomly selected to measure the gas exchange rate using a portable photosynthesis system (LI-6400, Li-Cor Inc., Lincoln, NE, USA). Gas exchange rate was assayed for the third fully expanded and intact leaves from 8.30 to 11.30. Leaves were first acclimated in the chamber for 15 min under following conditions: CO₂ concentration, 380 µmol mol⁻¹; photosynthetic photon flux density, 1200 µmol m⁻² s⁻¹; leaf temperature, 25°C; relative air humidity, 70%. Once the apparent steady-state gas exchange was achieved, the steady-state net photosynthetic rate (P_n), stomatal conductance (G_s), intercellular CO₂ concentration (C_i) and transpiration rate (T_r) were recorded. Stomatal limitation value (L_s) was calculated as 1 − C_i/C_a (Farquhar and Sharkey 1982), where C_a was CO₂ concentration in leaf chamber.

Measurement to hydrogen peroxide and malondialdehyde

At the end of experiment, a cutting was selected randomly from five cuttings of each replicate, and thus there were five cuttings in total in each treatment used for the following measurements. The fourth fully expanded and exposed young leaf of each cutting was sampled and then stored under −70°C. Following physiological or biochemical parameters were measured within 2 weeks after sampling in order to obtain accurate data. For determination of H₂O₂ content Brennan and Frenkel (1977) procedure was followed of. Fresh leaf sample (0.5 g) was homogenized in 5% (m/v) trichloroacetic acid and centrifuged at 10 000 g for 10 min. 100 µl of 20% titanium tetrachloride in hydrochloric acid was added to 1 ml of the clear supernatant extract, and 200 µl of ammonium hydroxide was then added. The precipitate was obtained by centrifugation at 5000 g for 10 min, and then dissolved in 3 ml of 1 mM sulfuric acid. Finally, H₂O₂ content was detected by the spectrophotometer method at wavelength of 410 nm.

The malondialdehyde (MDA) content was measured according to method of Hodges et al. (1999). Fresh leaf sample (1 g) was homogenized and extracted in 10 ml of 10% trichloroacetic acid. After centrifugation at 12 000 g for 10 min, 2 ml of clear supernatant extract was added to 2 ml of 0.6% thiobarbituric acid and dissolved in 10% trichloroacetic acid. The reaction solution was incubated in boiling water for 15 min, cooled to room temperature and then centrifuged at 12 000 g for 10 min. The absorbance was measured at 450, 532 and 600 nm using a spectrometer.

Measurement of activities of the antioxidant enzymes

Activities of superoxide dismutase (SOD, EC 1.15.1.1), peroxidase (POD, EC 1.11.1.7) and catalase (CAT, EC 1.11.1.6) were measured by following the method of Chen et al. (2016). Fresh leaf sample (0.3 g) was ground in liquid N and homogenized in 6 ml of 50 mM potassium phosphate buffer (pH 7.8), which including 0.2% (v/v) Triton X-100, 0.1 mM ethylene diaminetetra acetic acid disodium salt (Na₂EDTA) and 1% (w/v) polyvinylpyrrolidone (PVP). The homogenate was then centrifuged at 12 000 g for 15 min. The clear supernatant extract was used to measure the activities of SOD, POD and CAT. The activity of SOD was determined by measuring its capacity to inhibit photochemical reduction reaction of nitro-blue tetrazolium. The reaction mixture with a total volume of 3 ml, constituted of 2 ml of 50 mM potassium phosphate buffer (pH 7.8), 0.3 ml each of 20 µM riboflavin, 150 mM l-methionine, 600 µM nitro-blue tetrazolium and 0.1 ml of enzyme extract. The reaction was initiated after the addition of riboflavin and continued for 30 min under an irradiance of 170 µmol m⁻² s⁻¹ provided by a white fluorescent lamp. A system without enzyme extract was served as a negative control. SOD was determined when detected at 560 nm with a spectrophotometer. SOD activity unit was defined as the amount of enzyme required to cause 50% suppression of the reduction of nitro-blue tetrazolium. The POD activity was assayed in a 3 ml of reaction solution, including 50 mM potassium phosphate buffer (pH 6.0), 50 mM H₂O₂ and 50 µl enzyme extract. The absorbance was measured at 470 nm for 3 min. Activity of POD was based on the rate of tetraguaiacol production using an extinction coefficient of 25.5 mM⁻¹ cm⁻¹. POD activity unit was interpreted as the amount of enzyme that oxidizes 1 mmol of guaiacol min⁻¹ per g of fresh leaf. The CAT activity was assayed in a 3 ml of reaction mixture, which was composed of 50 mM potassium phosphate buffer (pH 7.0), 10 mM H₂O₂ and 100 µl enzyme extract and absorbance was noted at 240 nm for 3 min. CAT activity unit was calculated using the H₂O₂ extinction coefficient of 40 mM⁻¹ cm⁻¹. CAT activity unit was interpreted as the amount of enzyme that oxidizes 1 µmol of H₂O₂ min⁻¹ per g of fresh leaf.

