In a previous study (Tazuke et al., 2015), a sudden decline in the respiration rate was observed in cucumber fruits on vines under prolonged darkness. The study suggested that this was caused by an abrupt cessation of photoassimilate translocation to the fruit because the time course of the decline in the respiration rate was similar to that of fruits treated by heat-girdling of the peduncle. Here, we examined this hypothesis by detailed analyses of the time course of fruit respiration rates in plants treated prolonged darkness, cutting the peduncle, heat-girdling the peduncle, and total defoliation of the plant. All these treatments upregulated candidate marker genes of sugar starvation, CsSEF1, asparagine synthetase gene, and CsFDI1. Respiration rates of the fruits of cucumber plants (Cucumis sativus L.; cv. Tokiwa and cv. Tokiwa Hikari 3A) were measured in a growth chamber controlled at 25°C. Under prolonged darkness, the fruit respiration rate began to decline at about 12 h after the start of darkness. After some transition period, the fruit respiration rate was described well by the equation: y = a exp(–bt) + c, where y and t are the respiration rate and time, respectively, and a, b, and c are constants. In other treatments, the fruit respiration rate after some transition period following the treatment was also described well by the same equation. The value of the time constant of the decay, b, was about 0.003 min–1 under prolonged darkness and defoliation, whereas it was about 0.007 min–1 after cutting and heat-girdling of the peduncle. The similarity of the value of b with prolonged darkness and defoliation supports an abrupt cessation of photoassimilate translocation to the fruit. Possible explanations of the higher values of b in cutting and heat-girdling the peduncle are discussed.
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