Wheat Straw Burial Improves Physiological Traits, Yield and Grain Quality of Rice by Regulating Antioxidant System and Nitrogen Assimilation Enzymes under Alternate Wetting and Drying Irrigation

Yousef Alhaj hamoud, Hiba Shaghaleh, Wang Ruke, Willy Franz Gouertoumbo, Amar Ali Adam hamad, Mohamed Salah Sheteiwy, Wang Zhenchang, Guo Xiangping

Research Paper

Wheat Straw Burial Improves Physiological Traits, Yield and Grain Quality of Rice by Regulating Antioxidant System and Nitrogen Assimilation Enzymes under Alternate Wetting and Drying Irrigation

Yousef Alhaj hamoud1, Hiba Shaghaleh2, Wang Ruke1, Willy Franz Gouertoumbo1, Amar Ali Adam hamad1, Mohamed Salah Sheteiwy3, Wang Zhenchang1, Guo Xiangping1

(College of Agricultural Science and Engineering, Hohai University, Nanjing 210098, China; Jiangsu Provincial Key Laboratory for the Chemistry and Utilization of Agro-Forest Biomass, College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China; Department of Agronomy, Faculty of Agriculture, Mansoura University, Mansoura 35516, Egypt)

Wheat straw burial has great potential to sustain rice production under alternate wetting and drying (AWD) irrigation. A field experiment was conducted with three wheat straw burial treatments, including without straw burial (NSB), with light straw burial of 300 kg/hm2(LSB) and dense straw burial of 800 kg/hm2(DSB), as well as three AWD regimes: alternate wetting/moderate drying (AWMD), alternate wetting/severe drying (AWSD) and alternate wetting/critical drying (AWCD). The rice growth and grain quality were higher in LSB and NSB than those in NSB under the same AWD regime. The AWMD × DSB treatment resulted in the highest yield, brown rice rate, milled rice rate, amylose content and protein content. Conversely, the AWCD × NSB treatment led to the lowest yield, brown rice rate, milled rice rate, amylose content and protein content. The active absorption area and nitrate reductase activity of roots were higher in the AWMD × DSB treatment than those in the AWCD × NSB treatment, as the former increased organic carbon and nitrogen contents in the rhizosphere, whereas the latter reduced their availability. Total soluble protein content and glutamine synthetase activity were greater in the AWMD × DSB treatment than those in the AWCD × NSB treatment. The activities of superoxide dismutase and catalase were higher in the AWMD × DSB treatment compared with the AWCD × NSB treatment, leading to the amelioration of oxidative cell injury, as shown by a lower malonaldehyde level. This study suggested that farmers should implement AWMD irrigation after leaving the straw residues in the field, followed by deep tillage to improve soil quality and mitigate the drought stress cycles of AWD. This approach can improve rice growth and grain quality and alleviate the problems of disposal of straw residues and water scarcity for sustainable rice production.

antioxidant enzyme activity; wheat straw burial; irrigation regime; nitrogen uptake;; grain quality; yield

Water scarcity and food demand are increasing simultaneously, along with further demands from stakeholders for environmental sustainability. Rice is one of the most important crops worldwide and is the leading staple food in Asia (Datta et al, 2017). Globally, rice farming uses 70% of the total agricultural water resources (Statista, 2017). In China, 23% of the cultivated area is used for paddy fields, accounting for 19% of the global cultivated area (Jiang et al, 2019). In addition, rice farming in China requires 65% of freshwater resources under flooded irrigation (Alhaj Hamoud et al, 2018). In light of climate change and environmental pollution, freshwater is becoming scarce, and affecting rice production (Pan et al, 2017). Therefore, rice productivity should be improved, and the demand for freshwater resources should be alleviated to ensure food security (Xu et al, 2018; Alhaj Hamoud et al, 2019a). Thus, irrigation technologies, such as alternate wetting and drying (AWD) irrigation, were introduced to cope with the shortage in water resources (Pan et al, 2017). However, increasing water scarcity can exacerbate con?icts on water resources over the coming decades (Oumarou Abdoulaye et al, 2019). Hence, rice production systems with high resource use e?ciencies must be developed (Tao et al, 2015).

Contiguously flooding (CF) irrigation uses large amounts of water, leading to low water productivity and threatening rice production (Lin et al, 2014; Liang et al, 2019). Instead, AWD irrigation has been applied where the water is supplied to the ?eld when a certain threshold of soil water content is reached during the growing season (Lampayan et al, 2015). Many indices are available for soil drying in AWD, including setting certain thresholds of leaf water potential (Ye et al, 2013) and the upper and lower limits of soil water content (Alhaj Hamoud et al, 2019b). Although the application of the AWD method improves water productivity compared with the CF method in rice (Wang et al, 2016), the application of AWD in clay soils causes high water losses and a sharp reduction in rice growth, yield and nutrient use efficiency (Al Aasmi et al, 2022). Thus, irrigation options should enhance water storage and maintain soil properties.

The utilization of straw residues has been the topic of numerous studies (Shaghaleh et al, 2019). In China, wheat straw production yields more than 9 × 108t/year, and 15% of wheat straw produced is utilized as feed for cattle and ?ber for the production of construction materials. The remaining straw quantity is burned to prepare the land for the next crop season (Shaghaleh et al, 2019). Mulching the soil surface using straw residue is a new tactic to reduce the negative effect of straw burning on the environment (Mu?oz et al, 2017). The bene?cial effects of straw mulch on soil organic matter, water retention, soil temperature and plant growth have recently been stated (Moreno-García et al, 2018; Yang et al, 2020; Li et al, 2022). In addition, returning straw residues to the soil surface improves crop quality and productivity (Zhang et al, 2017; Li et al, 2020). However, weed and pest are difficult to control under straw mulch cultivation, which can be dangerous to crops, as straw attracts insect pests and interrupts rainfall reach into the soil (Turmel et al, 2015).

Burying a straw layer in soil positively affects soil management (Rasool et al, 2019). The straw layer serves as a barrier, inhibiting water movement from the subsoil to the topsoil (Li et al, 2020), and limiting water loss (Chen et al, 2019). Straw burial also improves water storage capacity and reduces nutrient leaching loss (Yang H S et al, 2017). Moreover, straw burial has been considered available in managing options for improving soil fertility biological activities and ultimately increasing the productivity, quality and sustainability of cropping systems (Li et al, 2020; Yang et al, 2020). Other bene?ts of a straw layer buried in the soil include decrease of soil pH (Fan et al, 2012) and increase of plant growth (Zhai et al,2021). Straw burial can modify soil properties. However, the effects of straw burial on nitrogen assimilation and antioxidant activity and their relations to yield and grain quality of rice under AWD irrigation remain unclear.