Nitrate reductase (NR, EC 1.6.6.1) activity was measured based on the method of Högberg et al. (1986). Fresh leaf sample (0.5 g) was ground in liquid N to a fine powder and extracted in 4 ml of 25 mM phosphate buffer (pH 7.5), including 5 mM cysteine and 5 mM Na₂EDTA. Then the homogenate was centrifuged at 4000 rpm for 15 min at 4°C. 0.4 ml of supernatant was added to 1.6 ml of the assay mixture, which was composed of 1.2 ml of 0.1 M KNO₃-phosphate buffer and 0.4 ml of 3 mM NADH, and then incubated at 25°C for 30 min. For the negative control, 0.4 ml of NADH was substituted by 0.4 ml of phosphate buffer. Then, 1 ml of 1% (w/v) sulfanilamide in 3 N HCl and 0.02% N-naphthyl ethylenediamine in water was added into the incubation solution. After incubation for 15 min, reaction solution was centrifuged for at 4000 rpm for 5 min, and the absorbance of the supernatant was measured at 540 nm. The NO₂⁻ concentration in each sample was calculated from a standard curve of known NO₂⁻ concentrations.

As to the assay of glutamine synthetase (GS, EC 6.3.1.2) activity, a total volume of 1.9 ml of reaction mixture was composed of 1.8 ml of phosphate buffer (pH 7.5) including 13 mM hydroxylamine hydrochloride,1 mM adenosine triphosphate, 20 mM MgCl₂, 50 mM glutamate, 20 mM sodium arsenate and 0.1 ml enzyme extract. After incubated at 37°C for 30 min, the reaction solution was centrifuged at 5000 g for 15 min under 4°C. The absorbance of supernatant was monitored at 540 nm (Gao et al. 2013a).

For measurement of the glutamate synthase (GOGAT, EC 1.4.1.14) activity (Gao et al. 2013a), fresh leaf sample (0.2 g) was ground in liquid N to a fine powder and homogenized in 2 ml of 50 mM phosphate buffer (pH 7.5) containing 0.5 mM Na₂EDTA, 5 mM l-cysteine and 0.5% PVP. The homogenate was centrifuged at 12 000 g for 10 min under 4°C. The supernatant was used to assay GOGAT activity. The reaction mixture (1.9 ml) was composed of 1.8 ml of phosphate buffer (pH 7.5) containing 5 mM α-ketoglutaric acid, 10 mM glutamate, 0.15 mM NADH and 0.1 ml of the supernatant. The decrease in absorbance was recorded at 340 nm for 3 min. Deaminating glutamate dehydrogenase (NAD-GDH, EC 1.4.1.2) activity was measured by recording the reduction of NAD. To determine the NAD-GDH activity, a reaction system consisted of 1.9 ml buffer (40 mM l-glutamic acid and 0.2 mM NAD) and 0.1 ml the supernatant. The absorbance was monitored at 340 nm for 3 min.

Measurement of NO₃⁻ and NH₄⁺

The measurement of NO₃⁻ concentration in leaves was based on the method described by Patterson et al. (2010). Fresh leaf sample (0.1 g) was grounded in liquid N to fine powder and then homogenized in 1 ml of deionized water, and then incubated at 45°C for 1 h. The homogenate was centrifuged at 5000 g for 15 min under 20°C. 200 µl of supernatant was mixed thoroughly with 0.8 ml of 5% (w/v) salicylic acid in concentrated H₂SO₄. The reaction solution was then incubated at room temperature for 20 min. 1.9 ml of 2 M NaOH was added to raise the pH of the solution. The NO₃⁻ concentration was monitored at 410 nm.

The NH₄⁺ concentration was determined according to the method of Gao et al. (2013a). The reaction system was composed of 0.1 ml of supernatant, 0.01 ml of 10% K–Na tartrate, 0.1 ml of Nessler reagent and 2.4 ml of distilled water. After incubation for 5 min, the absorbance was recorded at 425 nm, and concentration of NH₄⁺ was calibrated with a standard curve generated using known concentration of NH₄⁺.

