Introduction

Echium plantagineum L. is a species of the genus Echium of the family Boraginaceae1. It is now widespread in the Mediterranean mainland. It is also found in Australia, New Zealand, South Africa, southern South America and the western United States2. The seeds of E. plantagineum contain highly polyunsaturated oil (approximately 14% linoleic acid, 10% γ-linolenic acid, 33% α-linolenic acid and 14% stearidonic acid), and almost half of this fatty acid is omega-3 fatty acid which cannot be synthesised by the human body3. Omega-3 long chain polyunsaturated fatty acids have certain beneficial effects on inflammatory and autoimmune diseases such as atherosclerosis, cancer, rheumatoid arthritis, asthma, and Alzheimer’s disease4. The United States Food and Drug Administration approved the use of E. plantagineum seed oil as a dietary ingredient in 2002, while the European Union classified it as a new type of food in 2008, allowing it to be added to various types of food5. As the value of echium oil in food, medical, and cosmetic products has been gradually developed, the corresponding market demand has increased6.

Photosynthetic efficiency plays an extremely important role in the yield formation of E. plantagineum. It not only affects the vegetative and reproductive growth of E. plantagineum, but also determines the filling rate and dry matter accumulation at the late grain stage. Therefore, understanding and improving the photosynthetic efficiency of the plant was of great importance for increasing the productivity and grain yield of E. plantagineum. Different sowing dates not only cause differences in ecological conditions such as photothermal resources during plant growth and development, but also alter photosynthesis and nutrient distribution during growth and development, thus affecting plant dry matter accumulation and yield7. Therefore, in order to make more effective use of local photothermal resources, improve light energy utilisation by E. plantagineum, and promote its growth and yield, it is of great practical significance to further strengthen the study of the effect of sowing date on the photosynthetic physiology and ecology of E. plantagineum. 90–95% of plant dry matter is derived directly or indirectly from photosynthesis8. Canopy light interception is the basis of plant photosynthesis. Canopy light interception and transmittance are important determinants of crop dry matter accumulation and subsequent yield9. Chlorophyll is the material basis of plant photosynthesis, and its content is closely related to the photosynthetic efficiency of crops10. The kinetic characteristic of chlorophyll fluorescence can directly reflect the photosynthetic performance of plants, and compared with the traditional “apparent” gas exchange index, the chlorophyll fluorescence parameters can better reflect the “intrinsic” characteristics of plant photosynthesis and can quickly, sensitively and non-destructively analyse the potential mechanism of environmental factor on photosynthesis11, 12. Photosynthesis is the material basis for the formation of biological yield and economic yield of plants, and improving the photosynthetic characteristics of plants is an important aspect to fully exploit the yield of plants13.

At present, the research on E. plantagineum mainly focuses on the development of its molecular substances and co-products such as honey, bee pollen, seed oil, shikonin and so on6, but there is little research on its cultivar. Different sowing dates will result in different environmental factors around the crop. By changing the sowing date, the utilisation rate and competition for natural climatic resources such as gas, water, heat and light of E. plantagineum can be adjusted, and its photosynthetic performance or yield composition can be improved to some extent, thus increasing its yield. However, the plant characteristics of different varieties of E. plantagineum are different. Therefore, regulating environmental factors by changing the sowing date has a good research prospect for improving flowering and fruiting and increasing seed yield of different varieties of E. plantagineum. In this experiment, the effects of sowing date on canopy light interception, chlorophyll content, photosynthesis, chlorophyll fluorescence and yield of E. plantagineum, in order to provide theoretical help and technical support for plantain production in this area.

