Effect of sowing date and accumulated temperature on seedling characteristics
Because the plants were sown on three different dates, there were 50-, 70- and 90-d-old seedlings on the day of transplantation. The seedlings with different sowing dates also experienced a different active accumulated temperature (AAT) and effective accumulated temperature (EAT) (Table 1) before transplantation, transplanted on 20 August was highest in both AAT and EAT, followed by transplanted on 9 September and 29 September. Seedling-age and accumulated-temperature have positive effect on seedling morphology with limits. The seedling morphology of the test cultivars, especially the pseudostem diameter, varied significantly among the sowing dates in both years (Table 2) for the seedling-age and accumulated-temperature reason.
Table 1
Seedling age, active accumulated temperature (AAT) and effective accumulated temperature (EAT) of ≥5° C and ≥10°C from the sowing date to 19 November, 2010 and 2011
Table 2
Plant height, pseudostem diameter and leaf number on 19 November, 2010 and 2011, for the tested cultivars sown on three different dates
Different temperatures in three transplant locations from 19 November to 30 April in the 2 successive production years
From the hourly temperature records, the daily average temperature was obtained and the active accumulated temperature (AAT), effective accumulated temperature (EAT) of ≥5°C, ≥10°C and ≥15°C, accumulated low temperature (ALT) and effective accumulated low temperature (EALT) of ≤13°C in different transplant locations were calculated from 19 November to 30 April for each year of the experiment. There was a significant difference in the four temperature indictors among the three different transplant locations (Table 3). The highest AATs and EATs of ≥5°C, ≥10°C and ≥15°C were in the plastic tunnel (In the first year, AAT of ≥5°C, ≥10°C and ≥15°C were 1885.8°C, 1539.6°C, and 1027.7°C, respectively; EAT of ≥5°C, ≥10°C and ≥15°C were 122.08°C, 659.6°C and 337.3°C, respectively. In the second year, AAT of ≥5°C, ≥10°C and ≥15°C were 1047.3°C, 1060.1°C, and 924.9°C, respectively; EAT of ≥5°C, ≥10°C and ≥15°C were 920.3°C, 490.1°C and 219.9°C, respectively) and the lowest were in the open field (In the first year, AAT of ≥5°C, ≥10°C and ≥15°C were 912.6°C, 611.5°C, and 390.5°C; EAT of ≥5°C, ≥10°C and ≥15°C were 487.6°C, 221.5°C and 75.5°C, respectively. In the second year, AAT of ≥5°C, ≥10°C and ≥15°C were 758.8°C 603.0°C, and 420.5°C, respectively; EAT of ≥5°C, ≥10°C and ≥15°C were 473.8°C, 233.0°C and 63.5°C, respectively) in both years. However, noticeable patterns among the ALT and EALT values of the three locations in the two year were: the lowest ALT (≤13°C) and the highest EALT (≤13°C) were in the open field (200.7°C and 1554.3°C in the first year and 445.3°C and 1384.7°C in the second year) and the highest ATL (≤13°C) and the lowest EALT (≤13°C) were in the plastic tunnel (801.3°C and 589.7°C in the first year and 618.0°C and 812.0°C in the second year).
Table 3
Active accumulated temperature (AAT), effective accumulated temperature (EAT) of 5°C, 10°C and 15°C, accumulated low temperature (ALT) and effective accumulated low temperature (EALT) at 13°C in different transplant locations from 19 November to 30 April, in the two succeeding production years
Comparison of the bolting characteristics of the ten cultivars
The number of days from the transplantation date to the initiation of bolting differed among the cultivars (Table 4). In both of the two years, with the three transplanted locations and the three sowing dates (except 29 September, 2011 sown and transplanted in open field) treatments, the bolting dates of C1 and C7 were significantly earlier and those of C3 and C10 were significantly later than the bolting dates of the other cultivars. There was no significant difference among the remaining six cultivars, and thus, it was inferred that C3 and C10 were highly resistant to bolting and were categorised as a strong bolting type, C6 and C8 were moderately susceptible to bolting and were classified as a moderate bolting type, C1 and C7 were highly susceptible to bolting and termed a weak bolting type. The same classification was used during the next year of the experiment, i.e., 2011–2012. To simplify the analysis, the C1, C3 and C6 cultivars were selected as representative of each of the above bolting types.
