The Japanese Society of Agricultural, Biological and Environmental Engineers and Scientists has been leading research guided by the Speaking Plant Approach (SPA) (Hashimoto, 1989), which means that plant eco-physiological information is obtained using sensing technologies and cultivation environment is optimally controlled on the basis of these diagnostic results. These technologies should be core factors driving realization of data-driven smart agriculture. Based on this viewpoint, we, Biological Measurement Subcommittee, planned this special issue “Sensing Technologies of Plant Ecophysiological Information Contributing to Data-driven Smart Agriculture.” One opinion paper, three original research papers, and one short communication have been published herein. The contents are as follows:

　Kitano et al. (2022) propose the original concept of an Internet of Plants (IoP), and provide a valuable opinion on agricultural innovation through visualization, utilization, and sharing of plant eco-physiological information. Fujiuchi et al. (2022) focus on real-time monitoring of photosynthesis and transpiration of whole plants, which can be regarded as the most important eco-physiological information, and examine new calculation methods for improved accuracy. Nurmalisa et al. (2022) also focus on improving accuracy of a photosynthetic measurement chamber method, where they also analyze and improve distribution of airflow in the chamber. Ono et al. (2022) focus on respiration of fruits as important eco-physiological information, and develop a new closed chamber system for easily measuring respiration rate of intact fruits. Yasutake et al. (2022) focus on the carbon partitioning characteristics in a plant body, as an eco-physiological correlate of plant growth, and show these characteristics' dependence on leaf growth stage.

　We hope that this special issue will provide the readers with broad insights into this research field. Finally, we would like to thank all authors who submitted their works to this special issue.

Agriculture can be defined as a kind of the “Monodzukuri” industry (manufacturing industry) that utilizes crop photosynthesis to generate products (photosynthates). Agricultural production entirely depends on crop physioecological processes, such as photosynthesis, transpiration, translocation, and vegetative and reproductive growth, which are strongly affected by environmental conditions and farming technologies in crop fields and greenhouses. Since the beginning of agriculture, visualizing time-series data on crop physioecological processes and applying these data in everyday farm work have been impossible because of the difficulties in measuring crop physioecological processes. This invisibility of crop physioecological processes hinders not only farmers’ inventive ideas but also Information and Communication Technology (ICT) and Artificial Intelligence (AI) to drive evolution and improvements in everyday farming to achieve demand-oriented crop production. Therefore, we propose an innovative concept, the “Internet of Plants (IoP),” as a regional information infrastructure for smart agriculture. This concept is driven by a cloud computing system (IoP Cloud), which is equipped with physioecological and farming support AI engines that visualize, functionalize and share highly explainable information about crop physioecological processes and farming technologies. The IoP is expected to facilitate the bottom-up evolution of agriculture, which will be driven by smart farmers empowered by IoP functions.

We developed an open-chamber system to monitor photosynthesis and transpiration of whole plants with high time resolution. The system may be promising for environmental controls based on plant stress diagnoses; however, the conventional method used to calculate net CO₂ exchange rate (NCER) should be improved to avoid overestimation and underestimation of NCER under greenhouse conditions. To calculate NCER, the conventional method uses the difference in CO₂ concentration between inlet and outlet air of the open-chamber system. The CO₂ concentrations are alternately recorded by a single sensor every 5 min. With this method, inherently delayed response of outlet CO₂ concentration causes overestimation and underestimation of NCER especially when CO₂ concentration in a greenhouse continuously increases or decreases. We developed a calculation method considering the number of air exchanges, and then compared the NCER calculated with the new method (NCER_ex) to that calculated with the conventional method (NCER_conv). During the night, when the CO₂ concentration in the greenhouse continuously increased, NCER_ex (with the average NCER_exs being －0.78, －0.71, and －0.96 μmol s^-1 at different timeframes) was lower than NCER_conv (with the average NCER_convs being －0.30, －0.32, and －0.65 μmol s^-1). The higher values of NCER_conv indicated overestimation by the conventional calculation method during the night. The total amount of night respiration calculated using NCER_ex was 2.2 times greater than that calculated using NCER_conv. The new calculation method is useful to measure the NCER of whole plants validly under greenhouse conditions.

A numerical simulation was performed to predict the distribution and uniformity of the airflow in a newly developed photosynthesis chamber. It is a semi-closed hanging-type chamber with three exhaust fans placed at the top to maintain airflow. The chamber's bottom (area: 0.55 m²) is fully opened for outside air inflow. For model validation, we measured both horizontal and vertical air velocity inside the chamber. After model validation, multiple sizes of transparent plates were applied just below the top of the chamber to investigate the effect of the plates on the uniformity of airflow. The simulation's results showed a diminishing stagnant area at the higher part of the plant, reaching a more uniform airflow distribution, with a Coefficient of Variation (CV) of 9.1% (full plate), 12.2% (half plate placed near the fans), 50.9% (without a plate), 45.5% (half plate placed on the opposite side of the fans), and 44.0% (small plate placed opposite with the fans). From simulation results, mounting a full-size transparent plate and a half-size one near the fans can significantly help to produce uniformair velocity distribution at the plant canopy.