Materials and methods

General situation of experimental site

The field trials were conducted in 2021 and 2022 in two growing seasons, and the experimental sites were located at the experimental base of Jilin Agricultural University in Changchun (43°53′N, 125°10′E), Jilin Province. The soil is a black loam with an obvious aggregate structure on the surface, rich in organic matter and high water-holding capacity, mainly containing montmorillonite and mica minerals. The basic fertility of the experimental field is as follows: organic matter 23.5 mg/kg, available phosphorus 24.3 mg/kg, available potassium 111.9 mg/kg, alkali-hydrolysable nitrogen 176.25 mg/kg, total nitrogen 1.645 g/kg, and soil pH value about 6.3. The temperature data in the experimental field of E. plantagineum during the whole growth period are shown in the following figures (the data corresponding to spring and autumn in the figure are in May and September, respectively). It can be seen that the phenomenon of short-term high temperatures often occurs in May in spring. Since the climatic difference during the growth period of two years was not significant, the experimental data of two years were averaged and analysed (Fig. 1).

Figure 1
figure 1

Historical temperature in growth period (2021 on the left and 2022 on the right). The daily range of reported temperatures (indicated by gray bars) and 24-h high (red checkmark) and low temperature (blue check) are placed above the daily average high (light red line) and low temperature (light blue line) and have 25–75% and 10–90% percentile bands. [Data quoted from: Weather Spark, website: https://zh.weatherspark.com].

Full size image

Experimental materials, design, and crop management

The seeds of E. plantagineum were obtained from a germplasm collection held at the School of Agriculture, Jilin Agricultural University, and the original collection was collected in England. Two cultivars of E. plantagineum were used in the study: Blue Bedder, which is characterised by slower growth and dense plant habit, and Mixed Bedding, which grows faster and has relatively few branches14. As there is little early spring rainfall in this area, soil moisture is insufficient before May, making it unsuitable for planting. In previous years, the local sowing date was around the beginning of May, so the sowing dates were split into 1 May (T1), 8 May (T2), 15 May (T3), 22 May (T4) and 29 May (T5). The experiment was designed using a random block of variety and sowing date, with a factor of 2*5, a total of 10 treatment groups, each treatment containing three replicates. Ridge width 65 cm, ridge height 10 cm. Each replicate consists of three ridges, each ridge is 5 m long, a corridor of 1 m is reserved, and the planting distance for hole sowing is 30 cm. Three for each acupoint. Test field peripheral protection row 1. Thinning is carried out after the seedlings have grown and one plant is reserved for each acupoint. Normal field management during the growing season. To ensure that the sowing date corresponds to the seed quality of E. plantagineum, the seeds of the lower third of the anterior flowers on the branches of the cyme are black (dry and fully mature), the seeds of the middle third are grey (physiologically mature), and the seeds of the upper third are green or the flowers are still mature for harvesting15.

Measurement items and methods

The phenological period was subdivided and the criterion for subdivision was to reach more than 70% of the plants in each plot, and the corresponding indices were measured in each phenological period. Seedling stage: unfolding of the cotyledon to the eighth true leaf. Rosette stage: Eight true leaves spread out to the main stem/lateral branches for 2 cm. Extensional branching stage: Main stem/lateral branches extend 2 cm to 45% of the plant buds. Flowering stage: 45% of the buds are now harvested.

Canopy light interception

Clear and cloudless weather was selected during the extensional branching and flowering stages of E. plantagineum. Canopy photosynthetic active radiation (PAR) was measured at 11:30–14:30 using a canopy analyser. PAR interception rate (CaR) and transmittance (PeR) at different levels were calculated according to the following formula16.

$${\text{CaR}} = \left( {{\text{PAR}}_{{\text{n}}} {\text{ - PAR}}_{{\text{n - 1}}} } \right)/{\text{PAR}}_{{\text{T}}} \times 100\% ,$$
$${\text{PeR}} = {\text{PAR}}_{{{\text{n}} - {1}}} /{\text{PAR}}_{{\text{n}}} \times {1}00\% .$$

In the formula: PART denotes photosynthetic active radiation under natural light; N respectively denotes h (the height from the ground to the top of E. plantagineum) and 1/2 h, respectively; N-1 denotes 1/2 h and 0 h. When calculating the total CaR and total PeR of the canopy, n and n-1 indicated h and 0 h9.

