A middle range portable scanning lidar was applied from a 10-m height place to estimate vertical plant area density (PAD) profiles of rice canopy at different growth stages. Voxel-based canopy profiling method was applied for the estimation. Resultant PAD estimates showed over estimation at middle to lower layers with the root mean square errors of 0.36, 0.45, 0.95, 1.81 m²m^-3 in each stage. Laser beam coverage index Ω was calculated and related to absolute errors of PAD to investigate causes of the errors. This showed that lack of the number of incident laser beams into the internal canopy was a major cause of the error. Although the lidar-derived PAD showed overestimation in middle to lower layers, the PAD estimates of upper layers agreed well with the actual values. The accurate PAD estimates of upper five layers could offer good correlation to dry weight of ears with R²=0.988.

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The residual product of anaerobic digestion, called digestate, contains high concentrations of inorganics like nitrogen and potassium, and it can be used as liquid fertilizer for crop production. When using digestate as nutrient solution for hydroponic culture, however, plant growth is often strongly inhibited due to root damage. This damage may be caused by several potential inhibitors contained in the digestate. In this study, we aimed to identify the main source(s) of inhibition when cucumber seedlings were grown hydroponically with digestate as nutrient solution, focusing on nitrogen form, particulates, microorganisms, and pH of the digestate. When inorganic nitrogen in the digestate was provided as a high concentration of NH₄⁺ (260 mg NH₄⁺-N L^-1), all the seedlings' roots died, regardless of whether particulates and microorganisms were present. The inhibition of shoot and root growth in NH₄⁺ was mitigated by decreasing the pH of the digestate, and this effect was enhanced by inactivating the microorganisms in the digestate. When the nitrogen form in the digestate was a low concentration of NH₄⁺ (20 mg NH₄⁺-N L^-1) and a high concentration of NO₃^- (110 mg NO₃^--N L^-1), the dry weights of shoots and roots were much greater than those grown with only NH₄⁺ (130 mg NH₄⁺-N L^-1). This effect was enhanced by removing particulates and microorganisms from the digestate by membrane filtration. These results indicate that decreasing NH₄⁺ and pH and increasing NO₃^- in filtered digestate can improve the suitability of digestate as a nutrient solution for plant hydroponics.

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Acoustic emission (AE) was measured at the stem of a cherry tomato in spray culture, and the change ratio of AE before and after irrigation RD was calculated. The AE event rate highly fluctuated, and the number of detected AEs was insufficient to calculate RD at all times. However, the change in RD was negatively related to the increase and decrease in irrigation time for a short duration (less than 5 h), while the change of RD contrarily showed positive relationship with the irrigation time in a long term. Furthermore, the yield and sugar content of the fruits were related to the average value of RD. These results suggest that RD might improve the efficiency of tomato cultivation in a horticultural facility.

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