On the cover

Long vase life and low ethylene sensitivity are important for dahlia to maintain their commercial value when they are used as a cut flower. We crossed cultivars with a long vase life and cultivars with a low degree of abscission to examine the possibility of producing lines with both traits. While offspring from the crossing showed various degrees of abscission, lines F182-10, F181-14, and F182-17 consistently showed no abscission throughout the evaluation. Thus, they were selected as lines with a low degree of abscission. Moreover, the vase lives of lines F182-10 and F182-17 were longer than that of the parental cultivar, ‘Port Light Pair Beauty’. This is the first report of breeding dahlia with a low degree of abscission, and it indicates the possibility of producing lines with both long vase life and a low degree of abscission. Accordingly, to improve vase life of the selected lines, further breeding is now in progress.

(Provided by T. Fujimoto: NARO)

Various attempts have been made to use abscisic acid (ABA) as a plant bioregulator (PBR). Recently, a new ABA formulation, produced through microbial fermentation, has been commercialized in Japan as a PBR for improving grape peel color. Nonetheless, the horticultural application of ABA remains limited compared to other plant hormones. Generally, the vital functions of ABA include the regulation of abiotic stress tolerance and plant dormancy via metabolic changes associated with the antioxidant system and the modulation of sugar biosynthesis/translocation, respectively. These metabolic changes are crucial for the quality of fruit, suggesting the potential of ABA for promoting the production of high-quality fruit. In non-climacteric fruit, ABA plays a pivotal role in anthocyanin pigmentation, a significant component of the antioxidant machinery. Studies in grapes and strawberries have shown that the responses to ABA differ depending on the type of ABA agonist used and the specific organs to which it is applied. Herein, the influence of ABA on climacteric fruit exhibiting ethylene-regulated ripening is discussed. ABA-mediated ethylene induction followed by ripening suggests a shared regulatory mechanism that underlies ripening in both climacteric and non-climacteric fruits that involves crosstalk between ABA and ethylene. This review firstly summarizes the historical challenges with ABA and its agonist for application as PBRs and discusses the role of ABA in the ripening of non-climacteric fruit, particularly grapes. Subsequently, the molecular background of ABA in both climacteric and non-climacteric fruit ripening is explained, with a focus on peel coloration, sugar synthesis, and aroma volatile synthesis.

Dahlia (Dahlia Cav.) is an important ornamental plant used as cut flowers. Recently, we bred the “Eternity series” cultivars with long vase life to overcome the difficulty of long-distance transport due to their short vase life. However, dahlias, including the Eternity series, are ethylene-sensitive, inducing petal abscission within a short period. Therefore, cultivars with long vase life and low degree of abscission were crossed to examine the possibility of producing lines with both traits. The flower life of offspring after treatment with 10 μL·L⁻¹ ethylene (time to ethylene response) was investigated in open-field cultivation during the summer–autumn of 2021. The degree of abscission was classified into three types according to the time to ethylene response and the occurrence of abscission: abscission within four days (high), abscission after four days (mid), and the absence of abscission during senescence (low). We selected 17 of the 105 lines with mid or low degrees of abscission. The degree of abscission of the selected lines was re-evaluated during greenhouse cultivation in the winter–spring of 2021–2022. Three lines (F182-10, F181-14, and F182-17) that showed no petal abscission in the winter–spring were selected. The drawing resistance force of the petals from lines F182-10 and F182-17 was maintained at 1.0 N even during the senescence stage (six days after harvest), which was related to their low degree of abscission. The degree of abscission of lines F182-10, F181-14, and F182-17 was stable during the re-evaluation in summer–autumn of 2023, and their times to ethylene response were 4.8, 5.8, and 4.6 days, respectively. Moreover, the vase lives of lines F182-10 and F182-17 were 5.8 and 5.6 days, respectively, while that of ‘Port Light Pair Beauty’ was 4.4 days. These results suggested that degree of abscission was inherited from ‘Port Light Pair Beauty’, and the vase life was improved after crossing with a cultivar with a long vase life. This is the first report about breeding dahlia with a low degree of abscission, and it indicates the possibility of producing lines with both long vase life and a low degree of abscission.

