2020 Volume 89 Issue 3 Pages 278-283
By using a stable isotope method, partitioning of photosynthates was examined in a modified arching (MA) technique that achieved a high cut flower productivity to clarify how photosynthates produced by bent shoots are distributed within a plant at different developmental stages of harvest shoots. The translocation ratios from bent shoots at four days after harvest, eight days after harvest, for 10 cm-shoots, flower budding, and blooming were 44.0, 40.2, 51.0, 38.3, and 23.4%, respectively, and plants at the blooming stage had lower translocation ratios compared to those at the other developmental stages. The distribution of photosynthates produced by the bent shoot parts varied with the plant stage of the harvest shoots. The partitioning ratios of roots and crowns at four days, eight days, for 10 cm-shoots, flower budding, and blooming were 63.9, 53.5, 17.5, 26.6, and 81.0%, respectively. From these results, it was revealed that in the period from blooming to the next sprouting, roots and crowns were strong sinks. The partitioning ratios of mother stems were 36.2, 46.4, 7.2, 15.0, and 8.2%, respectively. We estimate that the mother stems are a strong sink right after harvest and absorb the photosynthates for the new sprouting. At the 10 cm-shoot and flower budding stages, photosynthates were partitioned to harvest shoots at 75.2% and 58.4%, respectively, indicating that they were distributed preferentially to vigorously growing harvest shoots. A total of 11.4% of the photosynthates were partitioned to harvest shoots at the blooming stage, suggesting that the photosynthates from the bent shoots were not involved directly in flower blooming. This study will contribute to improving the MA technique and cut rose production.
Since the late 1980s, the technique of cut rose production has evolved from the traditional cut back technique to the shoot bending technique (“arching” technique) (A technique) (Shigeoka, 1999). In the A technique, shoots sprouting from crowns are divided into harvest shoots (sink) and bending shoots (source). By bending the shoots in the shape of an arch, photosynthates originating from bent shoots promote strong harvest shoot development from the crown. However, the number of harvest shoots produced in the A technique was lower than that of the traditional cut back technique (Nimura et al., 1997).
In order to solve this problem, the high-rack technique was developed in Japan (Kajihara and Katsutani, 2008; Shigeoka, 1999). The high-rack technique is an improved method of the A technique, in which some thick shoots (mother stems) developing from a crown are cut and left at a certain length when harvested; the growers then harvest the shoots that develop from those mother stems. Kajihara et al. (2009) reported the requirements to calculate the partitioning ratio of carbon gained by photosynthesis to harvest shoots, roots, crowns, and mother stems etc. by feeding 13CO2 to bent shoots. However, they only fed 13CO2 to the plants just before blooming in the high-rack technique. It is thought that depending on the developmental stage of harvest shoots, the sink ability is different, and so the translocation and partitioning ratios to plant parts may differ.
In the Netherlands, a modified arching technique (the MA technique) (Fig. 1 and described in detail in Materials and Methods) as a hybrid of these cut back, A and high-rack techniques has been developed and has begun to be adopted around the world. In fact, the number of cut flowers produced using the MA technique is higher than that of the A technique by 20–30% without reducing the cut flower quality (Mercurio, 2007). Shimomura et al. (2003) and Doi et al. (2009) investigated the relationship between the productivity of cut flowers and trimming management and clarified that increasing the number of harvested shoots promoted their sink ability.
Drawing of the partitioning experiment using 13CO2.
With regard to maintenance of the bending shoots in the MA technique, high photosynthate absorption by harvest shoots can be expected, although how to optimize this process has not been specified. In this study, in order to gather further knowledge to discuss both plant architecture and greenhouse environments, we delivered 13CO2 to rose plants with harvest shoots at different stages, and investigated the partitioning ratio of photosynthates produced by bent shoots to other plant parts using the MA technique.
All experiments were carried out in a ventilated greenhouse at Okayama University (Okayama, Japan). Rosa hybrida hort. ‘Sweet Avalanche’ cuttings grafted on rootstocks of Rosa gigantea Collet ex Crép. ‘Odorata’ were used. These were planted in 9 cm polyether pots on April 2, 2015, and individually repotted in 18 cm plastic pots filled with perlite on May 11. Silver polyether film (Sun Silver; Mitsubishi Chemical Agri Dream Co., Ltd., Japan) was used to cover the pots. Plants were transferred to another greenhouse heated at 15°C and ventilated at 30°C during the growing period. OAT A solution (N:P2O5:K2O = 10.5:4:13.5; OAT Agrio Co., Ltd., Japan) adjusted to N: 90 ppm was used every 2 days. After adequate rooting was observed, the solution was delivered automatically for 2 min eight times a day. The amount of solar radiation was measured by Weather Station Vantage Pro (Davis Instruments, CA, U.S.A.), and recorded every 5 min. Air temperature was measured by an Ondotori TR-71 (T&D Corporation, Japan) with a sensor attached to a ventilation pipe in the greenhouse and recorded every 5 min.