Rice roots respond to soil variations (Alhaj Hamoud et al, 2019c), and root-soil interactions regulated by the availability of water play a vital role in nutrient uptake from the soil (Al Aasmi et al, 2022). Accordingly, vigorous rice roots support great shoot biomass (Li et al, 2021). However, data on the root growth traits and their relations to rice grain quality with straw burial under AWD irrigation are limited. Burying straw layer below the soil surface can extend water residence time in the layer above, thereby maximizing the amount of water in the soil as the water moves slowly down the pro?le. Soil water content alters soil properties, which eases straw degradation, enhances ion exchange/absorption, and increases the amount of total dissolved nutrients (Alhaj Hamoud et al, 2019b; Jin et al, 2020). Hence, the integration of AWD irrigation and straw burial will affect the dynamics of water distribution in the soil, as well as the role of soil redox potential (Eh) and soil acidity (pH) in modifying the dynamics of nutrients in the root zone. This phenomenon improves plant growth by regulating the antioxidant system and nitrogen assimilation enzymes, and thus increasing the yield and grain quality of rice. This study assumed that straw burial can increase soil organic matter input in AWD-irrigated paddy fields. The guiding of this study will be of the significant development of sustainable agriculture to identify the mechanism by which AWD irrigation will increase the yield and grain quality of rice and elucidate the synergistic interaction between AWD irrigation and straw burial on the productivity and antioxidant activity of rice. Moreover, detailed knowledge of the biochemical interactions regulating nitrogen assimilation and the antioxidant system under straw burial and AWD irrigation provides new insights into the actual water needs of plants under limited freshwater resources. To test the hypothesis mentioned above, this study quantified the antioxidant activities and nitrogen assimilation enzymes and the physiological parameters of rice under different AWD regimes and wheat straw burial rates in a ?eld trial.

RESULTS

Straw burial improved environmental parameters of soil in root zone under AWD irrigation

Under the same straw burial rate and compared to alternate wetting/critical drying (AWCD) regime, the alternate wetting/moderate drying (AWMD)resulted in a substantial decrease in the soil pH and Eh, whereas significant increases were observed in the total nitrogen (TN), easily oxidizable carbon (EOC), total organic carbon (TOC), and dissolved organic carbon (DOC) contents in the root zone (Table 1). Under the same AWD regime and compared to without straw burial (NSB, 0 kg/hm2), light straw burial (LSB, 300 kg/hm2) and dense straw burial (DSB, 800 kg/hm2) decreased the soil pH and Eh while enhanced the TN, EOC, TOC and DOC contents in the root zone (Table 1). Due to the interaction, the AWMD × DSB treatment exhibited the lowest values of pH and Eh and the highest contents of TN, EOC, TOC and DOC of the soil, whereas the AWCD × NSB treatment showed the highest values of pH and Eh and the lowest contents of TN, EOC, TOC and DOC of the soil.

Table 1. Function of different water regimes and straw burial rates on soil chemical environmental parameters in root-zone.

AWMD, Alternate wetting/moderate drying; AWSD, Alternate wetting/severe drying; AWCD, Alternate wetting/critical drying; NSB, Without straw burial; LSB, Light straw burial; DSB, Dense straw burial; AWD, Alternate wetting and drying; Eh,Redox potential; TN, Total nitrogen; EOC, Easily oxidizable carbon; TOC, Total organic carbon; DOC, Dissolved organic carbon; ns, Not significant.

The means followed by different uppercase letters are signi?cantly different among different AWD regimes (≤ 0.05). Under the same AWD regime, the means followed by different lowercase letters are signi?cantly different among different straw burial rates (≤ 0.05) according to the Duncan’s multiple range tests. *, ** and *** denote significant differences at the 0.05, 0.01 and 0.001 levels, respectively, among treatments.

Root physiological traits improved by subsoil straw burial under AWD irrigation

As shown in Fig. 1, the AWMD regime offered greater root active adsorption area (RAAA), root nitrate reductase activity (RNR) and root oxidative activity (ROA) than AWSD and AWCD under the same straw burial rate. The decline in RAAA, RNR and ROA was evident for roots when the straw burial rate increased during LSB and DSB. Furthermore, plants grown under the AWMD × DSB treatment exhibited the greatest values of RAAA (6.48 m2/plant), RNR [7.02 mg/(g?h)] and ROA [55.70 mg/(g?h)], whereas the plants grown under the AWCD × NSB treatment showed the lowest values of RAAA (2.64 m2/plant), RNR [4.37 mg/(g?h)] and ROA [10.23 mg/(g?h)]. The maximum RAAA was obtained at the milky stage, and the maximum RNR and ROA were obtained at the heading stage (Fig. 1-A to -C).

Fig. 1. Root physiological traits affected by straw burial rate and alternate wetting and drying (AWD) regime.

A?D, Root active adsorption area (RAAA) (A), root nitrate reductase activity (RNR) (B), root oxidative activity (ROA) (C), and aerenchyma percentage (D) under the treatments of wheat straw burial rate and AWD regime. AWMD, Alternate wetting/moderate drying; AWSD,Alternate wetting/severe drying; AWCD, Alternate wetting/critical drying; NSB, Without straw burial; LSB, Light straw burial; DSB, Dense straw burial.Data are Mean ± SE (= 3). At each period, the means are signi?cantly different among different AWD regimes (≤ 0.05) when followed by different uppercase letters above the bars. Under the same AWD regime, the means are signi?cantly different among different straw burial rates (≤ 0.05) when followed by different lowercase letters above the bars.

Under the same straw burial level, the aerenchyma percentage increased gradually, with increases in water supply during AWCD, AWSD and AWMD. For the same AWD regime, increasing the straw burial level signi?cantly enlarged the aerenchyma percentage, and the highest values were recognized in DSB, followed by LSB, whereas the lowest values were found in NSB. Due to the interaction, the AWMD × DSB treatment presented the greatest aerenchyma percentage. However, the AWCD × NSB treatment presented the lowest aerenchyma percentage. Moreover, the maximum aerenchyma percentage was recorded at 20 cm from the root tip (Fig. 1-D).