Measurement of biomass accumulation and partitioning

At the end of experiment, five cuttings from each treatment were harvested, and divided into three parts, i.e. leaf, stem and root. All plant parts were cleaned with tap water and deionized water, then dried at 105°C for 30 min in an oven and finally plant organs were dried to a constant weight under 70°C for 48 h. The total biomass is the sum of dry weight of each organ. The ratio of root mass to shoot mass (R/S) was also calculated.

Statistical analysis

All data were analyzed in statistics using Statistical Package for the Social Sciences (SPSS) software version 22.0. Two-way analysis of variance was employed to evaluate the interactive effects of drought and soil type on morphological and physiological parameters. All data were tested for normality and the homogeneity of variances, and were log-transformed to correct deviations from these assumptions if needed. Tukey’s test was employed to compare the means at a significant level of α = 0.05.

RESULTS

Difference in gas exchange rate affected by interaction of drought and soil type

In case of gas exchange rate, drought stress induced a significant decrease in C_i and T_r under RS (Fig. 1c and d), but nonsignificant effects were seen in both P_n and G_s (Fig. 1a and b). Under YS, P_n, G_s, C_i and T_r of poplars were all declined by 36.30%, 57.63%, 55.27% and 48.05%, respectively, when exposed to water deficit. However, under BS, drought did not affect significantly in such parameters compared with those under control conditions. As for L_s (Fig. 1e), compared with the control, this parameter increased by 54.9% and 91.9% under water deficit when the plants were cultivated under RS and YS, respectively, whereas drought did not significantly increase L_s under BS. Based on statistical analysis, all photosynthetic parameters were strongly affected by drought (Fig. 1). P_n, C_i and L_s were significantly affected by soil type. Moreover, both C_i and T_r were significantly affected by the interaction of drought × soil type.

Difference in antioxidant status affected by interaction of drought and soil type

Concentration of H₂O₂ in poplar leaves under RS and YS was increased by 48.10% and 36.89%, respectively, as compared with the control (Table 2). As to MDA, there was a significant increase when plants exposed to water deficit under RS, while drought did not significantly affect the level of MDA under YS and BS.

Under RS, drought induced a decline of 42.0% and 33.7% in activities of POD and CAT, respectively, but did not affect the activity of SOD (Table 2). In contrast, water deficit induced an increase of 21.42% in the SOD activity under YS conditions. Interestingly, the activities of SOD and POD in leaves increased by 22.8% and 42.4% under BS conditions, respectively, when compared with the control. Statistical analysis revealed that (Table 2), the concentration of both H₂O₂ and MDA and the activities of SOD and CAT were significantly affected by drought, and the activities of SOD, POD and CAT were affected significantly by soil type and the interactive of drought × soil type.

Difference in nitrogen metabolism affected by interaction of drought and soil type

Decreased trend was seen by 51.7%, 47.6% and 58.1% in activities of NR, GS and GOGAT under RS, respectively, when exposed to drought conditions as compared with the control. Similar results were also noticed in activities of NR, GS and GOGAT under YS, which was declined by 46.0%, 28.4% and 52.2%, respectively (Fig. 2a–c). However, drought did not significantly affect the NR, GS and GOGAT activity of poplars when planted under BS. Additionally, drought reduced NAD-GDH activity by 38.2% under RS, but increased the activity of this enzyme by 109.8% under BS (Fig. 2d).

In terms of free inorganic nitrogen, compared with the control conditions, NO₃⁻ concentration significantly increased by 23.0% and 11.6% under RS and YS, respectively, but decreased by 8.1% under BS, when exposed to drought conditions (Fig. 2e). Contrastingly, drought induced an increase in the NH₄⁺ content by 22.5% and 10.1% under RS and YS, respectively, but a significant decrease under BS (Fig. 2f). Based on the results of statistical results (Fig. 2), except for NAD-GDH and NH₄⁺ concentration, other indexes related to nitrogen metabolism were significantly affected by drought. With the exception of NR, other parameters were significantly affected by soil type and interaction of drought × soil type.

Difference in biomass accumulation and partitioning affected by interaction of drought and soil type

Under drought conditions, leaf biomass, stem biomass and root biomass decreased by approximately 47.1%, 44.4% and 45.7%, respectively, under RS when compared with the controls, while 48.8%, 62.3% and 62.7%, respectively, under YS (Fig. 3a–c). In contrast, under drought conditions, leaf biomass and stem biomass were declined by 15.4% and 23%, respectively, but root biomass was increased by 50.2% under BS. Thus, drought did not significantly affect the total biomass of poplars in BS, but significantly decreased total biomass of those under both RS and YS (Fig. 3d).

