Reducing nitrogen (N) application can improve nitrogen use efficiency (NUE) and mitigate environmental pollution. However, this reduction often adversely affects photosynthesis and leads to inhibition of wheat growth. Thus, exploring effective agronomic measures to improve wheat’s tolerance to low N is essential for balancing the conflict between N reduction and wheat growth. Drought priming has been proven to enhance plant stress tolerance, but its role and mechanisms in improving photosynthetic capacity under low N stress remain unclear. The effects of drought priming on wheat growth and photosynthesis were investigated under low N stress using hydroponic experiments with two wheat varieties, YM158 (low N-tolerant) and YM25 (low N-sensitive). The results showed that low N stress significantly inhibited biomass accumulation, whereas drought priming effectively alleviated this growth inhibition. Drought priming significantly increased the net photosynthetic rate (Pn) of both cultivars compared to non-primed treatments under low N stress, which was primarily associated with enhanced Rubisco maximum carboxylation rate (Vcmax) and activities of Rubisco and Rubisco activase (RCA). Furthermore, drought priming promoted triose phosphate (TP) utilization and upregulated the expression of sugar transporter genes, which reduced sucrose accumulation in leaves and consequently alleviated its feedback inhibition on photosynthesis. In summary, drought priming enhances phloem loading-driven sucrose transport, reduces leaf sucrose accumulation, and improves Rubisco activation, collectively alleviating photosynthetic feedback inhibition and sustaining stronger photosynthetic capacity under low N stress.

Global climate warming is characterized by diurnal and seasonal asymmetry, with greater increases at nighttime and in winter and spring. Growing evidence has recognized that night-warming in winter and spring significantly impacts winter wheat production. Pre-crop straw returning is the principal method for straw utilization, but the interactions between straw returning and night-warming on wheat yield and N use efficiency (NUE) remain unclear. Here, a consecutive three-year field experiment with two straw treatments (S0, straw removal; S1, straw returning) and two warming treatments (W0, no warming control; W1, night-warming) found that both S1 and W1 improved wheat grain yield and NUE, with W1 exhibiting more pronounced improvements. Notably, the interaction between S1 and W1 (S1W1) further enhanced yield and NUE by 13.0 and 16.5%, respectively, compared to S0W0 through increasing grain number and 1,000-grain weight (threeyear average). Additionally, root growth and topsoil inorganic N content decreased in S1 before jointing, thereby reducing plant dry matter and N accumulation. However, W1 exhibited an opposite trend, thereby mitigating these negative effects. Simultaneously, under S1W1, increased N translocation to grain and post-anthesis dry matter accumulation, driven by greater N distribution to leaves and higher N metabolism enzyme activity, enhanced both yield and NUE. This improvement was supported by better root morphology and biomass, particularly in the 0-40 cm soil layer, boosting plant N absorption. Additionally, elevated soil N-acquiring enzyme activity after jointing increased the net N mineralization rate and microbial biomass N, enhancing soil N-supply capacity. As a result, post-jointing inorganic N content rose in the 0-20 cm layer while decreasing at 20-60 cm, thus reducing the apparent N surplus. Collectively, straw returning, night-warming, and their interactions enhanced root distribution and N-supply capacity after jointing in the topsoil layer, thereby increasing plant N uptake and its translocation to grains, along with post-anthesis dry matter accumulation, ultimately improving grain yield and NUE.