Plant height and tiller number increased by subsoil straw burial under AWD irrigation

Straw burial resulted in significant improvements of plant shoot physiological traits under all the AWD regimes after a range of growth periods after transplanting (Table 2). This positive effect was greater at a higher straw burial rate in the order of DSB > LSB > NSB. Under the same straw burial rate, decreasing the amount of water applied led to a reduction in plant height and the number of tillers per m2(Table 2). The AWMD × DSB treatment also showed the maximum plant height and the number of tillers. By contrast, the lowest plant height and the number of tillers were obtained by the AWCD × NSB treatment. Furthermore, the highest plant height was noted at 70 d after transplanting, whereas the maximum number of tillers was recorded in the AWMD × DSB treatment (Table 2).

Table 2. Effects of AWD regime and straw burial rate on plant height and tiller number at different treatments.

AWMD, Alternate wetting/moderate drying; AWSD, Alternate wetting/severe drying; AWCD, Alternate wetting/critical drying; NSB, Without straw burial; LSB, Light straw burial; DSB, Dense straw burial; AWD, Alternate wetting and drying; ns, Not significant.

The means followed by different uppercase letters are signi?cantly different among different AWD regimes (≤ 0.05). Under the same AWD regime, the means followed by different lowercase letters are signi?cantly different among different straw burial rates (≤ 0.05) according to the Duncan’s multiple range tests. *, ** and *** denote significant differences at the 0.05, 0.01 and 0.001 levels, respectively, among treatments.

Stem diameter and stem length increased by subsoil straw burial under AWD irrigation

The straw burial had a positive effect on the stem growth of rice (Table 3). The LSB and DSB treatments increased the stem diameter, stem length at different stem parts and total stem length compared with the NSB treatment under the same AWD regime. The highest values of the stem diameter, the stem length at different stem parts and the total stem length were also recorded with AWMD, followed by AWSD, whereas the lowest values were observed with AWCD (Table 3). Furthermore, the highest mean values of the stem diameter, stem length at the lower, middle and upper parts of the stem, and the total stem length were noted in the AWMD × DSB treatment. However, the lowest mean values of the stem diameter and stem length at the lower, middle and upper parts of the stem, and the total stem length were documented in the AWCD × NSB treatment (Table 3).

Leaf physiological traits improved by subsoil straw burial under AWD irrigation

Straw burial led to significant enhancement of plant shoot physiological traits under all the AWD regimes, mainly under the dense rate of straw burial (Fig. 2). Under the same straw burial rate, the AWD regimes presented significant effects on photosynthesis rate (Pn), stomatal conductance (gs) and relative water content (RWC) of the leaf, where these photosynthetic rates increased with the amount of water applied. The AWMD × DSB treatment showed the highest Pn, gs and RWC. At the other end of the range of these measurements, the AWCD × NSB treatment showed the lowest Pn, gs and RWC (Fig. 2-A to -C).

The contents of hydrogen peroxide (H2O2), superoxide anion radical (O2·?) and hydroxyl ion (OH?) in leaf tissues significantly respond to the variations in AWD regimes and straw burial rates at the different growth stages (Fig. 2-D to -F). The H2O2, O2·? and OH?contents in leaf tissues increased with the decrease of soil water content and straw burial rate. The highest H2O2, O2·? and OH?contents were detected at the AWCD × NSB treatment, whereas the lowest values were observed in the AWMD × DSB treatment. The maximum H2O2, O2·? and OH?contents in leaf tissues were observed with NSB when combined with the AWCD regime at the different growth stages (Fig. 2-D to -F).

Table 3. Effects of AWD regime and straw burial rate on stem diameter and stem length at different treatments.

AWMD, Alternate wetting/moderate drying; AWSD, Alternate wetting/severe drying; AWCD, Alternate wetting/critical drying; NSB, Without straw burial; LSB, Light straw burial; DSB, Dense straw burial; AWD, Alternate wetting and drying; ns, Not significant.

The lower stem part represents the third and the fourth internodes, the middle stem part represents the second internode, and the upper stem part represents the first internode. The means followed by different uppercase letters are signi?cantly different among different AWD regimes (≤ 0.05). Under the same AWD regime, the means followed by different lowercase letters are signi?cantly different among different straw burial rates (≤ 0.05) according to the Duncan’s multiple range tests. *, ** and *** denote significant differences at the 0.05, 0.01 and 0.001 levels, respectively, among treatments.

Fig. 2. Shoot physiological traits affected by straw burial rate and alternate wetting and drying (AWD) regime.

A?F, Photosynthesis rate (Pn) (A), stomatal conductance (gs) (B), relative water content (RWC) (C), hydrogen peroxide (H2O2) content (D), superoxide anion radical (O2·?) content (E), andhydroxyl ion (OH?) content (F) respond to the variations in AWD regimes and straw burial rates at the different growth stages. AWMD, Alternate wetting/moderate drying; AWSD, Alternate wetting/severe drying; AWCD, Alternate wetting/critical drying; NSB, Without straw burial; LSB, Light straw burial; DSB, Dense straw burial.

Data are Mean ± SE (= 3). At each period, the means are signi?cantly different among different AWD regimes (≤ 0.05) when followed by different uppercase letters above the bars. Under the same AWD regime, the means are signi?cantly different among different straw burial rates (≤ 0.05) when followed by different lowercase letters above the bars.

Soluble protein content and leaf nitrate reductase activity (LNR) improved by subsoil straw burial under AWD irrigation

Significant differences among treatments were observed for the soluble protein content at different periods of plant sampling (Table 4). For AWD regimes, the soluble protein content tended to increase with the soil water content, and the highest values were obtained in AWMD, followed by AWSD and AWCD. The straw burial rates showed similar trends, where the enhancement was toward the highest straw burial rate. LSB and DSB significantly improved the soluble protein content compared with NSB. Due to the interaction, the AWMD × DSB treatment led to the maximum soluble protein content. However, the lowest protein content was measured in the AWCD × NSB treatment. The soluble protein content gradually increased over time after transplanting. The maximum value was recorded at 90 d after transplanting under the AWMD × DSB treatment, whereas the minimum value was recorded at 30 d after transplanting under the AWCD × NSB treatment (Table 4). LNR increased significantly with the water supply and straw burial rate and peaked in the AWMD × DSB treatment at the flowering stage. However, the minimum activity was obtained in the AWCD × NSBtreatment at the post- flowering stage (Table 4). LNR increased with the straw burial application under all the AWD regimes during the reproductive stage up to the flowering stage; then, it decreased sharply at the maturity stage. At the same straw burial rate, LNR increased with the water supply. Under the same AWD regime, LNR increased with the straw burial rate (Table 4).