Table 4
Days from transplant to initial bolting for ten cultivars, three sowing dates and three locations in 2011
The influence of cultivar, sowing date and transplant location on Welsh onion bolting
The number of days from the transplantation to the initial bolting, the number of days to the 50% bolting rate and the bolting rate by 30 April were selected as indicators to evaluate the bolting behaviour. To simplify the analysis, the C1, C3 and C6 cultivars were selected as representative of different bolting types as described above. The results of each treatment are listed in Table 5 for 2011 and 2012. The effects of each factor and their interactions were analysed.
Table 5
Days from transplant to initial bolting, to 50% bolting and bolting rate by 30 April for each treatment and year
Main effects of cultivar, sowing date and transplant location on the indicators of bolting
All the factors, including the cultivar, sowing date and transplant location, had a significant influence in both years on the days from transplant to initial bolting, the days from transplant to 50% bolting and the bolting rate by the 30 April.
The interactions of cultivar * sowing date (C*D) and sowing date * transplant location (D*L) had a significant influence on the days from transplant to initial bolting (2011, 2012), the days from transplant to 50% bolting (2011) and the bolting rate by 30 April (2011, 2012). Because some of the treatments did not attain 50% bolting in 2012, there was no significant difference in 2012 among the treatments in the days from transplant to 50% bolting. The interaction of cultivar * transplant location (C*L) on the days from transplant to 50% bolting were not significant in either of the two years.
The interaction of variety * sowing date * transplant location (C*D*L) on the days from transplant to 50% bolting was not significant in either year, but there was a significant influence on the days from transplant to initial bolting and the bolting rate by 30 April in both 2011 and 2012 (Table 6).
Table 6
The effects of cultivar, sowing date, and transplant location on days from transplant to initial bolting, to 50% bolting and bolting rate by 30 April in 2011 and 2012
Among the three analysed cultivars in 2011, the JinGuan (C1) cultivar was the first to bolt and, on average, did so by 10 May, only 111 days after sowing, which was 18 days earlier than the YeFu (C6) cultivar and 34 days earlier than the XiaHei (C3) cultivar. Moreover, JinGuan (C1) attained 50% bolting by 126 days after transplant; YeFu (C6) required 131 days, and XiaHei (C3) did not reach 50% bolting until 30 April. As for the bolting rate, JinGuan (C1), YeFu (C6) and XiaHei (C3) attained levels of 54, 36 and 29%, respectively, by 30 April.
In 2011, the plants sown on 20 August (D1) bolted by approximately 17 March. This date was 8 days earlier than that of the plants sown on 9 September (D2) and 24 days earlier than that of the plants sown on 29 September (D3). A period of 120 days was required from transplant to 50% bolting when the plants were sown on 20 August (D1), which was 27 days fewer than was required by the plants sown on 9 September (D2). The plants sown on 29 September (D3) did not reach 50% bolting until 30 April. The bolting rates were 65, 40 and 15%, respectively, for the earliest, middle and latest of the three sowing dates.
The use of the open field (L1) as the Welsh onion transplant location delayed bolting by approximately 49 and 21 days, respectively, compared with the use of the plastic tunnel (L3) and the cold shed (L2). The open field (L1) treatment produced bolting on approximately 23 April. The time required from transplantation to 50% bolting was 124 days in the plastic tunnel (L3) and 133 days in the cold shed (L2). In the open field, the Welsh onions did not reach 50% bolting until 30 April. The final bolting rate by 30 April in the open field (L1), cold shed (L2) and plastic tunnel (L3) was 9, 45, and 65%, respectively.
In 2012 compared with 2011, the dates when the bolting started and reached the 50% level were slightly later, and the bolting rate on 30 April was lower. However, the overall effects on bolting of the different cultivars, sowing dates, transplant locations and factor interactions were similar in the two years.
Thus, except the interaction of cultivar * transplant location (C*L) and the interaction of variety * sowing date * transplant location (C*D*L) on the days from transplant to 50% bolting were not significant in either of the two years, other factors, including the cultivar, sowing date and transplant location and their interaction, had a significant influence in both years on the days from transplant to initial bolting, the days from transplant to 50% bolting and the bolting rate by the 30 April.