MA training techniquesThe first shoots were bent and were considered to be bent shoots (BS) for photosynthesis. Next, two vigorous shoots were harvested leaving 10 cm stems from the crown, and these became two mother stems. Only one shoot was allowed to grow from a mother stem and it was cut leaving only 2 cm when harvested. Other sprouted buds were disbudded when they reached 10 cm. Shoots developed from the BS were removed if they were close to the base, and others were bent after disbudding. We prepared the plants at the following developmental stages: 4 days after harvest (4 days) with 2 to 4 buds of 0.2 to 2.0 cm length, 8 days after harvest (8 days) with 2 to 6 buds of 0.5 to 8.7 cm length, when harvest shoots (S) grew to 10 cm (10 cm), flower bud visible (flower budding), and fully bloomed (blooming).
13CO2 feeding and analysesGas containing 1% of 99 atom % 13CO2 (SI Science Co., Ltd., Japan) was used in this study. On sunny days, October 3 (flower budding), October 6 (blooming), October 16 (10 cm), October 20 (4 days), and October 25 (8 days) in 2015, feeding was conducted at 9:30 a.m. in another ventilated greenhouse to prevent contamination from 13CO2 leakage. Just before feeding, BS were pruned to leave a leaf area of 1500–2000 cm2 and covered with a thick clear vinyl bag (65 cm × 45.7 cm × 0.1 mm, 20 liter volume), with a W cap attached (AS ONE Corporation, Japan) with vinyl tape, and sealed with vaseline if necessary. Twenty liters of the 13CO2 gas mixture was introduced from a cylinder to each vinyl bag. The, the plants were allowed to stand and the vinyl bag was removed 5 h after the start of feeding. The minimum temperature in the vinyl bag was in a range from 20.0° (October 25) to 29.5°C (October 6) and the maximum temperatures were 31.0°C (October 25) and 46.0°C (October 6). Plants were grown for 72 h after feeding.
The whole plants were harvested and divided into parts as shown in Figure 1, then dried in a forced-air oven at 80°C for more than 80 h. The BS leaves fed with 13CO2 were photocopied just before drying and the leaf area was measured. After drying individual samples, each was weighed and ground by an oscillation cup mill. For the BS and S, leaves and stems were weighed together and ground. Samples of 0.1 ± 0.01 mg of each plant part were weighed in a tin cup and combusted in an elemental analyzer (Flash EA 1112; Thermo Fisher Scienthific, Germany), interfaced through a Conflo II (Thermo Fisher Scienthific, Germany). The quantity of 13C in plant materials was determined with an isotope ratio mass spectrometer (Delta Plus Advantage; Thermo Fisher Scienthific, Germany) at the NARO Western Region Agricultural Research Center in Fukuyama on November 16, and 17, 2015. The value of natural 13C% is 1.079. The 13C‰ excess, quantity of 13C, 13C translocation ratio, and 13C partitioning ratio were calculated from the following formulae (Ito et al., 2002);
Each experiment was conducted with three replicate plants (n = 3). The data means were arcsine transformed before analysis by ANOVA and comparison by Tukey’s HSD test (EXCEL-toukei 2010; Social Survey Research Information Co., Ltd., Japan) when a significant difference was observed.
Cumulative amounts of solar radiation during the time of feeding (9:30–14:30) to the plants at the stage of 4 days, 8 days, 10 cm shoots, flower budding, and blooming were 7.84, 7.98, 8.79, 7.29, and 8.78 MJ·m−2, respectively. The total cumulative amounts of solar radiation from the start of feeding until sampling were 21.8, 22.5, 24.2, 17.2, and 24.5 MJ·m−2, respectively.
Although the thermometer showed 46.0°C on October 6, no damage was observed. The average day (7:00–17:00) temperatures 72 h after feeding to the plants at 4 days, 8 days, 10 cm shoots, flower budding, and blooming were 25.6, 21.6, 23.9, 25.0, 23.9°C, respectively, and the average night (17:00–7:00) temperatures were 17.2, 17.6, 16.4, 17.1, 15.2°C, respectively. It is reported that the optimal growth day and night temperatures for roses are 23–27°C and 15–18°C, respectively (Mercurio, 2007; Ohkawa, 1999). Average day and night temperatures from feeding to harvest in this experiment were within these ranges.