Superoxide dismutase (SOD), catalase (CAT) and glutaminesynthetase (GS) activities, and malondialdehyde (MDA) content enhanced by straw burial under AWD irrigation

Signi?cant differences of SOD and CAT activities in plants were found across AWD regimes (Fig. 3). For the same straw burial rate, the SOD and CAT activities increased gradually during AWCD, AWSD and AWMD. Under the same AWD regime, the SOD and CAT activities followed the same trend of increasing when the straw burial rate increased, where the highest SOD and CAT activities were recorded by the highest straw burial rate, DSB. Under the combination of AWD regime and straw burial rate, plants grown in the AWCD × NSB treatment exhibited the lowest activities of SOD and CAT. Conversely, the plants grown in the AWMD × DSB presented the greatest activities of SOD and CAT. Furthermore, the maximum activity levels were achieved at the milky stage for SOD and at the heading stage for CAT. However, the minimum activity levels for SOD and CAT were achieved at the tillering stage (Fig. 3-A and -B). The straw burial treatments under different AWD regimes had positive effects on the GS activity. LSB and DSB treatments increased the GS activity compared with the NSB treatment under the same AWD regime. The lowest GS activity was observed in AWCD, followed by AWSD and AWMD (Fig. 3-C). Under the integration of AWD irrigation and subsoil straw burial rate, plants grown in the AWCD × NSB treatment displayed the lowest GS activity. However, the plants grownin the AWMD × DSB treatmenthad thehighest GS activity (Fig. 3-C).

Fig. 3. SOD (A), CAT (B) and GS (C) activities, as well as MDA content (D), in leaf tissues affected by straw burial rate and alternate wetting and drying (AWD) regime.

AWMD, Alternate wetting/moderate drying; AWSD,Alternate wetting/severe drying; AWCD, Alternate wetting/critical drying; NSB, Without straw burial; LSB, Light straw burial; DSB, Dense straw burial; SOD,Superoxide dismutase; CAT, Catalase; GS, Glutaminesynthetase; MDA, Malondialdehyde.

Data are Mean ± SE (= 3). At each period, the means are signi?cantly different among different AWD regimes (≤ 0.05) when followed by different uppercase letters above the bars. Under the same AWD regime, the means are signi?cantly different among different straw burial rates (≤ 0.05) when followed by different lowercase letters above the bars.

Table 4. Effects of AWD regime and straw burial rate on soluble protein and nitrate reductase activity at different treatments.

AWMD, Alternate wetting/moderate drying; AWSD, Alternate wetting/severe drying; AWCD, Alternate wetting/critical drying; NSB, Without straw burial; LSB, Light straw burial; DSB, Dense straw burial; AWD, Alternate wetting and drying; DAT, Days after transplanting;ns, Not significant.

The means followed by different uppercase letters are signi?cantly different among different AWD regimes (≤ 0.05). Under the same AWD regime, the means followed by different lowercase letters are signi?cantly different among different straw burial rates (≤ 0.05) according to the Duncan’s multiple range tests. *, ** and *** denote significant differences at the 0.05, 0.01 and 0.001 levels, respectively, among treatments.

The MDA content at various rice growth stages was significantly reduced by straw burial treatments under different AWD regimes and their combinations (Fig. 3-D). Rice plants in the AWCD treatment showed higher MDA content than AWSD and AWMD under the same subsoil straw burial rate. The reduction in the MDA content was significant when the straw burial rate increased. Furthermore, under the combination of AWD irrigation and subsoil straw burial rate, plants grown in the AWCD × NSB treatment showed the highest MDA content. However, the plants grown in the AWMD × DSB treatment presented the lowest MDA content (Fig. 3-D). Furthermore, the lowest level of MDA was recorded at the milky stage.

Rice yield attributes and biomass production under straw burial and AWD irrigation

The effects of the AWD regime, straw burial rate and their interactions on the shoot development were significant (Fig. 4). The number of panicles per m2, panicle length, grain filling rate and grain yield of rice cultivated at the same straw burial rate significantly increased with water application increasing. Increasing the straw burial rates significantly improved the rice shoot development under the same water management option. The number of panicles per m2, panicle length and grain yield increased dramatically with straw burial rate increasing. Due to the interaction, the AWMD × DSB treatment resulted in the maximum number of panicles per m2, panicle length, grain filling rate, 1000-grain weight and grain yield. However, the lowest number of panicles per m2, the shortest panicle length, as well as the lowest grain filling rate, 1000- grain weight and grain yield were found in the AWCD × NSB treatment (Fig. 4-A to -E).

Fig. 4. Number of panicles per m2(A), panicle length (B), grain filling rate (C), 1000-grain weight (D), grain yield (E) and total biomass (F) as affected by straw burial rate and water regime.

AWMD, Alternate wetting/moderate drying; AWSD,Alternate wetting/severe drying; AWCD, Alternate wetting/critical drying; NSB, Without straw burial; LSB, Light straw burial; DSB, Dense straw burial; AWD, Alternate wetting and drying.

Data are Mean ± SE (= 3). The means are signi?cantly different among different AWD regimes (≤ 0.05) when followed by different uppercase letters above the bars. Under the same AWD regime, the means are signi?cantly different among different straw burial rates (≤ 0.05) when followed by different lowercase letters above the bars.

The total biomass production was found to vary depending on the AWD regime, straw burial rate and their interactions (Fig. 4-F). The average total dry weight increased with the water supply increasing under the same straw burial rate. Moreover, under the same water regime, the lowest total dry weight was in NSB, followed by LSB, whereas the highest was in DSB. The total dry mass increased significantly with an increasing soil water supply and soil straw burial rate, and peaked in the AWMD × DSB treatment. By contrast, the minimum values were obtained in the AWCD × NSB treatment (Fig. 4-F).