Effects of cultivar and sowing date on days from transplant to initial bolting, days to 50% bolting and bolting rate by 30 April in the two years
The interaction between the cultivar and the sowing date significantly influenced the initial bolting date (2011, 2012) and the bolting rate by 30 April (2011). In 2011, the earliest initial and 50% bolting rate occurred when the JinGuan cultivar was sown on 20 August (C1*D1). It only took 100 (27 February) and 116 days (15 May) for the initial and 50% bolting levels to occur, respectively, and the bolting rate reached 80%, which was the highest in all of the treatments. By contrast, the treatment showing the latest bolting was the XiaHei cultivar, which was sown on 29 September (C3*D3). This cultivar required 155 days (23 April) for the initial bolting and did not reach the 50% bolting level until 30 April, on average. The final bolting rate of this cultivar was lower than 3%, which was the lowest among all of the treatments (Table 7).
Table 7
Effects of cultivar and sowing date on days from transplant to initial bolting, to 50% bolting and bolting rate by 30 April in 2011 and 2012
The days to initial bolting were similar in 2011 and 2012. In 2012, however, the 50% bolting date did not differ significantly among the treatments because none of the treatments reached the 50% bolting rate by 30 April. Surprisingly, the XiaHei (C3) cultivar did not reach 50% bolting in any of the three sowing dates in either 2011 or 2012.
Effects of cultivar and transplant location on days from transplant to initial bolting, days to 50% bolting and bolting rate by 30 April in two years
There was a significant difference among the treatments in the days from the transplant to the initial bolting (2011, 2012), and in the bolting rate by 30 April (2011, 2012) because of the interaction of the cultivar and the transplant location (Table 8). In particular, the JinGuan cultivar transplanted in the plastic tunnel location (C1*L3) took only 84 days in 2011 and 106 days in 2012 to initial bolting, which were the shortest times of any of the treatments. The JinGuan (C1) bolting rates were also the highest, i.e., 85% and 29% in 2011 and 2012, respectively. The treatment that took the longest to initiate bolting was the XiaHei cultivar that was transplanted in the open-field location (C3*L1); no bolting was observed before 30 April. Because no treatment reached 50% bolting in 2012, it was difficult to show an interaction of the cultivar and the transplant location, a situation similar to that encountered when exploring an interaction between the cultivar and the sowing date. The XiaHei (C3) cultivar did not reach 50% bolting in any of the three transplant locations in 2011.
Table 8
Effects of cultivar and transplant location on days from transplant to initial bolting, to 50% bolting and bolting rate by 30 April in 2011 and 2012
Effects of sowing date and transplant location on days from transplant to initial bolting, days to 50% bolting and the bolting rate by 30 April in the two years.
An interaction between the sowing date and the transplant location significantly affected the bolting parameters of the Welsh onion cultivars (Table 9). In 2011, the days to initial bolting increased from 95 with the sowing on 20 August and the transplant location of the plastic tunnel (D1*L3) to above 165 with the sowing on 29 September and the open-field transplant location (D3*L1). A similar pattern appeared with the same two treatments in 2012, i.e., the days needed for initial bolting increased from 111 to 162. The D1*L3 treatment combination required the fewest days to attain the 50% bolting rate (2011) and exhibited the highest final bolting rate, whereas the D3*L1 treatment combination never reached 50% bolting (2011, 2012) and showed the lowest final bolting rate. The final bolting rates for the D1*L3 and D3*L1 treatment combinations, respectively, were 91 and 0% in 2011 and 47 and 0.44% in 2012.
Table 9
Effects of sowing date and transplant location on days from transplant to initial bolting, to 50% bolting and bolting rate by 30 April in 2011 and 2012
On 29 September (D3), the latest sowing date, no treatment reached the 50% bolting rate in any of the three transplant locations in either 2011 or 2012, so there was no difference among these treatments.
The influence of seedling age, AAT and EAT on bolting of the cultivar of XiaHei
Cultivar XiaHei sown on 29 September, transplanted on cold shed could not bolt in the first year compared with sown on 9 September, we can deduce that pseudostem diameter around 4.06 mm, AAT (≥5°C) of 1156.1°C and EAT (≥5°C) of 796.6°C before transplant were necessary for cultivar 3 to response the cold environment and begin the process of vernalisation, then bolting in the next year.
Taken together, sowing date affected the seedling morphology when transplanting, which would further determine Welsh onion’s bolting behaviour together with the duration of low temperature. Specifically, all three factors (cultivar, sowing date and transplant location) and their interaction had significant effects on the initial and final rate of bolting observed on 30 April. The earliest bolting date and highest bolting rate occurred when the JinGuan cultivar was sown on 20 August and transplanted in a plastic tunnel, whereas the latest date and lowest rate of bolting occurred when the XiaHei cultivar was sown on 29 September and transplanted in an open field.