Leaf photosynthesis is a powerful indicator for evaluating growth and tolerance to environmental stresses in tropical fruit trees; however, its measurement requires an expensive system. To examine the applicability of a new, low-cost, closed system for leaf photosynthetic measurement in tropical fruit trees, the photosynthetic rates of each two genotypes of 16 species were measured simultaneously using closed and open systems following accuracy evaluation by regression analysis with values from a standard open system as an explanatory variable. For all measurement combinations, significant and strong positive correlations were obtained with “Fair” accuracy. Photosynthetic rates tended to be overestimated in the closed system. Significant and strong positive correlations were observed for each species. Accuracy differed among species and measurement periods, with a tendency toward inferior accuracy at lower rates. The accuracy was sometimes inferior (“Poor” or “Fair”) for seven species, whereas the remaining species showed generally higher accuracy (“Good” or “Excellent”). Although the accuracy was “Good” for three species, the relationship between the two measurements varied between genotypes and periods. For seven species, the universal regression line was adopted for the relationship between two measurements, consequently demonstrating that the closed rapid gas-exchange system facilitated accurate (“Good”) and rapid ≈40 s/record) measurements.

　The reutilization of hydroponic nutrient solutions in agriculture is important for increasing the utility of limited water resources. It is necessary to remove the contaminating plant pathogen before reusing the culture solution. Damping-off, caused by Pythium aphanidermatum, is a major disease in hydroponic and soil cultivation systems. Strategies to control P. aphanidermatum primarily involve chemical fungicides that are not effective in hydroponic cultivation systems. Therefore, novel methods are needed to control this pathogen. In this study, we investigated the inhibitory effects of ultraviolet-light emitting diode (UV-LED) irradiation (265, 280, and 300 nm) on P. aphanidermatum. UV-LED irradiation significantly and permanently inhibited the growth of P. aphanidermatum mycelia in water. Fungal DNA degradation was observed as early as 5 min after irradiation with UV-LED (280 nm), and fungal mycelia were unable to infect cucumber plants after being irradiated for 5 min with UV-LED (280 nm). These results suggest that UV-LED irradiation can control damping-off caused by P. aphanidermatum and be used for the recycling of hydroponic nutrient solutions.