Gain quality of rice enhanced by straw burial under AWD irrigation

The brown rice rate, milled rice rate, head rice rate, amylose content, protein content and chalkiness degree were significantly affected by the AWD regime, straw burial rate and their interactions (Table 5). Under the same straw burial rate, AWMD increased the brown rice rate, milled rice rate, head rice rate, amylose content and protein content while reducing the chalkiness degree compared with AWSD and AWCD. Moreover, under the same AWD regime, the lowest brown rice rate, milled rice rate, head rice rate, amylose content and protein content were in NSB, followed by LSB, whereas the highest were in DSB. Due to the interaction, the AWMD × DSB treatment exhibited the highest brown rice rate, milled rice rate, head rice rate, amylose content and protein content and the lowest chalkiness degree. However, the AWCD × NSB treatment showed the lowest brown rice rate, milled rice rate, head rice rate, amylose content and protein content and the highest chalkiness degree.

Table 5. Effects of water regime and straw burial rate on grain quality indicators of rice at different treatments. %

AWMD, Alternate wetting/moderate drying; AWSD, Alternate wetting/severe drying; AWCD, Alternate wetting/critical drying; NSB, Without straw burial; LSB, Light straw burial; DSB, Dense straw burial; AWD, Alternate wetting and drying.

The means followed by different uppercase letters are signi?cantly different among different AWD regimes (≤ 0.05). Under the same AWD regime, the means followed by different lowercase letters are signi?cantly different among different straw burial rates (≤ 0.05) according to the Duncan’s multiple range tests. *, ** and *** denote significant differences at the 0.05, 0.01 and 0.001 levels, respectively, among treatments.

DISCUSSION

Chemical and environmental parameters of soil in root zone

Straw burial prolongs the water residence time in soil, thereby retaining more water in the topsoil layer (Li et al, 2020; Yang et al, 2020). It also increases the accumulation of soil organic carbon in soil (Yang H S et al, 2017; Jin et al, 2020). In this study, DSB under the AWMD regime decreased the soil pH and Eh while increasing the TN, EOC, TOC and DOC contents of soil, because the increase of soil water content increases the straw decomposition rate and the amount of total dissolved nutrients. Moreover, wheat straw is easy to degrade, which increases the TN, EOC, TOC and DOC contents. Thus, the role of soil water variation, Eh and pH in enhancing straw degradation and modifying the dynamics of nutrients in the soil can be used to improve rice production. Recent studies have reported that straw burial with deep tillage increases organic matter and water storage and improves soil fertility and crop productivity (Moreno-García et al, 2018; Yang et al, 2020; Zhai et al, 2021).

Root physiological traits

Roots developed under the AWMD × DSB treatment via creating large roots in contact with the soil by a large RAAA, indicating abundant water and nutrients available to roots. Similarly, the RAAA of rice relies on the variations in the soil water content and nutrient availability (Alhaj Hamoud et al, 2019b). Moreover, a direct relationship between root growth and rice yield has been revealed (Xu et al, 2018; Li et al, 2021). The water stress and nutrient deficiency under AWCD × NSB can restrict the production of vigorous root growth, and thus reduce RAAA, which was in agreement with the results of recent studies (Sheteiwy et al, 2019; Al Aasmi et al, 2022). Increasing straw burial rate improves the ability of the soil to absorb water and allows rice plants to develop a larger root system in connection to the soil. Thus, a superior RAAA improves the overall plant growth. Likewise, straw burial increases the root growth performance of many crops (Chen et al, 2019; Rasool et al, 2019; Li et al, 2020).

Rice roots generate aerenchyma under CF conditions, allowing the transport of O2from shoots to roots (Yamauchi et al, 2017). Aerenchyma in rice is further increased under anaerobic conditions (Abiko and Obara, 2014). In this study, under the AWMD × DSB treatment, rice transferred a large amount of O2from shoots to roots over aerenchyma, whereas under the AWCD × NSB treatment, rice transferred a small amount of O2over aerenchyma. Similarly, rice root growth and ROA affect the O2quantity supplied in the subsoil (Panda and Barik, 2021). Moreover, rice root traits and ROA are affected by the availability of nutrients under various water regimes (Xu et al, 2018). Thus, a greater ROA is essential to achieve a higher rice yield (Alhaj Hamoud et al, 2019b). The variation in the ROA under different straw burial rates indicated that roots developed in DSB received more O2from the shoots than the roots developed in LSB and NSB. The high ROA is attributed to the large roots with the great RAAA produced in soil under the DSB treatment, transferring a high amount of O2to shoots. Similarly, rice root growth is influenced by the availability of soil nutrients under different AWD regimes (Al Aasmi et al, 2022).

A greater RNR denotes adequate root N content compared with a lower RNR. A great root N content is also elucidated by a high available N in the root environment related to a high plant N uptake under a low pH in the root zone (Alhaj Hamoud et al, 2019b; Chen Y Y et al, 2021). Thus, a high RNR under the AWMD × DSB treatment revealed increasing N availability to the roots and high N source from roots to shoots. Conversely, the low RNR quantified reduced N accessibility and reduced N source from roots to shoots under the AWCD × NSB treatment.

Shoot physiological traits and biomass production

The rice growth under AWMD was greater than that under AWCD. The higher numbers of tillers and panicles of rice are achieved in the CF than those in the AWD conditions (Dou et al, 2016; Alhaj Hamoud et al, 2018), as the CF conditions enhance the availability of nutrients, whereas the AWD conditions reduce their availability (de Almeida Carmeis Filhoet al, 2017). Moreover, the physiological improvements of the plant shoots are closely associated with water and nutrient uptake ability, thereby, increasing the rice yield (Chen et al, 2020). DSB can provide favorable growth conditions, which increased the growth of tillers, stems and panicles, and thus the total biomass, compared with NSB. Consistently, increasing the straw burial rates increase the rice growth and productivity (Zhai et al, 2021). The modifications in the water and nutrient contents of soil affect the nutrient supply, roots, and shoot physiological traits (Chen et al, 2020; Li et al, 2021), and thus the rice yield (Chen L M et al, 2021). The decline in the vegetative growth of rice under the AWCD × NSB treatment was caused by the limited supply of water and nutrients within the root zone with the absence of the straw layer. Likewise, the unfavorable nutrient regime for plant nutrients is created under the AWD conditions (Sahrawat, 2012), whereas the rice yield is dependent on the numbers of tillers and panicles (Gouda et al, 2020). Thus, the rice yield decreases under the AWD conditions (Carrijo et al, 2017). Our study suggested that the combination of AWMD and DSB plays a key role in the high production of rice through efficient transport of nutrients from the greater roots to the larger shoots.