R/S of poplars seems to be highly flexible when they were planted under distinct soil types (Fig. 3e). When compared with the control, R/S showed nonsignificant trends under RS. In contrast, R/S decreased by 24.8% under YS, but increased by 95.8% under BS. Statistical analysis revealed that, both biomass of all plant parts and R/S were significantly affected by drought and soil type (Fig. 3). Except for leaf and stem biomass, other parameters related to biomass and its allocation were significantly affected by the interactive effect of drought × soil type.

DISCUSSION

Under natural conditions, stomatal conductance, morphology and density in leaves of plants are very sensitive to soil or air water deficit as a result of seasonal fluctuation of precipitation (Gao et al. 2017; Gao and Tian 2019; Hernandez-Santana et al. 2016). Stomatal opening reduces quickly once guard cells perceive and transduce messenger molecule’s signal, such as abscisic acid, which can be rapidly biosynthesized in roots or in leaves under water depletion (Ikegami et al. 2009). Stomatal reaction can happen even earlier than changes in leaf water potential (Trejo and Davies 1991). Decrease in stomatal conductance could effectively control transpiration rate, maintain hydraulic integration in leaves, and thus alleviate adverse effects from water shortage. In our study, we found G_s decreased to a different degree when P. deltoides were exposed to water deficit under all soil types, especially under RS and YS, indicating that leaf water imbalance under RS and YS was more serious than that under BS. On the other hand, according to the principle on stomatal and nonstomatal limitation proposed by Farquhar and Sharkey (1982), decrease in C_i along with decline in G_s under RS and YS suggested that stomatal limitation might be the major factor for the decrease in photosynthesis rate in our study. Similarly, the significant increase in L_s showed that stomatal limitation in leaves of P. deltoides was obvious under RS and YS. The increase of physical limitation caused by stomatal closure can lead to resistance of CO₂ conductance into mesophyll cells, and thus a decrease in photosynthetic rate. In contrast, under BS, drought did not significantly affect G_s, T_r and P_n of P. deltoides, which is probably a consequence of optimal biomass allocation and high root production, benefiting for water absorption and carbon balance under dry conditions.

In our study, under BS, a rise in R/S and in the absolute amount of root biomass when water is limiting is a convincing evidence in support of optimal partitioning theory, that plants could allocate relatively more carbon and nutrients to root growth than to aboveground growth when plant growth is limited by water and/or nutrient shortage (Bloom et al. 1985). The increase in R/S can be considered to be an adaptive trait, thereby promoting water absorption capacity of plants (Wang et al. 2018) and keeping water equilibrium in leaves. On the other hand, under RS and YS, P. deltoides showed a completely distinct reaction in root growth and biomass partitioning under water shortage. These results suggest that root growth response of P. deltoides saplings to water deficit is dependent on additional factors, which are not considered in optimal partitioning theory. One factor seems to be nutrient availability, in particular the supply of nitrogen or phosphorus, which probably affects the size and morphology of root systems besides water availability (Lambers et al. 2006; Nadelhoffer 2000). In addition, under RS and YS, the reason for failure of optimal resource partitioning may be the decreased P_n along with drought-induced reductions in G_s, and consequent carbon limitation of growth (Hertel et al. 2013). Therefore, our results suggested whether increasing or reducing C allocation to root growth depends not only on water availability but also on the soil environments.

Many previous studies have indicated that, drought stress could induce oxidative damage to plants because of over-production of ROS (Halliwell 2006). Excessive ROS can cause lipid peroxidation, membrane instability, protein degradation and nucleotide injuries (Bailey-Serres and Mittler 2006). The final product of lipid peroxidation is MDA, which is usually used as an index to estimate the extent of oxidative stress (Hodges et al. 1999). In our study, under drought stress, significantly increase in H₂O₂, a strong oxidants in plant cells, under RS and YS may be the main factor for the increase of MDA, which signifies the increase of membrane lipid peroxidation as well as occurrence of cross-linking polymerization of macromolecules, such as cell membranes, proteins and nucleic acids (Bao et al. 2020). However, under BS, both H₂O₂ and MDA content were not significantly affected when P. deltoides was exposed to water deficit. Previous literatures also reported that, supplement with N or P can reduce production of ROS by enhancing osmoregulation ability of plants under water deficit (Law 2019; Saneoka et al. 2004). On the other hand, enzymatic degradation mediated by antioxidant enzymes, like SOD, POD and CAT, is key for quenching of ROS (Chen et al. 2014, 2015). In our study, under drought stress, the increase of both SOD and POD may make important contributions to the ROS scavenging by disproportionation O₂^·− and hydrolysis of H₂O₂, under BS. In contrast, under other two soil types, activities of most enzymes were inhibited (except for the increase in SOD under YS) to a different degree, especially for those under RS. Such results suggested that genes related to above antioxidant enzymes may be downregulated or their encoding proteins be inactivated, which induce disorder of ROS homeostasis in leaves of P. deltoides.