Flower longevity is essential to maintain the quality and value of ornamental crops. The distribution of high-quality ornamental flowers has been established by developing various preservatives, especially for cut flowers. However, genetic improvements to promote flower longevity have not progressed, except for some ornamental crops. This is partly due to a lack of analytical methods to evaluate flower longevity in large-scale trials. This study showed that high-throughput image analysis using deep learning models for computer vision, which has rapidly progressed, can quantify the flower longevity of multiple lines in greenhouses. Seven lines of Portulaca umbraticola, a one-day-flowering crop for summer gardening, were photographed at regular intervals from the top of the greenhouse. Using trained object detection models, including the YOLO series, multiple flowers and pots were accurately detected in each image. The opening degree of flowering was calculated by trained image classification models, such as Densenet and VGG. Although the images generally contained many flowers, multiple object tracking models, such as ByteTrack and StrongSORT, enabled us to track each flower and estimate its longevity. Additionally, video classification sorted the tracked flowers into seven P. umbraticola lines. Each line’s flower longevity could be quantified and it differed each day and within the same flower line. Therefore, applying the proposed method using deep learning models could dramatically accelerate research and breeding of flowers with increased longevity. The applicability of this method to various ornamental crops and traits is also discussed.

Japanese pear [Pyrus pyrifolia (Burm. F.) Nakai] has a S-RNase-based self-incompatibility system and inactivation of S-RNase causes self-incompatibility breakdown in this system. Copper ions (Cu⁺⁺) strongly inhibited the RNase activity of crude stylar proteins, but did not overcome self-incompatibility in the Japanese pear ‘Kosui’ (S₄S₅). To elucidate the cause of this phenomenon, the present study aimed to separate stylar RNases and clarify their responses to Cu⁺⁺ in ‘Kosui’. Using an anion exchange chromatograph equipped with a Mono-Q column, one strong non-S-RNase was separated, and at least six non-S-RNases together with 2 S-RNases were isolated, by cation exchange chromatography with the Mono-S column. S-RNases were identified by dot blot analysis using S-RNase antiserum, and hereafter, the seven non-S-RNases isolated are designated as NS1 to NS7 in eluting order. NS1 and NS2 were abundant RNases with strong activities, and their activities were reduced to 6.6 and 4.5% of the control by 1 mM CuSO₄, respectively. NS3, NS4, NS5, NS6, and NS7 were weak RNases, and NS7 showed intermediate inhibition (31%) with Cu⁺⁺, whereas inhibition of other RNases was weak, from 44 to 74% of the control. Meanwhile, the degree of inhibition was quite low in S₄- and S₅-RNase (86 to 89%). Thus, RNase inhibition of crude stylar proteins by Cu⁺⁺ is due to strong repression of NS1 and NS2 activities. These results indicate that active S-RNase can cause a self-incompatibility reaction despite large decreases in non-S-RNase activities in the pear style.

To accelerate the early development of superior apple varieties, it is crucial to establish techniques to expedite the transition from the juvenile phase to the adult phase. Clarifying the physiological mechanisms underlying this phase transition will contribute to the development of an apple-seedling growing system that ensures early phase transitions. Herein, the apomictic crabapple Malus hupehensis (Pamp.) Rehd. was hydroponically cultivated under controlled conditions to explore its phytohormone dynamics during the phase transition. Flowering was confirmed in 15 of 57 seedlings at approximately 10 months after germination. The flowering rate was 26.3%. The average height and average stem circumference of the flowered seedlings were 27 cm and 0.56 cm greater, respectively, than those of the unflowered seedlings. The ABA concentration in mature leaves at the main stem tip of the flowered seedlings exceeded that in the unflowered seedlings up to the 70th node, and then dropped below that of the unflowered seedlings by the 90th node’s developmental stage. The concentrations of GA₄ and cytokinins in the mature leaves at the main stem tip showed no significant difference between the flowered and unflowered seedlings. These results suggest that employing hydroponics in a controlled setting is beneficial for facilitating early phase transition in M. hupehensis. Furthermore, maintaining low ABA concentration levels in the mature leaves at the main stem tip could stimulate phase transitions in M. hupehensis.