Nitrogen assimilation enzymes and antioxidant defense system

The AWMD × DSB treatment led to the highest soluble protein content and LNR among all the treatments (Table 4). These findings can be attributed to the fact that LNR and GS are the key enzymes for N assimilation in the plant cells that catalyze the first step in the reduction of nitrate N to organic forms within the plant. The LNR and GS enzymes play the main roles in controlling the amount of nitric oxide (Chamizo-Ampudia et al, 2017)and can catalyze the nitrate-to-nitrite reduction process in plants. Further, reductions of nitrate to nitrite and nitrite to ammonium by nitrite reductase require nicotine adenine dinucleotide phosphate (NADPH) and reduced ferredoxin as reducing energy, respectively. Moreover, ammonium assimilation into amino acids by GS also requires adenosine triphosphate and NADPH or reduced ferredoxin (Yanagisawa, 2014). This process can enhance sucrose phosphate synthetase activity and sustain carbon remobilization from vegetative tissues to sink organs and grain filling by enhancing the sink activity (Yang J C et al, 2017) via regulation of the key enzymes involved. Under the water stress conditions, the low leaf water content can decrease CO2assimilation, leading to the over-reduction of the photosynthetic electron chain due to the high production level of reactive oxygen species (ROS). Consequently, plants must adapt themselves to such conditions by scavenging ROS through up-regulation of antioxidant enzymes, such as SOD and CAT (Medina et al, 2021). In this study, the SOD and CAT activities were positively affected by the increase of the straw burial rate and negatively influenced by the water shortage. These findings can be attributed to the fact that the AWCD conditions cause damage to plant cells by inducing ROS production and disrupting the scavenging sequences that eliminate the ROS detrimental effects. Moreover, the activation of the antioxidant system in plants results in ameliorating the oxidative cell injury by reducing the MDA level in the plant cells (Sheteiwy et al, 2018).

Grain quality

Grain quality is evaluated by appearance, milling, cooking and nutritional quality indices (Luo et al, 2020). Yan et al (2018) indicated that straw return improves the nutritional quality of rice. Li et al (2021) found that the rice grain quality can be improved by increasing nitrogen availability. In this study, the AWMD × DSB treatment not only increased the grain protein and amylose contents but also reduced rice chalkiness, which is the opaque white part of rice and generated by the poor filling of amyloplast and proteasome in endosperm and the existence of air between them (Mitsui et al, 2016). Accordingly, high chalkiness in rice grains enhances the probability of breakage during milling, thereby decreasing the milling quality of rice (Kargbo et al, 2016), which was consistent with this study. The decrease in chalkiness could be attributed to the increase in the enzyme activities of the panicles, which promotes the synthesis of grain protein and improves the biological process of grain filling. Yan et al (2018) found that increasing the nitrogen availability increases the amylose and protein contents and reduces the chalky white grain ratio, which improves the rice grain quality. To support the obtained results of this study, we modeled the mechanism by which the integration of AWMD and straw burial can improve physiological growth trait, yield and grain quality of rice (Fig. 5).

The AWMD irrigation combined with DSB reduced the Eh and pH of the soil while increased the organic carbon and nitrogen contents of the soil. The AWMD irrigation combined with DSB also improved the plant physiological traits, because the antioxidant enzymes can achieve the balance for ROS release into the plant cells and thus reduced oxidative stress. In addition, CO2assimilation and the loss of the leaf water can be regulated by controlling the stomatal closure, which increased the photosynthesis process and thus increased the rice yield and grain quality. By contrast, performing AWCD irrigation with NSB led to a major reduction in the yield and grain quality of rice, because the antioxidant enzymes did not offer substantial protectionagainst lipid peroxidation. Moreover, nitrogen availability decreased due to increase in soil Eh and pH, as well as the inhibition of nitrogen assimilation and oxidation activity under critical drying conditions. Our study proposed that farmers should perform AWMD irrigation after leaving the straw residues of the prior rotation in the field, followed by deep tillage, which can improve the soil properties and growth, yield and grain quality of rice. The study also suggested that the combination of AWMD irrigation with straw burial should be adopted to solve the problems of straw residues and water scarcity without major yield loss for sustainable rice production.

Fig. 5. Comprehensive model showing mechanism by which integration of alternate wetting/moderate drying (AWMD) and subsoil straw burial can improve rice growth and grain yield.

A plus symbol above the arrows indicates an increase, whereas a minus symbol above the arrows indicates a reduction.

EOC, Easily oxidizable carbon; DOC, Dissolved organic carbon; TOC, Total organic carbon; Eh, Redox potential; NiR, Nitrite reductase; NR, Nitrate reductase; SOD, Superoxide dismutase; CAT, Catalase; RAAA, Root active adsorption area; RNR, Root nitrate reductase activity; ROA, Root oxidative activity; ROS, Reactive oxygen species; N, Nitrogen.

Table 6. Monthly mean humidity, temperature, sunshine and solar radiation throughout the season in Nanjing, China in 2020.

The weather data came from the Nanjing Metrological Bureau of China.

METHODS

Experimental site description

A field experiment was conducted from June to November 2020 at the experimental farm of Hohai University (35.31o N 113.87o E, 76 m above the sea level), Nanjing, China. The area has a humid subtropical climate. The absolute maximum temperature reached 43 oC in July, and the absolute minimum temperature was -17 oC in January. The average annual rainfall in the area was nearly 1 062 mm. The climate data during the year of the experiment are shown in Table 6. An area of 27 m2was selected for the trial, and the main properties of the soil before the experiment were as follows: clay, 27.8%; silt, 53.7%; sand, 18.5%; pH, 7.0; bulk density, 1.25 g/cm3; total porosity, 41.0%; total N, 1.3%; total P, 307.5 mg/kg; available N, 47.5 mg/kg; available P, 13.2 mg/kg; available K, 92.6 mg/kg; and total organic matter, 1.18%.