Relative chlorophyll content

A hand-held SPAD-502 chlorophyll meter (manufactured in Japan)17 was used to the SPAD value of chlorophyll of E. plantagineum at seedling, rosette, branching and flowering stages. Three plants with uniform growth were selected from each plot, and the functional leaves with complete development in the middle canopy in a uniform light receiving direction were selected for measurement from each plant. The average of three measurement points on each leaf was taken, and the data were measured when the plants entered the corresponding phenology under the latest sowing date.

Photosynthetic characteristics

The photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), ambient CO2 concentration (Ca) and intercellular CO2 concentration (Ci) of the leaves were measured from 10:00 to 12:00 on a clear and windless day at the mid-flowering stage using a LI-6400 portable photosynthetic apparatus. The water use efficiency (WUE) was calculated as Pn/Tr18 and the stomatal limit value (Ls) was calculated as 1-Ci/Ca19. For the determination, three plants of similar plant size were selected from each cell except for the edge, and the fully developed functional leaves in the middle canopy with the same light receiving direction were selected for each plant for the determination20.

Chlorophyll fluorescence

Chlorophyll fluorescence parameters of the leaves at the mid-flowering stage of E. plantagineum were measured using a model PAM-2500 fluorometer (WALZ, Germany). Three plants of similar size, except for the margin, and the fourth fully expanded branch and leaf (inverted four-leaf) on the upper part of the main stem were selected. Dark adaptation was performed for 20 min, followed by light adaptation. The fluorescence parameters measured included initial fluorescence (Fo), maximum fluorescence (Fm), maximum photochemical efficiency (Fv/Fm), actual photochemical chemical efficiency (ΦpsII), and electron transfer rate (ETR)21.

Yield

At the late stage of maturation, the complete plants of three E. plantagineum plants from each community were selected and brought back to the laboratory. The E. plantagineum seeds in the collected samples were dried in the sun, threshed and weighed to obtain the economic yield per plant and calculate the yield22. The seeds were then evenly mixed and three 500-grain weights were randomly selected to calculate the 1000-grain weight.

Statistical analysis

Microsoft Excel2016 was used for basic data processing, spss was used for analysis of variance and correlation, and origin 2018 was used for mapping.

Ethical approval

Experimental and field studies, including the collection of plant material (seeds), of E. plantagineum were conducted in accordance with relevant institutional, national and international guidelines and legislation. This experiment has been licensed for the collection of E. plantagineum and the corresponding scientific research.

Results and analysis

The plant canopy light interception

Photosynthetic Active Radiation (PAR) is the amount of solar radiation that can be used by green plants for photosynthesis. Under the same solar radiation conditions, the amount of photosynthetic active radiation (PAR) directly reflects the light absorption and utilisation of plants. CaR and PeR are important indicators of light interception in the crop canopy. As shown in Table 1, the total canopy CaR of the two E. plantagineum cultivars gradually increased with advancing growth period, and overall, the total canopy CaR of Blue Bedder was slightly higher than that of Mixed Bedding in each growth period, and the change in total canopy PeR was opposite. For Blue Bedder, the CaR of the total canopy in the extensional branching stage and that of the upper canopy in the flowering stage both tended to decrease with delay in sowing date, but the CaR of the total canopy in the flowering stage first increased and then decreased, and the interception rate reached its maximum at sowing date T2. The CaR and PeR of the lower canopy in the flowering stage of Blue Bedder were the lowest at T1 sowing date and the highest at T5 sowing date. Total canopy CaR and the CaR and PeR of the upper canopy in flowering stage of Blue Bedder were significantly different at T1 sowing time and T5 sowing dates, but the CaR and PeR of the lower canopy in flowering stage were not significantly different at each sowing date. There was no significant difference in the PeR of the lower canopy at flowering stage and the upper canopy at flowering stage among the treatments of each sowing date for Mixed Bedding. With the delay in sowing date, the CaR of total canopy of Mixed Bedding was T1 > T3 > T2 > T4 > T5 in the extensional branching stage, but it presented as T2 > T3 > T1 > T4 > T5 in the flowering stage, and the change in PeR was opposite to that of CaR. In the T2 and T3 sowing date treatments, the total canopy CaR of the Mixed Bedding at flowering stage was significantly different from that in the T5 treatment. From the above analysis, it could be concluded that delayed sowing would reduce the total canopy CaR of E. plantagineum to some extent and increase the total canopy PeR, and there was a certain difference in the total canopy CaR and PeR between the two cultivars. Sowing date and variety had significant effects on total canopy CaR and PeR, but their interaction had no significant effect.