Fruit cracking is a fruit peel physiological disorder. In citrus, it typically results in fruit splitting and abscission. Cracking is related to plant water factors and the irrigation regime; heavy or non-regular irrigation is correlated with elevated cracking incidence. In addition, cracking is more common in larger fruit than in smaller ones. Some cultivars are more prone to cracking, suggesting that the phenomenon has a genetic background. However, it has become evident that climatic conditions also affect fruit cracking. As a result, some years are characterized by heavy cracking incidence, and this may be associated with yearly climate variations. Top netting is a useful tool to mitigate microclimates, and it can potentially reduce cracking intensity. We conducted a five-year experiment in a commercial ‘Valencia’ orange orchard, using three different photoselective nets and three distinct irrigation regimes: the recommended level (100%), and two reduced regimes (80% and 60%). Out of the five experimental years, cracking occurred in three years with various intensities. As expected, reduced irrigation in control trees resulted in decreased cracking intensity, and there was a positive relationship between the fraction of cracking and larger fruit size. In netted trees, the cracking intensity was reduced regardless of net color, irrigation regime or fruit size. During the summer, canopy temperature was reduced under the nets, and relative humidity increased. The possible involvement of climatic parameters, modified under the nets, in altering cracking intensity is discussed. Results of this experiment demonstrate the potential of top-netting technology to mitigate cracking intensity in citrus.

In Japan, white asparagus is produced by blanching with soil mounds or film-cover shading methods. However, with either method, white asparagus is produced only in the spring and not during the winter months, when labor and facilities can be more effectively utilized. Recently, a forcing-culture method for green asparagus using polyethylene bags was developed to reduce the workload required in green asparagus production during winter. In this study, we selected a medium composition that reduces bag weight while maximizing yield under a white asparagus forcing culture, as well as cultivars with superior yield and quality. In the first experiment, asparagus was cultivated under forcing culture in polyethylene bags using three different mixed-composition media: lightweight media and horticultural media (1:1, v/v), lightweight media and bark compost (1:1, v/v), and horticultural media and bark compost (1:1, v/v), comprising lightweight media with a bulk density of 0.45 g·mL⁻¹, horticultural media with a bulk density of 0.8–0.9 g·mL⁻¹, and bark compost of 0.4 g·mL⁻¹. The lightest bag (weighing 8.0 kg) contained a combination of lightweight medium and bark compost. No differences were observed between the three combinations in terms of asparagus growth, yield, or spear weight. In the second experiment, the cultivars ‘Taihowase’, No. 056, ‘Suguderu II’, and ‘Welcome’ were grown in a combination (1:1, v/v) of lightweight medium and bark compost. The spear weight of No. 056 was greater than that of ‘Welcome’. No differences in sugar content were observed among the four cultivars. The protodioscin content of ‘Suguderu II’ was higher than that of ‘Taihowase’, No. 056, and ‘Welcome’. These results indicated that forcing culture using bags is practical and that a combination of lightweight medium and bark compost is suitable for this culture method. The best asparagus cultivars among those tested were No. 056, which produced heavy young stems, and ‘Suguderu II’, which developed spears with a high protodioscin content.

Pollinator insects are required to pollinate flowers in the production of some fruits and vegetables, and strawberries fall into this category. However, the function of pollinators has not been clarified by quantitative metrics such as the duration of pollinator visits needed by flowers. Due to the long activity time of pollinators (approximately 10-h), it is not easy to observe the visitation characteristics manually. Therefore, we developed software for evaluating pollinator performance using two types of artificial intelligence (AI), YOLOv4, which is an object detection AI, and VGG16, which is an image classifier AI. In this study, we used Phaenicia sericata Meigen (green blow fly) as the strawberry pollinator. The software program can automatically estimate the visit duration of a fly on a flower from video clips. First, the position of the flower is identified using YOLO, and the identified location is cropped. Next, the cropped image is classified by VGG16 to determine if the fly is on the flower. Finally, the results are saved in CSV and HTML format. The program processed 10 h of video (collected from 07:00 h to 17:00 h) taken under actual growing conditions to estimate the visit durations of flies on flowers. The recognition accuracy was approximately 97%, with an average difference of 550 s. The software was run on a small computer board (the Jetson Nano), indicating that it can easily be used without a complicated AI configuration. This means that the software can be used immediately by distributing pre-configured disk images. When the software was run on the Jetson Nano, it took approximately 11 min to estimate one day of 2-h video. It is therefore clear that the visit duration of a fly on a flower can be estimated much faster than by manually checking videos. Furthermore, this system can estimate the visit durations of pollinators to other flowers by changing the YOLO and VGG16 model files.