Experimental design and treatment application

The experiment was conducted in a split-plot design in a factorial experiment (two factors), with 3 replications and 27 experimental plots. The main treatment was the water regime, including three irrigation options managed as follows: flooding soil with 5 cm water depth once the soil water content reached 90%?100% of saturation (AWMD), flooding soil with 5 cm water depth once the soil water content reached 70%?80% of saturation (AWSD), and flooding soil with 5 cm water depth once the soil water content reached 60%?70% of saturation (AWCD). Soil water content was monitored by a time-domain re?ectometer (Mini Trace System-Soil Moisture Equipment Corporation, Santa Barbara, CA, USA), and water was pumped from the pond nearby and induced over pipes to the plots.AWD cycles were applied during the vegetative and reproductive stages, whereas the soil was naturally dried during the maturity period.

The subtreatment was the buried layer of wheat straw at three densities, namely, 0, 300 and 800 kg/hm2corresponding to NSB, LSB and DSB, respectively. The chemical compositions of straw were evaluated by the procedure suggested by Hames et al (2008). The straw organic matter was 95.35% of dry matter, which included 36% cellulose, 20.3% hemicellulose, 2.2% acid- soluble lignin, and 18.7% acid-insoluble lignin, and this structurewas in good agreement with previous assessment (Shaghaleh et al, 2019). The straw was chopped into 10 cm pieces, the soil was ploughed, and plots were prepared. Each plot measured 1 m width × 1 m length, was isolated by ridges covered with plastic film as barriers between the treatments.Burial ditches were arranged by plowing the soil at intervals of 40 cm distance. Afterward, a thickness of 5 cm of the straw layer was embedded at a depth of 40 cm, and the plowed soil was then re?lled and ?attened to a bulk density consistent with the initial value. Straw layers were buried in the soil at 10 d before transplanting.

Cropping and cultural practices

A local rice variety Nanjing 44 was used. Seedlings grown in a nursery for 30 d were then transplanted at a density of 20 hills/m2, with three plants per hill on June 2020. A space of 30 cm between plants was used. Fertilizers applied were urea (46% N) as N fertilizer, superphosphate (12% P2O5) as P fertilizer, and potassium chloride (60% K2O) as K fertilizer. Fertilizers were applied at rates of 250 kg/hm2N, 90 kg/hm2P, and 80 kg/hm2K, where 40% of N and the total P and K were supplied as a basal dose mixed with the soils. The remaining N was delivered as 20% at tillering, 20% at heading, and 20% at milky ripening. Plots were hand-weeded until canopy leaves were crowded. The final harvest was done in November 2020.

Measurement of chemical and environmental parameters of soil in root zone

The soil samples were collected from the root zone at the tillering, booting, heading and milky stages. Soil pH was measured in 1 : 5 (soil : water) extract using a calibrated pH meter. Soil Eh was determinedwith a redox potential meter (AZ8601, Shenzhen, China). A soil sample was directly digested in a mixture of selenium sulfate and salicylic acid by using a test tube heater (YKM-36, Shanghai, China) at 100 oC for 30 min and then increased to 380 oC for 3 h (Jackson, 2005).TN content in the digest was measured using a spectrophotometer (UV1901, Kejie, Nanjing, China) (Searle, 1984). One soil sample was situated in a fridge at 4 oC to quantify DOC content. Another soil sample was dried and sieved through 100 mm mesh screen to measure TOC and EOC contents. TOC content was measured by oxidation with potassium dichromate and titration with ferrous ammonium sulfate (Lu, 1999). DOC content was determined in 1.0 : 2.5 (soil : water) extract at 25.8 oC after shaking for 60 min and centrifuging at 4 500 r/min for 10 min (Jiang et al, 2006). EOC content was measured by reacting ground air-dried soil samples with 333 mmol/L of KMnO4through shaking at 60 r/min for 1 h. The suspension was then centrifuged at 2 000 r/min for 5 min. The supernatant was diluted and measured by the spectrophotometer at 565 nm (Blair et al, 1995).

Determination of root physiological traits

Soil root cores were collected at the tillering, heading and milky stages. Fresh roots were isolated by washing the soils. The RAAA of fresh samples was estimated by the methylene blue dyeing method (Zhang et al, 1994). The root RNR was determined by the assay mixture containing 1 g of fresh roots located in 25 mmol/L of K3PO4buffer (pH 7.5), 10 mmol/L of KNO3, 0.2 mmol/L of NADH, 5 mmol/L of NaHCO3and 5 μL of extract in a final volume of 0.5 mL. The reaction was terminated by adding 50 μL of 0.5 mol/L Zn(CH3COO)2, and excess NADH was oxidized by adding 50 μL of 0.15 mmol/L phenazine methosulphate. The mixture was centrifuged at 10 000 ×for 5 min. The NO2level was measured by merging 500 μL of supernatant with 250 μL of 1% sulfanilamide prepared in 1.5 mol/L of HCl and 250 μL of 0.02%-(1-naphthyl)ethylene- diamine dihydrochloride. The absorbance at 540 nm was read by the spectrophotometer. ROA was measured through the oxidation of alpha-naphthylamine (α-NA) (Zhang et al, 1994). In short, 1 g of fresh roots and 50 mL of 20 mg/L α-NA were incubated for 2 h in an end-over-end shaker. Then, 2 mL of the resultant ?ltered aliquots were mixed with 1 mL of sodium nitrate (1.18 mmol/L) and 1 mL of sulphanilic acid. The color produced was read at 530 nm by the spectrophotometer.

Root segments of 1-cm-long collected from 10, 20 and 30 cm behind the tip of the roots were embedded in 5% agar. Cross sections were prepared by cutting agarose blocks containing root segments. The achieved sections were treated with 92% lactic acid saturated with chloral hydrate for 1 h at 70 oC. Then, the sections were washed with water and viewed with a microscope (XSP-16A, Suzhou, China) ?tted with a digital camera linked to a computer. The percentage of each cross section engaged by aerenchyma was estimated using an Image J software (Ver.1.39u; NIH, Bethesda, MD, USA).

Determination of shoot morphophysiological performance

Three plants were selected from the middle of each plot for measurements. The plant height was measured from the soil surface to the highest blade tip before heading and from the soil surface to the top of the highest spike after heading. The number of tillers per m2was regularly recorded. The main stem was separated into four internodes (the first, second, third and fourth internodes). The length of each internode was measured by ruler reading, and the total stem length was then determined. The stem diameter was measured using a digital caliper (CD-20APX, Mitutoyo, Japan) at different internode positions in two (right-angled) directions: the long and short axes of an approximate ellipse.

