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The effects of cyclic adenosine 3':5'-monophosphate (cAMP) on the growth of Avena coleoptile segments over 4 to 10 hours were monitored with a position sensing transducer. At pH 6, cAMP (0.1 mm with and without 2.5 mm glucose; or 2 mm alone) or dibutyryl cAMP (0.1 mm) was added at the beginning of the experiment, or after about 1 hour or after about 6 or 7 hours. Under all conditions tested, cAMP compounds had little or no effect on coleoptile segment elongation. Inasmuch as cAMP does not duplicate the rapid and vigorous elongation obtained with 2 mum auxin, the hypothesis that cAMP is a mediator of auxin activity is not supported by experimental evidence in this system. This conclusion is dependent upon the assumption that the cAMP compounds penetrated the tissue.

The comparative effects of metabolic inhibitors on acid- and auxininduced growth in oat (Avena sativa L. var. Victory) coleoptile segments have been examined. Acid (pH 4)-induced growth in both peeled and unpeeled segments is inhibited by 1 millimolar KCN when added at the time of acidification. KCN inhibits total acid-induced growth by 59 and 76%, respectively, in peeled and nonpeeled segments during the first 60 minutes. The growth rate of cyanide-treated tissue drops to zero or near zero in both peeled and nonpeeled segments during this period. Cyanide inhibition of total acid-induced growth in peeled segments at pH 5 is even more severe, amounting to about 80% during the first 60 minutes. The possibility that inhibition by cyanide may be caused by some nonspecific effect of the inhibitor on a process other than respiration, e.g. turgor reduction due to membrane damage, has not been ruled out. Acid-induced growth is also inhibited by 3 millimolar sodium fluoride and by anoxia. In unpeeled segments total pH 4-induced growth is inhibited 73% by sodium fluoride and 38% by anoxia during the 1st hour. Possible corrections to the above inhibition percentages which may be necessary due to the sensitivity of basal growth to inhibitors are discussed. Cyanide was found to inhibit auxin-induced growth much more rapidly than acid-induced growth. These data suggest that acid growth may be dependent on respiratory metabolism but to a lesser degree than is auxin-induced growth. If the acid growth theory of auxin action is correct, it appears that there may be two steps in the growth process which are dependent on respiratory metabolism: (a) auxin-induced proton pumping which is highly sensitive to respiratory inhibitors; and (b) acid-mediated wall loosening which is moderately and perhaps indirectly sensitive to respiratory inhibitors.

The effects of cordycepin (3'-deoxyadenosine), an RNA synthesis inhibitor, on auxin-induced elongation in Avena coleoptile segments were studied with a position-sensing transducer. Cordycepin rapidly inhibited auxin-stimulated growth in the coleoptile segments whether added before, at the same time as, or after, the 2 mum auxin treatment. Midcourse additions of 100, 50, and 25 mug/ml cordycepin inhibited auxin-promoted elongation in an average of 18, 22, and 35 minutes, respectively. Additions of cordycepin before or at the same time as the auxin treatment partially inhibited the magnitude of the subsequent auxin-promoted growth but did not appreciably alter the latent period of the auxin response. It was concluded that if cordycepin is inhibiting the synthesis of RNA required for growth, the decay time for this RNA may be considerably shorter than that suggested in the literature from actinomycin D experiments. Preliminary kinetic evidence indicated that cordycepin does not inhibit auxin-induced elongation by acting as a respiratory inhibitor. Studies in mung bean shoot mitochondria demonstrated that cordycepin has no effect on respiration, respiratory control, or ADP/oxygen ratios.

An angular position-sensing transducer was used to make continuous measurements of acid-induced elongation of Avena sativa coleoptile segments. Elongation rates at pH 4.5 (5 mm succinate buffer) were about 5-fold greater than those at pH 6.0. Buffered 0.1 mm abscisic acid produced a partial decrease of the growth rate. Pretreatments with abscisic acid buffered at pH 6.0 usually caused a further reduction of the elongation response when the coleoptile segments were subsequently placed in buffer at pH 4.5 containing abscisic acid. Abscisic acid did not completely prevent the pH effect in any of these experiments, and the brief latent period of the pH response was not affected by abscisic acid treatments. At pH 4.5, where the inhibitory effect of ABA was maximum, low pH-induced elongation was also inhibited by KCN and HgCl(2). These results suggest that pH-(4.5) induced elongation in this system may be dependent on some metabolic processes and that abscisic acid-induced inhibition of this elongation may involve an interaction with these processes.

Experiments on Xanthium leaf discs were carried out to determine whether concomitant photoreactivation could be detected and if so, to compare its effects with photoreactivation due to post-irradiation treatments. Attempts mere made to simulate certain intensities of unfiltered solar radiation. A small but definite amount of simultaneous reactivation was observed. The dose reduction factor averaged 0.57. It was not clear whether the reactivation was due mainly to direct photoreactivation, photoprotection, or to heating effects. A much larger amount of reactivation due to post-irradiation treatments (direct photoreactivation) was noted with the dose reduction factor averaging 0.30.

An angular position sensing transducer was used to make continuous measurements of elongation of a column of Avena sativa coleoptile segments. Elongation stimulated by 2 mum indoleacetic acid was inhibited by 0.1 mm abscisic acid with a latent period of about 4 or 5 minutes at pH 6.0, 30 C. Full growth inhibition was not established until about 1 hour after the addition of the abscisic acid. The same degree of final growth inhibition could be obtained under the above conditions using 10 muM and 1 muM abscisic acid, but the latent period was longer. Pretreatments with abscisic acid affected the growth rate but did not extend the latent period of a subsequent response to auxin. The short term kinetics of inhibition by abscisic acid were not similar to those of any of the inhibitors of RNA and protein synthesis tested in this system.