Optimizing source-sink dynamics is fundamental to understanding crop yield formation and achieving high productivity. This study aims to elucidate the physiological mechanisms by which source-sink traits influence wheat yield and to classify cultivars based on their limiting factors. Field experiments were conducted using three wheat cultivars (YM1, YM25, and ZM27) under two nitrogen (N) application rates to assess their source-sink characteristics. Compared with YM1, the cultivars YM25 and ZM27 exhibited significantly enhanced source capacity, characterized by larger leaf area, higher specific leaf weight (SLW), extended SPAD duration, and photosynthetic duration. Consequently, YM25 and ZM27 achieved significantly higher grain yields and sink capacities. The yield advantage was primarily driven by an increased number of kernels per spikelet and higher 1000-kernel weight (TKW), which resulted from a faster grain-filling rate, earlier onset of peak filling, and improved grain fullness. In contrast, YM1 displayed limited source strength (smaller leaf area, lower SLW) and poor sink activity, characterized by slower grain filling and reduced grain weight. Source-sink classification analysis revealed that YM25 and ZM27 are source-limited cultivars, possessing high source and sink strengths but a relatively high sink-source ratio. Conversely, YM1 is a sink-limited cultivar, exhibiting low source and sink strengths with a relatively low sink-source ratio. Furthermore, N fertilization effectively regulated source-sink relationships, specifically by mitigating source limitations in high-yielding cultivars. These findings provide a theoretical basis for cultivating high-yield wheat varieties by coordinating source-sink interactions.

Increasing spike number is essential for achieving high wheat yield under dense planting conditions. However, dense planting reduces the ratio of red to far-red light (R/FR) in the canopy, which inhibits productive tiller formation and spike development. Sufficient assimilate supply is essential for spike differentiation and seed setting, but the mechanism of low R/FR affects spike development remains unclear. In this study, a pot experiment was conducted with supplemental FR light from wheat stem elongation to heading to simulate a low R/FR environment and to analyze its impact on young spike differentiation and the physiology of leaves and stems. The results showed that low R/FR significantly reduced wheat yield, primarily due to a decrease in both spike number and grains per spike. The developmental failure of high-position tillers (tiller III) contributed to over 65% of the total yield loss. Under low R/FR, stem elongation and spike differentiation of tiller III were synchronously arrested, with most tillers aborting during the stamen and pistil primordium stage. Low R/FR treatment significantly reduced the net photosynthetic rate (Pn) of the fully expanded top leaves of the main stem and tillers, thereby decreasing the overall supply of assimilates. Furthermore, low R/FR inhibited the export of carbohydrates from leaves to stems, leading to a significant increase in soluble sugar content in leaves and a marked decline in stems. The limited assimilate supply intensifies nutrient competition among tillers, causing more carbohydrates to be allocated to low-position tillers and ultimately leading to the developmental failure of tiller III spikes. Hormonal analysis revealed that low R/FR significantly reduced cytokinin (CTK) levels in young spikes of tiller III by inhibiting its synthesis and promoting degradation, while simultaneously inducing abscisic acid (ABA) synthesis, thereby directly inhibiting the developmental progression of young spikes. In summary, low R/FR signaling alters the hormonal balance of CTK and ABA in the young spikes of high-position tillers, coordinately regulating the export and allocation of carbohydrates, ultimately leading to the termination of spike development in high-position tillers.

Although the source-sink relationship is an intrinsic factor affecting crop photosynthesis, the response of different source-sink types to nitrogen (N) availability in photoprotection and photosynthetic efficiency is still unclear. This study investigates the physiological mechanisms underlying differences in photosynthetic capacity and photoprotection in wheat with different source-sink relationships under varying N levels. Field experiments revealed that compared to sink-limited wheat (YM1), source-limited wheat (YM25 and ZM27) maintained higher chlorophyll content, maximum net photosynthetic rate (Pnmax), and light energy utilization efficiency, and alleviated photoinhibition by reducing non-photochemical quenching (NPQ) and increasing the proportion of cyclic electron flow (CEF). Additionally, source-limited wheat flag leaves had higher carotenoid content and soluble protein content and stronger antioxidant capacity, which enabled them to scavenge reactive oxygen species, reduce membrane lipid peroxidation, and delay leaf senescence. N fertilization significantly improved wheat’s photosynthetic capacity and light energy utilization efficiency, alleviating photoinhibition. Source-limited wheat can still maintain the integrity of the photosynthetic apparatus under low N conditions by enhancing photoprotective mechanisms, showing stronger environmental adaptability. Therefore, proper N fertilization and optimization of source-sink relationships help improve wheat’s photosynthetic capacity and yield potential.

