Gradient of Growth, Spontaneous Changes in Growth Rate and Response to Auxin of Excised Hypocotyl Segments of Phaseolus aureus

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Abstract
Spontaneous growth was studied in excised mung bean (P. aureus Roxb.) hypocotyl segments. Measurements were made with a growth-recording apparatus using displacement transducers on single 5-6 mm samples excised from the growth zone immediately below the hook. Even for a given zone and under controlled experimental conditions, there are differences in the spontaneous growth of individual explants. In every case, 2 phases of endogenous acceleration are found at 15-20 min, and 120-150 min after excision. Accelerations were separated by steady growth phases. Knowledge of the spontaneous growth curve appears important for the choice of the time of application of experimental stimuli. Auxin was added at various times after excision (0-6 h). The classical biphasic response to auxin was obtained when the hormone was added during a steady phase of growth. The response was difficult to interpret when the hormone was added during an acceleration phase. Spontaneous and IAA induced growth were studied along the hypocotyl. Spontaneous growth rate and growth potential revealed by IAA changed markedly along the growth gradient. The nature of spontaneous changes according to experimental time and state of differentiation of the cells is discussed.