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The plants of Nicotiana tabacum L. cv. NC2326 were germinated in 10 ㎝ D×20 ㎝ H polyethylene pot, and sand-cultured with Hoagland solution near by the window of laboratory room(26±5℃). The growing plants were sprayed with various concentrations of ABA around 9:00 a.m. once in every two days for 12 weeks in summer. As the results, frequency of stomatal number, stomatal opening, chlorophyll content, respiration rate, and protein content in the leaves were decreased with the increasing of concentrations of ABA, respectively. The plant growth was inhibited by exogenous ABA, but leaf abscission was not found during the experimental period. The ratio of three to one in chlorophyll a to b was not altered by exogenous ABA. All the stomata were closed within three minutes by 100 ㎍ ㎖^-1 ABA and within seven minutes by 1-10 ㎍ ㎖^-1 ABA after the spraying of ABA, and then reopended after a few hours in 1-10 ㎍ ㎖^-1 ABA and after 24 hours in 100 ㎍ ㎖^-1 ABA. The polar movement of chloroplast within the guard cells was found in the higher concentrations of 10 and 100 ㎍ ㎖^-1 ABA, but not found in the lower concentrations than 1 ㎍ ㎖^-1 ABA. During the night and weak light, it was found that the inhibition of respiration rate by the higher concentration of ABA was owing to firstly the stomatal closure by the spraying of ABA and secondly the decrease of stomatal frequency by the inhibition of stomatal development with exogenous ABA for the long period of 12 weeks. In the band number of leaf protein by the electrophoresis, most of the protein bands were disappeared by the higher concentration of 100 ㎍ ㎖^-1 ABA, but were not altered by the lower concentraiton of ABA in comparison with control.
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