Chromium (Cr)-contaminated in irrigation water poses a significant threat to the safety of wheat (Triticum aestivum L.) production safety. Recent studies suggest that melatonin (MT) could enhance crop tolerance to Cr pollution. This study aimed to investigate the effects of foliar spraying MT on alleviating Cr toxicity and accumulation in wheat irrigated with K2Cr2O7 solution at concentrations of 5, 10, and 20 mg/kg Cr in the soil. Our results showed that Cr-contaminated water irrigation significantly reduced dry weight, grain numbers, grain weight, yield, harvest index, net photosynthetic rate (Pn), maximum and actual photochemical efficiency of photosystem II (Fv/Fm and ΦPSII), chlorophyll contents, and the a/b ratio. It also increased PSII photodamage and oxidative stress in wheat leaves, resulting in high Cr accumulation in roots, leaves, and grains. Foliar spraying of MT alleviated Cr toxicity by improving Pn, Fv/Fm, and ΦPSII, enhancing chlorophyll content, promoting dry matter accumulation and yield, and reducing oxidative stress and Cr translocation. Furthermore, MT application enhanced transcriptional regulation, alleviated oxidative stress by boosting antioxidant enzyme activities, and restricted Cr translocation from roots to leaves and grains by increasing the accumulation of secondary metabolites, such as lignin and metallothionein. These findings suggest that MT application could serve as a viable strategy for reducing Cr contamination in cereals and supporting phytoremediation efforts.

Understanding the contributions of source-sink relationships to photosynthesis will help achieve high wheat grain yields. A single-factor field experiment was conducted to quantify the regulatory effects of different sink-source ratios on wheat photosynthetic characteristics, including two wheat cultivars with different source-sink relationships as materials for detailed source-sink manipulations through flag leaf removal (LR) and removal of spikelets on one side of each spike (SR). Compared with a control (CK), LR increased the sink-source ratio (23.84%) and significantly reduced the yield (16.17%), 1000-kernel weight (11.73%), and kernels per spike (7.33%). LR increased the leaves' net photosynthetic rate (Pn) (4.27-15.82%), the electron transfer rate (3.97-14.93%), and the Rubisco activity (2.16-12.25%) in the short term, and LR increased sucrose synthesis-related enzyme activities (3.96-19.95%) and gene expressions (SPS1, SUS1, CIN1, and SUT1). Compared with CK, SR reduced the sink-source ratio (44.12%) and significantly increased the 1000-kernel weight (10.02%) but reduced the yield (43.93%) and kernels per spike (49.31%). SR reduced the leaves' Pn (8.54-21.41%), the electron transfer rate (3.51-16.71%), and the Rubisco activity (5.96-21.51%), and the photosynthetic process was limited. SR decreased sucrose synthesis-related enzyme activities (5.12-29.09%) and gene expressions (SPS1, SUS1, CIN1, and SUT1). Therefore, a higher sink-source ratio is an important indicator of high photosynthetic efficiency, which can be used as a screening and judgment index in variety selection and cultivation regulation.

Late sowing is a critical factor that hinders achieving high-yield, good-quality wheat under rice-wheat rotation. Understanding the physiological basis and regulatory pathways that lead to high yield and sound quality late-sown wheat is crucial for developing effective cultivation strategies. A 2-year field experiment was conducted to investigate the effects of sowing date, nitrogen (N) application rate, and planting density on wheat yield, grain quality, population characteristics, and the underlying physiological factors. The results revealed significant interactions among the sowing date, planting density, and N application in regulating both yield and quality. Late sowing reduced grain yield primarily by reducing the number of spikes and kernels. However, the latter was improved by increasing N application and the planting density, thus mitigating the yield losses caused by late sowing. Moreover, the grain protein content (GPC) and wet gluten content (WGC) increased with delayed sowing dates and higher N rates but decreased with increased planting densities. For wheat yields over 9,000 or 7,500 kg ha-1, the latest sowing date should not be later than Nov. 4 or 15, respectively. In addition, specific criteria should be met, including a maximum of 1.5 and 1.0 million stems and tillers ha-1, a maximum leaf area index of 6.7 and 5.5, and a dry matter accumulation (DMA) at anthesis of 14,000 and 12,000 kg ha-1, respectively. For high-yield, good-quality late-sown wheat, the optimal combination is a 25% increase in the N rate (300 kg N ha-1) and a planting density of 2.25 million (N300D225) or 3.75 million (N300D375) plants ha-1 for 10-or 20-day delays in sowing, respectively. These combinations result in a higher leaf net photosynthetic rate, higher activities of leaf nitrate reductase, glutamine synthetase, grain glutamic pyruvic transaminase, and a lower sugar-N ratio during post-anthesis.