Leaf areas of 13CO2 fed BS and dry weights of plant partsTable 1 shows the mean dry weights of each plant part used for 13CO2 analysis. BS was the heaviest among all plant parts at any plant stage, although there was no significant difference among stages. Leaf areas of 13CO2-fed BS were 1691 cm2, 1471 cm2, 1891 cm2, 1851 cm2, and 1793 cm2 for the plants at 4 days, 8 days, 10 cm shoots, flower budding, and blooming, respectively.
Dry weight of rose plants parts at different plant stages.
Table 2 shows the residual 13C‰ excess per plant part after 72 h of 13CO2 feeding. MS tended to have the highest 13C‰ excess when harvest shoots started to grow, that is, at the 4-day and 8-day stages, respectively, a 5.4 and 6.8 13C‰ excess were measured and thereafter the 13C‰ excess of MS decreased gradually. The 13C‰ excess of S was marked at the 10 cm stage. This represented the maximum value of 67.1 13C‰ excess among all plant stages and all plant parts.
13C‰ excess of rose plant parts at different plant stages.
Table 3 shows the 13C quantities in each plant part. Total 13C absorption quantities per plant at 4 days, 8 days, 10 cm shoots, flower budding, and blooming were 5.42, 6.20, 6.17, 4.70, and 5.95 mg, respectively. The retained 13C quantities in the BS in each plant stage were 3.04, 3.71, 3.02, 2.89, and 4.54 mg, respectively.
13C quantities of rose plant parts at different plant stages.
Figure 2 shows the translocation ratios from BS to the other plant parts. 13C translocation ratios at 4 days, 8 days, 10 cm shoots, flower budding, and blooming were 44.0, 40.2, 51.0, 38.3, and 23.4%, respectively, and there were significant differences among the plant stages. It was highest at the 10 cm stage and lowest at the blooming stage.
Translocation ratios of photosynthates at different plant stages. Vertical bars indicate SE of the mean (n = 3).
Table 4 shows the 13C partitioning ratios of plant parts. At the 4-day stage, the partitioning ratio was high in CR, R, and MS, and there was no significant difference among CR, R, and MS. At the 8-day stage, the partitioning ratio of MS was highest among CR, R, and MS; at this stage, that of R decreased. When harvest shoots grew to 10 cm, the partitioning ratios to CR and R decreased to less than 20%, and over 70% of photosynthates were partitioned to S. At this stage, the ratio to MS was small and was less than 10%. At the flower budding stage, about 60% of photosynthates were still partitioned to S. However, at this stage, the partitioning ratios to CR and R increased to over 25%. At the blooming stage the partitioning ratios of S and F markedly decreased to 11% and those of CR and R increased significantly. MS had almost the same ratio as S.
Partitioning ratios of 13CO2 fed to the bent shoots of roses at different plant stages.
We fed 13CO2 to bent shoots at different harvest shoot stages of rose plants trained by the MA technique. The 13CO2 concentration at the start of feeding was set at about 10,000 ppm, following Kajihara et al. (2009). It is predicted that the stable isotope ratio fluctuates according to environmental conditions and respiration. However, since we fed 13CO2 at a very high concentration, it can be assumed that the stable isotope ratio was almost at the same level in all treatments in this study.
There was a difference in the translocation ratio of photosynthates from BS among the plant stages (Fig. 2). The highest ratio of 51.0% was detected when the harvest shoots grow to 10 cm in length, and the lowest ratio of 23.4% was detected at the blooming stage. Since all the experiments were conducted in October 2015, there was little difference in the amount of solar radiation during the time of feeding. The differences in average day and night temperatures from feeding to harvest were also small. The translocation ratio of photosynthates is known to be affected by environmental conditions, and especially by the night temperature in roses (Khayat and Zieslin, 1986). It was reported that the translocation part between 18°C and 12°C was different, but that there was no difference between a constant 18°C and 18/12°C (alternating every 2 h). In our experiment, although the difference in the average night temperature between the 8-day stage and the blooming stage was 2.4°C, these temperatures were within the night temperature range for optimal growth. In addition, the differences in the translocation ratios at the 4-day, 8-day, 10 cm shoot and flower budding stages were small. In another experiment, we confirmed that the translocation ratio of photosynthates produced by BS was lowest at the blooming stage among the developmental stages (data not shown). Therefore, it was assumed that in this study the difference in the plant stage was the main factor affecting the sink ability of plant parts other than bent shoots.
