Delays in sowing have significant effects on the grain yield, yield components, and grain protein concentrations of winter wheat.  However, little is known about how delayed sowing affects these characteristics at different positions in the wheat spikes.  In this study, the effects of sowing date were investigated in a winter wheat cultivar, Shannong 30, which was sown in 2019 and 2020 on October 8 (normal sowing) and October 22 (late sowing) under field conditions.  Delayed sowing increased the partitioning of ¹³C-assimilates to spikes, particularly to florets at the apical section of a spike and those occupying distal positions on the same spikelet.  Consequently, the increase in grain number was the greatest for the apical sections, followed by the basal and central sections.  No significant differences were observed between sowing dates in the superior grain number in the basal and central sections, while the number in apical sections was significantly different.  The number of inferior grains in each section also increased substantially in response to delayed sowing.  The average grain weights in all sections remained unchanged under delayed sowing because there were parallel increases in grain number and ¹³C-assimilate partitioning to grains at specific positions in the spikes.  Increases in grain number m^–2 resulted in reduced grain protein concentrations as the limited nitrogen supply was diluted into more grains.  Delayed sowing caused the greatest reduction in grain protein concentration in the basal sections, followed by the central and apical sections.  No significant differences in the reduction of the grain protein concentration were observed between the inferior and superior grains under delayed sowing.  In conclusion, a 2-week delay in sowing improved grain yield through increased grain number per spike, which originated principally from an increased grain number in the apical sections of spikes and in distal positions on the same spikelet.  However, grain protein concentrations declined in each section because of the increased grain number and reduced N uptake.

Delayed sowing mitigates lodging in wheat.  However, the mechanism underlying the enhanced lodging resistance in wheat has yet to be fully elucidated.  Field experiments were conducted to investigate the effects of sowing date on lignin and cellulose metabolism, stem morphological characteristics, lodging resistance, and grain yield.  Seeds of Tainong 18, a winter wheat variety, were sown on October 8 (normal sowing) and October 22 (late sowing) during both of the 2015–2016 and 2016–2017 growing seasons.  The results showed that late sowing enhanced the lodging resistance of wheat by improving the biosynthesis and accumulation of lignin and cellulose.  Under late sowing, the expression levels of key genes (TaPAL, TaCCR, TaCOMT, TaCAD, and TaCesA1, 3, 4, 7, and 8) and enzyme activities (TaPAL and  TaCAD) related to lignin and cellulose biosynthesis peaked 4–12 days earlier, and except for the TaPAL, TaCCR, and TaCesA1 genes and TaPAL, in most cases they were significantly higher than under normal sowing.  As a result, lignin and cellulose accumulated quickly during the stem elongation stage.  The mean and maximum accumulation rates of lignin and cellulose increased, the maximum accumulation contents of lignin and cellulose were higher, and the cellulose accumulation duration was prolonged.  Consequently, the lignin/cellulose ratio and lignin content were increased from 0 day and the cellulose content was increased from 11 days after jointing onward.  Our main finding is that the improved biosynthesis and accumulation of lignin and cellulose were responsible for increasing the stem-filling degree, breaking strength, and lodging resistance.  The major functional genes enhancing lodging resistance in wheat that are induced by delayed sowing need to be determined.