Increasing drought events pose a significant threat to crop yields. Drought priming has emerged as a promising technique for enhancing plant tolerance to post-anthesis drought stress; however, its effectiveness in combination with other interventions remains underexplored. A pot experiment was conducted to evaluate the effects of drought priming combined with additional potassium (K) application on wheat (Triticum aestivum L.) tolerance to post-anthesis drought stress. The study examined the influence of these treatments on gas exchange parameters, leaf moisture metrics, osmotic adjustment, anatomical traits, gene expression levels, and grain yield. The results indicated that a seven-day post-anthesis drought markedly reduced grain yield, primarily due to stomatal limitations on photosynthesis resulting from leaf water loss. Drought priming and K application improved wheat performance by enhancing osmotic adjustment (OA) and leaf hydraulic conductance (KL), respectively, although the underlying mechanisms differed. Both treatments promoted OA by increasing the accumulation of proline and glycine betaine through the upregulation of TaP5CS and TaBADH. Drought priming enhanced KL via the upregulation of AQP genes, whereas K application increased KL by enlarging the bundle sheath size of the minor veins. Moreover, the combination of drought priming and K application synergistically promoted OA and KL, thereby minimizing leaf water loss and reducing stomatal limitations on photosynthesis, which maximized grain yield under drought conditions. These findings demonstrate that additional K application following drought priming can significantly enhance priming-induced drought tolerance in wheat, suggesting that additional K after drought priming is a promising strategy for improving crop yield under water-limited conditions.

Context: The rise in winter and spring nighttime temperatures is a hallmark of global climate change, and warming has been proven to stimulate N2O emissions from wheat fields. However, it remains elusive whether this increasing effect is influenced by straw return, a practice considered globally as a future climate-smart agricultural strategy. Objectives or methods: A 3-year field experiment (2020-2023) was conducted with two straw treatments (S0: straw removal; S1: straw return) and two warming treatments (W0: no-warming; W1: night-warming) to quantify the effects of straw return and night-warming on N2O emissions from wheat fields in a rice-wheat rotation system. Results: Straw return (S1) boosted post-jointing N2O production, whereas night-warming (W1) stimulated N2O emissions before the booting stage. Notably, the interaction between straw return and night-warming significantly affected seasonal cumulative N2O emissions, with W1 causing an 11.1 % increase under S0 and a more substantial 18.1 % increase observed under S1. Moreover, both S1 and W1 increased N2O warming potential, yield-scaled, and biomass-scaled N2O emissions. Compared to S0W0, soil dissolved organic C and inorganic N content increased in S1W1, while pH declined. Both S1 and W1 enhanced soil nitrification enzyme activity, nitrate reductase activity, and nitrite reductase activity in comparison to their respective controls. Additionally, S1W1 increased N2O production and inhibited N2O reduction by upregulating AOB-amoA and nirS gene abundances and downregulating nosZ gene expression, as evidenced by the elevated (nirS+nirK)/nosZ ratio. Random forest analysis identified that denitrification enzyme activity was the most important factor influencing N2O emissions. Conclusions or implications: Our findings suggest that rice straw return may amplify the increasing effect of nightwarming on N2O emissions from wheat fields. From an environmental protection perspective, straw return under the context of future warming will lead to an increased risk of N2O emissions, which may further exacerbate climate warming.