Table 1 Effects of different treatments on interception rate and transmittance of photosynthetic active radiation in different canopy layers (2020–2022).
Full size table

The chlorophyll content (SPAD value)

Table 2 shows that the sowing date had a significant effect on the chlorophyll content of E. plantagineum at the seedling, rosette and extensional branching stages. Except for the flowering stage of Mixed Bedding, the chlorophyll content of E. plantagineum at different growth stages of the two cultivars gradually decreased with the delay in sowing date, reaching the maximum at sowing date T1 and the minimum at sowing date T5. In the flowering stage, the chlorophyll content of Mixed Bedding showed T1 < T3 < T2 < T5 < T4, and reached the maximum at T4 sowing date, which was significantly different from T1. The chlorophyll of Blue Bedder was significantly different from that of T4 and T5 at seedling, rosette, branching and flowering stages and at T1 sowing date. The chlorophyll of the T1 plant of Mixed Bedding was significantly different from those of T2, T3, T4 and T5 at the seedling and flowering stages. Therefore, it could be concluded that delayed sowing was not conducive to improving the chlorophyll content of plants of Blue Bedder and Mixed Bedding at any growth stage, but moderate late sowing of Mixed Bedding at T4 stage was conducive to improving the chlorophyll content of E. plantagineum at flowering stages. The variety had a significant effect on the chlorophyll at the rosette stage and the flowering stage, and the interaction of sowing date and variety had a significant effect on the chlorophyll content at the flowering stage of E. plantagineum.

Table 2 Effect of sowing date on chlorophyll content of different E. plantagineum varieties at different growth stages. (SPAD value) (2020–2022).
Full size table

The leaf photosynthetic characteristics

As shown in Table 3, the differences in leaf Tr and WUE of the two varieties at each sowing date were not significant. With the delay in sowing date, leaf Pn, Tr and Ls of Blue Bedder tended to decrease, while Ci and Gs tended to increase, reaching their maximum and minimum values respectively at sowing date T1. There were significant differences in Pn, Ci and Ls of Blue Bedder with those of T1, T2 and T3 at sowing date T5. With the delay in sowing date, the leaf Pn and Tr of Mixed Bedding first increased and then decreased, then increased and then decreased, reaching the maximum at sowing date T4. At sowing date T1, there were significant differences in leaf Pn, Ci and Ls of Mixed Bedding and those of T2, T3, T4 and T5. Therefore, it could be concluded that normal sowing of Blue Bedder and moderately late sowing of Mixed Bedding were conducive to improving the photosynthetic characteristics of the corresponding E. plantagineum at the flowering stage. In addition, sowing date had significant effects on leaf Pn, Ci and Ls, and there were significant differences in Pn, Ci, Tr and Ls between the two cultivars. The effect of cultivar on photosynthetic characteristics was greater than the difference at sowing.

Table 3 Effects of sowing date on photosynthetic characteristics of different E. plantagineum varieties at flowering stage. (2020–2022).
Full size table