To develop a novel highly robust humidification technology that requires minimal adjustment of operational methods, we investigated the vapor pressure deficit (VPD) conditions that cause water stress in strawberry (Fragaria × ananassa Duch.) plants. The relationship between the rate of relative change in fruit-stalk diameter (RCFSD_t) and VPD conditions, including VPD and rate of change in VPD (VPD′; hPa·min⁻¹) were analyzed by performing multiple regression. A negative RCFSD_t was defined as an indicator that the plant was experiencing water stress. Regression of RCFSD_t on VPD and VPD′ revealed that VPD′ greater than 0.069 hPa·min⁻¹ led to negative RCFSD_t when VPD was 8.0 hPa or higher. In addition, we investigated whether the rate of change in VPD′ (VPD″; hPa ·min⁻²) could be used as an indicator to cease humidification, employing both VPD and VPD′. Analysis of the relationship between RCFSD_t and VPD″ revealed that even if VPD and VPD′ exceeded 8.0 hPa and 0.069 hPa·min⁻¹, respectively, RCFSD_t could not be negative when VPD″ was less than −0.00426 hPa·min⁻². We measured the relative change in fruit-stalk diameter (CFSD_t) of strawberry under the conditions generated by a novel humidification system. The novel humidification system humidified the air when VPD, VPD′, and VPD″ exceeded each threshold for avoiding the condition, where VPD, VPD′, and VPD″ exceeded 8.0 hPa, 0.069 hPa·min⁻¹, and −0.00426 hPa·min⁻², respectively. In practice, there were periods when humidification control was not executed because while VPD exceeded its control threshold, VPD′ and VPD″ were below their respective control thresholds. In addition, the novel humidification system prevented a decrease in CFSD_t during the daytime under the non-humidification treatment.

Autophagy is a conserved system responsible for the degradation of intracellular components and remobilization of nutrients in eukaryotes. Autophagy-like processes have been observed in senescing petals of several plant species. Further, significant expression levels of autophagy-related genes are induced along with an increase in ethylene production in flowers that show ethylene-dependent senescence. However, knowledge of autophagy-like processes in ethylene-independent flower senescence remains limited. In this study, we isolated autophagy-related genes (LhATG5, LhATG6, five LhATG8 homologues, and LhATG10), and analyzed their expression levels during tepal senescence in the Oriental hybrid lily, which shows ethylene-independent senescence. The transcript levels of LhATG5, LhATG6, and LhATG8s, but not LhATG8e, increased as tepals senesced. Furthermore, we observed cellular structures stained with monodansylcadaverine (MDC) dye, which stains acidified vesicular compartments, in lily tepals. MDC-stained structures were observed more frequently in the mesophyll cells of senescing tepals than in pre-senescent tepals. These results suggest that autophagy-like processes are induced as lily tepals senesce. Moreover, the amino acid content gradually increased in tepal mesophyll as the flowers senesced. Such increased amino-acid content was accompanied by a decrease in protein content and an increase in the transcript levels of autophagy-related genes, suggesting the involvement of autophagy-like processes in the production of nitrogenous components resulting from bulk protein degradation. Overall, our data indicated that autophagy-like processes were induced during lily tepal senescence. Further studies on the regulatory mechanisms of autophagy-like processes in ethylene-independent petal senescence may contribute to improvements in flower quality.

Winter heating energy saving is an important issue, not only for reducing production costs, but also for reducing CO₂ emissions during cultivation. This study investigated the effect of root-zone heating using a new root-zone environmental control system (N.RECS) in winter on the growth and flowering of pot flowers and the reduction in heating costs. Bulbous begonia, geraniums, dwarf gerberas, and New Guinea impatiens, which require relatively high temperatures, were used in the present study. This experiment consisted of two plots: a conventional plot (control) with a minimum air temperature of 16°C and no root-zone heating, and a root-zone (RZ) heating plot with a minimum air temperature of 12°C and root-zone heating of 24°C. The results showed that RZ heating significantly promoted the growth and flowering of all tested pot flowers. Heating costs for bulbous begonia, geranium, and dwarf gerbera were reduced by 28–45% in the RZ heating plot compared to that of the control. In New Guinea impatiens, heating costs were not reduced by RZ heating because this experiment started late, in mid-March, and heating costs in the control were low. CO₂ emissions were roughly proportional to energy costs. In conclusion, root-zone heating with the N.RECS enabled the early production of high-quality plants in winter cultivation of pot flowers with relatively high temperature requirements and could contribute significantly to the reduction of CO₂ emissions during the cultivation period by reducing heating costs.