Reducing nitrogen (N) application is crucial in addressing the low N utilization efficiency (NUE) and the risks of environmental pollution in wheat production. Improving low N (LN) tolerance in wheat can help balance the conflict between wheat growth and reduced N fertilization. Hydroponic experiments were conducted using Yangmai158 (LN-tolerant) and Zaoyangmai (LN-sensitive) cultivars to study whether LN priming (LNP) in the 3-leaf stage can improve the photosynthetic capacity of wheat seedlings under N-deficit stress at the 5-leaf stage. LNP increased the net photosynthetic rate (Pn), stomatal conductance (Gs), electron transfer rate (ETR), carboxylation efficiency (CE), maximum carboxylation rate (Vcmax), and the content and activity of Rubisco and Rubisco activase (RCA) in both cultivars, with Yangmai158 showing a greater increase than Zaoyangmai. After 14 days of N-deficit stress, the decreases in Pn, Gs, ETR, CE, Vcmax, and the content and activity of Rubisco and RCA of the two cultivars treated with LNP were significantly lower compared with those of the treatments without LNP. LNP improved the allocation proportion of leaf N to photosynthetic machinery, with the greatest increase in the carboxylation machinery. These results indicate that LNP can allocate more N to the photosynthetic apparatus, improving Rubisco content and activity to enhance the photosynthetic capacity and NUE of leaves under N-deficit stress.

The genetic diversity in tetraploid wheat provides a genetic pool for improving wheat productivity and environmental resilience. The tetraploid wheat had strong N uptake, translocation, and assimilation capacity under N deficit stress, thus alleviating growth inhibition and plant N loss to maintain healthy development and adapt to environments with low N inputs. Tetraploid wheat with a rich genetic variability provides an indispensable genetic pool for improving wheat yield. Mining the physiological mechanisms of tetraploid wheat in response to nitrogen (N) deficit stress is important for low-N-tolerant wheat breeding. In this study, we selected emmer wheat (Kronos, tetraploid), Yangmai 25 (YM25, hexaploid), and Chinese spring (CS, hexaploid) as materials. We investigated the differences in the response of root morphology, leaf and root N accumulation, N uptake, translocation, and assimilation-related enzymes and gene expression in wheat seedlings of different ploidy under N deficit stress through hydroponic experiments. The tetraploid wheat (Kronos) had stronger adaptability to N deficit stress than the hexaploid wheats (YM25, CS). Kronos had better root growth under low N stress, expanding the N uptake area and enhancing N uptake to maintain higher NO3− and soluble protein contents. Kronos exhibited high TaNRT1.1, TaNRT2.1, and TaNRT2.2 expression in roots, which promoted NO3− uptake, and high TaNRT1.5 and TaNRT1.8 expression in roots and leaves enhanced NO3− translocation to the aboveground. NR and GS activity in roots and leaves of Kronos was higher by increasing the expression of TANIA2, TAGS1, and TAGS2, which enhanced the reduction and assimilation of NO3− as well as the re-assimilation of photorespiratory-released NH4+. Overall, Kronos had strong N uptake, translocation, and assimilation capacity under N deficit stress, alleviating growth inhibition and plant N loss and thus maintaining a healthy development. This study reveals the physiological mechanisms of tetraploid wheat that improve nitrogen uptake and assimilation adaptation under low N stress, which will provide indispensable germplasm resources for elite low-N-tolerant wheat improvement and breeding.