The chlorophyll fluorescence

As shown in Table 4, Fo, Fv/Fm, qP, ETR and ΦpsII showed significant differences between the two varieties, and the sowing date had no significant effect on chlorophyll fluorescence parameters. The interaction between sowing date and varieties had significant effects on qP, and the influence of varieties on chlorophyll fluorescence parameters was greater than that of sowing date. Fv/Fm, ETR and ΦpsII of Blue Bedder were significantly higher than those of Mixed Bedding, while Fo, Fm, Fv/Fm, ETR and ΦpsII of Blue Bedder and Mixed Bedding did not differ significantly from those of their respective varieties at sowing date. With a delay in sowing date, Fo and Fm of Blue Bedder first increased and then decreased, and reaching a maximum at T3, while NPQ showed the opposite. QP, ETR and ΦpsII of Blue Bedder decreased with the sowing date, reaching their maximum values at sowing date T1. The Fo, Fm, qP, ΦpsII and ETR of Mixed Bedding initially increased, then decreased, then increased and then decreased with the delay in sowing date, reaching the maximum at sowing date T4, but the corresponding NPQ value was the minimum at that time.

Table 4 Effects of sowing date on chlorophyll fluorescence parameters of different E. plantagineum varieties at flowering stage (2020–2022).
Full size table

The grain yield

Table 5 shows that the variety and sowing date have significant effects on 1000 grain weight and grain yield, and the interaction between variety and sowing date has a significant effect on grain yield. From Fig. 2 it can be seen that the yield of Blue Bedder was significantly higher than that of Mixed Bedding at each sowing date. The thousand grain weight of Mixed Bedding at T1 was higher than that of Blue Bedder, and the thousand grain weight of Blue Bedder at other sowing dates was significantly higher than that of Mixed Bedding. As the sowing date was delayed, the 1000 grain weight of Blue Bedder increased and the yield decreased, reaching a maximum of 996.72 kg/hm2 at T1. The 1000 grain weight of Blue Bedder at sowing dates T3, T4 and T5 was significantly different from that at T1 and T2 treatments, while the yield at sowing date T1 was significantly different from that at T2, T3, T4 and T5 treatments. The delayed 1000 grain weight of Mixed Bedding first decreased and then increased with the sowing date, and the minimum 1000 grain weight was 2.68 g at sowing date T2. In addition, the grain yield of Mixed Bedding in the different sowing date treatments was T2 < T1 < T3 < T5 < T4, the minimum yield at T2 was 344.4 kg/hm2, and the maximum yield at T4 was 489.17 kg/hm2. The 1000 grain weight of Mixed Bedding at T5 was significantly different from T1, T2 and T3 treatments, while the yield at T4 was significantly different from T1 and T2 treatments. The maximum yield of Blue Bedder was significantly different from that of Mixed Bedding, and the maximum yield of Blue Bedder was 203.76% higher than that of Mixed Bedding. From the above analysis, it could be concluded that delaying the sowing date could significantly increase the 1000 grain weight of E. plantagineum, while Mixed Bedding at T4 sowing date and Blue Bedder at T1 sowing date contributed to the significant increase in grain yield, and Blue Bedder had yield advantages over Mixed Bedding.

Table 5 Analysis of variance of 1000-seeds weight and grain yield (F value) (2020–2022).
Full size table
Figure 2
figure 2

Effects of sowing date on 1000-grain weight and grain yield of different E. plantagineum Varieties (2020–2022). Histograms capped with different letters indicate significant difference (P < 0.05).

Full size image

The correlation analysis

As shown in Table 6, the total canopy CaR, SPAD, Pn, Tr, Ls, Fv/Fm, qP, ETR and ΦpsII of E. plantagineum at the flowering stage had a linear positive correlation with yield, while the total canopy PeR, Ci and Fo had a significant negative correlation with yield. The decision coefficients R2 of leaves SPAD, Pn, Ci, Tr, Ls, ETR and ΦpsII were greater than 0.7, which proved the improvement of the photosynthetic characteristics of the plant and had a positive effect on the improvement of the crop yield. Table 7 shows that the varieties had a significant positive correlation with 1000 grain weight and qP, an extremely significant positive correlation with yield, SPAD, Pn, Tr, Ls, Fv/Fm, ETR, ΦpsII and total canopy CaR at the flowering stage, and an extremely significant negative correlation with Ci, Fo and total canopy PeR at the flowering stage. Sowing date had a very significant positive correlation with 1000 grain weight and total canopy PeR at flowering stage, and a very significant negative correlation with total canopy CaR at flowering stage. Total canopy CaR at flowering stage had a very significant positive correlation with Pn, Ls, Fv/Fm, ETR, ΦpsII, SPAD and yield, a significant positive correlation with SPAD, a very significant negative correlation with Ci, and a significant negative correlation with Fo, while the correlation with the total canopy PeR at flowering stage was the opposite. This indicated that sowing date had a direct effect on canopy structure and thus indirectly affect photosynthesis, fluorescence characteristics and grain yield.

