Stem cuttings of Populus nigra were Ircated with 10 and 100 mg/1 each of IAA, IBA, 2,4-D and NAA at one month intervals and observations were recorded for the morphophysiological status of the liranches, their starch content and their rooting response. -The first phase characterized by delayed, sbort and scarce roots and the high starch content of cuttings coincided with Ibe onset of winter dormancy in November lasting througb February. It was followed by a pbase of vigorous rooting and low starch content of cuttings coinciding with the renovation of growth activity in February lasting tbrongh October, except in April and May when rooting was more or less completely nullified. -Tbe ])oor rooting in winter was caused by low activity of bydrolyzing enzymes not mobilizing starch into soluble sngars; and profuse rooting during active growth period by high activity of hydrolyzing enzymes caused by endogenous anxin, resulting in mobilization of reserved food materials necessary for the initiation and development of roots. The low rooting in Aprit and May is ascribed to the fact tbat l)ulk of tbe mobilized food was nsed up in tbe growtti of sprouted branches leaving very little for rooting wtien tbese cuttings were planted. -The seasonal changes in the effectiveness of exogenonsty applied auxins also appear to be related with tbe level of endogenous auxin. In June endogenous auxin was high dne to high meristematic activity, tbe exogenously ajiplied auxins raising it lo supra-optimal levels that were inhibitory. On the other hand, in October exogenously applied auxins enbanced rooting by raising it to an o])timal level as the jjroduclion of endogenous auxin had been decreasing gradually due lo lowering temperatures. -The results demonstrate that anxin effect on differential rooting with season in this plant is determined by the physio-morphological status of the brandies tbat govern the prodnction of endogenous auxin and is mediated primarily through its effect on mobilization of reserve food materials caused by enhanced activity of liydrolytic enzymes.• i. u .-«"
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