Straw returning is an efficient straw usage strategy in rice-wheat rotation, but nitrogen (N) use efficiency (NUE) was decreased due to incorrect straw and N fertilizer managements. To investigate the effects of straw and N fertilizer management on root growth, N fertilizer fates, grain yield and NUE of wheat, a two-year field and micro-plot N-15-labelled experiment under three levels of N application rate (0, 180 and 240 kg N ha(-1)) with two basal N application stages [seeding (BN), and 3-leaf stage (TN)] and three straw treatments [no straw return (NS), straw return by rotary tillage (SR) and straw return by ploughing (SP)] was conducted. The results indicated that SP increased grain yield and NUE, and the increase was highest under TN180. SP increased N uptake by enhancing root extension and soil N supply capacity, and TN decreased N-15 residual in 60-100 cm soil layer. SP and TN180 both decreased N-15 fertilizer loss and increased N-15 recovery. Reducing basal N and applied at third-leaf stage (TN180) under SP had the same grain yield level as conventional N management (BN240) under NS, while highly improved NUE due to more root extension in deep soil layer and less N fertilizer loss.

Source-sink relationships influence photosynthesis. So far, the limiting factors for photosynthesis of wheat cultivars with different source-sink relationships have not been determined. We aimed to determine the variation patterns of photosynthetic characteristics of wheat cultivars with different source-sink relationships. In this study, two wheat cultivars with different source-sink relationships were selected for photosynthetic physiological analyses. The results showed that YM25 (source-limited cultivar) had higher photosynthetic efficiency compared to YM1 (sink-limited cultivar). This is mainly due to a stronger photochemical efficiency, electron transfer capacity, and Rubisco carboxylation capacity of YM25. YM25 accumulated less soluble carbohydrates in flag leaves than YM1. This is mainly due to the stronger sucrose synthesis and transport capacity of YM25 by presenting higher sucrose-related enzyme activities and gene expression. A PCA analysis showed that Rubisco was the main factor limiting the photosynthetic capacity of YM25. The soluble sugar accumulation in flag leaves and sink limitation decreased the photosynthetic activity of YM1. Increased N application improved source-sink relationships and increased grain yield and source leaf photosynthetic capacity in both two wheat cultivars. Taken together, our findings suggest that Rubisco and sucrose synthesis and translocation are involved in the regulation of photosynthesis of wheat cultivars with different source-sink relationships and that source and sink limitation effects should be considered in photosynthesis.

Although tetraploid wheat has rich genetic variability for cultivar improvement, its physiological mechanisms associated with photosynthetic productivity and resilience under nitrogen (N) deficit stress have not been investigated. In this study, we selected emmer wheat (Kronos, tetraploid), Yangmai 25 (YM25, hexaploid), and Chinese Spring (CS, hexaploid) as materials and investigated the differences in net photosynthetic rate (Pn), carboxylation capacity, electron transfer capacity, photosynthetic product output, and photosynthetic N allocation under normal N (CK) and low N (LN) through hydroponic experiments. Tetraploid emmer wheat (Kronos) had a stronger photosynthetic capacity than hexaploid wheat (YM25, CS) under low N stress, which mainly associated with the higher degree of PSII opening, electron transfer rate, Rubisco content and activity, ATP/ADP ratio, Rubisco activase (Rca) activity and Rubisco activation state, and more leaves N allocation to the photosynthetic apparatus, especially the proportion of N allocation to carboxylation under low N stress. Moreover, Kronos reduced the feedback inhibition of photosynthesis by sucrose accumulation through higher sucrose phosphate synthetase (SPS) activity and triose phosphate utilization rate (VTPU). Overall, Kronos could allocate more N to the photosynthetic components to improve Rubisco content and activity to maintain photosynthetic capacity under low N stress while enhancing triose phosphate output to reduce feedback inhibition of photosynthesis. This study reveals the physiological mechanisms of emmer wheat that maintain the photosynthetic capacity under low N stress, which will provide indispensable germplasm resources for elite low-N-tolerant wheat improvement and breeding.

