Aerial O2 transport through leaves for root respiration was examined in relation to water uptake in cucumber plants (Cucumis sativus L.) by using an airtight hydroponic system and by 18O2 tracing. Root respiration clearly responded to aerial O2 to which, in darkness, the leaves were exposed. Root respiration in an O2-deficient nutrient solution increased with a higher aerial O2 concentration and decreased with a lower aerial O2 concentration. Water uptake was promoted in darkness with aerial O2, and appeared simultaneous in dynamics with root respiration. For tracing aerial O2 transport through leaves for root respiration, leaves were exposed to aerial 18O2, and H2 18O produced by respiration in roots was analysed. A significant differ ence In 18O2 isotopic abundance in roots was noted between 18 and untreated plants at the 1% level. On the other hand, no aerenchyma was present in the cortex of cucumber roots. From these facts, we conclude that aerial O2 can be transported through leaves and gas-filled intercellular spaces among cells for root respiration in cucumber plants grown in an O2 root environment, and this results in improvement of water uptake in darkness.
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