Table 6 Effects of photosynthetic characteristics and fluorescence characteristics of leaves at flowering stage on the yield of E. plantagineum.
Full size table
Table 7 Correlation between sowing date and photosynthesis, chlorophyll fluorescence parameters and yield characteristics of E. plantagineum at flowering stage.
Full size table

Discussions

The yield potential of E. plantagineum depends on the amount of photosynthate accumulated during the growing season or on the proportion of the total biomass allocated to the grain at harvest. CaR and PeR in the canopy are important factors that determine the dry matter accumulation and subsequent crop yield9. The results of this study showed that under different sowing dates and varieties, the changing trends of CaR and PeR were opposite. As the growth period progressed, the total canopy CaR increased, and the upper canopy CaR at the flowering stage was much higher than that of the lower canopy, while the total canopy CaR of Blue Bedder was relatively higher than that of Mixed Bedding. This was consistent with the fact that Blue Bedder was more densely branched than Mixed Bedding23. The results showed that sowing date and variety had significant effects on total canopy CaR and PeR, but their interaction had no significant effect.

Chlorophyll is the material basis of photosynthesis in plants24. The results of this experiment showed that the chlorophyll content of the main functional leaves of E. plantagineum was the highest during the flowering stage. Except for the flowering stage of Mixed Bedding, the chlorophyll content of leaves of E. plantagineum at each growth stage decreased with the delay in sowing date. The chlorophyll value before late sowing is relatively low, mainly because the corresponding functional leaves are delayed in spreading. In this study, the chlorophyll of T1 plant of Mixed Bedding was significantly lower than that of T2, T3, T4 and T5 at flowering stage and the specific reason required further investigation. In addition, this study also found that the chlorophyll content of Blue Bedder and Mixed Bedding was significantly different at the flowering stage, while the sowing date had a significant effect on the chlorophyll content at the seedling, rosette and extensional branching stages. The interaction between the sowing date and cultivar had a significant effect on the chlorophyll content at the flowering stage.

Photosynthesis is the basis of plant dry matter accumulation and yield formation, and improving photosynthesis is of great importance for improving plant productivity and grain yield. Chlorophyll fluorescence can quickly, sensitively and non-invasively study and detect the true behaviour of photosynthesis of intact plants in different environments, and is widely used to evaluate the effects of photosynthetic apparatus function and environmental changes on it25. The results showed that sowing Blue Bedder at T1 was beneficial to increase the plant leaves Pn, Tr, Ls, qP, ETR and ΦpsII while reducing Ci and Gs, and sowing Mixed Bedding moderately later at T4 was beneficial to increase the leaves Pn, Tr, Fo, Fm, qP, ΦpsII and ETR. There were significant differences in Pn, Ci, Tr, Ls, Fo, Fv/Fm, qP, ETR and ΦpsII between the two cultivars, while the Pn, Tr, Fv/Fm, ETR and ΦpsII at flowering stage of Blue Bedder were significantly higher than that of Mixed Bedding. It indicated that sowing Blue Bedder at T1 and sowing Mixed Bedding at T4 could improve the photosynthetic performance of plants, and the photosynthetic performance and fluorescence performance of Blue Bedder were significantly higher than that of Mixed Bedding. In addition, Fv/Fm is the maximum primary photochemical quantum efficiency of PSII and an important parameter of photochemical reaction, which reflects the potential maximum photosynthetic capacity of plants26. Fv/Fm of most higher plants ranged from 0.8 to 0.85, and when Fv/Fm decreased, it represented that the plants were under stress27,28,29. In the experiment, the Fv/Fm of Blue Bedder was in the range of 0.8–0.85, while the Fv/Fm of Mixed Bedding was less than 0.8 in the range of 0.75–0.78, which could be due to the different responses of the two varieties to the same environmental change.