Early-stage low nitrogen priming promotes root growth and delays leaf senescence through gene expression, enhancing nitrogen absorption and assimilation in wheat seedlings, thereby alleviating growth inhibition under nitrogen deficit stress and supporting normal seedling development. Verifying the strategies to reduce the amount of nitrogen (N) fertilizer while maintaining high crop yields is important for improving crop N use efficiency (NUE) and protecting the environment. To determine whether low N (LN) priming (LNP) can alleviate the impact of N-deficit stress on the growth of wheat seedlings and improve their tolerance to N-deficit stress, we conducted hydroponic experiments using two wheat cultivars, Yangmai 158 (YM158, LN tolerant) and Zaoyangmai (ZYM, LN sensitive) to study the effects of LNP on wheat seedlings under N-deficit stress. N-deficit stress decreased the plant dry weight, leaf area, and leaf N content (LNC), while LNP could significantly reduce this reduction. Distinct sensitivities to N-deficit stress were observed between the wheat cultivars, with ZYM showing an early decrease in leaf N content compared to YM158, which exhibited a late-stage reduction. LNP promoted root growth, expanded N uptake area, and upregulated the expression of TaNRT1.1, TaNRT2.1, and TaNRT2.2 in wheat seedlings, suggesting that LNP can enhance root N uptake capacity to increase N accumulation in plants. In addition, LNP improved the activity of glutamine synthase (GS) to enhance the capacity of N assimilation of plants. The relative expression of TaGS1 in the lower leaves of priming and stress (PS) was lower than that of no priming and stress (NS) after LNP, indicating that the rate of N transfer from the lower leaves to the upper leaves became slower after LNP, which alleviated the senescence of the lower leaves. The relative expression of TaGS2 was significantly increased, which might be related to the enhanced photorespiratory ammonia assimilation capacity after LNP, which reduced the N loss and maintained higher LNC. Therefore, LNP in the early stage can improve the N absorption and assimilation ability and maintain the normal N supply to alleviate the inhibition of N-deficit stress in wheat seedlings.

Although lipids are involved in plant adaptation to phosphorus (P) deficiency, the underlying relationships are not fully understood, especially in wheat. Thus, a comparative study of changes in leaf lipid content, composition and implicated gene expression during low-P was conducted hydroponically using two wheat cultivars differing in low-P tolerance. Low-P reduced phospholipid content but increased digalactosyldoacylglycerol (DGDG) and sulfoquinovosyldiacylglycerol (SQDG) content in older fully unfolded leaves (OL), with ND2419 (tolerant) showing greater changes than ZM366 (sensitive), suggesting a greater capacity to recycle internal P in ND2419. Interestingly, a novel lipid remodeling pattern was detected in the youngest unfolded leaf (YL) of ND2419, which reduced less in phospholipid content but accumulated more DGDG and SQDG with higher unsaturation levels than ZM366. These changes favored better chloroplast development, functionality and membrane integrity of ND2419, which together improved low-P tolerance. The expression of genes encoding glycolipid synthases and enzymes involved in phospholipid biosynthesis and degradation explained the lipid changes. Furthermore, our findings indicate that phospholipid loss in OL is primarily caused by enhanced degradation, whereas that in YL is driven by decreased biosynthesis and increased degradation. The detailed description of the changes in lipid profiles and gene expression under low P provides a basis for improving wheat low-P tolerance.

Increasing nitrogen (N) topdressing ratio may be an effective cultivation method to enhance wheat yield and N use efficiency (NUE) under global asymmetric warming due to the lower N demands of wheat at seedling stage. A two-year field experiment was conducted to investigate the effects of different basal and topdressing N ratios under winter and spring night-warming on grain yield and NUE of winter wheat. Increasing N topdressing ratio under winter and spring night-warming improved grain yield by enhancing grain number per spike and 1000-grain weight, and winter night-warming exhibited a greater increase than spring night-warming. Increasing N topdressing ratio under night-warming facilitated post-anthesis dry matter accumulation by enhancing post-anthesis net photosynthetic rate and chlorophyll fluorescence parameters of flag leaves, leading to improved grain yield. Moreover, increasing N topdressing ratio under night-warming enhanced the capacity of pre-anthesis N uptake and post-anthesis N metabolism, improved the post-jointing upper root growth, and post-anthesis soil inorganic N content while reduced the apparent N surplus, together distributed more N to leaves at anthesis to improve the NUE. Our findings suggest that increasing N topdressing ratio under winter and spring night-warming can improve grain yield and NUE caused by enhanced photosynthetic capacity, roots growth, and N uptake and assimilation.

