The phloem exudate (PE) prepared from cotyledons of Ipomoea tricolor seedlings that had been exposed to a single 16 h dark period induced flowering in cultured apices excised from non-induced seedlings. When the PE was separated to 4 fractions by dialysis, low (‹1,000) and high (10,000–25,000) molecular weight (HMW) fraction had flower-inducing activity. To characterize the HMW fraction, the fraction was treated with some disintegrating enzymes. The flower-inducing activity was disappeared by proteinase K treatment. Therefore, HMW active substance(s) may be protein. When the HMW fraction was fractionated by gel filtration chromatography, the molecular weight of active fraction was ca. 23,000 dalton. Simultaneous addition of low molecular weight fraction could not promote inducing activity of HMW fraction. The PE prepared from I. tricolor seedlings grown under continuous light conditions (CL-PE) inhibited flowering in cultured apices excised from induced seedlings. When the CL-PE was added to assay medium simultaneously with HMW fraction, flowering was inhibited.

View full abstract

Under climate change, frosts in agricultural fields often bring serious damage to crops. Accordingly, it is keenly desired to improve frost protection methods based on the quantitative evaluation of their effects. The method for quantifying the thermal effect brought by operation of frost protection methods was newly developed based on analysis of leaf heat balance, where leaf boundary layer conductance was continuously determined in field conditions by applying artificial model leaves and a mathematical model. Reliability of the newly developed method was demonstrated in a tea field under the operation of a frost protective fan. The frost protective fan operating at the upper layer of a thermal inversion with strong radiative cooling enhanced convective heat transfer with increased the leaf boundary layer conductance and the ambient air temperature by supplying leaves with warm air. As a result, sensible and latent heat fluxes were significantly increased, which brought a rise of leaf temperature. This thermal effect brought by the fan operation was remarkably fluctuated due to the fan oscillation and advection from the surrounding field, while increasing gradually until just before sunrise when the lowest leaf temperature was observed. Thus, the newly developed method based on the heat balance analysis can be expected to contribute to the improvement of frost protection methods.

View full abstract

Ralstonia solanacearum is a soil-borne pathogen that causes bacterial wilt disease in many plants worldwide. The wild-type strain undergoes a phenotypic conversion (PC) spontaneously, shifting from a pathogenic to a non-pathogenic form. There are still uncertainties about the growth behavior of the wild-type strains and PC mutants when both strains are mixed in cultures. We investigated the bacterial competition between both strains in liquid medium. When bacteria were mixed and cultured statically in BG broth, the growth of only the wild-type strain was reduced at the later period (16–24 d) of culture. However, this bacterial competition was not observed in 100-fold diluted BG broth, suggesting that there are factors other than competition for nutrients. When each strain was individually inoculated in bacterial culture filtrates, the PC mutant could grow in the culture filtrates. However, the wild-type strain could not grow only in the culture filtrate of the PC mutant. The growth inhibition of the wild-type strain was also observed in the heat-treated culture filtrate of the PC mutant. These results indicate that the bacterial competition between the wild-type strain and the PC mutant in BG broth was due to changes in the culture components by the PC mutant.

View full abstract

Ralstonia solanacearum causes bacterial wilt in tomatoes and undergoes spontaneous phenotypic conversion (PC) from a wild-type pathogenic to non-pathogenic form. We investigated the suppressions of bacterial wilt disease of the tomato cv. Micro-Tom by using PC mutants. Tomato seedlings were pre-inoculated with a PC mutant suspension and then challenge-inoculated with the wild-type strain. Seedlings pre-inoculated with the selected PC mutant showed suppression of bacterial wilt in both soil and hydroponic tests. We also analyzed the expressions of pathogenesis-related (PR) protein genes in the tomato to elucidate the mechanisms underlying the disease control by the PC mutant. The expressions of basic PR proteins were induced in tomato roots by inoculation with the PC mutant. Our results suggested that an induced defense response is involved in the biological control of bacterial wilt in tomatoes by the PC mutant. In addition, there was no effect on the growth, yield or quality of the tomato plants inoculated with the PC mutant. We also used 18 PC mutants to investigate the suppression of bacterial wilt in tomato plants. Some strains showed effective suppression against the disease. We suspect that the suppression of bacterial wilt in tomatoes by PC mutants could become a practical biological control method.

View full abstract

Winter ‘fusekomi’ forcing culture of asparagus is becoming popular in Japan because the method can make production of asparagus possible during cold season. However, there are some problems such as color of the spear is pale, and rutin content is lower compared to spring harvest due to the low light intensity, especially in the production area which has much snow and short sunshine. The objective of this study was to clarify the effect of supplemental lighting on the yield, rutin content, sugar component (fructose, glucose, sucrose), and the color of spears. The experiments were conducted by using different irradiation time and different numbers of fluorescent lamps hanging on the tunnel poles over the cultivation bed on the winter ‘fusekomi’ forcing culture. Compared to the control, rutin content was significantly increased under supplemental lighting plots. No significant difference or negative impact was observed in sugar contents and yield on each plot. Moreover, spear color also appeared to be better under supplemental lighting than that of the control. These results suggested that supplemental lighting was effective to improve the quality of asparagus spears (such as rutin contents, spears color), especially for the production area that has low light intensity or in short day conditions.

View full abstract

This study aimed to investigate the effects of hydroponics culture involving deep flow technique (DFT) on the growth of edible Opuntia, and drought stress on spine occurrence on daughter cladodes. Edible Opuntia cladodes were grown by hydroponic and pot culture. In pot culture experiments, plants were watered at different frequencies (once a week, once every 2 weeks, and once every 4 weeks). The total fresh weight of cladodes grown by DFT increased and daughter cladodes appeared, thus indicating that edible Opuntia can be grown by DFT. The number of daughter cladodes did not significantly differ between hydroponic and pot culture, but the growth rate of first daughter cladodes under DFT and high drought stress became slower than that with other treatments. We counted the number of spines on daughter cladodes, which is one of the most undesirable characteristics in edible cacti, and it was the greatest on cladodes under high drought stress and lowest on cladodes under DFT. Our results suggest that drought stress affects daughter cladode growth and the number of spines on daughter cladodes. Thus, controlling water availability is important for improving edible cactus quality.

View full abstract