We found that there was a significant difference in the partitioning ratios of photosynthates from bent shoots to the other plant parts. When the harvest shoots grew to 10 cm in length, over 70% of photosynthates translocated from BS were partitioned to S (Table 4). At this stage, a large amount of photosynthates was needed to grow the harvest shoots and as a result these plants exhibited not only a high translocation ratio of photosynthates, but also a high partitioning ratio of S. On the other hand, at the blooming stage, S and MS did not require such a large amount of photosynthates. From these results, harvest shoots worked as a strong sink at the 10 cm shoot length to flower budding stage, and thereafter the partitioning ratio of S, as well as the translocation ratio, dramatically decreased. Between the flower budding stage and the blooming stage, there is a turning point at which the harvest shoots change from a sink to an autotrophic part.
Kajihara et al. (2009) studied the partitioning of 13CO2 using the blooming stage of rose plants trained by a high-rack technique, and reported that 51–70% of photosynthates produced by the bent shoots were translocated to other plant parts within 72 h. Most of them were partitioned to roots, crowns, and mother stems. The translocation rate in this study was relatively low compared to the reports of Kajihara et al. (2009) and Mor and Halevy (1980). Kajihara et al. (2009) did their experiment from the end of June to the beginning of July. On the other hand, we treated the plants with 13CO2 in October when the amount of solar radiation and temperature were relatively low. Shishido et al. (1987) reported that the translocation ratio of photosynthates in cucumber is affected by temperature, not irradiation. As described above, Khayat and Zieslin (1986) reported the translocation ratio of photosynthates in roses is affected by environmental conditions, and especially by night temperature. The amount of solar radiation and temperature may be factors related to the lower percentage of translocation. Ushio (2008) reported that the photosynthesis ability and the quality of cut flowers were greatly affected by temperature during the growth period of the harvest shoots. It may be necessary to consider the effect of growth season on the translocation of photosynthates produced by bent shoots.
In this study, buds and shoots at 4 days and 8 days were too small to extract the minimum amount of dry sample for analysis. Therefore, we sampled these buds and shoots with MS and analyzed them together. At the 4 day- and 8 day-stages, photosynthates were partitioned mainly to CR and R and we confirmed that these underground parts had a strong sink activity. However, since MS with buds also exhibited a relatively high partitioning ratio, MS can be considered as a sink that presumably stores the photosynthates for subsequent bud development. Mor and Halevy (1979) reported that young harvest shoots at 10 cm showed a strong sink activity for photosynthates from the mature leaves attached to the mother stems and that harvest shoots with 4 mm flower buds (at the stage just after flower budding) lost sink activity and became a source. These findings regarding the conventional cut back technique are consistent with our results for the MA technique, and suggest that the harvest shoots changed from a sink to a source after the budding stage. Jiao et al. (1989) reported that the developing flower bud was the biggest sink by studying the sink and source relationship of roses using 14CO2. Mor and Halevy (1979) also reported that just after the flower budding stage in the conventional cut buck technique, the photosynthates for the growth of the harvest shoots and flower buds were provided by leaves on the harvest shoots themselves.
It is interesting that at the blooming stage the partitioning ratios of MS, S, and F became quite low (Table 4), whereas those of CR and R were high. Kohl and Smith (1970) reported that photosynthates mostly partitioned to roots after harvesting, and as harvest shoots develop, they partitioned to plant parts above ground including harvest shoots, suggesting that the sink ability fluctuated between the underground (R and CR) and aboveground (MS, S, and F) parts. The high partitioning ratio of R and CR from the blooming stage to the early stages of harvest shoot development may play a role in storing photosynthates for the next growth cycle. We assumed that the overall translocation ratio from BS decreased because not only harvest shoot photosynthesis increased, but also some new lateral shoots started to sprout from BS at this stage. These results suggest that retranslocation of assimilates stored in roots and crowns to the harvest shoots should be considered.
Considering the developmental process of roses by the developmental index (DVI) from zero (start of harvest shoot growth) to 1 (harvesting stage), we assume that the partitioning of photosynthates to aboveground parts (MS and S) at different shoot stages correlates with DVI by a quadratic equation with the peak at the plant stages between 10 cm and flower budding. Based on these facts, it is considered that photosynthates produced by bent shoots were effectively stored and used for harvest shoot growth according to the developmental stage of the harvest shoots.
In conclusion, our results show that in the MA technique, translocation and partitioning of the photosynthates produced by bent shoots differs among growth stages of the harvest shoots; the strong sinks were both underground parts and mother stems after harvesting, harvest shoots in the period from shoot elongation until flower budding, and finally crowns and roots at flowering. It is estimated that the direct contribution of photosynthates produced by bent shoots is highest in the period between the 10 cm stage and flower budding. Therefore, rose growers should pay attention to optimizing the plant architecture, photosynthesis environment and translocation during these plant stages. In the future, the effect of periodic CO2 feeding on the cut flower yield and quality should be investigated based on this hypothesis.