N metabolism is one of the most fundamental processes of plant physiology. NO₃⁻, as the main N form absorbed from soil, and is quickly reduced to NO₂⁻ in the cytosol by NR, which is positively regulated by NO₃⁻ (Navarro et al. 1996). Then, reduction of NO₂⁻ into NH₄⁺ is catalyzed by NiR in the chloroplasts or plastids. Afterward, NH₄⁺ came from NO₃⁻ reduction or from soil is converted to glutamate by GS and GOGAT cycle or the alternative GDH pathway (Sánchez-Rodríguez et al. 2011). In angiosperms, assimilation of NH₄⁺ resulting from NO₃⁻ reduction and from photorespiration mainly depends on the GS/GOGAT cycle (Weber and Flügge 2002), and glutamine and glutamate from this cycle are precursors for synthesis of N-containing organic compounds, such as amino acids, nucleic acids, polyamines and chlorophyll in plants. Thus, decrease in GS or GOGAT activity may negatively affect normal metabolism in N-containing compounds under RS and YS. Otherwise, in view of the decrease in GS activity, GDH catalysis is an alternative biochemical pathway for N assimilation under unfavorable conditions (Zhang et al. 2014a). For example, 2-ketoglutaric acid produced by deamination of glutamate through GDH provides tricarboxylic acid cycle with a carbon skeleton, and ensures continuity of nitrogen metabolism (El-Shora and Abo-Kassem 2001). In our study, in response to drought stress, the decrease in NR activity and activities of enzymes in GS–GOGAT cycle hindered reduction of NO₃⁻ and assimilation of NH₄⁺, respectively, under RS and YS conditions. Thus, accumulation of inorganic nitrogen in the leaves would result in N deficiency and even toxic effect of NH₄⁺. Disorder in N metabolism would inevitably inhibit photosynthetic rate and growth of poplars (Gao et al. 2013b). However, under BS conditions, drought did not inhibit activities of NR and GOGAT of poplars, but promote activities of GS and NAD-GDH, which eventually accounted for the decrease in NH₄⁺ and NO₃⁻. Previous studies have indicated that many secondary metabolites, including alkaloids, polyamine and polyphenols, produced by normal N metabolism play a vital role in cell detoxication by regulating cell ion balance, inhibiting membrane lipid peroxidation and promoting cell repair (Sarker and Oba 2019; Yamaguchi et al. 2007). Therefore, acceleration of N assimilation may help P. deltoides resist against drought stress under BS.

In conclusion, the responses of P. deltoides in biomass accumulation and partitioning and physiology to drought stress depend on soil environmental types. Poplars cultivated under RS and YS were more vulnerable to drought stress than those cultivated under BS. Drought exerted more significantly negative effects on photosynthesis and growth of poplars cultivated under RS and YS, while poplars exhibited a better strategy in biomass partitioning under BS. When exposed to water deficit, the poplars under BS demonstrated a higher ability to scavenge ROS and maintain normal N assimilation compared with those under RS and YS. These results suggested that more sufficient nutrient provision and favorable soil texture are beneficial to poplars’ acclimation against water scarcity. Therefore, BS might be preferable for cultivation of P. deltoides, relative to those of RS and YS. Additional studies in field conditions, such as difference in P. deltoides plantation’s productivity and stock volume under RS, YS and BS, are needed to verify the present results and to estimate their implications for afforestation planning and management.

Funding

This research was supported by Sichuan Science and Technology Program (no. 2016NYZ0035-07, 2019YJ0416 and 2019YJ0427).

Acknowledgements

We are grateful to the Collaborative Innovation Center of Ecological Security in the Upper Reaches of the Yangtze River.

Conflict of interest statement. None declared.

Authors’ Contribution

Senlin Yang and Jian Shi conducted the experiments to obtain the analytical data. Lianghua Chen interpreted the results and wrote the paper. Li Zhang, Hanbo Yang and Tiantian Lin modified some experimental methods, helped to collect experimental data and contributed to the manuscript and to its correction. Yang Liu and Peng Zhu coordinated the study and carried out data analysis and interpretation. Jiujin Xiao and Yu Zhong supervised the research work. Jian Zhang, Danju Zhang and Zhenfeng Xu provided advice for planning of the experiments, helped us classify different types of soil and revised the manuscript.

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