In this study, sowing date significantly affected the Pn, Ci, Tr and Ls of E. plantagineum leaves, but had no significant effect on Gs, Tr and chlorophyll fluorescence parameters. Delaying the sowing date caused a decrease in Pn, Ls, qP, ETR, ΦpsII and an increase in Ci of Blue Bedder. However, the rule of Mixed Bedding was not obvious. According to previous studies30,31,32,33, the decrease in Ci and increase in Ls meant that the decrease in stomatal conductance was the reason for the decrease in Pn, while the increase in Ci and decrease in Ls indicated that the decrease in photosynthetic activity of mesophyll cells might be the reason for the decrease in Pn. The sowing date mainly improved the Pn of Blue Bedder by improving the photosynthetic activities of mesophyll cells such as qP, ETR, and ΦpsII. Considering that the Fv/Fm of Mixed Bedding was less than 0.8 at each sowing date, and its growth was subjected to external stress, the specific photosynthetic mechanism may be affected, and whether the effect of the specific sowing date on Mixed Bedding was the same as that of Blue Bedder needs to be further investigation.

The sowing date was an important factor influencing the yield formation of E. plantagineum. Król et al.34 had conducted planting trials in Poland and found that under the dense planting condition of 45 cm row spacing and 25 plants per square metre, the early sowing in April was conducive to increasing in the yield and 1000 grain weight of E. plantagineum in the region. However, in this study, except for the T1 sowing date of Mixed Bedding, both Mixed Bedding and Blue Bedder showed the increasing trend of delayed 1000-grain weight along with the sowing date, while the yield of Blue Bedder was the highest at the T1 sowing date, and the yield of Mixed Bedding was the highest at the T4 sowing date. This shows that density, sowing date and variety have certain effects on the yield characteristics of E. plantagineum. Correlation analysis showed that sowing date had a very significant positive correlation with total canopy PeR at the flowering stage and 1000 grain weight, and a very significant negative correlation with total canopy CaR at the flowering stage. Total canopy CaR and PeR at the flowering stage were significantly correlated with yield and photosynthetic traits (Pn, Ci, Ls, Fo, Fv/Fm, ETR, ΦpsII, SPAD, etc.). In addition, sowing date had a significant effect on total canopy CaR, PeR, Pn, Ci and Ls of E. plantagineum, as well as on 1000 grain weight and yield. Therefore, different sowing dates can improve the photosynthetic capacity and thus the yield by adjusting the canopy structure of the population and the photothermal resources (Supplementary Information).

Conclusion

The results showed that the photosynthetic, fluorescence and yield characteristics of the two E. plantagineum cultivars showed some differences. Under the experimental design conditions, the yield and photosynthetic performance of Blue Bedder were better than that of Mixed Bedding under the same sowing date. Sowing date had significant effects on the total canopy CaR, PeR, Pn, Ci and Ls, as well as on the 1000 grain weight and yield of E. plantagineum. Under the same cultivar conditions, with a delay in sowing date, the leaf SPAD, Pn, Tr, Ls, qP, ETR, ΦpsII and yield of Blue Bedder decreased and reached a maximum at T1, whereas the SPAD, Pn, Tr, WUE, Ls, Fo, Fm, qP, ETR, ΦpsII and yield of Mixed Bedding reached a maximum at T4. Therefore, different sowing dates can optimise the population structure and photothermal resources by adjusting the growth process to improve the photosynthetic capacity and thus increase the yield. The analysis showed that the sowing yield of Blue Bedder was the highest at T1 period and that of Mixed Bedding was the highest at T4 period. Therefore, cross sowing could improve the yield of E. plantagineum, and the sowing dates of the highest yield for different varieties were different.