Understanding of how combinations of agronomic options can be used to improve the grain yield and nitrogen use efficiency (NUE) of winter wheat is limited.  A three-year experiment involving four integrated management strategies was conducted from 2013 to 2015 in Tai’an, Shandong Province, China, to evaluate changes in grain yield and NUE.  The integrated management treatments were as follows: current practice (T1); improvement of current practice (T2); high-yield management (T3), which aimed to maximize grain yield regardless of the cost of resource inputs; and integrated soil and crop system management (T4) with a higher seeding rate, delayed sowing date, and optimized nutrient management.  Seeding rates increased by 75 seeds m^–2 with each treatment from T1 (225 seeds m^–2) to T4 (450 seeds m^–2).  The sowing dates were delayed from T1 (5th Oct.) to T2 and T3 (8th Oct.), and to T4 treatment (12th Oct.).  T1, T2, T3, and T4 received 315, 210, 315, and 240 kg N ha^–1, 120, 90, 210 and 120 kg P₂O₅ ha^–1, 30, 75, 90, and 45 kg K₂O ha^–1, respectively.  The ratio of basal application to topdressing for T1, T2, T3, and T4 was 6:4, 5:5, 4:6, and 4:6, respectively, with the N topdressing applied at regreening for T1 and at jointing stage for T2, T3, and T4.  The P fertilizers in all treatments were applied as basal fertilizer.  The K fertilizer for T1 and T2 was applied as basal fertilizer while the ratio of basal application to topdressing (at jointing stage) of K fertilizer for both T3 and T4 was 6:4.  T1, T2, T3, and T4 were irrigated five, four, four and three times, respectively.  Treatment T3 produced the highest grain yield among all treatments over three years and the average yield was 9 277.96 kg ha^–1.  Grain yield averaged across three years with the T4 treatment (8 892.93 kg ha^–1) was 95.85% of that with T3 and was 21.72 and 6.10% higher than that with T1 (7 305.95 kg ha^–1) and T2 (8 381.41 kg ha^–1), respectively.  Treatment T2 produced the highest NUE of all the integrated treatments.  The NUE with T4 was 95.36% of that with T2 and was 51.91 and 25.62% higher than that with T1 and T3, respectively.  The N uptake efficiency (UPE) averaged across three years with T4 was 50.75 and 16.62% higher than that with T1 and T3, respectively.  The N utilization efficiency (UTE) averaged across three years with T4 was 7.74% higher than that with T3.  The increased UPE with T4 compared with T3 could be attributed mostly to the lower available N in T4, while the increased UTE with T4 was mainly due to the highest N harvest index and low grain N concentration, which consequently led to improved NUE.  The net profit for T4 was the highest among four treatments and was 174.94, 22.27, and 28.10% higher than that for T1, T2, and T3, respectively.  Therefore, the T4 treatment should be a recommendable management strategy to obtain high grain yield, high NUE, and high economic benefits in the target region, although further improvements of NUE are required.

The quality or structure of a wheat population is significantly affected by the compositions of tillers. Little has been known about the physiological basis for the differences of productive capacity among tillers. Two winter wheat cultivars, Shannong 15 (SN15) and Shannong 8355 (SN8355), were used to investigate the differences of productive capacity among tillers and analyze the physiological mechanisms that determine the superior tiller group. Low-position tillers (early initiated tillers) had a higher yield per spike than high-position tillers (late initiated tillers) in both cultivars, which was due to their more grain number per spike, more fertile spikelet per spike, less sterile spikelet per spike and higher grain weight. According to cluster analysis, tillers of SN15 were classified into 2 groups: superior tiller group including main stem (0), the first primary tiller (I) and the second primary tiller (II); and inferior tiller group including the third primary tiller (III) and the first secondary tiller (I-p). Tillers of SN8355 were classified into 3 groups: superior tiller group (0 and I), intermediate tiller group (II and III) and inferior tiller group (I-p). In comparison with other tiller groups, the superior tiller group had higher photosynthetic rate of flag leaves, higher antioxidant enzyme (SOD, POD and CAT) activities and lower levels of lipid peroxidation in leaves, higher grain filling rate in both superior and inferior grains during grain filling, higher single-stem biological yield and larger single-stem economic coefficient. Correlation analysis showed that yield per spike was positively and significantly correlated with the flag leaf photosynthetic rate, grain filling rate, the antioxidant enzyme activities and soluble protein content (except for SN15 at 5 days post-anthesis (DPA)) of flag leaf, the single-stem biological yield, and the single-stem economic coefficient. Remarkable negative correlation was also found between yield per spike and MDA content of flag leaf. These results suggested that superior tiller group had stronger leaf photosynthetic capacity, more predominance in terms of grain filling, slower senescence rate, higher biological yield and larger economic coefficient, and therefore, showed greater productive capacity than other tiller groups.