The Pn and gs of leaves were determined at the tillering, heading and milky stages between 9:00 am and 11:00 am on a sunny day under the condition of an opened flow gas exchange system using a portable photosynthesis system (LI-6400XT, LI-COR, USA) (Alfadil et al, 2021).

Determination of soluble protein content in rice leaves

To determine the soluble protein, 0.5 mL of trichloroacetic acid (TCA)-homogenized leaf sample supernatant was mixed with 5 mL of C45H44O7S2Na. The absorbance of the supernatant was measured by the spectrophotometer at 595 nm. The content of protein was calculated according to Sheteiwy et al (2017).

Determination of antioxidant enzyme activities in rice leaves

A frozen flag leaf (0.5 g) was ground in pestle using liquid nitrogen and homogenated with 5 mL of 50 mmol/L PBS buffer comprised 1% polyvinylpyrrolidone and centrifuged at 12 000 ×for 15 min at 4 oC. The enzyme extract in the resulting supernatant was obtained using 1.5 mL of 50 mmol/L PBS, 0.3 mL of 130 mmol/L dl-methionine, and 0.3 mL of 100 mmol/L EDTA-Na2mixture. Subsequently, a mixture of 7.5 mL of pure water, 2.5 mL of EDTA-Na2, 0.3 mL of 750 mmol/L tetrazolium blue chloride, and 0.3 mL of 20 mmol/L riboflavin vitamin B2was added to 0.01 mL of the enzyme extract to start the photoreduction of nitro blue tetrazolium. The absorbance of the formed purple formazan was read at 560 nm, and the SOD activity was expressed by U/mg protein (Beauchamp and Fridovich, 1971). To measure the CAT activity, the enzyme extract was prepared by homogenizing fresh samples in 50 mmol/L of 0.5% Tris-NaOH, 2% PVP and 0.5 mmol/L EDTA and then centrifuged at 4 oC for 10 min at 22 000 ×. CAT assay was conducted by reading the absorbance at 240 nm of the mixture containing 50 mmol/L of potassium phosphate buffer and 250 μL of enzyme extract initiated by 60 mmol/L of H2O2(Sheteiwy et al, 2018). To measure GS activity, 0.5 g of the grounded frozen flag leaf using liquid nitrogen was extracted in a mixture of 1 mL of Tris-HCl, 1 mmol/L of EDTA, 15% of glycerol, 14 mmol/L of 2-mercaptoethanol, and 0.1% of Triton X-100 and then centrifuged. The GS activity was determined by measuring glutamylhydroxamate at 540 nm formed after the reaction with FeCl3(10%) : TCA (24%) : HCl (6 mol/L) at a ratio of 1 : 1 : 1 (Elliott, 1953).

For the LNR assay, the leaf samples were collected at heading [50 d after transplanting (DAT)], pre-flowering (70 DAT), flowering (80 DAT) and post-flowering (90 DAT). A 0.2 g of leaf sample was incubated 30 min in the dark at 30 oC in 9 mL of medium consisting of 0.1 mol/L of PBS, 0.2 mol/L of KNO3and 25% of isopropanol. The nitrite released was measured spectrophotometrically at 540 nm (Baki et al, 2000).

Determination of MDA and ROS contents

MDA content was measured according to Zhou and Leul (1999). Approximately 0.25 g frozen flag leaves were ground using liquid nitrogen, and then adding 5 mL of 10% TCA. The homogenate was centrifuged at 4 000 ×for 15 min at 4 oC, and 2 mL of supernatant was mixed with 2 mL of 0.67% thiobarbituric acid (TBA), incubated at 100 oC for 30 min. The samples were then centrifuged at 2 000 ×for 5 min at 4 oC. The absorbance of the homogenized supernatant was read by the spectrophotometer at 532, 600 and 450 nm. To determine H2O2content, 0.5 g leaves were homogenizedwith 5.0 mL of 0.1% TCA using an ice bath, and the homogenate was then centrifuged for 15 min at 12 000 ×(Velikova et al, 2000). The H2O2content in the supernatant was measured spectrophotometrically at 390 nm. O2·? content was measured according to Jiang and Zhang (2001). RWC and OH?contents were determined according to Sheteiwy et al (2019).

Determination of yield components and grain quality

At the harvest, total biomass and grain yield were measured. The panicle length was measured by a ruler, and the number of panicles per m2was calculated. Spikelets were dried at 80 oC for 48 h to determine the yield components. Harvested rice was air-dried and stored at room temperature for three months to measure the grain quality according to Pan et al (2013). The THU35C husker (Satake Manufacturing Co., Ltd., Suzhou, China) was used to measure the brown rice rate. The milled rice rate was measured using a rice grader (1-200tpd, Hubei Pinyang Technology, China). Broken rice was removed by a combination of 1.8 mm and 2.0 mm round hole sieves, weighing and determining the head rice rate. Chalkiness was determined by a rice appearance quality detector (Zhejiang Top Cloud-Agri Technology Co., LTD, Hangzhou, China). The grain was ground into ?our using a centrifuge mill (Retsch ZM200, Germany) with a mesh of 0.75 mm using a centrifuge speed of 10 000 r/min. The grain protein content was based on 13.5% moisture and was measured by a near-infrared re?ectance spectroscopy using an Infratec 1241 Grain Analyzer (FOSS, Hilleroed, Denmark). The grain amylose content was determined using an Amylose/Amylopectin Kit (Megazyme, Ireland), according to the manufacturer’s instructions.

Statistical analysis

The data were statistically analyzed using the IBM-SPSS statistical package (IBM SPSS Inc., USA). A general linear model procedure was used to perform the analysis of variance (two-way ANOVA). Whenvalues were significant (≤ 0.05), mean values were compared by applying the Duncan’s multiple range test at the 0.05 level of significance. All the obtained values are the means of three replicates.

ACKNOWLEDGEMENTS

This study was supported by the Water Conservancy Science and Technology Project of Jiangsu Province, China (Grant Nos. 2020049 and 2021055).

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http://dx.doi.org/

8 December 2021;

30 March 2022

Guo Xiangping (xpguo@hhu.edu.cn)

(Managing Editor: Wu Yawen)