We designed and constructed a light-shielding, closed fruit chamber system that can easily measure the respiration rate of intact fruits. Intact strawberries (Fragaria×ananassa Duch. cv. Fukuoka S6) were placed in the newly-developed system, and the change in CO₂ concentration was measured. It was observed that this method was a valid means of determining the change in the CO₂ concentration. Also, the respiration rate of the intact fruits could be evaluated appropriately by fitting a theoretical equation to the change in CO₂ density for 180 s after closing the chamber measured by newly-developed system and obtaining the initial slope. This method was applied to intact strawberries at different ripening stages, and the effect of on the respiration rate was investigated. As the ripening stage progressed and the volume of fruits increased, the respiration rate per fruit at each ripening stage also significantly increased. However, there was no significant difference in the respiration rate per unit fruit volume at each ripening stage. These results suggest that newly-developed system is effective in measuring the respiration rate per fruit. This system would contribute to the comprehension of intact fruit respiration and photosynthate translocation to fruits.

In order to analyze the carbon partitioning characteristics of different organs (leaf, bulb, and root) of Chinese chive and their dependence on leaf growth stage, we applied the ¹³C-labelled tracer method to plants grown at two growth stages (mature and immature) in a commercial, soil-based greenhouse. The ¹³C concentration of plants sampled just before harvest was almost similar among the three organs, indicating a strong dependence of the ¹³C content on the dry weight of the respective organs. As a result, the ¹³C partitioning ratios for the leaf, bulb, and root were calculated as 43%, 17%, and 40%, respectively. On the other hand, the ¹³C concentration was slightly higher in the leaves of immature plants compared to the bulbs and roots. Therefore, the ratio of the ¹³C concentrations of the underground (bulbs and roots) to shoot organs (leaves) was significantly lower for the immature plants compared to mature plants. These results suggest that carbon is more effectively distributed in the leaf compared to the bulb or root of immature plants owing to active leaf growth. Thus, the characteristics of carbon partitioning in Chinese chive varied depending on the stage of leaf growth.

Broccoli (Brassica oleracea L., cv. MKS-B107) is a highly perishable vegetable and it has a relatively short shelf life. Storage conditions and postharvest treatments are the important factors to determine the postharvest quality and the shelf life of broccoli. This study indicated the effects of 1-methylcyclopropene (1-MCP) and temperature on the quality of broccoli during storage period. Broccoli was treated with air (as control) and 1.0 μL L^-1 1-MCP for 15 h at 15℃, and then was stored at 10℃ and 0℃ for 25 days and 50 days, respectively. The results showed that 1-MCP treated broccoli was in good quality during 50 days of storage at 0℃ and 20 days of storage at 10℃ compared to the control broccoli. The present study indicated that 1-MCP treatments suppressed the respiration rate and ethylene production, reduced the chlorophylls and ascorbic acid degradation and inhibited the color changes of broccoli during storage. Broccoli treated with 1.0 μL L^-1 1-MCP and stored at 0℃ represented remarkably better quality compared to broccoli of the other treatments.

This study identified the effects of temperature variations during the light period on the growth of hydroponic leaf lettuce grown under artificial lighting. The temperature during 16-h light period and 8-h dark period was set at 23℃ and 18℃, respectively, as a control. In experiment 1, the temperature was quickly risen (QR) at the start of the light period (SL), kept at 23℃ during the first half of light period, and then slowly decreased during the second half of light period, as QRSL treatment. In experiment 2, the temperature was increased gradually during the first half of light period, kept at 23℃ during the second half of light period, and then quickly dropped (QD) at the end of the light period (EL), as QDEL treatment. The temperature during the dark period was set to 18℃ in both treatments. Both treatments increased fresh and dry weights of shoot. The QRSL treatment increased the growth rate and decreased the tipburn incidence. Meanwhile, the QDEL treatment decreased the growth rate because of the tipburn occurrence before harvesting. It was possible that these effects of temperature variations include the effects of vapor pressure deficit fluctuations since humidity varied with temperature.

In this study, we investigated the differences between Japanese tomato cultivar ‘CF Momotaro York’ and Dutch tomato cultivars ‘Endeavour’ and ‘Managua’ in characteristic of growth rate and temperature of fruit with single-truss using the highdensity planting system. Total dry weight in Dutch was higher than that in Japanese cultivars. Although the fresh fruit yield was not significantly different between Japanese and Dutch cultivars, Japanese cultivar showed higher total soluble solids content and higher distribution ratio of dry matter to the fruit than those in Dutch cultivars. No differences were observed in the required time for fruit development between the Japanese and Dutch cultivars. Fruit temperature at the end of development was higher in Dutch cultivars than Japanese cultivar, which induce the yield reduction in Dutch cultivars. These results indicated that the Japanese cultivars have the characteristics of efficient fruit production compared to Dutch cultivars under low-node order pinching and high-density planting system.