Context: Improving the photosynthetic capacity of crops is key to achieving synergism between high crop yield and nitrogen (N) efficiency, and it is also an important target for future cultivar breeding and cultivation management. Although photosynthetic N use efficiency (PNUE) is commonly used to determine the N economy of leaves, it is unclear whether the PNUE of wheat has improved through genetic engineering of wheat cultivars. Objectives: The objectives of this study were to investigate the development of net photosynthetic rate (Pn) and PNUE, and to identify the key factors that limited Pn and PNUE in the tested cultivars. Methods: A 2-year field experiment with three different N fertilizer application rates using five wheat cultivars developed between 1950 s and 2010 s was conducted to examine the evolution and physiological processes of wheat Pn and PNUE. Results: Grain yield, N recovery efficiency (NRE), leaf Pn, leaf area, the content of N in leaves (LNC), and the content of N per unit area in leaves (NA) increased through genetic engineering of wheat cultivars. However, the increase in Pn was lower than that in NA, resulting in a decrease in PNUE. Modern cultivars had a higher Pn, which was attributed to the improvement of NA, which likely enhanced the maximum ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation rate (Vcmax), maximum electron transport rate (Jmax), electron transport rate from photosystem II (JF), and electron transport rate from the cytochrome b6f complex (Jc). The Rubisco content and activity of modern cultivars were higher than those of early cultivars, but the activation state decreased, which was related to the downregulation expression of TaRca1 alpha and TaRca2 beta-A and the decrease of the adenosine diphosphate/adenosine triphosphate (ADP/ATP) ratio in modern cultivars, resulting in decreased Rubisco activase (Rca) content and activity. Increasing N application increased NA and Pn but decreased the PNUE. N fertilizer had a greater effect on Rubisco content and PNUE than genetic engineering, while it had a smaller effect on NA, leaf mass per area (LMA), Rubisco activity, and Rca content and activity than genetic engineering. Conclusions: Our results suggest that lower Rubisco activation is a major reason for the lower PNUE of modern wheat cultivars, and improving Rca content and activity in the future could enhance Pn and PNUE while improving the N utilization of leaves to enhance crop yield and N efficiency of wheat under current or lower leaf N conditions.

Low temperature (LT) during germination of late sown wheat can severely inhibit germination and seedling establishment, and it’s important to explore approaches to improve LT tolerance in wheat germination. Yangmai16 (LT tolerant) and Xumai33 (LT sensitive) wheat cultivars were primed with 100 µmol/L melatonin (MT) to investigate the effects of exogenous MT on hormone and substance metabolism during seed germination under LT stress. LT delayed the germination time of both cultivars and reduced the vigor index (VI), but these changes were more pronounced in Xumai33 than in Yangmai16. MT improved VI of seeds of different cultivars, reduced germination time, and promoted radicle and coleoptile growth under the LT treatment. MT improved the amylase activity, soluble sugar, and free amino acid contents and up-regulated the relative expression of GA synthesis genes (TaGA20ox1 and TaGA3ox2) and ABA catabolic genes (TaCYP707A1 and TaCYP707A2) while down-regulated the relative expression of GA catabolic genes (TaGA2ox6 and TaGA2ox9) and ABA biosynthesis genes (TaNCED and TaABI3) leading to an increase in GA content and a decrease in ABA content to improve GA/ABA ratio under LT treatment. Moreover, Yangmai16 had stronger material mobilization and osmoregulation abilities than Xumai33 under LT stress. These results proved that MT priming could be a potential method to improve GA/ABA ratio, starch metabolism, and osmoregulation to enhance reserve mobilization and hormone metabolism to promote seed germination and seedling growth in wheat.