To understand the effects of plant growth regulators (PGRs) on the aromatic compounds of ‘Kyoho’ grapes, changes in fruit quality and aroma compound content at different growth and developmental stages were investigated after treatment with gibberellin A₃ (GA₃) at the flowering stage and with abscisic acid (ABA) at veraison. Thirty compounds were identified in peels and pulps, with 3-hexenal, (E)-2-hexenal, ethyl acetate, ethyl butyrate, and limonene being the main compounds. GA₃ treatment inhibited ester synthesis, including ethyl acetate, ethyl butyrate, and terpenes, as well as limonene, while promoting the synthesis of C₆ compounds, including 3-hexenal and (E)-2-hexenal in peels compered to no treatment. ABA treatment could promote the synthesis of most aroma compounds, especially esters, terpenes, and alcohols in peels and pulps at the ripening stage. Previously, it has been suggested that ABA reduce the negative influence of GA₃ on aroma compound synthesis. The combination of GA₃ and ABA can obtain high quality berries in ‘Kyoho.’ These results provide a reference for further studies on the effects of plant regulators on aroma compounds synthesis in grapes.

The regular pruning in the cultivation of kiwifruits (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson) is necessary to maintain fruit productivity and quality, and the pruning generates a significant amount of wastes. In this study, allelopathic activity of pruning branch wastes was investigated to discover beneficial applications of the wastes. An aqueous methanol extract of pruning branch wastes inhibited the growth of cress (Lepidum sativum L.), lettuce (Lactuca sativa L.) and alfalfa (Medicago sativa L.) and weed species, Lolium multiflorum Lam., Vulpia myuros (L.) C. C. Gmel., and Echinochloa crus-galli (L.) P.Beauv. Significant reductions in the growth were observed as the extract concentration was increased. The effectiveness of the extract varied by these plant species. The extract was then separated by several chromatographies with monitoring the inhibitory activity and the most active substance in the extract was isolated. The substance may work as an allelopathic agent because of its growth inhibitory activity. The present research suggests that kiwifruit pruning branches wastes may have allelopathic activity and be potentially useful for weed control in several agricultural field conditions. Further investigation is, however, necessary in the field conditions to develop the practical application of the pruning wastes.

We examined the effects of aluminum chloride (AlCl₃) on the organogenesis of Cymbidium Sweet Waffle ‘Tarte’ from inoculated protocorm-like bodies (PLBs) and of Cymbidium kanran ‘Murotonishiki’from rhizomes to determine the appropriate concentration of AlCl₃ for organogenesis. The explants were cultured in modified MS media supplemented with various concentrations of AlCl₃ and maintained at 25±1℃ and a 24 h light period for 6 weeks for C. Sweet Waffle‘Tarte’and 10 weeks for C. kanran‘Murotonishiki.’ In C. Sweet Waffle ‘Tarte’ 1.0 mg L^-1 AlCl₃ significantly increased PLB formation, shoot formation from PLBs and root formation from shoot. In C. kanran ‘Murotonishiki,’ 1.0 mg L^-1 AlCl₃ induced shoot from rhizome and developed roots from shoot. On the other hand, high concentration of AlCl₃ (10 mg L^-1) resulted the formation of protocorm-like shoots in both Cymbidiums. We concluded that the concentration of 1.0 mg L^-1 AlCl₃ is optimum for the root and shoot formation of the Cymbidium orchids.

Seasonal variations of health-promoting properties of fruits have been known to depend upon environmental conditions and plant growth parameters; therefore, their trend lines need to be predicted. This study was aimed at finding out the impact of seasonal variations, induced by changes in the environmental conditions, to health-promoting properties of cherry tomato in long-term cultivation and at developing their statistical predictive models. Both total ascorbic acid content and antioxidant activity increased gradually during research period regardless the experimental site or year. Polyphenol content was the lowest at the initial date of study but it depended on experimental years. In addition, by the stepwise multiple linear regression analysis, we were able to develop a predictive model for health-promoting properties by using environmental data and plant growth/agronomic quality traits. In the best-fitted model, the combinations of data set and the data period were different for each compound. Furthermore, health-promoting properties can be predicted with high accuracy by using several environmental data 2 or 3 weeks before the harvest time and the growth/agronomic quality data at the harvest day. Thus, we conclude that constructed models may contribute to a good practice for the prediction of health-promoting properties of tomato.